Tillage with Crop Residue Returning Management Increases Soil Microbial Biomass Turnover in the Double-Cropping Rice Fields of Southern China

Abstract

The variety of soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN) content, and the flux turnover rate of SMBC and SMBN for 0–10 cm and 10–20 cm layers in a paddy field in southern China with different tillage practices were studied. The tillage experiment included conventional tillage and crop residue returning (CT), rotary tillage and crop residue returning (RT), no–tillage and crop residue returning (NT), and rotary tillage with all crop residues removed from the paddy field as a control (RTO). The result showed that the SMBC and SMBN contents at 0–10 cm and 10–20 cm layers in the paddy field with CT, RT, and NT treatments were significantly increased. This result indicates that the flux turnover rate of SMBC and SMBN for 0–10 cm and 10–20 cm layers in the paddy field with CT treatment were increased by 65.49%, 39.61%, and 114.91%, 119.35%, compared with the RTO treatment, respectively. SMBC and SMBN contents and the flux turnover rate of SMBC and SMBN for the 0–10 cm layer were higher than that of the 10–20 cm layer in paddy fields under the same tillage condition. Therefore, applying rotary tillage or conventional tillage and crop residue returning produced beneficial management for increasing soil microbial biomass content and its turnover under a double–cropping rice system in southern China.

1. Introduction

Soil microbial biomass (SMB) is usually regarded as a sensitive indicator for various levels of soil fertility and soil quality [1], including soil carbon (C), nitrogen (N), phosphorus (P), and sulfur (S) [2]. It is generally believed that the SMB content is an early indicator of varying soil quality and soil fertility that it is sensitive to external environmental change [2]. Soil microbial biomass C (SMBC) comprises only 1 to 4% of organic C due to its fast turnover time, and the microbial biomass plays a key role in controlling soil nutrient cycling and energy flow [3]. Therefore, the contents of SMB (SMBC, soil microbial biomass nitrogen (SMBN)) are closely related to the soil environment [4], which is clearly influenced by applying different forms of field management, such as crop, tillage, crop residue, and fertilizer treatment [5,6].

The effects of different tillage practices on the amounts and quality of soil organic carbon (SOC) and SMB contents have received the utmost attention, as they are considered an early indicator of changing soil quality [7]. It is usually believed that soil organic matter (SOM) and SMB contents are closely related to its turnover in agricultural soil, which is clearly influenced by different tillage conditions [8,9]. Some results indicate that the SMB content with conventional tillage (CT) treatment is significantly lower than that of no-tillage (NT) treatment [10,11]. However, some results indicate that SOC concentration, stock and sequestration, and its SMB content with NT treatment are higher than that of CT treatment [9,12]. Meanwhile, other results have found that there is no clear difference in the SMB content between NT and CT treatments [13]. Some results indicate that the soil microbial is a significant process in increasing C and N and converting it into SOM with different tillage management under rice cropping system conditions [14]. The SMB contents (SMBC, SMBN) and its microbial processes at the surface soil layer with plow treatment were clearly lower than that of NT treatment, while reverse results at a deeper soil layer were found [15]. Many studies have been conducted to investigate the effects of different tillage practices on SOC, SMBC, and SMBN contents in the field [16], but less information about SMB microbial processes in the field was obtained [17]. However, there is still the need to further study the change in the SMB content and its turnover in the double-cropping rice field under different tillage and crop residue returning conditions.

At present, the main rice (Oryza sativa L.) planting pattern in southern China uses a double–cropping rice system (early rice and late rice) [18]. Recently, crop residues returning to the paddy field combined with applied rotary tillage or NT treatments were accepted by people in this region, which is beneficial for increasing soil quality and soil nutrients for rice growth, thus obtaining a higher rice yield [19]. In a previous study, our results demonstrated that the SOC and SMB contents in the 0–20 cm layer in the paddy field significantly change with different tillage managements [20,21]. However, under the double–cropping rice system, the effects of different tillage management on the SMB content, flux, and turnover rate of SMBC and SMBN at 0–20 cm soil layers in the paddy field still need to be further investigated. Therefore, the aims of this study were as follows: (1) to analyze varied SMB contents at 0–20 cm layer with different tillage practices, (2) to investigate the flux and turnover rate of SMBC and SMBN in the double–cropping rice field under different tillage conditions. We hypothesized that the (i) SMB content in the paddy field would increase with tillage and crop residue returning managements, and the (ii) flux and turnover rate of SMBC and SMBN in the paddy field would increase under tillage and crop residue returning conditions.

