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Article

Changes in Soil C and N Stocks and Their Effects on Rice Yield under Long-Term Upland-Paddy Rotations

by
Mengjia Wang
1,†,
Xiangqian Feng
1,2,†,
Hengyu Ma
1,
Danying Wang
1 and
Song Chen
1,*
1
China National Rice Research Institute, Chinese Academy of Agricultural Sciences, Hangzhou 310006, China
2
College of Agriculture, Yangtze University, Jingzhou 434025, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agronomy 2023, 13(4), 1028; https://doi.org/10.3390/agronomy13041028
Submission received: 7 March 2023 / Revised: 28 March 2023 / Accepted: 30 March 2023 / Published: 30 March 2023
(This article belongs to the Section Soil and Plant Nutrition)
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Abstract

Observations of the effects of long-term paddy-upland rotations on soil carbon (C) and nitrogen (N) stocks are scarce. This experiment aimed to examine soil C and N stock characteristics and their relationships with rice yield using four long-term rotation systems. The results showed that in the topsoil at 50 cm, compared to rice-fallow (RF), rice-potato with rice straw mulch and return (RP) and rice-Chinese milk vetch (RC) increased soil organic carbon (SOC) stock by 19.67% and 15.29%, total nitrogen (TN) stock by 20.40% and 18.54%, and available nitrogen (AN) density by 61.54% and 34.44%, respectively; while rice-wheat (RW) increased AN density by 30.24%. Furthermore, the increases in densities of soil C and N at 20–30 cm were likely to increase rice above biomass and yield. The highest soil C and N densities were observed in RP, which was the best choice for increasing soil nutrients and rice yield. In addition, SOC, TN, and AN had obvious stratification and surface aggregation characteristics, and more than 70% of the total stocks were concentrated at the top 30 cm. These results provide a theoretical basis for the high-yield cultivation of rice in the middle and lower reaches of the Yangtze River.

1. Introduction

The stocks of soil carbon (C) and nitrogen (N) represent the total mass of soil C and N in a certain area and at a certain depth [1], and these stocks have an impact on soil structure and soil water-nutrient-crop productivity relationships [2], thus affecting crop production. Bauer and Black [3] reported that the contribution of 580 kg ha−1 soil organic carbon (SOC) in the upper 30 cm to inherent soil productivity was equivalent to a wheat grain yield of approximately 15.6 kg ha−1. However, stagnation or decline in rice yield has been observed in recent years due to the loss of SOC and total nitrogen (TN) [4]. Therefore, it is urgent to improve the C and N stocks of farmland soil. In recent years, many studies have concluded that this objective can be obtained using different rotation systems [2,5] and possible corresponding technical measures, such as crop residue mulch and/or return [5,6].
In addition to lowering the carbon footprint for crop production [7] and improving the efficient utilization of natural resources and nutrients [2], reasonable rotation systems can affect the community structure, diversity, and metabolites of microorganisms involved in the C and N cycles by changing the soil environment and further promote the decomposition and utilization of soil nutrients and the accumulation of soil C and N [8] compared with monocropping. In particular, the paddy-upland rotation system results in an alternation of water and drought in soil due to crop changes, creating a unique soil ecological environment and unique C and N stocks [9]. Cha-un and Towprayoon [10] found that the SOC stocks at the 0–15 cm soil depth increased in rotation systems of rice-rice, corn-rice, and sweet sorghum-rice when crop residues were incorporated compared with fallow-rice, and a similar result was found in tobacco-rice rotation, which contributed more to conserving the SOC and TN stocks than did tobacco monoculture in the 0–20 cm soil layer [11]. Furthermore, the stocks of soil C and N under paddy-upland cropping systems might vary with different crops and agronomic practices. The rice-oilseed rape rotation system promoted SOC stocks at a depth of 0–20 cm more than the system of rice-wheat under crop straw mulch after harvesting, as proposed by Liu et al. [12], while Zhang et al. [13] reported that compared with rice-fallow, rice-potato with rice straw mulch significantly increased the SOC and TN, and rice-ryegrass and rice-Chinese milk vetch increased the SOC. Therefore, illustrating the soil C and N stocks in paddy-upland rotation systems with different winter crops not only contributes to arranging the cropping system reasonably and improving land productivity but also aids in the efficient utilization of soil resources. However, the investigations of soil C and N in current studies have mostly focused on the top 30 cm of farmland, and the patterns of soil C and N stocks in deeper soil layers are not clear when different rotation systems are used, as the deeper layers have low available nutrient contents but a high stabilization capacity [14]. Moreover, the remaining challenge is that the research about the effect of paddy-upland rotations on soil C and N stocks in light of long-term field observations is limited.
In the current study, we hypothesized that different types of paddy-upland rotations would affect soil C and N stocks and rice yield. The aim of the study was to evaluate (1) the difference in soil C and N stocks among rotations and (2) the vertical distribution characteristics of SOC, TN, and available nitrogen (AN) affected by rotations and their relationships with rice yield.

