1. Introduction
Fine roots (<2 mm in diameter) are the main organs of the plant for nutrient and water uptake, allowing for material exchange and energy flow with the above-ground parts of the plant [1,2,3]. Plants can take up soil nutrients via fine roots and adjust their nutrient uptake strategies accordingly, especially when there is below-ground competition between species for soil resources. Fine root traits determine a plant’s ability to absorb nutrients, such as adaptation to the environment through changes in the allocation and morphology of fine root biomass, which are key indicators of plant growth to ensure survival [4]. FRB, SRL, RAD, RLD and other indicators reflect both the morphological structure of the root system and the role of roots in nutrient uptake, transportation and other functions [5,6,7]. Makita et al. found that the SRL plays a vital role in fine root resource allocation and nutrient uptake in their studies [8]. It has been shown that FRB and RLD changed significantly with increasing soil depth, and thus fine root function [9,10]. Therefore, it is necessary to explore response of soil nutrients and vertical distribution of underground fine roots to fertilization.
It has been shown that fine root traits are closely related to changes in soil nutrient effectiveness caused by fertilization [11,12]. Sofia et al. studied whether N, P and K fertilizers could effectively improve root morphology in cash crops, whereas the lack of N, P and K can inhibit root biomass [13]. Yang et al. investigated the effects of substrate and moisture on the root growth of Sapindus mukorossi seedlings, and the results showed that the root biomass, total root length, and root surface area of the seedlings decreased with the decrease in moisture content [14]. The reaction of fine root traits to fertilization is still controversial. For example, the application of moderate amounts of nitrogen fertilizer can significantly increase the root length of trees and improve the FSRL of fine roots [15]. However, applying too much fertilizer reduces root biomass and weakens the overall absorption capacity of the root system [16,17,18]. Therefore, further research is needed on the effects of fertilization on root traits, especially how to affect fine root function by changing soil properties.
Balanced fertilization is an application technology of fertilizer in a reasonable amount, which maintain a balance between soil fertilizer performance and nutrients demand of crops, based on the fertilizer characteristics [19,20]. Relevant studies have shown that unscientific fertilization measures can cause problems such as reduced soil quality, acidification and crusting [21,22]. In addition, unreasonable fertilization can lead to poor plant growth and reduce flowering and fruiting [20]. Proper N, P and K fertilizer ratios and application rates can significantly enhance nutrient uptake of plants [19,23,24]. Fertilization can change soil productivity by directly or indirectly changing the physical and chemical properties of the soil [25,26,27,28]. Li et al. found that fertilization had a significant effect on soil quick-acting nutrients and yield of citrus (Citrus reticulata Blanco), but soil quick-acting nutrients and yield showed a tendency to increase and then decrease with the increase in fertilization.
Sapindus mukorossi is a widely distributed energy plant in tropical and subtropical regions of China, and is widely used in bioenergy, biomedicine and cosmetics with high economic value. Previous studies have found that the most direct and effective way to increase yields and economic value is scientific fertilization [29,30,31]. However, unbalanced fertilization has caused serious problems in Sapindus mukorossi plantations, resulting in low yield and quality. Therefore, exploring scientific fertilization ratios is essential to improve soil nutrients and fruit yield and improve the sustainability of Sapindus mukorossi plantations [19]. Furthermore, balanced fertilization is very important to the physical and chemical properties of soil and the growth of root system in the forest floor of Sapindus mukorossi plantations which, in turn, can lead to the formation of the most suitable soil conditions for the root growth of Sapindus mukorossi. For this reason, the study was conducted with 6-year-old Sapindus mukorossi as the research object, and the “3414” fertilization experiment was adopted. This study aimed to achieve two objectives: (1) to investigate the response of fine root traits and soil properties of Sapindus mukorossi to different nitrogen, phosphorus and potassium (NPK) fertilizers; and (2) to reveal the relationship between fine root traits and soil properties of Sapindus mukorossi under different fertilizer treatments.
