Residue decomposition dynamics in mixed ratios of two warm-season cover crops

Abstract

Cover crop residue management has gained attention over the past few decades due to the potential of cover crop residues to provide soil cover and nutrient availability for subsequent crops. Nutrients released from cover crop residues depend on the nutrient composition of plant tissue and ensuing rates of mineralization. To evaluate the rate of carbon (C) and nitrogen (N) mineralization from cover crops, a field decomposition experiment was conducted in a humid subtropical agricultural field. Residues of two warm-season cover crops, sunn hemp (Crotalaria juncea L.) and sorghum sudangrass (Sorghum bicolor L. × S. bicolor var. Sudanese), were placed in litterbags at various mixture ratios on the soil surface to quantify the amount of N and C released from the residues over 56 d. Residue biomass decomposition followed decay patterns that were similar despite treatment mixtures of cover crop ratios. The initial ratio of cover crop material in the litter mixture influenced early N release where ratios with increasing sunn hemp increased N decomposition. The majority of residue breakdown (50–75%), and C (50–75%) and N (65–75%) decomposition occurred during the first 7 d for all the treatments. A minor fraction of residue remained (10–14%) at Day 56, with 10–14% C and 7–25% N. The 25% N remained in the sole sorghum sudangrass treatment. These results indicate that N from legume cover crops may not be available to subsequent crops in humid subtropical soils lacking the capacity to retain nutrients leached out of cover crop residue, though small fractions of dry matter may remain.

Abbreviations
• C
• carbon
• DAP
• day after placement
• DM
• dry matter
• N
• nitrogen
• OM
• organic matter
• TREC
• UF/IFAS Tropical Research and Education Center

INTRODUCTION

Cultivation of cover crops can provide numerous ecosystem services and reduce environmental impacts from agriculture (Snapp et al., 2005; Wittwer, et al., 2017; Wortman et al., 2012). Cover crops are planted to strengthen the soil surface, suppress weeds, facilitate nutrient cycling for essential plant nutrients such as nitrogen (N) and carbon (C), and enhance soil organic matter (OM) when left in the field to decay (Schwartz, 2014; Wang et al., 2003). As a result, cover crop treatments have been integrated in cropping system rotations over the past few decades (Muchanga, et al., 2020).

Cover crops are commonly used with the intent to supply nutrients to the soil, often through nitrogen-fixing legumes (Blanco-Canqui et al., 2012). Leguminous cover crops convert atmospheric N into forms that can be used by plants (e.g., NH₄⁺, NO₃⁻) through biological N fixation. Besides legumes, several other types of nonlegumes (e.g., grasses, forbs) can be used as cover crops. Nonleguminous cover crops play an important role in scavenging nutrients in the soil by trapping them in their tissues, which will be released and used by subsequent crops once the cover crop residues break down (Kaur et al., 2017). Nutrient scavenging is especially beneficial when residual nutrients are captured from the previous cropping cycle, which would otherwise be lost through nutrient leaching or runoff.

Management practices including cover crops for soil nutrient retention can further contribute to soil health (Li et al., 2015; Marchi et al., 2016). Together, legume and nonlegume cover crops can supply OM to the soil with nutrients available for subsequent cash crops (Möller et al., 2008). Such a response in OM accumulation has been observed in the long-term along with increased C availability and reduction of the risk of other nutrient losses through leaching and erosion (McDowell et al., 2019). However, during the decomposition processes, N released from cover crop residues can be leached beyond the root zone making nutrient mineralization timing a critical factor in cover crop benefit and nutrient management (Brust, 2019). The ultimate benefits of cover crops are best achieved when they are treated as an integral part of the cropping system (Cogger et al., 2014).

Some important factors to consider for integrating cover crops in a cropping system are cover crop selection (Wang et al., 2004; Wortman et al., 2012), cover crop termination method and timing (Jani et al.,2016; Jani & Mulvaney, 2019; Mulvaney et al., 2017), and cover crop management in monocultures or mixtures. These factors have the potential to influence the rate of nutrient cycling and the amount returned to soils (Sainju et al., 2008). Yet, residue decay can occur quickly in warm and porous soil as the biological processes for decomposition are promoted in these conditions allowing for only short-term residence times of available nutrients from cover crop decay to subsequent crops.

Soil C and N cycling are two biogeochemical processes that play a significant role in improving soil fertility (Cai et al., 2016). Their ratio (C/N) in plant residues has a direct impact on C and N cycling in soils (Hoorman, 2009). Understanding C and N dynamics of cover crop residues can make cover crop selection and practices more effective at improving soil C and N content. Monitoring cover crops residue composition and decay is a vital component in understanding the capacity of the soil to store and cycle these nutrients.