2. Materials and Methods

2.1. Study Site and Experimental Design

The field experiment was established at the experimental farm of the China National Rice Research Institute in Hangzhou (120.2° E, 30.3° N) of China since 2003. The test site was located on the middle and lower reaches of the Yangtze River, which is part of the subtropical monsoon climate area, with an average annual temperature of approximately 17.5 °C, an annual sunshine duration of more than 1500 h, and an average annual precipitation of approximately 1400 mm. The soil is divided into Hydragric based on the world reference base for soil resources (WRB). The basic physical and chemical properties of the topsoil in all rotations prior to rice transplanting in 2019 are provided in Table 1.
In this study, a randomized complete block design with three replicates (plot area was 40 m2) was adopted, with summer rice as the main crop of the paddy-upland rotation mode and winter crop straw and residue returning. The four rotation systems in the long-term experiment were rice-winter fallow (RF, as a control), rice-potato with rice straw mulch (RP), rice-wheat (RW), and rice-green manure (Chinese milk vetch, RC). The winter straws and residues (no residue, potato and mulched rice straw, wheat straw, and Chinese milk vetch for RF, RP, RW, and RC, respectively) were incorporated into the soil.
The agronomic practices in the winter seasons were as follows: (1) RF, rice straw from the summer season was removed, and no crops were planted during winter. (2) RP, potato seed tubers were placed on the field in late January, covered with 8–10 cm of rice straw, and had compound fertilizer (N: P2O5: K2O 15: 15: 15) inputs of 600 kg ha−1. The rotten rice straw, potato stem, and root system were incorporated into the soil after harvesting in early May. (3) RW, wheat seeds were sown directly in mid and late November. Approximately 600 kg ha−1 of compound fertilizer was applied as basal fertilizer at the seedling stage, and 130 kg ha−1 of urea was applied at the heading stage. The wheat residue and straw were crushed and plowed back to the field in mid-May after harvest. (4) RC, Chinese milk vetch was sown in mid and late October and incorporated into the soil in early April. The straw and residual were incorporated with a micro tiller (Dongfeng 151/121, Changzhou, Jiangsu, China) [15]. The straw incorporation in 2019 and rice biomass and grain yield in the last four years in the treatments were shown in Tables S1 and S2, respectively.
For the rice season, rice seeds were sown in a seedbed on May 20, and seedlings were transplanted manually to the field on June 17, with a row plant spacing of 20 cm × 20 cm. Urea was used as the N fertilizer in the rice season, with an application amount of 67.5 kg N ha−1. Forty percent of the total was applied as basal fertilizer before transplanting seedlings, while the remaining N fertilizer was applied at the rice tillering and booting stages at a rate of 30% at both times. The application rates of potassium fertilizer and phosphorus fertilizer were 165 kg K2O ha−1 and 97.5 kg P2O5 ha−1, respectively, in which potassium fertilizer was applied before transplanting seedlings and in the booting stage at a rate of 50%, respectively, while all phosphorus fertilizer was applied as basal fertilizer before transplanting seedlings. After the rice harvest, all straw was removed from the paddy and stored for RP.