2. Materials and Methods

2.1. Field Site and Cropping System

The field experiment was set up in November 2015. This field tillage experiment was located in Ningxiang City (112°18′ E, 28°07′ N) of Hunan Province, China. The cropping system of this field experiment included Chinese milk vetch (Astragalus sinicus L.) and early rice and late rice (Oryza sativa L.). At the beginning of this field experiment, soil characteristics at the 0–20 cm layer were as follows: soil organic carbon (SOC) at 22.07 g kg⁻¹, total nitrogen (N) at 2.14 g kg⁻¹, available N at 192.20 mg kg⁻¹, total phosphorous (P) at 0.82 g kg⁻¹, available P at 13.49 mg kg⁻¹, total potassium (K) at 13.21 g kg⁻¹, available K at 81.91 mg kg⁻¹, and pH 5.79. The more detailed information about climate conditions (annual mean precipitation and evapotranspiration, monthly mean temperature) during the field experiment region and other soil characteristics were described by Tang et al. [20].

2.2. Experimental Design

This field experiment included four the following tillage treatments: (1) conventional tillage and crop residue returning (CT), (2) rotary tillage and crop residue returning (RT), (3) no-tillage and crop residue returning (NT), and (4) rotary tillage with all crop residues removed from the paddy field as a control (RTO). Each tillage treatment in the paddy field was laid out with three replications based on a randomized complete block design. CT treatments were tilled once with a moldboard plow at a depth of 15–20 cm and then rotated twice to a depth of 8–10 cm before transplanting the rice seedling. RT and RTO treatments were rotated four times at a depth of 8–10 cm before transplanting the rice seedlings. Soil tillage practices with RTO treatments were similar to that of RT treatments except that all crop residues were removed from the paddy field both in the early rice and late rice growing season. The field experiment ensured all treatments received the same total number of N, phosphorus pentoxide (P₂O₅), and potassium oxide (K₂O) (the total amount of N, P₂O₅, K₂O and included mineral fertilizer from crop residue) during the early rice and late rice growing season, respectively. The kinds of chemical fertilizers included urea, ordinary superphosphate, and K₂O. The quantity of Chinese milk vetch, the early and late rice straw residue added into the paddy soil for the CT, and RT and NT treatments were 22,500, 2000, and 2000 kg ha⁻¹, respectively. The quantity of Chinese milk vetch and the early and late rice straw residue removed from the paddy soil for the CT, RT, and NT treatments were 29,500, 3400, and 4000 kg ha⁻¹, respectively, and the quantity of the early and late rice straw residue removed from the paddy soil for the RTO treatment were 5400 and 6000 kg ha⁻¹, respectively. And the carbon (C) content of Chinese milk vetch with early and late rice straw residues were 386.4 g kg⁻¹, 395.3 g kg⁻¹, and 400.5 g kg⁻¹, respectively. Chinese milk vetch was sown at the end of October and returned to the paddy field in early April of the following year. The early and late rice seedlings were manually transplanted to the paddy in April and July and harvested with a combine harvester in July and October, respectively. Early rice and late rice used Xiangzaoxian 45 and Xiangwanxian 13 as materials during the field experiment period, respectively. Paraquat (1, 1′-dimethyl-4, 4′ bipyridiniumion) was applied with 6.0 kg ha⁻¹ to control weeds for NT treatment and 1.5 kg ha⁻¹ for RT, CT, and RTO treatments before the early and late rice transplanting. Other more detailed information about field management is described by Tang et al. [20].