An effective method to monitor residue decomposition is the litterbag approach (Karberg et al., 2008; Stallings et al., 2017). Monitoring bags of fresh leaf litter for decomposition in-situ retains exposure to environmental conditions and soil organisms while minimizing excessive particle losses. As a result, nutrient content and decomposition can be quantified to provide residue nutrient trends that occurred during the decay processes.

Core Ideas

• Cover crop residue management for C and N in subtropical agriculture should consider rapid decomposition.
• More than 50% of residue breakdown and nutrient mineralization occurred in the first 7 d of decomposition.
• Ten to 14% residue remained across treatments at 56 d after planting.
• Sorghum sudangrass residue decomposition was slower than when mixed with sunn hemp residue.

In South Florida, sunn hemp (Crotalaria juncea L.) and sorghum sundangrass (Sorghum bicolor L. × S. bicolor var. Sudanese) have been used as warm-season cover crops both in monocultures and mixtures (Wang et al., 2015). They are respectively used to supply and scavenge N into the soil for subsequent crops. Through expected contributions of plant growth and incorporation into the soil, these cover crops have been shown to improve yields and quality in several vegetable crops in South Florida (Wang et al., 2003). It remains unclear if the benefits from these cover crops are from contributions of OM and nutrients. Nutrients and OM can easily be leached through the soil in South Florida due to heavy rains and coarse soil texture. Research has shown that rainfall, a key feature of the South Florida warm-season, influences soil moisture and temperature thereby impacting soil biological activities and increasing nutrients losses because the soil has low-water and nutrient-holding capacities (Laporte et al., 2002; Salamanca et al., 2003). As a result, precipitation's leaching effect increases litter decomposition and nutrient losses, a challenge to cover crop management. One aspect we consider for cover crop management is the planting of cover crop mixtures.

The decomposition rate of mixed cover crop residues were investigated in this study by quantifying the amount of C and N released from the residues over time in a subtropical agricultural environment. This study was further designed to evaluate mixed cover crop residue decomposition dynamics in the warm rainy season of a humid subtropical environment. Understanding C and N decomposition rates of the cover crop mixture ratios can provide information on the residue of OM and the availability of nutrients for subsequent crop uptake. Thus, the objectives of this experiment were to measure the amount of C and N remaining from different cover crop mixture ratios over time as well as to identify the potential relationship between different cover crop mixture ratios and the availability of C and N during the mineralization process. We hypothesized that the mixture of a legume and nonlegume would influence greater retention of C and N in cover crop residue.

MATERIELS AND METHODS Site description

The study was conducted at the UF/IFAS Tropical Research and Education Center (TREC) in Miami-Dade County, Homestead (25°28′7.3992″ N, 80°28′39.2052″ W, 2.028 m elevation) on Krome gravelly loam (pH∼8 and slope∼0), which consists of very shallow, moderately well drained, moderately permeable soils over limestone in cropland, a soil series unique to this region. This area has a subtropical climate with hot and humid summers (33 °C), with an average annual precipitation of 1,500 mm (NOAA, 2022). The soil is primarily rocky (40–45%) with an OM content <2% and is relatively low in water and nutrient-holding capacities (Crane et al., 2006; Uzoma et al., 2011). Soil samples were collected prior to the study at 0-to-6-cm depth and analyzed for N content, including 1,758 mg kg^-1 total Kjeldahl N, 3.25 mg kg^–1 NH₄, and 7.42 mg kg^–1 NO₃.

Experimental design

An in-field residue decomposition experiment was set up from 7 July to 26 Aug. 2020 utilizing litterbag methodology. This approach is used to study decomposition at the soil surface (Karberg et al., 2008; Stallings et al., 2017). Two warm-season cover crops (sunn hemp and sorghum sundangrass) were used in multiple mixture ratios for this litterbag study. The nylon bags, 0.02 m² area with 0.005–0.006 mm openings (Ankom Technology), were placed into a completely randomized experimental design with four replicates of five cover crops ratios: 100% sorghum sudangrass (SS-100), 75% sorghum sudangrass, and 25% sunn hemp (75SS-25SH), 50% sunn hemp and 50% sorghum sudangrass (50SS-50SH), 25% sorghum sudangrass and 75% sunn hemp (25SS-75SH), 100% sunn hemp (SH-100). The litterbag material was composed of the aboveground residues collected from the two warm-season cover crops terminated 60 d after planting and then placed in the field with the other remaining residue for 56 d.