2.2. Sampling and Analyses

Soil samples were collected from a depth of 0–50 cm at intervals of 10 cm with an 8 cm diameter soil drill. For each plot, 5 subsamples were collected for bulk density (BD) determination, while another 5 subsamples were collected for determining the SOC concentration (SOCC), TN concentration (TNC), and AN concentration (ANC). After removing the plant residues and stones, the fresh soil samples were air-dried and ground through a 2-mm sieve. The SOCC, TNC, ANC, soil pH, and BD were determined and evaluated according to Lu [16]. SOC, TN, and AN were determined by the potassium dichromate oxidation-volumetric method, Kjeldahl method, and alkali hydrolyzed diffusion method, respectively. Soil pH was determined (1:5 water suspension) by a pH meter, and BD was calculated based on the soil dry weight and the volume samplers.
The calculation of soil C and N density at each layer was as follows:
SOCDi (kg C m−2) = SOCCi (g·kg−1) × BDi (g·cm−3) × H (cm)/100
TNDi (kg N m−2) = TNCi (g·kg−1) × BDi (g·cm−3) × H (cm)/100
ANDi (g N m−2) = ANCi (mg·kg−1) × BDi (g·cm−3) × H (cm)/100
where i represents the soil layers of 0–50 cm with 10-cm intervals (i.e., 0–10 cm, 10–20 cm, 20–30 cm, 30–40 cm, and 40–50 cm), and H is the sampling depth for each layer, i.e., 10. The stocks of soil C and N in the 0–50 cm layer were calculated as the density sum of each layer.
To explore the stratification and surface accumulation degree of soil C and N density under the different rotations, the stratification ratios (SRs) were calculated as follows:
SOC-SRi = SOCD0–10 cm/SOCDi
TN-SRi = TND0–10 cm/TNDi
AN-SRi = AND0–10 cm/ANDi
where i represents the soil layers from 10–50 cm with 10-cm intervals (i.e., 10–20 cm, 20–30 cm, 30–40 cm, and 40–50 cm), and SOCDi, TNDi, and ANDi are the densities of SOC, TN, and AN in layer i, respectively.
For rice yield, rice plants were harvested from approximately 5 m2 in the center of the experimental plot at the maturity stage. After threshing and impurity removal, the water content of the rice grain was measured by a grain moisture detector (PM-8188-A, Beijing, China). The final yield was calculated and adjusted with a water content of 13.5%. The sampled aboveground rice plant was deactivated by drying at 105 °C for 30 min and then dried at 75 °C for 48 h, and the total dry matter was weighed after cooling.

2.3. Data Analyses

All data in this paper were collected and pretreated in Microsoft Excel 2010 and subjected to analysis of variance (ANOVA) using SAS software (version 8.0, SAS Institute, Cary, NC, USA). The statistical significance of differences among rotations within the same soil layer was assessed by LSD. The correlation analysis using OriginPro 2021.

3. Results

3.1. The Differences in the Total Soil C and N Stocks within the 0–50 cm Soil Depth

The soil C and N stocks within the 0–50 cm soil depth are shown in Table 2. The soil organic carbon density (SOCD), total nitrogen density (TND), and available nitrogen density (AND) values among the rotations were 8.56–10.24 kg m−2, 0.87–1.07 kg m−2, and 37.22–60.13 g m−2, respectively. In general, crop rotations with winter crops improved the C and N stocks (except for the SOCD and TND in RW), and the increase depended on the type of winter crop. Compared with RF, both RP and RC increased the SOCD by 19.67% and 15.29%, the TND by 20.40% and 18.54%, and the AND by 61.54% and 34.44%, respectively. The AND of RW increased by 30.24% over that of RF, but there was no significant difference in the SOCD and TND between them. The RP and RC treatments had relatively higher C and N stocks among the rotations in the order of RP > RC > RW/RF.

