Cover Crops and Tillage: Effects on Soil Chemical Properties and Rice Yield

Abstract

Mechanical soil intervention with a chisel in cover crops (CCs) is a promising strategy for minimizing the superficial compaction of soil in a no-tillage system (NTS). The objective of this study was to evaluate the effect of mechanical chiseling associated with successive crops and preceding cover crops to assess grain yield and changes in soil chemical properties after rice cultivation for two consecutive years. The experimental design comprised randomized blocks in a 5 × 2 factorial scheme with four replications. The treatments consisted of five cover crops (Cajanus cajan, Crotalaria juncea, Urochloa ruziziensis, Pennisetum glaucum, and fallow), with or without soil mechanical chiseling. Soil samples from the 0–5, 5–10, 10–20, and 20–40 cm layers were collected, and the chemical properties were analyzed after rice cultivation. Under U. ruziziensis, the chemical properties were improved by mechanical scarification; however, the initial acidity increased in the 5–40 cm layer. The cultivation of P. glaucum and C. juncea prior to rice, regardless of scarification, improved the soil chemical properties in the 0–40 cm layer. Mechanical soil scarification and the use of C. cajan as a cover crop increased the grain yield of upland rice by approximately 552 and 1454 and 1330 kg ha⁻¹ compared to the P. glaucum and U. ruziziensis, respectively.

1. Introduction

Due to its widespread daily consumption, rice cultivation is essential for economic, social, and nutritional reasons. In Brazil, the annual per capita consumption of rice varies from 29 to 50 kg⁻¹ per year [1]. Rice grains are excellent sources of carbohydrates in their natural form. In addition, it can be an essential source of proteins, minerals (mainly phosphorus, iron, and calcium), and B vitamins, B1 (thiamine), B2 (riboflavin), and B3 (niacin). In the 2022/2023 growing season, an estimated total area of 1.7 million hectares was used for rice production in Brazil, with an output of 10.6 million tons and an average grain yield of 6300 kg ha⁻¹ CONAB [1].

Widespread compaction problems in the soil surface layers have been observed in different production systems under the no-tillage system (NTS), where the soil is only plowed in the seed furrow Salomão et al. [2]. Soil compression occurs mainly because of the heavy traffic of machinery during farming activities, such as planting, harvesting, and other soil treatments, especially when the soil moisture level is higher than necessary. This problem is more common in Rhodic Haplustox soil and when the soil water retention capacity is high, limiting the time when the soil has enough moisture for mechanized operations. Franchini et al. [3].

The yield potential of rice is highly variable and relative in the different production environments cultivated in the world, with rice being predominant in the irrigated (flooded) system in relation to highland rice, which has lower yield potential, as was found in this research. The yield potential reported in the state of Rio Grande do Sul, Brazil, for rice was estimated at 14,800 kg ha⁻¹ (irrigated rice), Ribas [4], and for highland rice in the Brazilian cerrado, it was estimated at 8000 kg ha⁻¹, with the national average grain yield of 6300 kg ha⁻¹, according to CONAB [1]. The yield potential is influenced by several factors, such as sowing time, cultivar, temperature, solar radiation, time of water entry into the crop, weed control, crop rotation and fertilizers; these are factors that are potentially related to the production potential gap.

Rice cultivation under the NTS has been associated with certain difficulties, particularly with regard to development and initial growth. Therefore, it is crucial to address these issues to ensure optimal yields and profitability. The growth of the aerial part of the upland rice decreased with increasing soil density from 1.2 Mg dm⁻³ and macroporosity below 0.10 m³ m⁻³; the roots in the compacted layer at depths from 0 to 20 cm became thicker due to the increase in soil density and reduction in microporosity, and superficial soil compaction reduced the number of roots present in this layer and in the lower uncompacted layer at depths of 20–40 cm Seidel et al. and Guimarães et al. [5,6].

Mechanical soil scarification is an agricultural and mechanized operation that involves breaking up compacted or dense layers of soil, both superficial and subsurface. Mechanical chiseling intervention is carried out by employing scarifiers or subsoilers with cutting discs in front of stems that prevent crop residues from being incorporated into the soil. However, the long-term effects of mechanical soil scarification are limited by time, ranging from a few months Nazari et al. and Boreta Junior et al. [7,8] to a few years Silva et al. [9], depending on the redisposition of the soil particles as a result of soil type, weather conditions, machines and implementation intervention, and predominant management practices in particular production systems [10]. Soil compaction has a direct effect on the physical and mechanical properties of the soil and consequently impairs plant growth and development. In general, soil compaction impairs water and nutrient uptake by limiting root length and penetration, which ultimately leads to poor plant growth and yield D’Or and Destain [10]. Scarification breaks down compacted soil layers and generates macropores to improve soil permeability and facilitate root penetration and corrective movement.

The cultivation of cover crops alone or in an intercropping system is a promising alternative that can be used to increase plant biomass yield and nutrient accumulation, according to the NTS Pereira et al. and Nascimento et al. [11,12]. The leftover straw of cover crops on the soil surface creates a physical barrier between machinery tires and the soil surface to minimize surface compaction Moreira et al. [13]. The plants of the Poaceae family, such as Pennisetum glaucum, Urochloa ruziziensis, and Urochloa brizantha, are fast-growing species that are capable of greater biomass production, as well as the promotion of nutrient cycling Pereira et al. and Piva et al. [13,14]. The inclusion of Fabaceae species (Cajanus cajan and Crotalaria juncea) in production systems, either alone or in association with Poaceae, has increased the yield of crops in succession by increasing nitrogen (N) availability Pacheco et al. [15] as a result of biological nitrogen fixation (BNF) and a low carbon/nitrogen (C/N) ratio in straw Pacheco et al. and Crusciol et al. [15,16].

Considering this research gap, the present study hypothesized that mechanical soil scarification associated with cover crop predecessors (Cajanus cajan, Crotalaria juncea, Urochloa ruziziensis, and Pennisetum glaucum) can improve the assessment of grain yield and changes in soil chemical properties after two years of rice cultivation. In this context, mechanical soil scarification and cover crops in no-tillage systems can positively impact the productivity of upland rice in succession and the chemical properties of the soil. Therefore, the objective of the present study was to evaluate the effect of mechanical soil scarification associated with successive and predecessor cover crops on the evaluation of grain yield and changes in soil chemical properties after rice was grown for two consecutive years under the NTS for twelve years at a low altitude in Brazil.

2. Materials and Methods

2.1. Changes to the Description of the Site

The study was carried out in Brazil during two growing seasons in the district of Selvíria in the state of Mato Grosso do Sul (MS) (51°22′ W, 20°22′ S; elevation 335 m), on Rhodic Haplustox soil with a clayey texture.

The annual means of the rainfall, temperature, and relative humidity in the region are 1370 mm, 23.5 °C, and 75%, respectively. According to the Köppen classification, the climate is Aw, i.e., humid tropical, with rainy summers and dry winters. The climatic rainfall and temperature data were recorded at the Meteorological Station of the Experimental Farm of UNESP (São Paulo State University), Brazil, during the experiment with cover crops and upland rice (Figure 1a,b). Water (14 mm) was sprinkled on the rice and cover crops every three days or when necessary by a center pivot irrigation system.

The experimental area was managed for twelve years in a no-tillage system (NTS), and in the two years before the installation of the experiment, maize was planted in the summer, and common bean was planted in the winter (Table 1).

Before the experiment, the chemical and physical properties of the soil were analyzed. For chemical analysis, a composite sample composed of 20 simple disturbed soil samples from across the experimental area from the stratified layers of 0–5, 5–10, 10–20, and 20–40 cm was used. In this investigation, we analyzed the following soil chemical attributes: phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg), potential acidity (H + Al), and aluminum (Al), and we calculated the sum of bases (SBs), cation exchange capacity (CEC), and base saturation. P and S were reported in mg dm⁻³; other elements, SB, and CEC were reported in mmol_c dm⁻³. The methodology used is described in Section 2.3.

At the study site, the mean soil contents are 392, 135, and 473 g kg⁻¹ of sand, silt, and clay, respectively. Before and after mechanical scarification, the mean soil density and macroporosity were 1.50 and 1.42 Mg m⁻³ and 0.08 and 0.12 m⁻³, respectively, in the 0–40 cm layer. The chemical properties can be summarized as follows: 25 mg dm⁻³ P (resin); 16 g dm⁻³ organic matter (OM); pH 4.7 (CaCl₂); K⁺, Ca⁺², Mg⁺², H + Al, SB, and CEC = 1.6, 13.5, 9.5, 35.5, 24.6, and 60.1 mmol_c dm⁻³, respectively; and a base saturation of 41% (Table 2). Soil acidity was corrected by liming 35 and 123 days before sowing the cover crops and rice, respectively, in the entire experimental area, as recommended by Raij et al. [17]. With a spreader, 1600 kg ha⁻¹ of dolomitic limestone (CaCO₃) with 85% effective calcium carbonate equivalent (ECCE) was applied until 70% base saturation was reached.