Residue collection and sampling

Prior to the experiment, a 50:50 cover crop mixture of sunn hemp and sorghum sudangrass was planted into furrows spaced 20.3 cm between row at rates of 16.81 kg ha⁻¹ per species. The average aboveground biomass production for sunn hemp and sorghum sudangrass from a multi-site experiment conducted the previous growing season was used to determine the amount of fresh biomass to be placed in each litter bag. The purpose of this study was to approximate the litter dynamics of a green cover crop mechanically terminated using a mower. The sample biomass for each bag contained approximately 50% leaf and 50% stem tissue. The average dry biomass production was determined to be 12.46 t ha⁻¹, which gives 25 g 0.02 m⁻² of total cover crop biomass in each litterbag.

At termination, fresh aboveground biomass for each cover crop species was collected and dried down in a passive-air drier at 75 °C to a constant weight. Dry leaves and stems samples were prepared to be representative of realistic litter composition and placed in litter bags based on treatment ratio: SS-100, 75SS-25SH, 50SS-50SH, 25SS-75SH, SH-100. Treatments (n = 5) were arranged in a completely randomized design with four replicates for each of the six sampling date, which conferred a total number of 120 observations for the study. Each bag was packed with residue, labeled with treatment identification, sealed, then pinned on the soil surface with ground staples so that there was direct contact between the bags and the soil surface on 7 July 2020. Daily temperature and precipitation were recorded for the duration of the experiment (Figure 1).

FIGURE 1. Average daily precipitation and temperature recorded from the Florida Automated Weather Network at UF/IFAS Tropical Research and Education Center (TREC) in Homestead, FL, during the study period 7 July (days after placement [DAP] = 0)–26 Aug. 2020 (DAP = 56)

Residue processing

Four bags (replicates) of each treatment were retrieved at 4, 7, 14, 28, 42, and 56 d after placement from the soil surface so that at each sampling date, 20 bags were retrieved from the field. Once collected from the field, bags were gently brushed to remove accumulated soil with contents oven-dried at 75 °C and weighed. Dried samples were then ground, sieved at 2 mm, weighed for dry matter, and delivered to the UF/IFAS North Florida Research and Education Center for analysis. Residue samples were analyzed for total C and N content using a Carbon Hydrogen and Nitrogen analyzer by the Dumas dry combustion method (Vario Micro Cube; Elementar). Residue from each bag was also prepared and ashed to identify soil mineral contaminants in a muffle furnace at 550 °C for 4 h. Litterbag content is reported on an ash-free dry weight basis.

Statistical analysis

The decomposition dynamics of the cover crops ratio treatments were analyzed using ANOVA (aov function) in R (Version 1.2.5033, 2019). The treatment variables were the ratio treatments and days after placement (DAP), including the interaction of treatment and DAP. The response variables evaluated were dry matter (DM), N content, C content, and C/N ratio. Tukey post-hoc test (p < .05) was used to determine significant differences among treatment means.

RESULTS AND DISCUSSION Climate condition

The average daily temperature was between 26 and 29 °C and average daily rainfall was between 0 and 58 mm at TREC during the experiment (Figure 1). Half of the precipitation across the period of the study occurred during the first 14 d of the experiment with two heavy rain events >2 cm in the 1st week and another two in the 2nd week. Leguminous cover crops have been demonstrated to contribute to increased N leaching aided by increased precipitation (Muñoz-Carpena et al., 2008).

Residue decomposition

Regardless of crop ratio treatment, 50–75% of residue breakdown occurred by 7 d after placement. From Day 28 until the end of the trial, a similar amount of dry matter remaining was observed for all ratio treatments (Figure 2). This could be the fact that the same ratios of stem and leaf fractions were identical in each litterbag (50% leaf/50% stem), of which fractions could decompose at similar rates despite cover crop identity (Table 1, Abiven et al., 2005). Yet, cover crop residue decomposition was significantly affected by treatment, DAP as well as their interaction as expected for a decay experiment (Table 2). Sole sorghum sudangrass (SS-100) had the slowest initial residue breakdown rate expressed as dry matter remaining followed by the remaining ratio mixtures and sunn hemp, perhaps due to the high C/N ratio (Table 1, Figure 2). In a manner similar to previously reported research, more rapid decomposition occurred during the early stage of the decomposition period (Jani et al., 2016; Jani & Mulvaney, 2019; Mulvaney et al., 2017).