2. Materials and Methods
2.1. Overview of the Trial Site
The study site was located in Jianning County, Sanming City, Fujian Province (116°47′20″ E, 26°40′3″ N), with an average annual temperature of 17.0 °C, an average annual rainfall of 1792 mm, and a relative humidity of 84%. The soil of the test site was sandy clay loam, The test material was the asexual line of Sapindus mukorossi raw material forest ‘Yuanhua’, the age of the forest was 6 years old, and the row spacing was 4 m × 4 m. The total experimental sample area was 3 ha, with an average tree height of 2.38 m, an average ground diameter of 6.97 cm, and an average crown width of 2.4 m × 2.2 m. Soil properties are detailed in Table 1.
2.2. Experimental Design
A “3414” randomized block design was used for the formulated fertilization trial; we conducted a field trial using a combination of different levels of nitrogen (N), phosphorus (P) and potassium (K) fertilizers (Level 0: N, P, K all 0 kg·ha−2; Level 1: N 300 kg·ha−2, P 250 kg·ha−2, K 200 kg·ha−2; Level 2: N 600 kg·ha−2, P 500 kg·ha−2, K 400 kg·ha−2; Level 3: N 900 kg·ha−2, P 750 kg·ha−2, K 600 kg·ha−2) with a control treatment of no fertilizer. That is, 3 elements (N, P, K), 4 fertilization levels, and 14 treatments. The experiment had 3 replications, and a total of 42 treatment plots were laid out. Fertilizer application rates are shown in Table 2, and isolation rows were set up between treatment plots. Fertilizer was applied three times throughout the year, April 10 (flowering fertilizer, accounting for 30% of the total), July 20 (strong fruit fertilizer, accounting for 30% of the total) and November 1 (post-harvest fertilizer, accounting for 40% of the total). Using furrow fertilization method to apply fertilizer, according to the fertilizer dosage mixed into the application of the soil immediately after mulching, and other maintenance and management measures are the same as those of the control group. The test fertilizers were urea (containing N 46.0%) as the only source of N, calcium superphosphate (containing P2O5 12.0%) as the only source of P, and potassium sulfate (containing K2O 60.0%) as the only source of K.
2.3. Sampling and Analysis of Fine Roots
Fine root collection: in the fruit expansion period of Sapindus mukorossi, 4 average standard wood sample plants were selected in each plot, and root samples (0–20 cm, 20–40 cm) were collected by root drill at a distance of 60–80 cm from the tree. Root samples were soaked in water and then rinsed under running water through a sieve with a pore size of 0.8 mm to separate the roots from most of the soil and other impurities. Live and dead roots were distinguished on the basis of color, elasticity, shape, and ease of peeling of the cortex from the mid-column [32], and all live roots were carefully picked up in deionized water using forceps and a mesh spoon. According to the traditional root classification criteria [9], fine roots and coarse roots are classified with the diameter ≤ 2 mm as a threshold. An Epson Twain Pro root scanning system (Seiko Epson Inc., Suwa, Japan) and a WinRhizo root image analysis system (Regent Instruments Inc., Quebec, QC, Canada) were applied to analyze the fine roots. And then, we oven-dried the roots at 65 °C to a constant weight to calculate fine roots dry mass (RDM, g). Specific root length (FSRL, cm.g−1), root length density (RLD, cm.cm−3), fine root biomass (FRB, mg.cm−3) and average diameter (RAD, mm) were calculated using the following formulas [4,32]:
FRB = RDM × 103/V
RLD = RL × 106/V
FSRL = RL/RDM
In the above equation, RDM is the dry mass of fine roots (g), RL is the total root length (cm) and V is the volume of soil sample (cm3).
2.4. Soil Sampling and Analysis
In each standard plot, soil profiles were excavated according to the five-point sampling method. Soil samples were collected at soil depths of 0–20 cm and 20–40 cm, and 1kg was taken out as a sample by tetrad method. Then, we naturally air-dried and sieved the 1kg sample for determining the contents of soil pH, SOC, TN, TP, TK, AK, AP and AN. SOC was determined by potassium dichromate oxidation-external heating method [33]; TN was determined by Kjeldahl nitrogen determination method using 2300 K jeltec analyzer (FOSS, Munkedal, Sweden). AN was determined by alkaline dissolution diffusion method; TP and AP were determined by molybdenum-antimony antimony colorimetric method; TK was determined by atomic absorption method; AK was determined by flame photometric method; and soil pH was determined by a pH meter in a mixture of soil and water (1:2.5).
