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Article

Straw Returning with Decomposition Agent Enhanced Rice Yield and Decreased Yield-Scaled N2O Emissions in Tropical Paddy Fields

by
Longwei Meng
1,2,†,
Qiqian Lu
2,†,
Tian Lin
1,
Lu Dong
1,
Xiaochen Wu
1,
Yixiu Zhuo
1,
Anfu Yang
1,*,
Qilin Zhu
2 and
Lei Meng
2
1
Hainan Research Academy of Environmental Sciences, Haikou 571126, China
2
School of Breeding and Multiplication (Sanya Institute of Breeding and Multiplication), Hainan University, Sanya 572025, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agronomy 2024, 14(12), 3060; https://doi.org/10.3390/agronomy14123060
Submission received: 25 November 2024 / Revised: 18 December 2024 / Accepted: 20 December 2024 / Published: 22 December 2024
(This article belongs to the Section Innovative Cropping Systems)
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Abstract

:
Straw returning (R) combined with the application of a decomposition agent (RD) can increase crop yield and soil carbon (C) storage. However, the effect of RD on soil nitrous oxide (N2O) emissions in tropical areas remains poorly understood. In this study, an in situ experiment was performed under different water management strategies (long-term flooding or alternate wetting and drying) with the R and RD treatments to evaluate soil N2O emissions and rice yield. The SOC and TN contents were significantly lower under the RD treatment than under the R treatment. The R treatment significantly increased rice yield; however, the yield was further significantly increased under the RD treatment. The soil N2O emissions and yield-scaled N2O emissions were higher under the R treatment than under the no-straw-returning treatment. However, the RD treatment greatly reduced soil N2O emissions and yield-scaled N2O emissions under various water management strategies compared with those under the R treatment. Moreover, yield-scaled N2O emissions were lower in the RD treatment than in the control. The soil N2O emissions and yield-scaled N2O emissions were distinctly higher under alternate wetting and drying than under long-term flooding. Our results indicated that long-term flooding and straw returning with decomposition agents can effectively increase rice yield and reduce soil N2O emissions in tropical areas.