2.2. Layout of the Field Experiment

The experiment was arranged in a randomized block design in a 5 × 2 factorial scheme with four replications. The treatments consisted of five cover crops (Cajanus cajan, Crotalaria juncea, Urochloa ruziziensis, Pennisetum glaucum, and fallow), with or without soil mechanical chiseling, which were fixed during the two years of the research. In the fallow treatment, the development of spontaneous vegetation was allowed, where the species Ipomoea acuminata, Bidens pilosa, Leonotis nepetaefolia, Conyza spp., Commelina benghalensis, and Zea mays (spontaneous maize regrowth) predominated. In the Supplementary (Figures S1–S7) there are figures illustrating details in the field about the experimental area. In each experimental unit of 12.0 × 7.0 m, an area of 10.0 × 5.0 m was used for the evaluations (Figure 2).

2.3. Soil and Plant Sampling, Measurements of Soil Properties, and Rice Yield

In part of the experimental area, mechanical soil scarification (MSC) was carried out on August 9 in the first year before sowing the cover crops (Table 1). A seven-shank chisel plow (three on the front bar and four at the rear) with an inclined shape and chisel tip was used, with rods spaced 300 mm apart and an attack angle of 22, equipped with a clod breaking roller, coupled to the tractor traction bar. The mean working depth was set at 30 cm, and the width of the cutting strip was 210 cm. The operation was carried out when the soil water content was close to the friability point.

All the cover crops were sown by hand on August 14 and September 5 in the first and second years, respectively, using seed drills without mineral fertilizer at a row spacing of 0.45 m. A sowing density of 60 kg ha⁻¹ was used for C. cajan, 30 kg ha⁻¹ for C. juncea and P. glaucum, and 12 kg ha⁻¹ for U. ruziziensis.

The cover crop and spontaneous plants of the fallow period were desiccated with the herbicides glyphosate (1440 g ha⁻¹ a.i.) + 2,4-D (670 g ha⁻¹ a.i.), 68 days after sowing (DAS) in the first year and 3 DAS in the second year. The herbicides were distributed with a boom sprayer set to apply 200 L ha⁻¹ of spray. Ten days after the desiccation of the cover and fallow plants, the plant residues (straw) of all the cover and fallow plants were processed with a horizontal mechanical straw chopper at 10 cm above ground level.

After the cover crops were cut with a mechanical chopper, the dry weight of the plant shoots was assessed. Random sampling was carried out within a square of 50 × 50 cm at four representative points per plot. Subsequently, the collected fragmented material was dried to a constant weight at 65 °C in a forced air circulation and renewal oven. The shoot dry weight was determined by the arithmetic mean between the two sampled points after the means were transformed to Mg ha⁻¹. After weighing, the material was returned to the sampling area.

The rice grain yield was determined by weighing the grains from the evaluated plot areas; then, it was corrected to a moisture content of 13% and converted to kg ha⁻¹.

Both rice sowing methods were carried out mechanically using the same experimental design and treatments. The rice plots consisted of twenty 12 m rows spaced 0.35 m apart, of which the 18 central rows were used for evaluation, while 0.5 m at either end of each row was disregarded. Fertilization in the sowing furrows and side dressing between the rows was calculated based on the soil chemical characteristics and the recommendations of Cantarella and Furlani [18]. Fertilization at sowing consisted of 250 kg ha⁻¹ of 06-30-10 NPK fertilizer in the first year and 280 kg ha⁻¹ of 04-14-08 NPK fertilizer in the second year. Nitrogen was side-dressed 30 days after the emergence (DAE) of the rice plants at the beginning of tillering at a rate of 60 kg N ha⁻¹ in the form of ammonium sulfate (20% nitrogen and 22% sulfur).

Rice cultivars IAC202 and IAC203 were planted in the first and second years, respectively. These cultivars have modern architecture, upright leaves, and low plant height (mean of 0.96 m) and are resistant to lodging; the cycle lasts on average 112 days, and 50% of the flowering occurs at 69 DAE Regitano and Gallo [19]. In both cultivation years, a population of 180 plants m⁻² was obtained using a certain number of certified seeds. The seeds were treated with pyraclostrobin + methyl thiophanate + fipronil (5 g + 45 g + 50 g a.i. per 100 kg of seeds) for both years of rice cultivation.

Weeds were controlled with herbicides at a rate of 200 L ha⁻¹, applied using a tractor sprayer. The preemergence herbicide pendimethalin (1400 g ha⁻¹ a.i.) was applied during both years of cultivation. At 15 and 24 DAE, the postemergence herbicides metosulfuron-methyl (2 g ha⁻¹ a.i.) and bentazon (720 g ha⁻¹ a.i.) were used, respectively, in the first year of cultivation. In the second year, the postemergence herbicide bentazon (720 g ha⁻¹ a.i.) was only applied to 13 DAE plantlets. The remaining weeds not controlled by herbicides were eliminated by hand hoeing.

Preventive blast control was carried out with a fungicide based on trifloxystrobin (75 g ha⁻¹ a.i.) + tebuconazole (150 g ha⁻¹ a.i.). One application was carried out at 58 DAE and another at 79 DAE in the first year of cultivation and at 60 and 82 DAE in the second year of cultivation. The other cultural and phytosanitary treatments were those normally recommended for upland rice cultivation in the region.

After rice harvest, soil samples from 10 points (simply disturbed samples) per plot were taken with a screw auger (0–5, 5–10, 10–20, and 20–40 cm) and blended in four repetitions per treatment. After removing the roots and crumbling the material, the samples were placed in plastic bags, identified, and taken to the laboratory for analysis, and the soil chemical properties were evaluated according to the methodology of Cantarella, Raij and Quaggio [20,21], and Raij et al. [17]. In the soil fertility laboratory, the composite samples were dried and sieved (<2 mm).

The soil pH of the soil suspensions was determined using a 0.01 mol/L CaCl₂ solution at a ratio of 1:2.5. OM was determined by oxidation with K₂Cr₂O₇ in the presence of H₂SO₄ and a titration of excess dichromate with a 0.4 mol/L solution of Fe(NH₄)₂(SO₄)2.6H₂O. Exchangeable aluminum (Al⁺³) was extracted with a 1.0 mol L⁻¹ KCl solution and then titrated with 0.025 mol/L NaOH. Exchangeable calcium (Ca⁺²) and magnesium (Mg⁺²) were extracted by ion-exchange resin and quantified by atomic absorption spectrophotometry (AAS, Model Varian SpectrAA-55B, Varian, CA, USA). Exchangeable potassium (K⁺) and phosphorus (P) were also extracted from the resin; K⁺ was determined by flame photometry, and P was determined by colorimetry. The potential acidity (H⁺ + Al⁺³) was estimated using the SMP buffer pH method. Sulfur (S) was extracted from a solution of 0.01 mol L⁻¹ Ca(H₂PO₄)₂, and turbidity was subsequently measured via the precipitation of sulfate by barium chloride via colorimetry. With these results, the sum of bases (SBs), cation exchange capacity (CEC) at pH 7.0, and base saturation (BS) were calculated. In the Supplementary you will find the list of reagents used to carry out soil chemical analyzes.

We utilized a 10 cm³ sample of prepared soil, which underwent sieving with a 2 mm sieve and a milling process, to determine the concentration of chemical soil elements. It is standard practice in soil chemical analysis to use volume rather than mass for measurement; hence, the reported results are in volume units [22].

The equipment used in the research to perform the analysis of the chemical properties of the soil is shown in Figure 3.

2.4. Statistical Analysis

All the data were assessed for homoscedasticity using Levene’s test (p ≤ 0.05). Following this, the data were examined for normality using the Shapiro–Wilk test, which demonstrated a normal distribution of the data (W ≥ 0.90). Subsequently, the data were subjected to an analysis of variance (F test) to compare the single and interactive effects of the cover crops (factor 1) with or without mechanical soil scarification (factor 2). Whenever a significant main effect or interaction was detected by the F test (p ≤ 0.05), additional comparisons were carried out using the Tukey test (p ≤ 0.05). The entire analytical process was performed using the ExpDes package in R software version number 4.4.0 [23,24].

3. Results

3.1. Dry Matter Yield of Cover Crop Shoots and Rice Grains

The dry matter yield of the CC shoots was greater than 7.99 and 2.53 Mg ha⁻¹ in the first and second study years, respectively, compared to that in the fallow treatment (control), regardless of MSC (Table 3). There was an overall decrease in dry matter yield in the cover crop shoots in all the treatments from the first to the second year, except for P. glaucum (Figure 4a,b). It is worth highlighting that the previous cultivation of P. glaucum, which was independent of scarification, produced the highest shoot dry matter yield in both years.