2.5. Statistics and Analysis of Data
Statistical analyses by SPSS21.0 (SPSS Inc., Chicago, IL, USA), one-way analysis of variance (ANOVA) and least significant difference multiple range test (p ˂ 0. 05) were performed to assess the effect of different NPK fertilizers on soil properties (TN, TP, TK, SOC, AN, AP, AK, pH), and fine root traits (fine root biomass, root length density, specific root length, and average diameter). Redundancy analysis (RDA) and mapping of the relationship between fine root traits and soil properties of Sapindus mukorossi were carried out using Canoco5.0 (Microcomputer Power, New York, NY, USA). Important soil factors affecting the overall variation in fine root traits in Sapindus mukorossi were explored. Pearson correlation analysis was used to determine the relationship between fine root traits and soil properties.
3. Results and Analysis
3.1. Effect of Different Fertilization Treatments on Fine Root Traits
In the 0–20 cm soil layer, fine root biomass, root length density, specific root length, and average diameter of Sapindus mukorossi were significantly increased under fertilized conditions as compared to control (Figure 1) (p ˂ 0. 05). The increase compared to CK was 26.31%–188.89%, 36.84%–331.57%, 5.18%–85.09% and 5.71%–37.14%. In comparison, in the soil layer of 20–40 cm, the fine root biomass, root length density and average diameter of Sapindus mukorossi reached the maximum under N3P2K2 treatment, which significantly increased by 161.73%, 188.36%, and 42.43%, respectively. Compared to the control, there was no significant difference in specific root length.
In the vertical direction, fine root biomass, root length density, and average diameter showed a decreasing pattern with increasing soil depth under different fertilization treatments, while the trend of specific root length with increasing soil depth was less obvious. In the 20–40 cm soil layer, the fine root biomass of Sapindus mukorossi was significantly reduced by 25.39%–63.01% under different fertilization treatments compared to 0–20 cm; the reduction in root length density ranged from 25.54%–69.12%; while the reduction in average diameter ranged from 12.86%–35.42%.
3.2. Effect of Different Fertilizer Treatments on Soil Properties
In the 0–20 cm soil layer, except for pH, SOC, TN, TP, TK, AP, AK and AN contents of Sapindus mukorossi soils were higher than those of the control group under the different fertilizer treatments, which increased by 14.25%–52.61%, 3.90%–39.84%, 9.52%–150%, 1.39%–29.67%, 118.58%–407.08%, 5.35%–39.69% and 3.63%–40.41%, respectively, compared with CK. The maximum value of each soil nutrient content was significantly increased by 50.81%, 42.85%, 126.68%, 25.13%, 233.64%, 33.49% and 47.09% in the 20–40 cm soil layer as compared to CK. The increase in TN and AN content in the 20–40 cm soil layer was slightly higher than that in the 0–20 cm, while the increase in the rest of the soil indicators was lower than that in the 0–20 cm. At the levels of P2K2, N2K2, and N2P2, SOC and TN contents showed a trend of increasing and then decreasing with the application of N, P and K in the 0–20 and 20–40 cm soil layers, and the N2P2K2 treatment was significantly higher than N0P2K2, N2P0K2, and N2P0K2. In vertical distribution, SOC, TN, TP, TK, AP, AK and AN contents of Sapindus mukorossi soils under different fertilization treatments decreased significantly with increasing soil depth (p ˂ 0. 05) (Figure 2).
3.3. Relationship between Fine Root Traits and Characteristic Properties of Soil Nutrients in Sapindus mukorossi
RDA analysis showed that 41.10% and 5.64% were explained by different fertilization treatments in the 0–20 cm soil layer; 20−40 cm soil layer were explained by 35.79% and 9.17% (Table 3); refer to the associated figure (Figure 3a,b). The vast majority of both fine root traits and soil physicochemical properties were located in the first axis, with FRB, SRL, RLD, RAD, TK, AK, AP, SOC, TP, TN and AN at one end of the spectrum, while pH was at the other. Only SRL was located in the second axis, but the contribution to both axes was small. As can be seen from the figure, root length density and fine root biomass tended to be higher in the high fertilization treatments, especially high N and P, and had a strong positive correlation with SOC, TN, AN, and TP; in the 20–40 cm soil layer, unlike in the 0–20 cm, SRL and TK were in the second axis. Interestingly, fine root biomass and average diameter tended to be higher in the high N and P fertilizer treatments and were mainly regulated by soil available phosphorus (AP) content (Figure 3b). Soil AP content was the main factor influencing fine root traits in Sapindus mukorossi.