2.3. Soil Sampling Collect

This field experiment was cultivated uniformly for 6 years. Soil samples were collected from the paddy field at the maturity stage of late rice in October 2021. Soil samples close to the rice plant were collected for 0–10 cm and 10–20 cm layers in the paddy field with a soil drill. Correspondingly, one composite soil sample from ten points was sampled from each plot. Thus, three soil composite samples were collected for each tillage treatment. The soil sample was transported to a laboratory, the soil sample was passed through a 2 mm mesh, and then stored at a 4 °C condition to investigate the soil microbial biomass content.

2.4. Soil Laboratory Analysis

2.4.1. Soil Bulk Density

Soil bulk density (BD) at 0–10 cm and 10–20 cm layers in the paddy field with different tillage treatments was measured using metallic cores of a known volume (internal diameter was 15.0 cm and length was 10.0 cm). These soil samples were oven-dried at 105 °C for 24 h to constant weight; more detailed information about this method is described by Blake and Hartge [22]. The number of replicates was conducted to investigate the soil BD of each tillage treatment with three replications.

2.4.2. Soil Microbial Biomass Content

The soil microbial biomass C (SMBC) and soil microbial biomass nitrogen (SMBN) contents were investigated using the fumigation–extraction method; more detailed information about these methods is described by Wu et al. [23] and Jenkinson [24], respectively. Briefly, two subsamples of preincubated soils (25 g) were placed in a vacuum desiccator and exposed to alcohol-free CHCl₃ vapor at room temperature for 24 h. These subsamples were then transferred to a clean desiccator, and the residual CHCl₃ was evacuated. In total, 80 mL of 0.5 M K₂SO₄ was added to the fumigated and non-fumigated samples, 10 mL extracts were taken to analyze organic C using an automated C analyzer (Phoenix 8000), and 20 mL extracts were digested, and ammonium nitrogen (N) was analyzed colorimetrically using a flow injection analyzer (FIASTAR 5000). SMBC was calculated using the equation SMBC = 2.22 × E_C, where E_C represents the increase in extractable C in the fumigated soil over that in the non-fumigated soil. K₂SO₄ extractable N from non-fumigated soil subtracted from that of fumigated soil was divided by a K_N value of 0.45. A number of replicates were conducted to investigate the SMBC and SMBN contents of each tillage treatment with three replications.

2.4.3. Flux Turnover Rate of SMBC and SMBN

The flux turnover rate of SMBC and SMBN of each soil sample was calculated according to the following equation [25]:

The flux of SMBC (SMBN) (g m⁻² a⁻¹) = (SMBC (SMBN) × BD × depth coefficient)/Turnover time

The flux turnover rate of SMBC (SMBN) (a⁻¹) = the um of dynamic decrease (or increase) in SMBC (SMBN) in a year/Average value of SMBC (SMBN) in a year

The flux turnover time of SMBC (SMBN) (a⁻¹) = 1/Flux turnover rate of SMBC (SMBN)

Here, the depth coefficient for every 10 cm of sampling depth was 1.

2.5. Statistical Analysis

Each investigated index with all tillage treatments was expressed as the mean and standard error. And the data of each measured item at the same soil layer among different tillage treatment means were analyzed using a one-way analysis of variance (ANOVA) according to the 5% probability level and was considered to indicate statistical significance. All the data of each measured item in this present paper were analyzed with the SAS 9.3 software package [26].

3. Results

3.1. SMBC and SMBN Contents

This result showed that the soil microbial biomass carbon (SMBC) content at the 0–10 cm and 10–20 cm layers with CT and RT treatments were significantly (p < 0.05) higher than that of the RTO and NT treatments (Figure 1a). At the 0–10 cm soil layer, the SMBC content with NT, RT, and CT treatments was significantly higher (p < 0.05) than that of the RTO treatment. Compared to the RTO treatment, the SMBC content at the 0–10 cm layer with NT, RT, and CT treatments increased by 28.99%, 15.78%, and 10.16%, respectively. At the 10–20 cm soil layer, the SMBC content with CT and RT treatments was significantly higher (p < 0.05) than that of the NT and RTO treatments. Compared to the RTO treatment, the SMBC content at the 10–20 cm layer with CT and RT treatments increased by 33.31% and 26.68%, respectively. This result proved that the SMBC content at the 10–20 cm layer was lower than that of the 0–10 cm layer under the same tillage treatment condition.