Mechanical soil scarification and cover crops significantly influenced rice yield in the first year, which increased by approximately 552 kg ha⁻¹ (12.2%) compared to that in the non-scarified soil (Table 3). In the second year of cultivation, the rice yield decreased in all treatments around the first year, owing to the rupture of the compacted layers. However, this effect decreased as time passed after the tillage operations due to natural soil settling.

Concerning the soil cover species (Table 3), C. cajans (5614 kg ha⁻¹) induced a greater rice yield than did P. glaucum (4154 kg ha⁻¹) and U. ruziziensis (4284 kg ha⁻¹) in the first study year. These results highlight the potential of C. cajans for shoot dry matter yield (9.7 Mg ha⁻¹), in addition to nutrient recycling and release for subsequent crops (rice). Management with MSC and CCs did not influence rice grain yield in the second year of cultivation.

3.2. Dynamics of Soil Chemical Properties

The interactions between mechanical scarification and the cover crops were significant in the 0–5 cm layer for P, Ca, SB, and CEC; in the 5–10 cm layer for S, Mg, H + Al, and V; in the 10–20 cm layer for P, S, H + Al, CEC, and Al; and in the 20–40 cm layer for H + Al and Al in the first and second years, respectively (Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10 and Table 11).

In the 0–5 cm layer (Table 4 and Table 5), mechanical soil scarification significantly affected the soil properties in terms of S, pH, K, Ca, CEC, and V (%). In general, the values were greater under NTS, except for S. Urochloa ruziziensis, which had a greater soil K content (3.76 mmol_c dm⁻³) than did C. cajans (2.79 mmol_c dm⁻³) in the first year. In the second year, a significant effect of MSC on S, pH, Mg, H + Al, and V% in the 0–5 cm layer was observed (Table 4 and Table 5). Among the studied CCs, those previously cultivated with P. glaucum exhibited the greatest increases in soil organic matter, K, Mg, and V (26 g dm⁻³, 2.36 mmol_c dm⁻³, 35 mmol_c dm⁻³, and 82%, respectively).

Table 4

Mean soil chemical properties OM, P, S, K, Ca, and Mg in the 0–5 cm layer after rice cultivation, cover crop (CC) cultivation, and mechanical soil scarification (MSC) in Selvíria, MS, Brazil, in two growing seasons.

Soil Chemical Attributes Harvesting Upland Rice (0–5 cm)
	MO		P		S		K+		Ca2+		Mg2+
	g dm−3		---------- mg dm−3 -------				------------------- mmolc dm−3 -----------------
Mechanical soil scarification (MSC)
	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year
Without	28	24	29	35	6 b	8 b	3.5 a	2.0	57 a	40	35	29 a
With	26	23	36	35	8 a	10 a	3.0 b	2.1	44 b	35	33	26 b
Cover Crops (CCs)
Fallow	27	22 b	32	26	7	10 ab	2.9 ab	1.9 ab	53	35	34	24 b
U. ruziziensis	27	24 ab	35	33	8	9 ab	3.8 a	2.0 ab	50	35	35	24 b
C. juncea	27	24 ab	31	38	6	11 a	3.2 ab	1.9 b	51	41	36	27 b
C. cajan	27	22 b	33	36	6	7 b	2.8 b	2.2 ab	47	34	30	24 b
P. glaucum	26	26 a	31	42	7	9 ab	3.7 ab	2.3 a	54	44	35	35 a
F-Value (5%)
MSC	3.83 ns	1.70 ns	32.70 *	0.0 ns	7.97 *	24.59 *	4.78 *	2.11 ns	11.05 *	18.89 *	0.36 ns	4.37 *
CC	0.36 ns	4.81 *	1.10 ns	8.13 *	1.33 ns	3.44 *	3.73 *	3.59 *	0.44 ns	11.75 *	0.64 ns	7.63 *
MSC × CC	0.59 ns	1.68 ns	4.90 *	3.14 *	0.74 ns	1.85 ns	0.46 ns	0.41 ns	0.38 ns	8.39 *	2.13 ns	2.56 ns
Minimum significant difference (5%)
MSC	--	--	2.75	--	3.11	1.32	0.42	--	7.99	--	--	2.99
CC	--	--	--	--	--	2.98	0.94	0.45	--	--	--	6.74
CV (%)	8.74	9.26	13.05	17.34	30.4	22.42	19.7	14.65	24.28	9.57	22.57	16.72
IC	24	24	29	29	3	3	3.5	3.5	41	41	25	25

^ns not significant. * significant at 5% probability according to the F test. Means followed by the same letter for mechanical soil scarification and cover crops did not differ statistically from each other according to the Tukey test at 5% probability. IC: chemical attributes of the soil in the experimental area before installing the experiment (Table 2); coefficient of variation (CV); -- Due to the studied variable not showing a significant difference, the minimum significant difference values (5%) were not presented.

Table 5

Mean soil chemical properties pH, H + Al, Al, SB, CEC, and V% in the 0–5 cm layer after rice cultivation, cover crop (CC) cultivation, and mechanical soil scarification (MSC) in Selvíria, MS, Brazil, in two growing seasons.

Soil Chemical Attributes Harvesting Upland Rice (0–5 cm)
	pH		H + Al		Al3+ (1)		SB		CEC		V(%)
	CaCl2		-------------------------------------------- mmolc dm−3 ----------------------------------------
Mechanical soil scarification (MSC)
	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year
Without	6.3 a	6.0 a	18	17 b	0.0	0.0	99.5 a	71.3	113.6 a	88.3	84 a	80 a
With	6.2 b	5.8 b	19	19 a	0.0	0.0	80.7 b	63.4	99.9 b	82.6	80 b	77 b
Cover Crops (CCs)
Fallow	6.1	5.8	19	18	0.0	0.0	90.1	61.4	108.9	79.1	83	77 b
U. ruziziensis	6.1	5.9	20	19	0.0	0.0	88.3	64.6	107.9	83.3	81	77 b
C. juncea	6.3	5.8	19	18	0.0	0.0	92.3	70.0	107.8	87.9	83	79 ab
C. cajan	6.2	5.8	19	19	0.0	0.0	89.9	58.9	98.3	78.5	79	76 b
P. glaucum	6.2	6.1	19	17	0.0	0.0	92.3	80.9	110.8	98.2	83	82 a
F-Value (5%)
MSC	4.52 *	4.93 *	1.59 ns	10.36 *	--	--	7.45 *	10.84 *	7.03 *	6.38 *	5.28 *	13.1 *
CC	1.42 ns	1.87 ns	0.31 ns	0.68 ns	--	--	0.65ns	9.99 *	0.71 ns	9.83 *	0.83 ns	4.55 *
MSC × CC	2.48 ns	0.66 ns	2.51 ns	0.65 ns	--	--	0.55ns	5.45 *	0.56 ns	5.32 *	0.40 ns	2.71 ns
Minimum significant difference (5%)
MSC	0.11	0.16	--	1.37	--	--	11.16	--	10.68	--	3.63	2.02
CC	--	--	--	--	--	--	--	--	--	--	--	4.55
CV (%)	2.76	4.13	11.33	11.69	-	-	19.53	11.34	15.41	8.51	6.82	3.93
IC	5.9	5.9	19	19	0	0	69.5	69.5	88.5	88.5	79	79

^ns not significant. * significant at 5% probability according to the F test. Means followed by the same letter for mechanical soil scarification and cover crops did not differ statistically from each other according to the Tukey test at 5% probability. ⁽¹⁾ Analysis of the data transformed into the square root of x + 0.5 for the aluminum variable. IC: chemical attributes of the soil in the experimental area before installing the experiment (Table 2); coefficient of variation (CV); -- Due to the studied variable not showing a significant difference, the minimum significant difference values (5%) were not presented.

With respect to the interaction between the cover crops subjected to mechanical soil scarification and the available soil P in the 0–5 cm layer (Figure 5a and Figure 6a) in the first year, the P. glaucum, U. ruziziensis, C. juncea, and fallow land soils subjected to mechanical soil scarification exhibited increased soil P contents. Fertilization at rice sowing may have contributed to the increase in the available soil P. Regarding the dry matter yield of the CCs, the scarified soil under U. ruziziensis again had a greater P content (42 mg dm⁻³) than that under C. juncea (33 mg dm⁻³). In the second year, there was a significant effect on soil P, SB, and CEC under the CCs, with 38 mg dm⁻³, 57 mg dm⁻³, and 80 mmol_c dm⁻³, respectively. In particular, P. glaucum induced higher values of these soil properties (Figure 5a and Figure 6a,c,d).