In the soil layer of 0–20 cm, the fine root biomass and root length density of Sapindus mukorossi were significantly and positively correlated with the soil organic carbon, available nitrogen and available phosphorus content, while soil total phosphorus and available phosphorus were negatively correlated with specific root length and positively correlated with the diameter (Table 4). Soil pH was significantly negatively correlated with the fine root biomass, positively correlated with the specific root length, and negatively correlated with the diameter in Sapindus mukorossi. In the 20–40 cm soil layer, organic carbon was significantly and positively correlated with total nitrogen, available phosphorus and fine root biomass, and root length density and average diameter were significantly and positively correlated with soil available phosphorus and total nitrogen content, while negatively correlated with pH and available potassium content.
4. Discussion
4.1. Effect of Fertilization on Fine Root Traits
Fine roots, providing the plant with mineral nutrient resources, are the main organs of nutrient uptake and the link between the soil and the plant. They are very sensitive to changes in the environment, often showing excellent plasticity, and their development has a major impact on plant nutrition [34,35,36]. They not only determine the utilization efficiency and potential of the soil, but also reflect the distribution pattern of mineral nutrients in the soil. In this study, it was found that FRB, RLD and RAD under different fertilization treatments showed a decreasing pattern with the increase in soil depth, and the fine root biomass of Sapindus mukorossi was significantly reduced by 25.39%–63.01% in the soil layer of 20–40 cm, compared to 0–20 cm. It indicates that the fine roots of Sapindus mukorossi were mainly distributed in the top soil layer and, similarly, other tree species such as poplar (Populus), moso bamboo (Phyllostachys edulis) and pomelo (Citrus maxima) have similar results [4,14,32]. The topsoil layer is the main provider of nutrients for plant roots [10]. The topsoil layer is rich in nutrients due to fertilizer application and decomposition of some of the upper residues in the forest floor, combined with suitable conditions of soil texture, temperature and nutrients. As a result, the topsoil layer of the forest floor becomes the concentrated distribution layer for fine roots of forest trees. And as the soil depth deepens, the distribution of fine roots decreases layer by layer due to the reduction in other resources compared to the topsoil layer [37,38].
In this study, the fine root biomass, root length density and specific root length of Sapindus mukorossi gradually increased with the increase in fertilizer application, which was especially most significant in the high-level fertilization treatment. This indicates that the absorption capacity of the root system was strengthened after fertilization. This may be due to increased nutrient availability in the soil due to fertilization, driving changes in root function [39,40,41]. High fertilization conditions can increase nutrient transport around the soil to a wider area where roots are present, thus promoting forest root growth more efficiently [32]. In addition, the growth of fine roots is still affected by the fertilization method, stand type, and stand age [32,42,43,44]. At present, Sapindus mukorossi is now at the first fruiting stage, and it has not formed a well-developed root system, which is very limited for absorbing and utilizing the nutrient resources in the deep soil. It absorbs the water and nutrient resources through a large number of fine roots concentrated in the topsoil layer. It has been shown that root length density and surface area density of fine roots both decrease with increasing stand age in shallow soil layers [45].