3.2. Spatial Differences in Soil C and N Stocks

To explore the detailed distribution characteristics of soil C and N, the proportion and change trend of soil C and N in the 0–50 cm soil layer at five 10 cm intervals was observed. It was found that 72.17–83.37% of the total SOCD, 75.70–80.48% of the total TND, and 75.69–81.08% of the total AND were concentrated in the top 30 cm of soil (Table S3), and the SOC, TN, AN, and soil BD were extremely sensitive to the change in soil depth. Overall, the SOCD, TND, and AND first increased and then decreased with increasing soil layer depth, which was slightly different from the changing trend of their concentrations (decreased with soil depth) because of the soil BD (Figure 1 and Figure 2). The soil BD increased with soil depth, which was opposite to the pattern of soil C and N. However, except for RC, which was 8.80% greater than RP in the 10–20 cm soil layer, there were no significant differences among the rotation systems in the other soil layers, which showed that the densities of soil C and N were mainly affected by their concentrations.
The impacts of rotations with cover crops in winter on SOC stocks were mainly reflected in the 30–50 cm soil layer (Figure 1). The SOC stocks in the RP, RW, and RC treatments within the 0–30 cm soil layer were not significantly different from that in RF, while the value in RW decreased by 16.31–24.65% at 20–30 cm compared with the others. At 30–40 cm, the RP, RW, and RC treatments significantly increased in SOC density compared to that in RF, with increases of 174.41%, 61.99%, and 121.95%, respectively, while RW and RC also increased by 66.85% and 58.85%, respectively, at 40–50 cm. In general, although the SOC density in the RP treatment decreased by 19.90–51.99% at 40–50 cm compared with the others, it had the greatest advantage within the 0–40 cm layer due to its high concentration of SOC, which increased by 9.86–171.07% compared with the others within the 0–40 cm layer (Figure 2). It is also worth noting that the SOC density in each rotation system was not significantly different at 0–10 cm, but the SOC concentration in RP (3.16%) was significantly higher by 26.87–31.28% than that in the other systems, which basically had the same concentration level (2.41–2.49%).
As with SOC, the TND in the four treatments had no significant difference at 0–10 cm, ranging from 0.21 to 0.24 kg m−2, whereas that in RP was 26.87–31.28% higher than the others. A larger difference in TN among the rotations was observed at 10–40 cm, and the TND in RP and RC at 10–20, 20–30, and 30–40 cm increased by 2.69–11.21%, 9.45–21.07%, and 20.02–31.87%, respectively, over that of RW and RF. The four treatments had similar TND values (0.07–0.10 kg m−2) at 40–50 cm, although RC and RW had values of 22.63% and 23.79% lower than that of RF. The variation trend of the TN concentration within the 20–50 cm layer was similar to its density.
As the actual available N form of crops, the exploration of the vertical change trend of AN is necessary. Slightly different from TN, the effects of rotations on AN were mainly reflected at the 0–30 cm layer in the order of RP > RC/RW > RF (Figure 1 and Figure 2). Compared with RF, the AND in the RP, RC, and RW treatments increased by 30.93%–58.42% at 0–10 cm, 57.62–90.83% at 10–20 cm, and 21.76–52.66% at 20–30 cm, respectively. The AND in RP at 30–40 cm was significantly higher than that in others but had no significant difference, while that in RW was reduced by 32.95–37.72% compared to the others at 40–50 cm. The ANC among rotations showed a similar trend. Therefore, according to the density and concentrations of AN, the four rotation systems could be classified into three grades, i.e., high (RP), medium (RC and RW), and low (RF) AN stock groups within the 0–40 cm layer (Figure 1 and Figure 2).

3.3. Stratification Ratios of Soil C and N

To characterize and quantify the stratification characteristics and surface accumulation degree of C and N under long-term rotation systems within the 0–50 cm soil layer, the stratification ratios of SOC, TN, and AN in the four treatments were calculated by density. Table S4 shows that all the SR10–20 cm and SR20–30 cm values varied from 0.79 to 1.08, indicating that SOC, TN, and AN had no obvious stratification and surface aggregation characteristics within the 0–30 cm layer, and the distributions were relatively homogeneous. The SR30–40 cm of SOC and TN were 1.12–3.20-fold those of SR10–20 cm and SR20–30 cm, but the SR30–40 cm values in RP, RW, and RC were significantly less than that in RF, suggesting that the decline in SOC and TN in the 30–40 cm layer decreased and that the stratification and surface aggregation characteristics within the 0–40 cm layer weakened after rotation with potato, wheat, and Chinese milk vetch in winter. The SR40–50 cm values of SOC, TN, and AN were 1.92–5.39, which meant that the distinct stratification and surface aggregation characteristics of soil C and N at depths of 0–50 cm were found but varied with rotation type. Compared with RF, the characteristics of SOC in RP and AN in RP, RW, and RC were all exacerbated.