The cover crop interactions during mechanical decompaction were significant for the soil P available in the 5–10 cm layer (Figure 5b and Table 6). The available P content increased in the soil under C. juncea and in the scarified soil under fallow vegetation due to soil movement, despite the low mobility of this nutrient. Regarding the dry matter yield of the CCs, the scarified soil had a greater available P content under C. juncea (48 mg dm⁻³) and C. cajans (42 mg dm⁻³) in the first year, both because of the MSC and because both species were Fabaceae, which are more rapidly decomposed after straw incorporation in the soil by mechanical scarification.

The interactions of the cover crops with mechanical decompaction were significant for soil pH and Al in the 5–10 cm layer (Figure 5c,d). After MSC, the pH values were greatest under C. cajan and P. glaucum (5.6 and 5.5, respectively).

Table 6

Mean soil chemical properties OM, P, S, K, Ca, and Mg in the 5–10 cm layer after cover crop (CC) cultivation, rice cultivation, and mechanical soil scarification (MSC) in Selvíria, MS, Brazil, in two growing seasons.

Soil Chemical Attributes Harvesting Upland Rice (5–10 cm)
	MO		P		S		K+		Ca2+		Mg2+
	g dm−3		------------- mg dm−3---------				----------------------mmolc dm−3 -----------------------
Mechanical soil scarification (MSC)
	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year
Without	20	21	32	36 a	12	22	2.2	1.6	26	28	16 b	20
With	21	21	37	31 b	14	21	1.8	1.7	30	29	21 a	21
Cover Crops (CCs)
Fallow	20	20	32	37 ab	11 b	28	1.9	1.5	30	25	18	19
U. ruziziensis	19	21	31	29 b	20 a	22	1.9	1.6	21	26	15	21
C. juncea	21	21	37	28 b	10 b	24	2.1	1.5	31	30	21	19
C. cajan	21	20	38	36 ab	11 b	19	1.7	1.8	33	30	22	20
P. glaucum	21	22	34	38 a	12 b	17	2.5	1.8	25	32	17	23
F-Value (5%)
MSC	0.82 ns	0.01 ns	32.70 *	6.89 *	1.79 ns	0.26 ns	2.54 ns	2.94 ns	2.17 ns	1.02 ns	5.35 *	0.11 ns
CC	2.38 ns	1.94 ns	1.54 ns	4.68 *	8.75 *	5.05 *	1.03 *	2.34 ns	2.86 ns	2.81 ns	2.01 ns	5.07 *
MSC × CC	0.88 ns	0.53 ns	6.59 *	1.99 ns	0.69 ns	6.34 *	0.78 ns	1.72 ns	1.48 ns	1.14 ns	0.94 ns	3.63 *
Minimum significant difference (5%)
MSC	--	--	--	3.99	--	--	--	--	--	--	3.86	--
CC	--	--	--	8.98	5.69	--	--	--	--	--	--	--
CV (%)	8.64	8.03	20.99	18.26	20.66	23.99	40.0	15.90	28.55	15.40	32.03	11.91
IC	17	17	6	6	6	6	4.9	1.8	17	17	12	12

^ns not significant. * significant at 5% probability according to the F test. Means followed by the same letter for mechanical soil scarification and cover crops did not differ statistically from each other according to the Tukey test at 5% probability. IC: chemical attributes of the soil in the experimental area before installing the experiment (Table 2); coefficient of variation (CV); -- Due to the studied variable not showing a significant difference, the minimum significant difference values (5%) were not presented.

The soil Mg content increased in both years of cultivation in the 5–10 cm layer, and the V% increased only in the first year in the soil treated with MSC (Table 7). This can be explained by the limestone reaction in this soil layer caused by mechanical soil scarification, which promoted a higher Mg content (12–21 mmol_c dm⁻³), with a concomitant increase in soil V (48–65%) in the first year. The CCs had a significant effect on soil H + Al, S, and V%. Under U. ruziziensis, soil H + Al and S increased and V% decreased. Under the other cover crops, V% exceeded 60% in the first year. The previous cultivation of U. ruziziensis, due to the greater dry matter yield, or of C. juncea, because it is a legume with a higher S content and had MSC that accelerated decomposition through soil/straw contact, increased the soil S (30 and 29 mg dm⁻³, respectively) in this layer in the second year of rice (Table 7).

Table 7

The mean soil chemical properties pH, H + Al, Al, SB, CEC, and V% at 5–10 cm after rice cultivation, cover crop (CC) cultivation, and mechanical soil scarification (MSC) were measured in Selvíria, MS, Brazil, during two growing seasons.

Soil Chemical Attributes Harvesting Upland Rice (5–10 cm)
	pH		H +Al		Al3+ (1)		SB		CEC		V(%)
	CaCl2		----------------------------------------- mmolc dm−3 ---------------------------------------
Mechanical soil scarification (MSC)
	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year
Without	5.3	5.7	28	25	1.0	0.0	44.6	49.6	74.8	74.9	59 b	66
With	5.4	5.8	27	23	1.0	0.0	52.2	51.4	79.2	74.0	65 a	69
Cover Crops (CCs)
Fallow	5.4	5.7	26 b	23	0.0	0.0	50.0	45.4 b	77.0	68.3 b	65 a	66
U. ruziziensis	4.9	5.7	32 a	24	4.0	0.0	37.9	48.8 ab	69.8	72.4 ab	53 b	67
C. juncea	5.6	5.8	27 ab	24	0.0	0.0	54.3	50.1 ab	83.3	73.5 ab	65 a	68
C. cajan	5.5	5.6	25 b	23	0.0	0.0	56.1	51.3 ab	82.2	77.5 a	65 a	66
P. glaucum	5.3	5.9	28 ab	26	1.0	0.0	43.9	56.8 a	73.1	80.2 a	60 ab	70
F-Value (5%)
MSC	0.27 ns	2.24 ns	0.69 ns	26.02 *	8.8 ns	--	2.89 ns	0.88 ns	1.21 ns	0.25 ns	4.53 *	12.30 *
CC	17.73 *	1.12 ns	5.51 *	4.31 *	529.6 *	--	2.27 ns	3.28 *	1.58 ns	4.61 *	3.82 *	2.30 ns
MSC × CC	4.11 *	0.41 ns	2.17 ns	5.10 *	111.3 *	--	1.26 ns	1.96 ns	0.92 ns	1.70 ns	1.52 ns	3.07 *
Minimum significant difference (5%)
MSC	--	--	--	--	--	--	--	--	--	--	5.11	--
CC	--	--	4.72	--	--	--	--	8.82	--	8.89	11.51	--
CV (%)	40.0	4.25	11.63	7.13	6.88	--	29.3	11.96	16.61	8.18	12.7	4.79
IC	1.8	4.9	33	33	2	0	30.8	30.8	63.8	633.8	3	48

^ns not significant. * significant at 5% probability according to the F test. Means followed by the same letter for mechanical soil scarification and cover crops did not differ statistically from each other according to the Tukey test at 5% probability. ⁽¹⁾ The data were transformed into the square root of x + 0.5 for the aluminum variable. IC: chemical attributes of the soil in the experimental area before installing the experiment (Table 2); coefficient of variation (CV); -- Due to the studied variable not showing a significant difference, the minimum significant difference values (5%) were not presented.

Regarding the relationships between mechanical soil scarification and the cover crops, U. ruziziensis on scarified soil promoted a reduction in pH (4.9) and increased the Al content of the soil (2.75 mmol_c dm⁻³) in comparison with the other CCs and the fallow treatment (control) in the first year. Regarding the interactions between MSC and the CCs in the second year in the 5–10 cm layer, for soil H + Al and V%, P. glaucum under the NTS promoted a reduction in potential acidity (23 mmol_c dm⁻³) and increased the base saturation (72%), indicating the potential of this species for recycling and the releasing of nutrients into the soil (Figure 7c,d).

In the 10–20 cm layer, in the first year, the CCs influenced the P, K, Mg, H + Al, SB, and V% (Table 8 and Table 9). In particular, the soil under U. ruziziensis had higher levels of P, K, Mg, SB, V%, and H + Al (35 mmol_c dm⁻³). On the other hand, under C. juncea, the levels of K, SB, and V% increased, but H + Al (31 mmol_c dm⁻³) decreased. There was an increase in the OM content of this layer in the NTS in the second year (Table 8 and Table 9) due to the absence of soil disturbance (no MSC), which preserved the soil OM.

Table 8

Mean soil chemical properties (OM, P, S, K, Ca, and Mg) at 10–20 cm after rice cultivation, cover crop (CC) cultivation, and mechanical soil scarification (MSC) in Selvíria, MS, Brazil, in two growing seasons.