4.2. Effect of Fertilizer Application on Soil Properties of Sapindus mukorossi Woodlands
The results of this study showed that different nitrogen, phosphorus and potassium fertilization treatments could significantly affect the soil properties of Sapindus mukorossi. Fertilization could effectively increase the SOC, TN, TP, TK, AP, AK and AN contents of forest soils, and effectively improve the soil fertility. This is consistent with results from citrus (Citrus reticulata Blanco) [19] and pine (Picea schrenkiana Fisch) [46]. The soil nutrient content of Sapindus mukorossi decreased with the increase in soil depth. Differential variations in soil nutrient content in different soil layers of Sapindus mukorossi plantation forests may be caused mainly by fertilization, apoplastic forms of plant above-ground parts and inputs from the root system and root secretions [47]. AP can be utilized directly by plants in inorganic forms and small molecule organic phosphorus. It is mainly from the dissolution of phosphate minerals and the release of adsorbed and immobilized phosphorus, including the dissolution of inorganic phosphorus, desorption of adsorbed phosphorus, mineralization of organic phosphorus and the reaction of phosphorus with other soil components during transport [48,49]. In this study, it was found that the TP and AP contents generally showed an increasing trend with increasing phosphorus addition. This is because phosphate fertilizers themselves contain large amounts of phosphorus, which can increase soil available P. Soil AP can easily combine with Al3+ activated by soil acidification to form low-soluble compounds, making the increase in soil phosphorus content. The increase in fertilizer application will lead to the increase in phosphorus in the soil, which reduces the fixation capacity of the soil for phosphorus, resulting in the increase in released phosphorus [50,51]. In addition, the higher Fe content in the soil in the southern region is easy to combine with available P, resulting in an increase in TP and AP content. AP is the fraction of phosphorus in the soil that can be directly absorbed and utilized by plants, and is an indicator of the level of soil P nutrient supply [48,49,51,52]. It was also found that the available phosphorus content of Sapindus mukorossi soil increased more than other indicators under different fertilization treatments. This may be due to the fact that this experiment is located in the subtropical zone, where phosphorus is a limiting nutrient factor [53,54,55]. In the southern region, acidic soil adsorption of phosphorus is strong, a large amount of phosphorus deposition will lead to soil adsorption rate to reach saturation. Thus, the rate of phosphorus adsorption by the soil is reduced, and after saturation the excess inorganic phosphorus will remain in the soil. [56,57]. Therefore, fertilization can effectively enhance the available phosphorus content of the soil.
Different fertilization treatments significantly affected soil nutrient characteristics. It was found that increasing N or P application at the P2K2 and N2K2 levels, respectively, significantly reduced soil pH. Probably because the urea treatment (used here for nitrogen application) contributed more to soil acidification [58]. Urea can be rapidly converted to NH4+ by soil bacteria. And, during the oxidation of NH4+, H+ is released into the soil. N application leads to the loss of cations (Ca2+, K+, and Mg2+) in the soil, which are taken up by plants, thereby accelerating soil acidification [59,60,61]. It was found that with the increase in N, P and K fertilizers, the soil organic carbon content showed an increasing and then decreasing trend. This may be due to the transformation process of mineralization and fixation of N, P and K in the soil, and fertilizer application accelerated the uptake of soil nutrients. Some studies have shown that the application of N, P and K fertilizers can increase soil organic carbon, and that the application of N, P and K in pairs or individually can promote the growth of plant roots and increase the input of inter-root organic matter which, in turn, can increase the content of organic carbon [61,62]. However, when fertilizers are applied in excess, it leads to a decrease in total organic carbon content, which may be due to the fact that excess fertilizer application affects the number and activity of soil microorganisms and affects the biodegradation of organic carbon sources [60]. Therefore, in energy forests, attention needs to be paid to the amount and proportion of nitrogen, phosphorus and potassium fertilizers applied.
4.3. Relationship between Morphological Characteristics of Fine Roots and Soil Environmental Factors
Fine root traits in response to changes in soil nutrients lead to changes in root nutrient acquisition strategies [63,64]. Increased phosphorus in the soil enriches low molecular organic acids secreted by the root system in the inter-root zone, which promotes the release of soil P. Plants are able to obtain nutrients quickly to maintain their physiological functions [65,66]. Redundancy analysis showed that soil AP content was the most important factor affecting root traits, which could significantly improve the absorption function of fine roots. The reason for this phenomenon may be that the soil available phosphorus content increased rapidly after fertilization in Sapindus mukorossi woodland, while Sapindus mukorossi adopted an acquisition-based nutrient acquisition strategy. That is, to further improve its own nutrient uptake capacity by increasing the traits such as the specific root length and the root length density, which was conducive to the growth and development of Sapindus mukorossi’s root system.