4. Discussion

4.1. Differences in Total C and N Stocks in the Top 50 cm Soil Layer among Rotations

Existing reports have suggested that rotation cropping is a valuable management practice that can lower the C footprint of crop production [7] and increase the accumulation of C and N in soil [11,17]; this result was partly confirmed in the current study, as it was found that the crop system rotation with certain winter crops (RP and RC) increased both SOC and TN stocks within the 0–50 cm depth by more than 15% compared to that in RF (Table 2). However, the soil C and N stock responses to cropping systems were influenced by the selection of winter crops. We also found that the soil C and N stocks in RW did not differ remarkably compared to those in RF.
The soil C and N stocks provide real-time feedback on the dynamic balance between C and N inputs and outputs in the rotation system. The stocks of soil C and N largely depend on the amount and quality of crop straw [18], which is the main source of exogenous C and N [19]. The input of fresh organic material into the soil results in the increase of the mineralization of native SOC, the phenomenon known as the “priming effect” (PE) [20]. A large amount of crop straw return can not only provide ample C and N in soil but also stimulate the mineralization of existing soil organic matter (SOM) through PE [21], promoting soil microbial activity and diversity and even ecosystem stability [22]. Both Sainju et al. [2] and Alves et al. [23] found that soil C and N could be increased by the return of crop residues. Landriscini et al. [24] and Huang et al. [25] also noted the increasing effect of straw mulch on TN and AN.
In addition, the crop straw returned in the RP and RC treatments was characterized by a low C/N ratio (Table S1) [15], which was more conducive to the sequestration of SOC and TN [26] and the increase in AN [27] in the soil (Figure 1 and Figure 2). Previous studies have identified the positive effect of potato and Chinese milk vetch in the RP and RC rotation systems on the accumulation of soil SOC, TN, and/or AN by short- or long-term field experiments [13,28,29,30]. Zhang et al. [13] evaluated the effects of certain winter crops on soil C and N under winter crop-double cropping rice systems and found the largest increase in SOC and TN in potato treatments. Hu et al. [28] suggested that after 6 years of continuous cropping of potatoes, the AN increased by 122.81% and 45.05% compared with 2 and 4 years, respectively. Lan et al. [30] proposed that rotation treatments involving Chinese milk vetch had the highest SOC, TN, and AN, which might be partly attributed to the ability of Chinese milk vetch to maintain higher soil fertility [31]. Therefore, the increase in soil C and N in RP and RC was ascribed to the characteristics of potato and Chinese milk vetch, straw return, and the resulting systematic diversity.
However, not all the rotation treatments had a positive impact on the accumulation of soil C and N. The total stocks of SOC, TN, and AN in the RW and RF treatments were all significantly lower than those in the RP treatment within the 0–50 cm soil layer. RW is a rotation system that consumes C and N nutrients, and its consumption includes not only a large amount of nutrient leaching loss [32] but also an output of both rice and wheat grains; moreover, the C/N ratio of wheat straw was about twice that of straw in the RP and RC treatments (Table S1). Additionally, the lower C and N stocks under RW treatment might also be related to the lower number of microorganisms [33]. For RF, due to the lack of an impact of crop replacement on soil nutrients and the application of fertilizer and crop straw in winter, which rely only on the supply of chemical fertilizer in the rice season, the SOC, TN, and AN densities in the soil were lower. The nutrient consumption characteristics of RW and RF were consistent with the conclusions of Hu et al. [32].