Soil Chemical Attributes Harvesting Upland Rice (10–20 cm)
	MO		P		S		K+		Ca2+		Mg2+
	g dm−3		------------- mg dm−3----------				---------------------- mmolc dm−3 ----------------------
Mechanical soil scarification (MSC)
	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year
Without	18	18 a	29	23	19	43	1.3	1.5	14	16	10	13
With	18	17 b	26	22	29	44	1.2	1.3	16	15	11	13
Cover Crops (CCs)
Fallow	17	17	25 b	24	20	48	0.8 b	1.5	15	15	10 ab	13
U.ruziziensis	18	17	35 a	21	31	44	1.5 a	1.2	18	14	13 a	12
C.juncea	18	18	28 b	25	24	45	1.5 a	1.4	17	16	11 ab	13
C.cajan	18	17	24 b	18	21	33	1.1 b	1.4	14	14	11 ab	12
P. glaucum	18	18	24 b	24	25	48	1.0 b	1.5	11	17	8 b	14
F-Value (5%)
MSC	0.01 ns	4.58 *	2.82 ns	1.20 ns	33.64 ns	0.47 ns	0.76 ns	2.78 ns	2.39 ns	2.18 ns	1.50 ns	0.52 ns
CC	0.29 ns	2.68 ns	7.72 *	2.83 ns	5.59 ns	5.10 *	9.58 *	1.00 ns	7.52 *	2.96 ns	4.76 *	2.46 ns
MSC × CC	0.98 ns	0.15 ns	1.25 ns	3.96 *	3.02 *	2.90	1.34 ns	0.86 ns	4.49 *	1.47 ns	0.52 ns	1.47 ns
Minimum significant difference (5%)
MSC	--	1.62	--	--	--	--	--	--	--	--	--	--
CC	--	--	6.74	--	--	--	0.40	--	--	--	3.39	--
CV (%)	6.6	6.04	16.88	23.05	21.90	17.47	23.35	20.46	19.83	13.42	21.51	20.46
IC	15	15	38	38	13	13	1.3	1.3	13	10	7	1.3

^ns not significant. * significant at 5% probability according to the F test. Means followed by the same letter for mechanical soil scarification and cover crops did not differ statistically from each other according to the Tukey test at 5% probability. IC: chemical attributes of the soil in the experimental area before installing the experiment (Table 2); coefficient of variation (CV); -- Due to the studied variable not showing a significant difference, the minimum significant difference values (5%) were not presented.

Table 9

The mean soil chemical properties of pH, H + Al, Al, SB, CEC, and V% in layers of 10–20 cm after rice cultivation, cover crop (CC), and mechanical soil scarification (MSC) were measured in two growing seasons in Selvíria, MS, Brazil.

Soil Chemical Attributes Harvesting Upland Rice (10–20 cm)
	pH		H + Al		Al 3+ (1)		SB		CEC		V(%)
	CaCl2		----------------------------------------- mmolc dm−3 ----------------------------------
Mechanical soil scarification (MSC)
	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year
Without	4.8	5.1	34	31	3.4	1.1	25.8	30	59.7	60.8	43	49
With	4.9	5.2	33	30	3.3	1.7	28.2	29	61.5	57.8	45	50
Cover Crops (CCs)
Fallow	4.8	5.2 ab	35 a	29	4.0	1.3	25.8 ab	29 b	60.4	57.9	43 ab	50 ab
U. ruziziensis	4.9	5.0 b	35 a	32	3.5	1.8	32.8 a	27 ab	67.2	58.9	48 a	46 b
C. juncea	4.9	5.3 a	31 b	31	2.6	1.1	29.9 a	30 ab	60.9	60.9	49 a	50 ab
C. cajan	4.9	5.1 ab	33 ab	30	2.4	2.1	26.6 ab	27 ab	59.3	61.0	45 a	48 ab
P. glaucum	4.8	5.3 a	35 a	28	4.1	0.6	20.0 b	32 a	55.1	57.3	36 b	54 a
F-Value (5%)
MSC	0.45 ns	0.30 ns	0.84 ns	7.88 *	0.20 ns	0.38 ns	2.56 ns	0.38 ns	1.49 ns	8.52 *	2.91 ns	0.76 ns
CC	0.27 ns	4.30 *	5.47 *	2.72 ns	9.73 *	3.17 *	7.93 *	3.17 *	6.74 *	2.54 *	10.0 *	3.19 *
MSC × CC	1.54 ns	1.42 ns	1.18 ns	3.24 *	5.35 *	1.05 ns	2.39 ns	1.05 ns	2.67 *	5.22 *	1.84 ns	0.50 ns
Minimum significant difference (5%)
MSC	--	--	--	--	--	--	--	--	--	--	--	--
CC	--	0.25	3.02	--	--	5.26	7.1	5.26	--	--	6.62	6.60
CV (%)	3.92	3.38	6.14	10.49	9.55	12.31	17.77	12.31	7.81	5.13	10.29	9.13
IC	4.5	4.5	38	6	6	18.3	18.3	18.3	56.3	56.3	33	33

^ns not significant. * significant at 5% probability according to the F test. IC: chemical attributes of the soil in the experimental area before installing the experiment. Means followed by the same letter for mechanical soil scarification and cover crops did not differ statistically from each other according to the Tukey test at 5% probability. ⁽¹⁾ The data were transformed into the square root of x + 0.5 for the aluminum variable. IC: chemical attributes of the soil in the experimental area before installing the experiment (Table 2); coefficient of variation (CV); -- Due to the studied variable not showing a significant difference, the minimum significant difference values (5%) were not presented.

The interactions between the cover crops and mechanical soil scarification were significant for soil Al at 10–20 cm (Figure 8c). In the first year, under the U. ruziziensis and fallow conditions, with scarification (4 mmol_c dm⁻³), and under P. glaucum in the NTS (5 mmol_c dm⁻³), the soil Al increased; this was most likely due to greater shoot and root dry matter yields, the decomposition of which generates more organic acids and consequently increases soil acidity. In the second year, P increased in the 10–20 cm layer (Figure 8a), and higher P levels were noted after the previous cultivation of C. juncea (27 mg dm⁻³) and P. glaucum (28 mg dm⁻³) in the NTS.

The interaction effect of cover crops on mechanical soil scarification was significant for soil S at 10–20 cm. In the first year (Figure 9a), all the CCs treated with MSC, except P. glaucum, resulted in greater soil S. In the second year, the scarified soil under U. ruziziensis had a greater sulfur content (51 mg dm⁻³). With mechanical decompaction in the cover crops, the previous cultivation of U. ruziziensis with MSC increased the soil S (39 mg dm⁻³) but did not differ from the soil under C. juncea in the first year.

The interactions of the cover crops with mechanical soil scarification were significant for soil CEC in the 10–20 cm layer (Figure 9b). Regardless of scarification, U. ruziziensis promoted an increase in soil CEC, but this increase did not differ from that of the fallow plants under the NTS in the first year. In the second year, the interactions between the cover crops and MSC and between soil H + Al and CEC (Figure 8b,d) showed that the previous cultivation of P. glaucum (31 and 64 mmol_c dm⁻³) and U. ruziziensis (35 and 62 mmol_c dm⁻³) in the NTS increased soil H + Al and CEC, respectively.

The sulfur content was greatest in the 20–40 cm layer under the NTS (54 mg dm⁻³). The cover crops had a significant effect on the soil contents of P, pH, Ca, Al, S, and V% (Table 10 and Table 11) in the first year. However, Urochloa ruziziensis promoted greater P availability, with no significant difference from C. juncea. However, P. glaucum and C. juncea induced a greater increase in soil pH than did U. ruziziensis.

In the second year, in the 20–40 cm layer, the V value was highest (44%) under the NTS (Table 10 and Table 11). The CCs had a significant effect on pH, K, Ca, Mg, SB, and V%. In general, the C. juncea cultivated previously exhibited an increase in the above chemical properties. The effect of the interactions between the cover crops and mechanical decompaction caused a reduction in soil H + Al (24 mmol_c dm⁻³) and Al (0.00 mmol_c dm⁻³) under the NTS of C. juncea (Figure 10a,b).

Table 10

Mean soil chemical properties OM, P, S, K, Ca, and Mg in the 20–40 cm layer after rice cultivation, cover crop (CC) cultivation and mechanical soil scarification (MSC) in Selvíria, MS, Brazil, in two growing seasons.