1. Introduction

Nitrous oxide (N2O) is one of the greenhouse gases and is an important material that depletes ozone in the stratosphere [1,2]. The main source of atmospheric N2O is human activities, and an increased atmospheric N2O concentration is closely related to agricultural production [1,3]. As the byproduct of nitrification and denitrification, soil N2O emissions are affected by agricultural practices, soil moisture, and soil physicochemical properties [4,5]. For reducing soil N2O emissions, and ultimately mitigating global warming, it is highly important to better understand the mechanisms underlying soil N2O emissions during agricultural processes in response to changes in agricultural practices and soil moisture.
Soil N2O emissions during agricultural processes are usually determined by soil moisture content (WC) [6]. Previous studies have confirmed that nitrification contributes the maximum to soil N2O emissions when WC is less than 40%, and soil N2O emissions increase with an increase in soil moisture [7]. Both nitrification and denitrification are the highest contributors of soil N2O emissions, contributing to 45–75% of the field capacity [8,9]. When the WC exceeds the field capacity, denitrification is the more significant process because an anaerobic environment is conducive to the activity of denitrifying bacteria; this in turn promotes soil N2O emissions [6]. WC varies due to alternate wetting and drying in the paddy field, which makes soil N2O emissions more complicated. Some studies have reported that alternate wetting and drying in paddy fields causes higher soil N2O emissions than continuous flooding [10,11]. The drying stage increases the amount of degradable organic carbon (C) and promotes nitrification, whereas the wetting stage is conducive to denitrification, leading to increased soil N2O emissions [3,8,12]. However, some studies reported that, under long-term water flooding in paddy fields, denitrification is more intense, leading to increased soil N2O emissions [13,14]. In addition, soil N2O emissions under high moisture conditions is not directly proportional to WC [15,16]. Instead, there is an optimal moisture content range in which soil N2O emissions are significantly promoted by affecting nitrification and denitrification processes. Therefore, reasonable management of soil moisture conditions can effectively regulate soil N2O emissions and, in turn, reduce the emission of greenhouse gases.
As an important agricultural resource, straw is widely used to improve soil physicochemical property, enhance the ability of soil to retain water and fertilizer, soil microbial activity, and crop yield [17,18,19]. Maintenance or increase of soil C content can be achieved by amending straw returning [20,21]. Since the biogeochemical cycles of C and nitrogen (N) are closely coupled, straw returning can affect the N transformation and soil N2O emissions by influencing soil C storage [12,19,21]. Increased soil organic matter content after straw returning greatly promotes the activity of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) that mediate ammoxidation, thereby increasing soil N2O emissions [22,23]. During straw decomposition, substances such as organic acids are released, which may change soil pH. This affects the activity of the nitrifying microorganisms, further affecting the N cycle rates [24,25]. For example, a pH range of 6–8 is suitable for the growth of AOB, and nitrification may be inhibited if straw returning causes the soil pH to deviate from this range [26]. Moreover, the organic matter released during straw decomposition can be used as the C source and energy source by the nitrifying microorganisms, promoting their growth and multiplication; this may accelerate nitrification [10,21,22]. However, higher organic matter content may lead to the massive multiplication of heterotrophic microorganisms in the soil; they compete with nitrifying bacteria for oxygen and nutrients, thereby inhibiting nitrification [27]. The formation of anaerobic or micro-anaerobic environments locally in soil under straw returning is conducive to the growth of denitrifying genes (nirK, nirS, and nosZ), thereby increasing soil N2O emissions [28]. Moreover, the soil organic C in fields under straw returning provides a rich C source for promoting the abundance of denitrifying genes [13,28]. However, some studies have suggested that straw returning increases the oxygen content in soil and reduces soil N2O emissions by inhibiting denitrification when the WC is too high [29,30]. Currently, the impact of straw returning on soil N2O emissions is still controversial due to inconsistent results because of differences in conditions, such as soil texture, type of crops, and water management strategies. Therefore, studying the effect of straw returning on soil N2O emissions requires a comprehensive evaluation by integrating various influencing factors.
A decomposition agent is a substance composed of different beneficial microorganisms (such as Bacillus subtilis, Trichoderma viridis, etc.), which promotes the rapid decomposition of organic matter [13,31,32]. A decomposition agent can accelerate the degradation of organic components, such as cellulose, hemicellulose, and lignin in straw; promote the rapid decomposition of straw; and thus release nutrients, providing a material basis for the growth of soil microorganisms and plants [32]. Reasonable utilization of a decomposition agent is of great significance for improving soil fertility and protecting the environment. The decomposition process of untreated straw in the soil is relatively slow, which may lead to unbalanced nutrient release and affect the structure and function of the soil microbial community [33,34]. Straw returning, with the application of a decomposition agent, can accelerate the decomposition and transformation of organic matter in straw [13,32]. This not only helps to improve nutrient availability in the soil and meet the needs of crop growth but improves soil structure and enhances the water- and fertilizer-retention capacities. However, straw returning with a decomposition agent application may affect the soil N2O emissions [35]. The structure of the soil microbial community is altered and the activity is enhanced during straw decomposition in soil; this, in turn, promotes nitrification and denitrification processes and may increase the soil N2O emissions to a certain extent [14,23,31]. However, straw returning can improve soil aeration and moisture conditions, which will inhibit soil denitrification and N2O emissions [30]. In addition, after straw returning with a decomposition agent application, factors such as water management and fertilization methods in paddy fields will have a complex effect on the soil N2O emissions [36]. However, under different water management conditions, comprehensive studies elucidating the relationship between straw returning with or without a decomposition agent and soil functional N-cycling genes and soil N2O emissions from paddy fields are still lacking.
In this study, we aim to investigate soil N2O emissions after straw returning combined with a decomposition agent under different water management conditions in a paddy field in a tropical region. Previous studies have indicated that straw returning can increase soil N2O emissions by enhancing microbial activity and abundance [22,30]. Moreover, under alternate wetting and drying, it can increase soil N2O emissions by promoting the soil nitrification and denitrification processes [10,15]. Therefore, we hypothesize that straw returning with a decomposition agent application will significantly promote soil N2O emissions under alternate wetting and drying in paddy fields. Considering that, after straw returning with a decomposition agent application in tropical regions, straw will rapidly decompose in a short period, we hypothesize that straw returning with a decomposition agent application will promote soil N2O emissions in the short term but will reduce soil N2O emissions in the whole rice growing season. In particular, we aim to address (i) how soil N2O emissions and N-cycling genes change under long-term flooding and alternate wetting and drying in paddy fields, and (ii) how straw returning, with or without a decomposition agent, affect soil N2O emissions and the related functional N-cycling genes under different water management conditions.