FIGURE 2. Aboveground biomass residue dry matter (DM) presented in (a) absolute value, and (b) percentage remaining over time (x axis; days after placement [DAP]) across each cover crop treatment ratio with sorghum sudangrass (SS) and sunn hemp (SH) by percentage. Error bars represent 95% confidence interval around the mean

TABLE 1 Cover crop ratio initial litter conditions (days after placement = 0) for nitrogen (N), carbon (C), and C/N ratio. Cover crops are sunn hemp (SH) and sorghum sudangrass (SS) in the mixed residue ratios reported

Cover crops ratio
SH	SS	N	C	C/N ratio
		g
100	0	0.624a	10.00a	16.0c
75	25	0.529b	9.94ab	18.8c
50	50	0.436c	9.85abc	22.6bc
25	75	0.343d	9.84bc	29.0b
0	100	0.214e	9.71c	46.6a

Note. Letters denote statistically significant differences (p < .05) as determined by a post-hoc Tukey test.

TABLE 2 Analysis of variance p value results for the cover crops residue decomposition measuring percentage mass, carbon (C) and nitrogen (N) remaining, and C/N ratio at various sample times during the trial

	Mass remaining	N remaining	C remaining	C/N
DAP	<.01	<.01	<.01	.02
Treatment	<.01	<.01	<.01	<.01
DAP × treatment	<.01	<.01	<.01	<.01

Note. DAP, days after placement.

The rapid decomposition of litter during the first 14 d of the study could be accelerated by precipitation as there were four major rain events >22 mm in that time period (Figure 2). Rate of decomposition has been shown to increase with greater precipitation (Austin & Vitousek, 2000; Salamanca et al., 2003; Swift, et al., 1979). In addition, higher moisture content is often associated with increased rates of decomposition of residues in litterbag field studies (Lynch et al., 2016). This is also the period that the leaf fraction of litter is likely to experience rapid decay, while the stem fraction may be slower (Abiven et al., 2005). Variability, as demonstrated by confidence intervals around the mean, is also greater during the first 2 wk, perhaps indicating active decomposition at slightly varying timing and rates.

Nitrogen decomposition

Each cover crop treatment ratio with sunn hemp (SH) by percentage (SH-100, 75SH-25SS, 50SH-50SS, 25SH-75SS) had higher N concentration than treatments with less SH (Table 1). Nitrogen content in crop residue dropped considerably for all ratio treatments from the beginning of the trial indicated by the statistically significant effect of the cover crop treatments, DAP, and their interaction (Figure 3, Table 2). Nitrogen concentration decreased nonlinearly for all residue treatments during the initial 7 d after the bags were placed on the soil surface. The N remaining from sorghum sundangrass residue by itself was slightly greater (10–20% from 0 DAP) than all other cover crop ratio mixtures.

FIGURE 3. Nitrogen (N) remaining presented in (a) absolute value, and (b) percentage over time (x axis; days after placement [DAP]) across each cover crop treatment ratio with sorghum sudangrass (SS) and sunn hemp (SH) by percentage: SS-100, 75SS-25SH, 50SS-50SH, 25SS-75SH, SH-100. Error bars represent 95% confidence interval around the mean

Furthermore, the N decomposition appeared to correspond with residue breakdown. Previous research on cover crop mineralization has reported residue N loss occurs at the initial stage of the decomposition (Lynch et al., 2016; Mulvaney et al., 2017; Wagger, 1989). This is an indication that N may not be available to subsequent crops in humid subtropical soils lacking the capacity to retain nutrients leached out of cover crop residue. Thus, to utilize N contributions from cover crops, planting of the subsequent crop needs to be synchronized with N release from fastest decaying plant parts containing most of the N or N would need to be maintained in the soil by some other means such as through the accumulation of OM. It is feasible to plant a crop within that timing window, nevertheless, the short-term allelopathic characteristics of sunn hemp may inhibit germination of the following crop (Skinner et al., 2012).

Nonlegume cover crops have the ability to scavenge N from the soil, hold N in their residues, and make them available for the next cropping cycle. The amount of N scavenged depends on the amount of N available in the soil, which was determined to be 7.42 mg kg^-1 NO₃ and 3.25 mg kg^-1 NH₄ prior to the experiment, though this concentration is scattered around 40–45% gravel. Treatment ratios with high sorghum sudangrass had a significant difference in N remaining in their residues during the trial (e.g., SS-100 had 22% N left compared with 8.33% for SH-100 at 56 DAP) (Figure 3).