Soil Chemical Attributes Harvesting Upland Rice (20–40 cm)
	MO		P		S		K+		Ca2+		Mg2+
	g dm−3		------------ mg dm−3 ---------				----------------------- mmolc dm−3 -----------------------
Mechanical soil scarification (MSC)
	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year
Without	16	15	9	8	54 a	77	0.9	0.8	11	11	8	9
With	16	15	8	9	48 b	78	0.8	0.8	10	10	8	9
Cover Crops (CCs)
Fallow	16	16	8 b	8	59 a	89	0.9	0.7 ab	10 ab	10 b	8	9 ab
U. ruziziensis	16	15	12 a	9	50 ab	76	0.8	0.6 b	10 ab	9 b	7	8 b
C. juncea	16	16	9 ab	9	47 ab	70	0.9	1.0 a	11 a	12 a	9	9 ab
C. cajan	17	15	8 b	8	46 b	78	0.9	1.0 a	10 ab	11 a	8	9 ab
P. glaucum	16	15	6 b	9	55 ab	75	0.7	0.8 ab	9 b	10 b	8	10 a
F-Value (5%)
MSC	0.52 ns	1.20 ns	0.13 ns	1.53 ns	3.97 *	0.13 ns	2.52 ns	0.13 ns	4.18 ns	4.01 ns	0.05 ns	0.07 ns
CC	1.58 ns	0.51 ns	6.45 *	1.71 ns	2.97 *	2.23 ns	0.79 *	2.23 ns	2.53 *	6.19 *	1.42 ns	4.14 *
MSC × CC	1.26 ns	1.01 ns	2.56 ns	1.35 ns	1.94 ns	0.11 ns	0.33 ns	0.11 ns	2.36 ns	1.86 ns	0.32 ns	1.33 ns
Minimum significant difference (5%)
MSC	--	--	--	--	5.81	--	--	--	--	--	--	--
CC	--	--	3.89	--	13.08	--	--	0.26	2.51	1.96	--	1.72
CV (%)	5.56	7.02	31.74	19.14	17.49	16.96	29.91	22.01	14.30	13.07	17.79	13.09
IC	13	13	7	7	43	43	1.4	1.4	10	10	8	8

^ns not significant. * significant at 5% probability according to the F test. Means followed by the same letter for mechanical soil scarification and cover crops did not differ statistically from each other according to the Tukey test at 5% probability. IC: chemical attributes of the soil in the experimental area before installing the experiment (Table 2); coefficient of variation (CV). -- Due to the studied variable not showing a significant difference, the minimum significant difference values (5%) were not presented.

Table 11

Mean soil chemical properties pH, H + Al, Al, SB, CEC, and V% in the 20–40 cm layer after rice cultivation, cover crop (CC) cultivation, and mechanical soil scarification (MSC) in Selvíria, MS, Brazil, in two growing seasons.

Soil Chemical Attributes Harvesting Upland Rice (20–40 cm)
	pH		H + Al		Al3+ (1)		SB		CEC		V(%)
	CaCl2		----------------------------------------- mmolc dm−3 ----------------------------------
Mechanical soil scarification (MSC)
	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year	1st Year	2nd Year
Without	4.9	5.1	31	26	2.9	1.3	19.3	20.6	48.5	47.0	39	44 a
With	4.8	5.0	31	28	3.1	1.5	18.7	19.6	49.8	48.0	38	41 b
Cover Crops (CCs)
Fallow	4.9 ab	5.0 ab	30	27	3.0 b	1.5	18.5	19 b	45.1	46.5	39 ab	41 b
U. ruziziensis	4.7 b	4.9 b	32	29	4.4 a	1.5	17.9	18 b	50.2	46.7	36 b	39 b
C. juncea	5.0 a	5.1 a	30	26	2.1 b	1.0	21.0	23 a	51.0	50.0	41 a	47 a
C. cajan	4.9 ab	5.0 ab	32	27	2.8 b	1.5	19.9	21 ab	52.0	47.0	38 ab	44 ab
P. glaucum	5.0 a	5.1 a	30	27	4.4 a	1.6	17.8	19 b	47.5	46.8	38 ab	42 b
F-Value (5%)
MSC	3.57 ns	1.52 ns	0.29 ns	11.63 *	0.56 ns	17.27 *	0.74 ns	1.44 ns	0.43 ns	0.87 ns	1.15 ns	5.19 *
CC	4.43 *	2.64 *	0.98 ns	2.62 *	0.91 *	14.56 *	2.29 ns	5.73 *	1.53 ns	2.31 ns	2.25 *	6.79 *
MSC × CC	0.88 ns	2.17 *	0.94 ns	3.38 *	0.45 ns	5.89 *	1.39 ns	1.69 ns	1.35 ns	2.04 ns	1.68 ns	2.20 ns
Minimum significant difference (5%)
MSC	--	--	--	--	--	--	--	--	--	--	--	2.25
CC	0.21	0.19	--	--	1.37	--	--	3.58	--	--	5.35	5.07
CV (%)	2.92	2.54	11.50	5.79	13.96	11.38	13.47	12.19	13.00	12.19	9.50	8.20
IC	4.8	4.8	29	29	2	2	19.4	19.4	48.4	19.4	40	40

^ns not significant. * significant at 5% probability according to the F test. Means followed by the same letter for mechanical soil scarification and cover crops did not differ statistically from each other according to the Tukey test at 5% probability. ⁽¹⁾ The data were transformed into the square root of x + 0.5 for the aluminum variable. IC: chemical attributes of the soil in the experimental area before installing the experiment (Table 2); coefficient of variation (CV); -- Due to the studied variable not showing a significant difference, the minimum significant difference values (5%) were not presented.

4. Discussion

While the dry matter yield in the shoots of the cover crops decreased in all treatments from the first to the second year, the greatest increase in shoot dry matter occurred in the P. glaucum treatment group in both years, regardless of the degree of scarification.

According to our results, C. cajans resulted in a greater increase in rice yield than did P. glaucum and U. ruziziensis. Rice yields in the second year were lower than those in the first year of cultivation in all the treatments. According to Pacheco et al. [25], Urochloa ruziziensis (1554 kg ha⁻¹) is the best option for the CC preceding rice, and the authors explained that the low yield was due to the occurrence of water stress during pre-flowering. However, the authors also reported that after the CC, the rice grain yield under the NTS was greatest when the rice was grown on the straw of P. glaucum and U. ruziziensis due to the potential of these species for biomass production and nutrient cycling.

According to Abreu et al. [26], “biological scarification”, which considers the hydraulic conductivity of the saturated soil, proves, in the medium term, to be more effective in breaking the compacted layer and establishing water-conducting pores than MSC. This effect is attributable to greater moisture retention, mobilization by plant residues, and OM addition and can therefore explain the greater productivity of P. glaucum in the NTS. This finding corroborated the results of Vieira et al. [27], who described soil scarification in the NTS as a viable alternative that can be used to minimize physical limitations to plant growth.

Positive yield differences may reflect improvements in soil physical, chemical, and biological quality that are associated with a high input of OM and nutrients into the soil, mainly N, as described by Spagnollo et al. [28] for Rhodic Haplustox soil in systems with fabaceas with a high dry matter yield. Therefore, in areas with compacted surface layers, high acidity, and low soil fertility, as in this study, the introduction of these species contributed to an improvement in the soil quality and an increase in the yield of cash crops.

The results of this study confirmed those of Kappes [29], who reported that millet + sunn hemp, as a previous crop, which is associated with a side-dressing rate of 90 kg N ha⁻¹ on maize under the NTS, increased crop yields. According to Stone [30], the mean rice yield under the NTS or plow tillage was 32% and 21% greater, respectively, than that after harrowing. The authors mentioned that the OM increase over time in the system is the result of cover crop roots restructuring the soil, which can reduce soil density.

However, Secco et al. [31] reported that mechanical soil scarification increases maize and wheat grain yields under the NTS compared with those under the NTS without scarification. According to Klein et al. [32], scarification in soil managed for six years under no tillage increased the grain yield of wheat sown seven months after scarification. These divergent results possibly occurred because plant productivity is not a function of the soil alone, i.e., better soil physical conditions do not necessarily lead to higher yields.

In the no-tillage system, compaction was observed in the surface layer [27]. In some areas, crop productivity is affected [28]. More pronounced effects of this problem are noted when the soil is cultivated with a sequence of crops.

In the 0–5 cm layer, mechanical soil scarification influenced the soil pH, K, Ca, SB, CEC, S, and V (%). In general, the values were greater in the absence of scarification (NTS), except for that of S. Of the CCs, the soil under Urochloa had a greater K content (3.76 mmol_c dm⁻³) than that under pigeon pea (2.79 mmol_c dm⁻³). Under the NTS, more nutrients, such as Ca, Mg, and K, accumulate in the surface layers. In addition to the absence of disturbance, nutrients accumulate in the tissues of crop plants and are subsequently decomposed and released in the surface layers [29].

Analysis of the interaction results for the P content of scarified soil in the 0–5 cm layer revealed that the P content under P. glaucum, U. ruziziensis, and C. juncea and under fallow conditions with MSC was greater. Moreover, under U. ruziziensis with MSC, the P content was greater (42 mg dm⁻³) than that under sunn hemp (33 mg dm⁻³). Another possibility is that the base fertilization carried out at rice sowing contributed to the increase in the soil P content. Some crops increase the amount of P-solubilizing microorganisms, e.g., C. juncea [30]. The P and K levels reported here were considered medium to high for annual crops (16–40 and 41–80 mg dm⁻³ for P) and (1.6–3.0 and 3.1–6.0 mmol_c dm⁻³ for K), according to Raij et al. [17]. In this sense, the proposed soil management practice promoted increases in the soil K content, mainly in the two surface layers.