The fine root biomass, root length density, SOC, AN and AP content of Sapindus mukorossi in the study area were significantly positively correlated. This indicates that the richer the soil root system is, the more soil organic matter and nutrients can be released into the soil by the root system through decomposition. It has also been shown that the more complex the root growth of a plant is, the more effective it is in preventing the loss of AP from the soil, thus increasing the AP content of the soil [67]. Soil TP and AP were negatively correlated with specific root length and positively correlated with diameter. The reason is that in the subtropical forest soil environment, fine roots absorb phosphorus mainly through the “outsourcing strategy” of cooperating with mycelium, instead of the “on their own” strategy [68]. Soil pH affects soil fertility and, hence, plant growth and fine root traits. In this study, soil pH was found to be significantly negatively correlated with fine root biomass, positively correlated with specific root length and negatively correlated with diameter of Sapindus mukorossi. This may be due to the relatively lower soil pH and the increased secretion of organic acids after fertilization, which caused stronger soil phosphorus activation and facilitated phosphorus uptake by the fine roots through “self-reliance”, reducing the dependence on mycelium, and thus the nutrient acquisition strategy of the root system may have been altered after fertilization and driven by soil nutrients. Vogt et al. found that soil structure and nutrient status are important determinants of root growth and development, and that fine root production is mainly controlled by nutrient conditions [69]. Meanwhile, in this study, the soil environmental conditions of the fertilized Sapindus mukorossi woodlands varied greatly. And the fine root morphology all showed strong correlation with soil nutrients. It is suggested that the morpho-plasticity is largely affected by soil nutrient heterogeneity, which is the same as the related research results of many scholars [4,9,70,71]. This study thus clarifies that for subtropical woody plants, soil AP is the main factor driving root trait variation.
5. Conclusions
This study investigated the response of soil nutrients and fine root traits to balanced fertilization with N, P and K in a Sapindus mukorossi raw material forest. Our study showed that fertilizer ratios of N, P and K had a strong effect on soil nutrients and fine root traits of Sapindus mukorossi. The effects of fertilization on fine roots in different soil layers are multi-faceted, fertilization enhanced the uptake capacity of the Sapindus mukorossi root system, and both surface and deep soil roots showed increased biomass, increased specific root length, and increased root length density. Adding nitrogen or phosphorus can lead to increased soil acidification. RDA analysis showed that soil available phosphorus was the main factor driving root trait variation, and correlation analysis showed that fine root biomass and root length density of Sapindus mukorossi were significantly positively correlated with soil organic carbon, available nitrogen, and available phosphorus content, and soil total phosphorus and available phosphorus were negatively correlated with specific root length and positively correlated with diameter. Therefore, the application of appropriate amounts of nitrogen, phosphorus and potassium fertilizers is an effective way to improve soil fertility, and scientific fertilization strategies should be developed to improve soil productivity and reduce environmental pollution. This study provides an important theoretical basis for the cultivation and management of subtropical woody plants.
J.L. and L.J., conceived the research and designed the experiments. J.L., L.J. and D.W. wrote the manuscript. Y.G, Q.C., J.L. and L.Z. analyzed the date. J.L., J.W., G.Z. and X.Y., performed the experiments. J.L., L.J., L.Z., G.Z., S.W. and Y.G. modified the language and revised the manuscript. All authors have read and agreed to the published version of the manuscript.
The original contributions presented in the study are included in the article material, further inquiries can be directed to the corresponding authors.
We sincerely thank Shuijing Luo for his tending of the plants. We are also grateful to Yiyong Chen for his guidance and help in the experiment. We gratefully acknowledge the assistance of Xiaoli Yan and Yuanyuan Xu in this study.
Author Jianhui Weng was employed by the Yuanhua Forestry Biotechnology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
Footnotes
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Enlarge this image.Figure 1. Variation in fine root traits under different fertilization conditions. Note: (a) fine root biomass; (b) root length density; (c) specific root length; (d) average diameter; different lower-case letters indicate significant difference between fertilization treatments at 0.05 level, same as in the later text.
Enlarge this image.Figure 1. Variation in fine root traits under different fertilization conditions. Note: (a) fine root biomass; (b) root length density; (c) specific root length; (d) average diameter; different lower-case letters indicate significant difference between fertilization treatments at 0.05 level, same as in the later text.