At the 0–10 cm and 10–20 cm soil layers, the soil microbial biomass nitrogen (SMBN) content with RTO treatment was significantly (p < 0.05) lower than that of NT, RT, and CT treatments. There were no significant (p > 0.05) differences in the SMBN content at 0–10 cm and 10–20 cm layers between CT, RT, and NT treatments. Compared to the RTO treatment, the SMBN content at the 0–10 cm layer with NT, RT, and CT treatments increased by 11.12%, 13.16%, and 17.84%, respectively. The SMBN content for the 10–20 cm layer with NT, RT, and CT treatments increased by 8.69%, 11.95%, and 16.52%, compared to the RTO treatment, respectively (Figure 1b). This result proved that the SMBN content at the 10–20 cm layer was lower than that of the 0–10 cm layer under the same tillage treatment condition.

3.2. Flux of SMBC and SMBN

This result showed that the flux of SMBC at 0–10 cm and 10–20 cm layers with CT and RT treatments was significantly (p < 0.05) higher than that of RTO and NT treatments. There were no significant (p > 0.05) differences in the flux of SMBC at the 0–10 cm and 10–20 cm soil layers between the RT and CT treatments. Compared to the RTO treatment, the flux of SMBC at the 0–10 cm and 10–20 cm layers with CT and RT treatments increased by 20.20%, 11.00% and 23.01%, 18.43% (Figure 2a). The results indicate that the flux of SMBC at the 10–20 cm layer was lower than that of the 0–10 cm layer under the same tillage treatment condition.

Compared to the RTO treatment, the flux of SMBN at the 0–10 cm and 10–20 cm layers with NT, RT, and CT treatments was significantly (p < 0.05) enhanced. There were no significant (p > 0.05) differences the in flux of SMBN at the 0–10 cm and 10–20 cm layers between the NT, RT, and CT treatments. Compared to the RTO treatment, the flux of SMBN at the 0–10 cm layer with NT, RT, and CT treatments increased by 5.24%, 6.20%, and 8.41%. The flux of SMBN at the 10–20 cm layer with NT, RT, and CT treatments increased by 4.20%, 5.78%, and 7.99%, compared with RTO treatment (Figure 2b). The result indicates that the flux of SMBN at the 10–20 cm layer was lower than that of the 0–10 cm layer under the same tillage treatment condition.

3.3. Flux Turnover Rate of SMBC and SMBN

Compared to the RTO treatment, the flux turnover rate of SMBC at 0–10 cm and 10–20 cm layers with NT, RT, and CT treatments was significantly enhanced (p < 0.05). There were no significant (p > 0.05) differences in the flux turnover rate of SMBC at the 10–20 cm layer between the RT and CT treatments. Compared to the RTO treatment, the flux turnover rate of SMBC at the 0–10 cm layer with NT, RT, and CT treatments increased by 16.20%, 49.30%, and 65.49%. The flux turnover rate of SMBC at the 10–20 cm layer with NT, RT, and CT treatments increased by 14.94%, 33.77%, and 39.61% compared with the RTO treatment (Figure 3a). The results prove that the flux turnover rate of SMBC at the 10–20 cm layer was lower than that of the 0–10 cm layer under the same tillage treatment condition.

This result proves that the flux turnover rate of SMBN at 0–10 cm and 10–20 cm layers with RTO treatment was significantly (p < 0.05) lower than that of the NT, RT, and CT treatments (Figure 3b). The order of the flux turnover rate of SMBN at 0–10 cm and 10–20 cm layers with all tillage treatments showed that RTO < NT < RT < CT. Compared to the RTO treatment, the flux turnover rate of SMBN at the 0–10 cm layer with NT, RT, and CT treatments increased by 1.37, 1.99, and 2.15 times; the flux turnover rate of SMBN at the 10–20 cm layer with NT, RT and CT treatments increased by 1.40, 2.00 and 2.19 times, respectively. The results prove that the flux turnover rate of SMBN at the 10–20 cm layer was lower than that of the 0–10 cm layer under the same tillage treatment condition.