Menezes and Leandro [22] reported increased P extraction by P. glaucum and U. ruziziensis, followed by decomposition and release of this nutrient in the surface soil layers. Nutrient accumulation occurs more frequently in the NTS, especially Ca, Mg, K, and P accumulation in the surface soil layers. The reason is that there is no soil disturbance, but nutrients can accumulate in the tissue of cultivated plants, with subsequent decomposition and release of these nutrients in the surface layers [30].

If the soil is acidic, dissolving limestone produces exchangeable Ca and Mg, which can move in the soil if there are free anions. Otherwise, the limestone remains on the soil surface [30]. Under the NTS, the accumulation of some nutrients (P, K, Ca, and Mg) increased, and the pH, base saturation, and CEC increased. This effect was more evident in the surface layer, mainly for P, due to its low mobility in the soil, organic P recycling, and reduced P fixation due to the higher pH and lower acidity [31].

An increase in Mg and V (%) was noted after MSC in the 5–10 cm layer, indicating that scarification contributed to an improvement in the effect of soil liming by increasing the Mg content (12 to 21 mmol_c dm⁻³), with a direct effect on base saturation (48 to 65%). Under the CCs, there was a significant effect on H + Al, S, and V%, indicating higher H + Al and S and reduced V% in the soil under Urochloa. This shows that this CC can acidify the soil and intensify S leaching [4].

Under the other CCs, the V (%) values were greater than 60%. According to Raij et al. [17], the results of this study are medium (51–70%) for V (%), high (>10 mg dm⁻³) for S, and high for Mg (>8.0 mmol_c dm⁻³). The higher Mg and V (%) contents under the other CCs are due to the addition of plant residues that can promote an increase in pH before they are decomposed. This occurs by the complexation of H and Al with compounds from plant residues, leaving Ca, Mg, and K in the soil solution, which can cause an increase in CEC saturation by these basic reaction cations [29].

The soil pH was highest in the mechanically scarified soil under C. cajan and P. glaucum. On the other hand, U. ruziziensis in scarified soil reduced the soil pH (4.9 mmol_c dm⁻³) and increased the Al content (2.75 mmol_c dm⁻³) in response to the fallow treatment and the other CCs. Urochloa ruziziensis acidified the soil more than the other CCs, where the pH varied from 5.2 to 5.6 and the soil acidity was considered to be intermediate (5.1–5.5) or low (5.6–6.0). This species induced the release of organic acids in the surface and subsurface soil layers due to the emission of root system exudates and plant residue decomposition, which increased the soil Al concentration. The high shoot and root production of U. ruziziensis can increase organic carbon levels in the soil surface after residue decomposition [31], resulting in the release of organic acids during humification [30].

Mechanical soil scarification and biological soil decompaction, followed by a sequence of crops (rice), increase the CEC, which is fundamental for the stability of the production system. This increased the negative soil charge through greater biomass yield and an increase in OM, which contributed to increased soil CEC [32]. This confirmed that the soil under U. ruziziensis, with or without scarification, contributed the most to the increase in this chemical property.

5. Conclusions

Mechanical soil scarification increases the yield of Pennisetum glaucum, Urochloa ruziziensis, and Cajanus cajan in the first year of research. Pennisetum glaucum provides greater dry mass yield of the aerial part, regardless of soil mechanical scarification. Mechanical soil scarification and cover crops in a no-tillage system have a positive impact on the yield of upland rice in succession and on the soil chemical properties.

In the previous cultivation of Urochloa ruziziensis the soil chemical properties were positively improved by the effect of mechanical scarification; however, the acidity of the initial soil increased in the 5–40 cm layer. The cultivation of Pennisetum glaucum and Crotalaria juncea before and after rice, regardless of scarification, provides positive improvements in the chemical properties of the soil in the 0–40 cm layer, mainly in the chemical attributes of V(%), SB, pH, and phosphorus in the ground. Mechanical soil scarification provides an increase in the grain yield of upland rice of the order of 552 kg ha⁻¹ in the first year. The predecessor and successive cultivation of Cajanus cajan provide increases in the yield of upland rice in the tropical regions of Brazil of the order of 1454 and 1330 kg ha⁻¹ compared to the cultivation of Pennisetum glaucum and Urochloa ruziziensis, respectively, in the first year of the research. Scarification increases the exploitation of the soil by the roots of upland rice and, with the use of Cajanus cajan, improves the absorption of the nutrients available in the cover crop residues for successive crops.

A limitation of the research would be that the practice of mechanical scarification has short-term or ephemeral effects, but it breaks the soil at natural breakpoints that must be quickly filled with fine roots to improve soil aggregation and structuring, minimizing the effects of soil compaction, expanding the formation of biogenic biopores, and providing improvements in the flows of water and gases in the soil.

The inclusion of mechanical soil scarification and cover crops in a no-till system has great potential to increase the upland rice yield in succession and improve soil chemical properties. Therefore, we recommend mechanical soil scarification and the use of Cajanus cajan, Pennisetum glaucum, and Crotalaria juncea in a no-tillage system for upland rice cultivation and for positive improvements in soil chemical properties and grain yield.

One possibility for future research development would be to study the formation and maintenance of biogenic biopores and to explore the volume, mass, and length of the root systems of upland rice crops over several years through the adoption of soil mechanical scarification associated with the management of cover crops in a no-tillage system.

Author Contributions

V.d.N., formal analysis, resources, original draft and writing; O.A. and M.C.A., conceptualization, data curation, validation and writing; E.J.d.S. and P.R.T.d.S. formal analysis and data curation; F.H.K., M.C.M.T.F., E.P.P., S.F., L.F.R.d.S. and F.S.G., investigation, review and editing. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding authors.

Acknowledgments

The authors are indebted to the São Paulo Research Foundation (FAPESP) for a doctoral scholarship from the first author (PROCESS: 2012/05945-0) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Coordination for the Improvement of Higher Education Personnel). The authors wish to thank São Paulo State University (UNESP), Faculty of Engineering, Campus Ilha Solteira, for making the infrastructure accessible and providing human resources.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Rainfall (mm) and maximum and minimum temperatures (°C) during the experimental period of cover crop and upland rice cultivation in (a) the 1st growing season and (b) the 2nd growing season in Selvíria, MS, Brazil.

Figure 1. Rainfall (mm) and maximum and minimum temperatures (°C) during the experimental period of cover crop and upland rice cultivation in (a) the 1st growing season and (b) the 2nd growing season in Selvíria, MS, Brazil.

Figure 2. Arrangement of cover crops in strip plots * with mechanical soil scarification (MSC) and without MSC.

Figure 3. Equipment used in research to perform the analysis of the chemical properties of the soil, detailing its model, manufacturer, and country of the manufacturer. (a) 55AA Atomic Absorption spectrometer, Agilent, Santa Clara, CA, USA. (b) 55B Atomic Absorption spectrometer, Agilent, USA. (c) Espectrofotômetro Varian Cary 50 bio visível UV, Alt American Laboratory Tranding, East Lyme, CT, USA. (d) 55AA Atomic Absorption spectrometer, Agilent, USA. (e) 55B Atomic Absorption spectrometer, Agilent, USA. (f) Espectrofotômetro Varian Cary 50 bio visível UV, Alt American Laboratory Tranding, USA. (g) Espectrofotômetro 600S, FEMTO, São Paulo, São Paulo, Brazil. (h) Espectrofotômetro 600S, FEMTO, Brazil. (i) Incoterm Digital Benchmark Phmeter, Instrusul, Esteio, Rio Grande do Sul, Brazil.

Figure 4. Interaction effects on the dry matter yield of the shoots of cover crops (CCs) after soil decompaction by mechanical soil scarification under NTS. The means followed by the same lowercase letter for CC with MSC (1.05 and 0.84 mg ha−1) and uppercase letters for MSC in CC (1.49 and 1.19 Mg ha−1) did not significantly differ according to Tukey’s test at the 5% level. Coefficient of variation (%) = 7.09 and 9.49, Selvíria, MS, Brazil, 1st year (a) and 2nd year (b).

Figure 5. Interactions between soil available P in the 0–5 (a) and 5–10 (b) cm layers and between soil aluminum (c) and soil pH (d) in the 5–10 cm layer after mechanical soil scarification (MSC) and the cultivation of cover crops (CCs) and rice. The means followed by the same lowercase letter for CC with MSC (6.2 and 10.5 mg dm−3, 0.2, 1.7 mmolc dm−3) and the same capital letter for MSC in CC (8.8 and 14.9 mg dm−3, 0.32, 2.4 mmolc dm−3) did not significantly differ according to the Tukey test at 5% significance; coefficient of variation, CV (%) = 13, 21, 3, and 7 in Selvíria, MS, Brazil, in the first growing season.