Carbon decomposition

Each cover crop treatment ratio with sunn hemp (SH) by percentage (SH-100, 75SH-25SS, 50SH-50SS, 25SH-75SS) had the highest C concentration respectively to start, but SH-100 rapidly declined to lowest remaining C at Day 7 (Figure 4). Slower early C mineralization rate compared with N mineralization rate was observed across treatments (Table 1, Figures 3 and 4). Carbon mineralization was significantly affected by treatment ratios, DAP and their interaction (Table 2). More than 50% of the C from the residues was mineralized around 7 DAP with 75% mineralized by 14 DAP. From Day 14 until the end of the trial, a uniform C remaining was observed for all the ratio treatments. Carbon mineralization was observed to have followed a similar trend as mass loss in agreement with previous research (Mulvaney et al., 2017). Observations were indicative of a rapid and nonlinear decline signifying that these warm-summer cover crops can introduce C to the soil potentially increasing soil quality and yield of subsequent crops, though leaching and soil respiration may result in transient C.

FIGURE 4. Carbon (C) remaining presented in (a) absolute value, and (b) percentage over time (x axis; days after placement [DAP]) across each cover crop treatment ratio with sorghum sudangrass (SS) and sunn hemp (SH) by percentage: SS-100, 75SS-25SH, 50SS-50SH, 25SS-75SH, SH-100. Error bars represent 95% confidence interval around the mean

Carbon/nitrogen ratio

Carbon/nitrogen ratio was significantly affected by treatment ratios, DAP, and their interaction, though these treatment effects were not consistent in pattern (Table 1, Figure 5). The different decomposition rates observed across ratios with high sunn hemp and high sorghum sundangrass content is likely due to the high baseline C/N ratio in sorghum sudangrass compared with that of sunn hemp. The treatments with high sorghum sudangrass content had the highest initial C/N ratio than the ones with higher amounts of sunn hemp. The ratios with the lowest initial C/N ratios mineralized N at a faster rate than those with higher initial ratios. However, variability in C/N ratio across time indicates a variable decomposition of litter, perhaps mechanically and chemically driven. Sorghum sudangrass C/N declined linearly in the first 14 d while sunn hemp increased which could be the fact that sunn hemp was easily decomposable, and N was not limiting to microbial growth. It may be that the mixture of cover crop species further influences the trajectory and pace of decomposition, though a pattern was not observed in this study.

FIGURE 5. Carbon/nitrogen (C/N) ratio over time (x axis; days after placement [DAP]) across each cover crop treatment ratio with sorghum sudangrass (SS) and sunn hemp (SH) by percentage: SS-100, 75SS-25SH, 50SS-50SH, 25SS-75SH, SH-100. Error bars represent 95% confidence interval around the mean

Overall, there was a high variability in C/N ratio across treatments. Numerous other researchers in laboratory and field analysis have shown that more rapid decrease in C/N ratio over time for residues with low C/N ratios (Austin & Vitousek, 2000; Lynch et al., 2016), which may explain the initial trends across 14 d in our results. Because the treatments with the lowest initial C/N ratios, such as SH-100, mineralized N and C faster, this treatment can be somewhat ineffective in protecting soil from erosive effects of rainfall in addition to the rapid loss of N. Nevertheless, those residues with high C/N ratio, such as SS-100, will possibly offer protection in a long term.

CONCLUSION

Greatest nutrient mineralization and residue breakdown occurred during the first 7 d for all crop residue treatments. The similar pattern of decomposition across treatments may be indicative of the similar composition of stem and leaf materials in litterbags. Residue composition for the litter bag was prepared consistently to represent the composition of stem and leaf tissue in the field prior to termination and sampling. The two cover crop species and resulting mixtures differed in C and N concentration where the mixtures with high sorghum sudangrass content had the greatest initial C/N ratio. Sunn hemp contained the highest N concentrations and released the most N to the system, albeit rapidly. Incorporation of cover crops into soil may increase biological N and C in the system in the short term, but other properties of the soil must be available to retain the nutrients following litter decay. In order to keep soil covered and increase nutrient supply to other subsequent crops it may be possible to employ a combination of cover crops at different ratios to improve the overall N status of the system or facilitate termination of cover crops at various stages in alignment with subsequent plantings. As such, cover crop termination should occur near planting of the subsequent crop so that rapid N and C release from the residues can be assimilated along with efforts to maintain biological sources of nutrients in the soil.

ACKNOWLEDGMENTS

LD was supported by U.S. Agency For International Development Support to Agricultural Research and Development Program. The project was supported by USDA-NIFA Hatch FLA-TRC-005661.

AUTHOR CONTRIBUTIONS

Larousse Dorissant: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Visualization; Writing – original draft; Writing – review & editing. Zachary T. Brym: Conceptualization; Data curation; Formal analysis; Funding acquisition; Investigation; Methodology; Project administration; Visualization; Writing – original draft; Writing – review & editing. Stacy Swartz: Methodology; Validation; Writing – review & editing.

CONFLICT OF INTEREST

The authors report no conflict of interest.