2. Materials and Methods

2.1. Site Description

The experimental area was situated in Hanlin Town, Ding’an County, Hainan Province, China (19.31° N, 110.27° E). It has tropical monsoon climate with abundant rainfall and heat. The rainy season is from May to October. The average annual temperature is 25.5 °C, the average annual precipitation is 2300 mm, and the elevation is 125 m. The soil type is lateritic soil developed from quaternary basalt.
The soil properties are as follows: pH 6.4, organic matter content 14.8 g·kg−1, total N (TN) content 3.49 g·kg−1, available phosphorus (P; AP) content 126 mg·kg−1, and available potassium (K; AK) content 213 g·kg−1. The straw used was the local straw of the current season, which was pulverized and mixed with the decomposition agent and then returned to the field in the full amount. The decomposition agent used was the Jiefengan Composting Agent (Hainan Jinyufeng Bioengineering Co., Ltd., Haikou, China) and mainly contains two kinds of bacteria (Bacillus subtilis and trichoderma viridis), which has the characteristics of fast heating rate. Generally, it can reach 50–60 °C in 3 days, and the composting temperature can reach 70 °C. The application rate was 30–45 kg ha−1. The straw type used was rice straw.

2.2. Experimental Design

The in situ experiment adopted a randomized block design, with 6 treatments, each treatment has 3 replications and a total of 18 plots. Each plot was 5 m × 6 m in size. The experiment included the conventional treatment (control; CK), straw returning (R), and straw returning with a decomposition agent (RD). For each treatment, two water conditions were set: long-term flooding (L) and alternate wetting and drying (A). Therefore, the experiment included six treatments in total: conventional treatment + long-term flooding (CKL); conventional treatment + alternate wetting and drying (CKA); straw returning + long-term flooding (RL); straw returning + alternate wetting and drying (RA); straw returning with a decomposition agent + long-term flooding (RDL); and straw returning with a decomposition agent + alternate wetting and drying (RDA). The rice variety of this study was Boyou 225. Rice seedlings was transplanted and planted on 24 July 2023, with plant spacing of about 15 cm and row spacing of 25 cm. Am amount of 750 kg/ha of compound fertilizer was applied as base fertilizer, and 300 kg/ha urea was applied during the rice filling period. For the L condition, a water layer of 3–5 cm was always maintained during the rice growth period, and the field was drained before harvest. For the A condition, a soil moisture tensiometer (Haian Huating Instrument Co., Ltd., Jiaxing, China) was installed in each treatment pot to monitor the water potential at a depth of 15 cm. Further, 7 days before rice transplantation, a water layer of 3–5 cm was maintained to ensure the survival of the transplanted plants. Thereafter, natural drying was conducted. When the soil negative pressure reached approximately −15 kPa, the field was re-flooded to a depth of 3–5 cm and further naturally dried again. This cycle was repeated until harvest.

2.3. Soil Sampling

The aboveground litter layer was removed before sampling. The subsamples (0–20 cm) were mixed to form composite samples and passed through a 2 mm sieve to remove litter, roots, and other impurities. Soil samples were divided into three equal parts. One portion was immediately stored at 4 °C to determine soil N2O emissions. The second portion was stored at −80 °C to estimate soil microbial abundance, while the third portion was air-dried to determine the physicochemical properties.

2.4. Analysis

Soil samples were dried in an oven at 105 °C for 24 h to determine the WC. After inorganic C was removed using 1 M HCl (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) solution, the samples were washed with deionized water to remove acid and bring the pH to neutral. Soil organic C (SOC) and TN were measured using the Sercon Integra2 element analyzer (Sercon Ltd., Crewe, UK). Soil available phosphorus was determined by molybdenum–antimony resistance colorimetry and measured using X-ray fluorescence spectrometry (XRF). NH4⁺ and NO3− in the soil were extracted with 2 M KCl (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) at a soil–KCl solution ratio of 1:5 and determined using a continuous flow analyzer (Skalar; Breda, The Netherlands). Plant determination method, accurately weigh a certain amount of dried and crushed samples into the boiling tube, add concentrated sulfuric acid (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and catalyst (copper sulfate, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), deboiling under high temperature conditions, so that the organic matter in the leaves are oxidized and decomposed into a soluble state. After that, the N content was determined by distillation, the phosphorus content by colorimetry, and the potassium content by flame spectrophotometry or atomic absorption spectrophotometry.