Figure 6. Interactions between soil available P (a), Ca (b), CEC (c), and SB (d) in the 0–5 cm layer after mechanical scarification, cover crops, and rice cultivation under NTS. The means followed by the same lowercase letter for CC with MSC (8.8 mg dm−3, 5.2, 10.5, and 11.0 mmolc dm−3) and capital letters for MSC with CCs (12.5 mg dm−3, 7.5, 15.0, and 15.8 mmolc dm−3) did not significantly differ according to the Tukey test at 5% probability; coefficient of variation CV (%) = 17, 9, 8, and 11 in Selvíria, MS, Brazil, in the first growing season.

Figure 6. Interactions between soil available P (a), Ca (b), CEC (c), and SB (d) in the 0–5 cm layer after mechanical scarification, cover crops, and rice cultivation under NTS. The means followed by the same lowercase letter for CC with MSC (8.8 mg dm−3, 5.2, 10.5, and 11.0 mmolc dm−3) and capital letters for MSC with CCs (12.5 mg dm−3, 7.5, 15.0, and 15.8 mmolc dm−3) did not significantly differ according to the Tukey test at 5% probability; coefficient of variation CV (%) = 17, 9, 8, and 11 in Selvíria, MS, Brazil, in the first growing season.

Figure 7. Interactions between soil S (a), Mg2+ (b), V% (c), and H+ + Al3+ (d) in the 5–10 cm layer after mechanical scarification and cover crop and rice cultivation under NTS. Means followed by the same lowercase letter for CCs with MSC (7.7 mg dm−3, 3.5 and 2.5 mmolc dm−3 and 4%) and capital letter for MSC with CCs (10.92 mg dm−3, 5.01 and 3.52 mmolc dm−3, and 7%) did not differ significantly according to the Tukey test at 5% probability; coefficient of variation CV (%) = 24, 12, 7, and 5 in Selvíria, MS, Brazil, in the second growing season.

Figure 8. Interactions between soil available P (a), Al3+ (b), H+ + Al3+ (c), and CEC (d) contents in the 10–20 cm layer after decompaction by mechanical scarification (MSC) and cover crop and rice cultivation under NTS. The means followed by the same lowercase letter for CCs with MSC (7.5 and 11.0 mg dm−3, 3.4, 0.6, and 4.4 mmolc dm−3) and capital letter for CCs with MSC (10.7 and 15.7 mg dm−3, 4.9, 0.9, and 6.3 mmolc dm−3) did not significantly differ according to the Tukey test at 5% probability; coefficient of variation CV (%) = 23, 5, 17, 7, 10, and 5 in Selvíria, MS, Brazil, in the second growing season.

Figure 9. Interactions between soil S (a) and CEC (b) in the 10–20 cm layer after mechanical soil scarification (MSC) and cultivation of cover crops (CCs). The means followed by the same lowercase letter for CC with an MSC of 6.9 mmolc dm−3 and 7.7 mg dm−3 and the capital letter for MSC in CCs (9.8 mmolc dm−3 10.1 mg dm−3) did not significantly differ according to Tukey’s test at 5% probability; coefficient of variation CV (%) = 7.8 and 21.9 in Selvíria, MS, Brazil, in the first growing season.

Figure 10. Interactions between soil H+ + Al3+ (a) and Al3+ (b) in the 20–40 cm layer after decompaction by mechanical chiseling (MSC) and CC and rice cultivation under NTS. The means followed by the same lowercase letter for CC with MSC (2.3 and 0.6 mmolc dm−3) and capital letters for MSC with CC (3.2 and 0.9 mmolc dm−3) did not differ significantly according to Tukey’s test at 5% significance; coefficient of variation CV (%) = 5 and 11 in Selvíria, MS, Brazil, in the second growing season.

Table 1

Cultivation sequence in the experimental area over five growing seasons, three before and two during the experimental period, Selvíria, MS, Brazil.

		Season
		Spring	Summer	Winter
	August	September-November	November–March	May–August
		Fallow	Maize	Bean
		Fallow	Maize	Bean
		Fallow	Maize	Fallow
1st year	MSC	CC	Rice *	Bean
2nd year		CC	Rice *	Bean

MSC: mechanical soil chiseling to a depth of 0.30 m; CCs: cover crops; * soil sampling after rice and bean cultivation for physical and chemical analyses.

Table 2

Soil chemical and physical properties in the studied layers of the experimental area before the experiment in Selvíria, MS, Brazil.

	P	S	OM	pH	K+	Ca2+	Mg2+	H + Al	Al3+	SB	CEC		V
Depth (cm)	mg dm−3		g dm3	CaCl2	-------------------------- mmolc dm−3 ------------------------------------								%
0–5	29	3	24	5.9	3.5	41	25	19	0	69.5	88.5		79
5–10	6	6	17	4.9	1.8	17	12	33	2	30.8	63.8		48
10–20	38	13	15	4.5	1.3	10	7	38	6	18.3	56.3		33
20–40	7	43	13	4.8	1.4	10	8	29	2	19.4	48.4		40
	Particle size					Physical properties
	Sand		Silt		Clay	Macroporosity		Microporosity		Total		BD
Depth (cm)	-------------- (g kg−1) ------------					----------------- (m3 m−3) --------------						(Mg dm−3)
0–5	403		157		440	0.08		0.36		0.44		1.49
5–10	389		127		484	0.06		0.35		0.41		1.56
10–20	385		120		495	0.07		0.35		0.42		1.54
20–40	352		121		527	0.10		0.36		0.46		1.42

Depth: soil depth; P: available phosphorus (resin); S: sulfur; OM: organic matter; exchangeable K, Ca, Mg, and Al; H + Al: potential acidity; SBs: sum of bases; CEC: cation exchange capacity; V: base saturation; Total: total porosity; Macro: macroporosity; Micro: microporosity; BD: soil bulk density.

Table 3

Average values of highland rice grain yield after mechanical soil scarification and shoot dry matter yield of the aerial part of the vegetation cover in the first and second years of the research in Selvíria, MS, Brazil.

	Shoot Dry Matter Yield		Grain Yield
	------------------ (Mg ha−1) ------------------		------------------ (kg ha−1) ------------------
	1st Year	2nd Year	1st Year	2nd Year
	Mechanical soil scarification (MSC)
Without	9.9	11.1	4539 b	4078
With	10.5	12.9	5091 a	4196
	Cover Crops (CCs)
Fallow	9.5	11.71	5000 b	3873
U. ruziziensis	10.8	13.74	4284 c	4155
C. juncea	10.9	12.19	5024 b	4204
C. cajan	9.7	12.19	5614 a	4143
P. glaucum	10.8	10.18	4154 c	4309
	F-Value (5%)
MSC	5.85 *	24.99 *	22.86 *	1.51 ns
CC	7.86 *	9.97 *	21.58 *	2.26 ns
MSC × CC	7.98 *	6.40 *	2.04 ns	1.87 ns
	Minimum significant difference (5%)
MSC	--	--	236.56	--
CC	--	--	532.60	--
CV (%)	8.02	9.5	7.57	7.35

* significant at 5% probability according to the F test. ^ns not significant. Means of mechanical soil scarification and cover crops followed by the same letter did not differ significantly from each other according to the Tukey test at the 5% significance level. Coefficient of variation (CV); -- Due to the studied variable not showing a significant difference, the minimum significant difference values (5%) were not presented.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su16104098/s1, S1. List of Reagents Used. Figure S1. Preparing the soil mechanically with a scarifier (A) and general view of the area after passing the scarifier along the defined tracks (B). Selvíria, Mato Grosso do Sul, Brazil. Figure S2. Manual sowing of legumes (C. cajan and U. ruziziensis) with the aid of ratchet equipment (A) and supplementary irrigation with a central pivot after sowing the cover crops (B). Selvíria, Mato Grosso do Sul, Brazil. Figure S3. Cover plant C. juncea at the full flowering stage (A) and C.cajans at the time of desiccation (B). Selvíria, Mato Grosso do Sul, Brazil. Figure S4. Cover plant U. ruziziensis at the time of desiccation (A) and millet at the time of desiccation (B). Selvíria, Mato Grosso do Sul, Brazil. Figure S5—Fallow treatment with no preparation (A) and scarified fallow with the presence of spontaneous vegetation (B), both at the time of desiccation. Selvíria, Mato Grosso do Sul, Brazil. Figure S6—Sowing of upland rice in the experimental area (A) and general view of the rice crop 4 days after emergence (B). Selvíria, Mato Grosso do Sul, Brazil. Figure S7—General view of the rice crop at 60 days after emergence (A) and general view of the rice crop at 103 days after emergence (B). Selvíria, Mato Grosso do Sul, Brazil.

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