Enlarge this image.Figure 1. Variation in fine root traits under different fertilization conditions. Note: (a) fine root biomass; (b) root length density; (c) specific root length; (d) average diameter; different lower-case letters indicate significant difference between fertilization treatments at 0.05 level, same as in the later text.
Enlarge this image.Figure 2. Soil physicochemical properties under different fertilization conditions. Note: (a) SOC: soil organic carbon; (b) TN: total nitrogen; (c) TP: total phosphorus; (d) TK: total potassium; (e) AN: available nitrogen; (f) AP: available phosphorus; (g) AK: available potassium; (h) pH: soil acidity and alkalinity; different lower-case letters indicate significant difference between fertilization treatments at 0.05 level.
Enlarge this image.Figure 2. Soil physicochemical properties under different fertilization conditions. Note: (a) SOC: soil organic carbon; (b) TN: total nitrogen; (c) TP: total phosphorus; (d) TK: total potassium; (e) AN: available nitrogen; (f) AP: available phosphorus; (g) AK: available potassium; (h) pH: soil acidity and alkalinity; different lower-case letters indicate significant difference between fertilization treatments at 0.05 level.
Enlarge this image.Figure 2. Soil physicochemical properties under different fertilization conditions. Note: (a) SOC: soil organic carbon; (b) TN: total nitrogen; (c) TP: total phosphorus; (d) TK: total potassium; (e) AN: available nitrogen; (f) AP: available phosphorus; (g) AK: available potassium; (h) pH: soil acidity and alkalinity; different lower-case letters indicate significant difference between fertilization treatments at 0.05 level.
Enlarge this image.Figure 2. Soil physicochemical properties under different fertilization conditions. Note: (a) SOC: soil organic carbon; (b) TN: total nitrogen; (c) TP: total phosphorus; (d) TK: total potassium; (e) AN: available nitrogen; (f) AP: available phosphorus; (g) AK: available potassium; (h) pH: soil acidity and alkalinity; different lower-case letters indicate significant difference between fertilization treatments at 0.05 level.
Enlarge this image.Figure 3. RDA analysis of fine root traits and soil properties in 0−20 cm (a) and 20−40 cm (b) soil layers of Sapindus mukorossi. Note: SOC: soil organic carbon; TN: total nitrogen; TP: total phosphorus; TK: total potassium; AN: available nitrogen; AP: available phosphorus; AK: available potassium; pH: soil acidity and alkalinity. Note: * indicates significant correlation at the 0.05 level; ** indicates significant correlation at the 0.01 level.
Soil properties in different soil layers.
| Soil Layer (cm) | SOC (g/kg) | TN (g/kg) | TP (g/kg) | TK (g/kg) | AK (mg/kg) | AP (mg/kg) | AN (mg/kg) |
|---|---|---|---|---|---|---|---|
| 0–20 | 10.61 | 1.9 | 0.39 | 27.86 | 58.63 | 1.81 | 43.35 |
| 20–40 | 7.4 | 1.26 | 0.35 | 27.72 | 49.93 | 1.12 | 34.26 |
| 40–60 | 5.24 | 1.05 | 0.34 | 28.27 | 35.91 | 1.03 | 30.43 |
Note: SOC, soil organic carbon; TN, total nitrogen; TP, total phosphorus; TK, total potassium; AK, available potassium; AP, available phosphorus; AN, available nitrogen.
Combination of factors and levels in each treatment.
| Processing Number | Fertilization Level (kg·ha−1) | ||
|---|---|---|---|
| N | P | K | |
| N0P0K0 | 0 | 0 | 0 |
| N0P2K2 | 0 | 500 | 400 |
| N1P2K2 | 300 | 500 | 400 |
| N2P0K2 | 600 | 0 | 400 |
| N2P1K2 | 600 | 250 | 400 |
| N2P2K2 | 600 | 500 | 400 |
| N2P3K2 | 600 | 750 | 400 |
| N2P2K0 | 600 | 500 | 0 |
| N2P2K1 | 600 | 500 | 200 |
| N2P2K3 | 600 | 500 | 600 |
| N3P2K2 | 900 | 500 | 400 |
| N1P1K2 | 300 | 250 | 400 |
| N1P2K1 | 300 | 500 | 200 |
| N2P1K1 | 600 | 250 | 200 |
RDA analysis of soil properties of Sapindus mukorossi in 0–20 cm and 20–40 cm soil layers.