2.5. DNA Extraction and Real–Time PCR

Total nucleic acids were extracted from 0.5 g of soil aggregate fractions. Soil DNA was extracted by the Fast DNA® Spin Kit for Soil (MP Biomedicals, Solon, OH, USA) according to the instruction, and subsequently stored at −20 °C until use. Soil DNA quality and concentration were determined using a NanoDrop 2000 spectrophotometer (Thermo Fisher Scientific, Wilmington, DE, USA). To examine the response of soil N2O emissions related to functional microorganisms, we performed real-time quantitative PCR assays to quantify each gene abundance, including AOA, AOB, nitrite-reducing bacteria and fungi (nirK, nirS, and fungi–nirK), and N2O reducing bacteria (nosZI). Real-time quantitative PCR was performed on a CFX–96 thermocycler (Bio-Rad Laboratories Inc., Hercules, CA, USA) to assess the abundances of genes. The analysis included three biological replicates, each with three technical replicates. The qPCR reaction mixture contained 10 μL of SYBR Premix Ex Taq (Takara, Dalian, China), 0.5 μM of each primer, and 1 μL of DNA template (7.0–23.5 ng). Amplification procedures were as follows: 95 °C for 1 min × 1 cycle; 95 °C for 15 s, 60 °C for 15 s, 72 °C for 30 s × 40 cycles; 95 °C for 5 s, 60 °C for 1 min, 95 °C, 50 °C for 30 s × 1 cycle.

2.6. Data and Statistical Analyses

Soil N2O emissions were calculated as follows:
F = ρ × ∆ C / ∆ t × 273.15 / ( 273.15 + T ) × h
where F is the N2O emission flux (μg N2O–N kg–1 h–1); ρ is the density of N2O–N in the standard state, which is 1.25 g L–1; ΔC/Δt is the rate of change of N2O concentration in the chamber over the sampling period (10–9 N2O–N h–1); h is the height of the sampling chamber (m), and T is the ambient temperature (°C).
f = ∑ i = 1 n   ( F i × 24 ) + ∑ i = 1 n   ( F i + F i + 1 ) / 2 × ( t i + 1 - t i ) × 24
where f is the calculation for N2O emission of the whole growth period, n and i are sampling times, and t is the sampling day (d).
The yield-scaled N2O emissions were calculated using the following equation:
Yield-scaled N2O emission = f/Yield
Statistical analyses were performed using Microsoft Excel 2010 and SPSS 19.0 software. The discrepancy in soil properties, the yield and plant nutrients, and soil N2O emissions at p < 0.05 were assessed by one-way analysis of variance (ANOVA) and least squares difference (LSD) multiple comparison test. A Pearson correlation analysis was used for evaluating the relationship between soil properties, the yield and plant nutrients, and soil N2O emissions. The Origin 2021 Pro was used for plotting, and the data in the chart represent mean ± standard deviation.

3. Results

3.1. Soil Physicochemical Properties

Under the L condition, the contents of SOC and TN were significantly higher in the RL and RDL treatments than in the CKL treatment. Under the A condition, the highest SOC was recorded in the RA treatment (Table 1). The soil C/N ratio was significantly higher in the R and RD treatments than in the CK treatments, and it was significantly higher in the RD treatments than in the R treatments. Under the L, the highest soil pH was observed in the RDL. The AP levels were significantly higher in the R and RD treatments than in the CK treatment. The highest AP level was recorded in the RDA treatment. Soil AK content was significantly higher in the RL and RDL treatments than in the CKL treatment; however, it was remarkably lower in the RA treatment than in the CKA treatment.

3.2. Yield and the Nutrients in Straw and Grain

The highest yields were recorded in the RD treatments under both L and A conditions, followed by the R treatments (Table 2). Under the L condition, the highest straw-N, straw-P, grain-P, straw-K, and grain-K were recorded in the RDL treatment, followed by the RL treatment, both treatments exhibiting significantly higher values than those in the CKL treatment. Under the A condition, the straw-N, grain-N, straw-P, grain-P, straw-K, and grain-K were higher in the RDA treatment than in the RA and CKA treatments; they were the lowest in the CKA treatment.