The results indicate that the flux turnover time of SMBC at the 0–10 cm and 10–20 cm layers with NT, RT, and CT treatments was significantly (p < 0.05) lower than that of the RTO treatment. There was no significant (p > 0.05) difference in the flux turnover time of SMBC at the 0–10 cm and 10–20 cm layers between RT and NT treatments. Compared to the RTO treatment, the flux turnover time of SMBC at 0–10 cm and 10–20 cm layers with NT, RT, and CT treatments decreased by 12.05%, 21.88%, 49.55% and 13.45%, 22.27%, and 42.86% (Figure 4a). The results proved that the flux turnover time of SMBC at the 10–20 cm layer was higher than that of the 0–10 cm layer under the same tillage treatment condition.

The results indicate that the flux turnover time of SMBN at 0–10 cm and 10–20 cm layers with NT, RT, and CT treatments was significantly (p < 0.05) lower than that of the RTO treatment. Compared to the RTO treatment, the flux turnover time of SMBN at the 0–10 cm layer with NT, RT, and CT treatments decreased by 17.92%, 27.17%, and 46.82%. The flux turnover time of SMBN at the 10–20 cm layer with NT, RT, and CT treatments decreased by 11.76%, 20.81%, and 42.99% compared with the RTO treatment (Figure 4b). The results prove that the flux turnover time of SMBN at the 10–20 cm layer was higher than that of the 0–10 cm layer under the same tillage treatment condition.

4. Discussions

4.1. Effects of Tillage Managements on Soil Microbial Biomass Content

Soil microbial biomass (SMB) is usually regarded as an early indicator of changes in soil fertility and quality [4], which is clearly affected by using different tillage and crop residue management. In this study, this result proved that the SMBC and SMBN contents at 0–10 cm and 10–20 cm layers in the paddy field with the application of crop residue returning treatments (NT, RT, and CT) clearly increased; these results are in accord with hypothesis 1 that the SMB contents for the 0–20 cm layer with tillage and crop residue returning practices increase. These results are in agreement with the previous findings [27], suggesting that Chinese milk vetch and straw returning into the paddy field provide a carbon substrate for soil microbial activities, and the soil carbon and nitrogen conversion rate were promoted under combined tillage conditions. In the present study, our results proved that SMBC and SMBN contents in the paddy field with RT and NT treatments were lower than that of the CT treatment (Figure 1); the reason was attributed to the fact that crop residue was returned to the plow soil layer in the paddy field with conventional tillage practices, providing higher soil nutrients and available carbon substrate for soil microbial growth and multiplication [16]. Secondly, the soil’s physical characteristics, organic carbon, and total nitrogen contents with conventional tillage practice were enhanced, providing a good soil ecology environment and more available substrates for soil microbial growth [20,21]. Meanwhile, our result demonstrated that the SMBC and SMBN contents in the paddy field with NT treatment were lower than that of RT treatment; the reason for this was attributed to the fact that crop residues were returned to the 0–10 cm soil layer with rotary tillage practice when the depth of tillage was 8–10 cm with crop residue decomposing and providing higher soil nutrients and carbon substrate for soil microbial growth and multiplying [28].

In this study, our results prove that compared with the 10–20 cm layer, the SMBC and SMBN contents at the 0–10 cm layer in the paddy field were significantly increased, which could be attributed to the fact that crop residue and chemical fertilizer were incorporated into the 0–10 cm soil layer combined with conventional tillage or rotary tillage, and the 10–20 cm soil layer was little disturbed in the present study. On the other hand, it benefits practices by providing abundant soil available nutrients and a soil ecology environment for rice root growth and soil microbial activities with crop residue and chemical fertilizer input management. Therefore, it benefited from increasing the SMBC and SMBN contents in the paddy field in Southern China with conventional tillage or rotary tillage.