4.2. The Distribution Characteristics of Soil C and N at the 0–50 cm Soil Depth

Another phenomenon was found in which the densities of SOC, TN, and AN generally increased with soil depth in the 0–30 cm soil layer and then decreased (Figure 1). Wang et al. [34] mentioned that 54% and 74% of the total SOC was maintained in the topsoil of 30 and 50 cm, respectively, within the top 1 m, and our results were similar to the finding in that more than 70% of the total SOCD, TND, and AN was concentrated in the top 30 cm (Table S3). The SOC, TN, and AN also had apparent stratification and surface aggregation characteristics in the 0–50 cm soil layer based on the stratification ratios (Table S4). There are many reasons for the above phenomenon, and it is speculated that crop residues remain one of the main factors. The accumulation of nutrient-rich straw and crop roots left in the topsoil were sources of C and N [35], and Shi et al. [36] considered that the vertical differences in soil-available potassium were caused by the increase in the available potassium content in the topsoil because of straw returning.
By using the 15N tracer method, Hu et al. [32] found that the fertilizer applied to the soil was largely concentrated in the top 10 cm of the soil. However, Guo et al. [37] reported that when fertilizer was sufficient, it would spread to deeper soil. Thus, the soil C and N at the 30–40 cm layer under the condition of low N fertilizer application in this study would likely come from the winter crop residues or rice straw. This process explained the weakening of the SOC and TN stratification and surface aggregation characteristics within the 0–40 cm layer in RP, RC, and RW compared with that in RF. For the SOC in RP and the AN in RP, RW, and RC, the prominent stratification and surface aggregation characteristics in the range of 0–50 cm were mainly attributed to their higher densities in the top 10 cm than those in RF.
In general, compared with RF, the impact of RP, RW, and RC on SOCD was mainly found in the subsoil of the 30–40 cm layer, TND in the middle of the 20–40 cm layer, and AND in the top 30 cm layer. These might be related to the characteristics of nutrient conservation. The SOC in deeper soil had a relatively higher stabilization capacity [14], and the cumulative SOC released by the decomposition of crop residues resulted in a higher SOC stock in RP, RC, and RW. TN was more stable than SOC, so the N nutrients provided by residues were well preserved in the soil, while the AN that could be absorbed and utilized by plants tended to aggregate in the crop root layer. Additionally, for the SOC, TN, and AN densities at 40–50 cm, there was no obvious regularity among the rotations, which might have been caused by the original stocks of C and N.
Through correlation analysis, we found that there was a significant positive correlation between rice biomass and stocks of soil C and N within 0–50 cm. In particular, the TND at 20–30 cm was significantly positively correlated with rice yield and biomass (Figure 3). Therefore, the RP and RC treatments in this area, with high TN and AN densities, might improve rice yield.

5. Conclusions

The total stocks of C and N in the upper 50 cm in the rotations with cover crops in winter and their residue retention increased to some extent. Rice-potato with rice straw mulch (RP) and rice-Chinese milk vetch (RC) significantly increased the total densities of SOC, TN, and AN compared with those of rice-fallow (RF), especially in SOC at 30–40 cm, TN at 20–40 cm, and AN in the top 30 cm; additionally, the increases in the TN and AN at 20–30 cm were likely to increase the rice yield. Among the rotations, RP treatment was an optimum rotation for local application with the highest C and N stocks, especially in the 0–40 cm range. These results could provide some information for the high-yield cultivation of rice in the middle and lower reaches of the Yangtze River. In addition, the densities of SOC, TN, and AN first increased slightly and then decreased rapidly with soil depth and had apparent stratification and surface aggregation characteristics, with more than 70% of the total stocks concentrated in the top 30 cm. The surface aggregation of soil C and N is beneficial to enhance soil fertility, but at the same time, it leads to nutrient loss. Therefore, future studies need to focus on the turnover and dynamic changes of soil organic matter to investigate the mechanism of enhancing soil fertility and rice yield.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy13041028/s1. Table S1: Residue properties of winter crops before rice season in 2019; Table S2: Rice biomass and grain yield in the last four years under rotations; Table S3: Percentage share of total soil C and N content in investigated layers in ratio total stock in 0–50 cm (%); Table S4: The stratification ratios of SOC, TN, and AN densities.

Author Contributions

Conceptualization, M.W., performed the experiments, M.W., X.F. and H.M., investigation, X.F. and H.M., data curation, S.C. and D.W., writing—original draft preparation, M.W., writing—review and editing, X.F. and S.C., visualization, M.W. and X.F., project administration, D.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported in part by Zhejiang Province Natural Science Foundation of China (LY22C130001), Key Research and Development Program of Zhejiang Province (2022C02008), the National Natural Science Foundation of China (32172106), the National Rice Industry Technology System (CARS-01-04A), the Central Public Interest Scientific Institution Basal Research Fund (N0. 2017RG004-1, N0. 2017RG004-5), the Agricultural Science and Technology Innovation Program (CAAS-ZDRW202001), and Quzhou Municipal Science and Technology Project (2021K13).