| Spil Layer | Soil Properties | Explains (%) | Pseudo-F | p |
|---|---|---|---|---|
| 0–20 cm | AP | 30.2 | 17.3 | 0.002 |
| TK | 6.5 | 4 | 0.024 | |
| TN | 4.6 | 2.9 | 0.046 | |
| SOC | 4.4 | 3 | 0.064 | |
| pH | 3.0 | 2.1 | 0.124 | |
| AK | 0.4 | 0.3 | 0.838 | |
| TP | 0.2 | 0.1 | 0.95 | |
| AN | 0.1 | 0.1 | 0.998 | |
| 20–40 cm | AP | 30.6 | 17.7 | 0.002 |
| TN | 7.0 | 4.4 | 0.006 | |
| TK | 3.8 | 2.5 | 0.082 | |
| TP | 2.9 | 1.9 | 0.138 | |
| pH | 1.2 | 0.8 | 0.47 | |
| SOC | 0.7 | 0.4 | 0.746 | |
| AK | 0.4 | 0.2 | 0.89 | |
| AN | 0.3 | 0.2 | 0.906 |
Correlation analysis between fine root traits and soil properties of Sapindus mukorossi at 0–20 cm (Upper right corner) and 20–40 cm (lower left corner) under different soil layers.
| FRB | RLD | FSRL | RAD | SOC | pH | TN | TP | TK | AN | AP | AK | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| FRB | 0.87 ** | −0.03 | 0.49 ** | 0.35 * | −0.65 ** | 0.71 ** | 0.42 ** | 0.42 ** | 0.56 ** | 0.71 ** | 0.11 | |
| RLD | 0.80 ** | 0.42 ** | 0.55 ** | 0.30 * | −0.61 ** | 0.67 ** | 0.35 * | 0.48 ** | 0.53 ** | 0.60 ** | 0.10 | |
| FSRL | −0.09 | 0.50 ** | 0.28 | 0.09 | −0.01 | 0.06 | −0.01 | 0.27 | 0.01 | −0.03 | 0.09 | |
| RAD | 0.52 ** | 0.48 ** | 0.02 | 0.21 | −0.27 | 0.36 * | 0.23 | 0.47 ** | 0.26 | 0.57 ** | 0.14 | |
| SOC | 0.43 ** | 0.3 | −0.13 | 0.31 * | −0.35 * | 0.50 ** | 0.45 ** | 0.57 ** | 0.33 * | 0.59 ** | 0.24 | |
| pH | −0.47 ** | −0.29 | 0.21 | −0.31 * | −0.22 | −0.48 ** | −0.64 ** | −0.33 * | −0.60 ** | −0.60 ** | −0.2 | |
| TN | 0.67 ** | 0.66 ** | 0.12 | 0.33 * | 0.57 ** | −0.38 * | 0.2 | 0.53 ** | 0.68 ** | 0.65 ** | 0.22 | |
| TP | 0.32 * | 0.22 | −0.09 | 0.36 * | 0.24 | −0.61 ** | 0.27 | 0.28 | 0.42 ** | 0.54 ** | 0.22 | |
| TK | 0.11 | 0.29 | 0.36 * | −0.07 | −0.09 | −0.04 | 0.23 | 0.35 * | 0.36 * | 0.40 ** | 0.38 * | |
| AN | 0.34 * | 0.27 | −0.05 | 0.30 * | 0.24 | −0.43 ** | 0.22 | 0.17 | 0.12 | 0.51 ** | 0.37 * | |
| AP | 0.70 ** | 0.62 ** | −0.04 | 0.58 ** | 0.40 ** | −0.53 ** | 0.57 ** | 0.36 * | 0.01 | 0.52 ** | 0.09 | |
| AK | 0.08 | −0.01 | −0.09 | −0.07 | 0.19 | −0.10 | 0.07 | −0.02 | 0.08 | 0.13 | −0.09 |
Note: Upper right corner is 0–20 cm soil layer, lower left corner is 20–40 cm soil layer. *, p < 0.05; **, p < 0.01.
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