4.2. Effects of Tillage Managements on Flux of SMBC and SMBN

Some results demonstrate that the flux of SMBC and SMBN clearly changed under different tillage and crop residue returning conditions [15]. In this study, our result proved that the flux and turnover rate of SMBC and SMBN in the paddy field with RT and CT treatments were higher than that of NT and RTO treatments; these results were in accord with hypothesis two that the flux and turnover rate of SMBC and SMBN in the paddy field increased under tillage and crop residue returning conditions. The reasons for this were attributed to the fact that the crop residue and chemical fertilizer were incorporated into the plow soil layer with conventional tillage or rotary tillage practices, providing higher soil nutrients and available carbon substrate for soil microbial growth and multiplying. On the other hand, the soil’s active organic pool and metabolism of soil microorganisms were also promoted; therefore, the transformation and supply capacity of soil organic nutrients were improved. Meanwhile, this result demonstrated that the flux turnover rate of SMBC and SMBN in the paddy field with NT treatment was higher than that of the RTO treatment; the reasons for this were attributed to crop residue decomposing and providing a soil available nutrient and carbon substrate for soil microbial multiply [28]. Therefore, it was beneficial to increase the flux and turnover rate of SMBC and SMBN in the paddy field in Southern China with conventional tillage or rotary tillage.

In this study, our results demonstrated that the flux turnover time of SMBC and SMBN in the paddy field with RT and CT treatments was lower than that of NT and RTO treatments, which is in agreement with previous results [29]; the main reason for this could be attributed to the fact that crop residues were returning into the plow soil layer with a paddy field protective effect on the microorganism and decomposition rate of microorganisms enhanced by combined tillage management [30]. Secondly, the SOC and soil nutrient contents were increased under conventional or rotary tillage and crop residue returning conditions [31]. Therefore, it resulted in benefit management for decreasing the loss rate of carbon and nitrogen and increasing soil fertility by applying conventional or rotary tillage and crop residue returning practices. Meanwhile, our results indicate that the flux turnover time of SMBC and SMBN in the paddy field with NT treatment was lower than that of RTO treatment; the main reason for this was attributed to the fact that crop residue decomposing and providing soil available nutrients and carbon substrate for soil microbial activities, and the flux turnover rate of SMBC and SMBN were enhanced. As a result, tillage and crop residue returning practices not only improved the soil’s active organic pool but also promoted the speed of soil microorganism metabolism, improving the transformation and supply capacity of soil organic and nitrogen nutrients.

In this study, our results demonstrate that the flux turnover rate of SMBC and SMBN at the 0–10 cm layer is higher than that of the 10–20 cm layer in the paddy field, but the flux turnover time of SMBC and SMBN at the 0–10 cm layer was lower than that of the 10–20 cm layer in the paddy field; the main reason for this was attributed to the soil ecology environment at the 0–10 cm layer with tillage (conventional tillage or rotary tillage) and crop residue returning in paddy field practices improved. The flux turnover time of SMBC and SMBN in the paddy field was also promoted. Secondly, the crop residue and chemical fertilizer returned to the 0–10 cm layer in the paddy field with tillage practices, providing higher soil nutrients and available carbon substrate for soil microbial growth and improving the transformation and supply capacity of soil organic carbon and nitrogen [31]. Therefore, this provided effective practices for improving the flux turnover rate of SMBC and SMBN at the 0–10 cm layer the in paddy field, decreasing the flux turnover time of SMBC and SMBN at the 0–10 cm layer in the paddy field with tillage and crop residue input managements.

5. Conclusions

In the present study, our results demonstrate the beneficial practices for promoting soil microbial biomass (SMBC, SMBN) contents and its flux of turnover rate at the 0–20 cm layer in the double-cropping rice field with tillage and crop residue returning managements. Conventional tillage and rotary tillage with crop residue retained in the paddy field resulted in a double-cropping rice system with less flux turnover time of soil microbial biomass than the plots subjected to rotary tillage without crop residue input. At different soil layers, the soil microbial biomass content and its flux turnover rate in the double-cropping rice field with an increase in the soil layer were decreased under the same tillage condition. Therefore, it was beneficial practice to increase the soil microbial biomass content and its turnover under the double-cropping rice system in southern China with rotary tillage or conventional tillage and crop residue returning into the paddy field. However, there is still the need to investigate how changes in carbon source utilization and the soil microbial structure influence the ecological function of rhizosphere soil microorganisms under long-term tillage conditions.