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The vertical difference of soil organic C (A), total N (B) and available N (C) densities among rotations. LSD bars represent least significant difference (p = 0.05) among rotations at the same soil depth. The same as below.
Figure 1. The vertical difference of soil organic C (A), total N (B) and available N (C) densities among rotations. LSD bars represent least significant difference (p = 0.05) among rotations at the same soil depth. The same as below.
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Figure 2. The vertical difference of soil organic C (A), total N (B) and available N (C) concentrations, (D) bulk density, and (E) C/N ratio among rotations.
Figure 2. The vertical difference of soil organic C (A), total N (B) and available N (C) concentrations, (D) bulk density, and (E) C/N ratio among rotations.
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Figure 3. The correlation analysis of rice grain and biomass yield with soil bulk density and C and N density of different soil layers. −1, −2, −3, −4, −5, -A represent 0–10 cm, 10–20 cm, 20–30 cm, 30–40 cm, 40–50 cm, 0–50 cm soil, respectively. * Means statistical significance at p = 0.05.
Figure 3. The correlation analysis of rice grain and biomass yield with soil bulk density and C and N density of different soil layers. −1, −2, −3, −4, −5, -A represent 0–10 cm, 10–20 cm, 20–30 cm, 30–40 cm, 40–50 cm, 0–50 cm soil, respectively. * Means statistical significance at p = 0.05.
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Table 1. Soil properties under different rotations before rice season in 2019 (0–20 cm).
Table 1. Soil properties under different rotations before rice season in 2019 (0–20 cm).
RotationSOCC 5 (g kg−1)TNC 6
(g kg−1)		ANC 7
(mg kg−1)		BD 8
(g cm−3)		pHClay
(%)		Silt
(%)		Sand
(%)
RF 122.0 ab 92.0 b165.3 bc1.00 a5.99 a11.24 a74.99 b2.28 ab
RP 223.9 a2.3 a186.7 a0.88 c6.02 a11.48 a74.79 b2.61 a
RW 321.2 b2.1 b162.8 c0.95 b5.95 a11.26 a75.44 ab2.30 ab
RC 421.9 ab2.2 b176.8 ab0.97 ab5.96 a11.22 a78.01 a1.34 b
1 RF: rice-fallow; 2 RP: rice-potato with rice straw mulch; 3 RW: rice-wheat; 4 RC: rice-green manure (Chinese milk vetch); 5 SOCC: Soil organic carbon content; 6 TNC: Total N content; 7 ANC: Available N content; 8 BD: Soil bulk density. 9 Different lower letters in the same column indicate statistical significance at p = 0.05. The same as below.
Table 2. The total soil C and N stocks within 0–50 cm soil depth under different rotation systems.
Table 2. The total soil C and N stocks within 0–50 cm soil depth under different rotation systems.
RotationSOCD 1 (kg m−2)TND 2 (kg m−2)AND 3 (g m−2)
RF8.6 ± 0.1 b 40.89 ± 0.00 b37.2 ± 1.9 c
RP10.2 ± 0.2 a1.07 ± 0.04 a60.1 ± 4.1 a
RW8.8 ± 0.4 b0.87 ± 0.02 b48.5 ± 2.8 b
RC9.9 ± 0.3 a1.05 ± 0.06 a50.0 ± 4.4 b
1 SOCD: Soil organic carbon density; 2 TND: Total nitrogen density; 3 AND: Available nitrogen density. 4 Different lowercase letters indicate the significant difference at p = 0.05 among rotations. The same as below.
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Wang, M.; Feng, X.; Ma, H.; Wang, D.; Chen, S. Changes in Soil C and N Stocks and Their Effects on Rice Yield under Long-Term Upland-Paddy Rotations. Agronomy 2023, 13, 1028. https://doi.org/10.3390/agronomy13041028

AMA Style

Wang M, Feng X, Ma H, Wang D, Chen S. Changes in Soil C and N Stocks and Their Effects on Rice Yield under Long-Term Upland-Paddy Rotations. Agronomy. 2023; 13(4):1028. https://doi.org/10.3390/agronomy13041028

Chicago/Turabian Style

Wang, Mengjia, Xiangqian Feng, Hengyu Ma, Danying Wang, and Song Chen. 2023. "Changes in Soil C and N Stocks and Their Effects on Rice Yield under Long-Term Upland-Paddy Rotations" Agronomy 13, no. 4: 1028. https://doi.org/10.3390/agronomy13041028

APA Style

Wang, M., Feng, X., Ma, H., Wang, D., & Chen, S. (2023). Changes in Soil C and N Stocks and Their Effects on Rice Yield under Long-Term Upland-Paddy Rotations. Agronomy, 13(4), 1028. https://doi.org/10.3390/agronomy13041028

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