3.3. Soil N2O Emissions

The highest soil N2O emissions were observed on 12 August and 29 August 2023, in all treatments (Figure 1). The N2O emissions on both these days were significantly higher in the RA treatment than in the other treatments; emissions were the lowest in the CKL treatment (Figure 1; p < 0.05). The R and RD treatments significantly enhanced cumulative soil N2O emissions compared with the CK treatments (Figure 2). The cumulative soil N2O emissions across the whole rice growth stage was significantly higher in the RA treatment than in the other treatments, whereas the lowest cumulative soil N2O emissions was observed in the CKL treatment (Figure 2; p < 0.05). The yield-scaled N2O emission was significantly higher in the R treatments than in the CK and RD treatments under both L and A conditions (Figure 2). The yield-scaled N2O emission was significantly decreased in the RD treatments compared with that in the CK treatments. The soil N2O emissions and yield-scaled N2O emissions were significantly higher under the A condition than under the L condition (Figure 2).

3.4. Soil NH4+ and NO3– Contents

The soil NH4+ contents decreased with the extension of rice cultivation time (Figure 3). The highest soil NH4+ content was recorded at the rejuvenation stage in all treatments. The R and RD treatments significantly increased soil NH4+ contents across various stages of rice growth. The highest and lowest NH4+ contents at the tillering stage were observed in the RDL and CKL treatments, respectively. Soil NO3− contents were low in the whole rice planting period and exhibited a decreasing trend with the extension of rice cultivation time (Figure 3). Soil NO3− contents in the CKL were significantly higher in the first month of the rice planted.

3.5. Abundance of N-Cycling Functional Genes

The abundance of soil N-cycling functional genes was much higher in the R and RD treatments than in the CK treatments (Figure 4). The abundances of nirK, AOA, and AOB genes were significantly higher in the RD treatments than in the CK treatments under A and L water management condition, followed by those in the R treatments. The highest abundance of nirS and nosZ was recorded in the R treatments, followed by the RD treatments under L and A water management condition. The abundance of fungi-nirK was significantly higher in the R and RD treatments than in the CK treatments under the same water management condition.

3.6. Dominant Factors Controlling Soil N2O Emissions and Rice Yield

The soil N2O emissions significantly correlated with soil chemical properties (SOC, TN, C/N ratio, pH, and AP), organic N (NO3−), and plant nutrients (straw-N, straw-P, grain-P, straw-K, and grain-K; zero-order in Figure 5a). Soil N2O emissions significantly correlated with NH4+ content, N-cycling genes (AOA, AOB, nirK, and fungi-nirK), and plant nutrients (straw-N, grain-P, straw-K, and grain-K) after controlling the role of soil chemical properties. Soil chemical properties (SOC, TN, C/N ratio, and pH) and plant nutrients were significantly and positively correlated with soil N2O emissions when inorganic N was controlled. After controlling for soil N-cycling genes, soil chemical properties, inorganic N, and plant nutrients were significantly positively correlated with soil N2O emissions. Soil chemical properties (SOC, TN, and C/N ratio), NO3− content, and N-cycling genes (AOB, nirK, and fungi-nirK) were greatly correlated to rice yield (zero-order in Figure 5b). The SOC and TN were positively correlated with rice yield after controlling for inorganic N, N-cycling genes, and plant nutrients, respectively. The SOC and TN were significantly positively correlated with soil N2O emissions and rice yield (Figure 6). The AOA, nirK, nirS, fungi-nirK, and nosZ were positively correlated with soil N2O emissions (Figure 6).

4. Discussion

4.1. Effects of Returning Decomposed Straw on Soil Properties and Rice Yield

Studying the effects of the R and RD treatments on soil nutrients and crop yield is an important agricultural research field. Our results were consistent with previous studies reporting that straw returning can significantly increase the contents of soil organic matter content, TN, AP, and AK and soil microbial activity [37,38]. Moreover, our results revealed that straw returning combined with a decomposition agent significantly decreased SOC and TN contents, but significantly increased the C/N ratio compared with the straw returning treatment (Table 1). Straw returning with a decomposition agent can accelerate the microbial activity to promote the rapid decomposition of straw, and the nutrients are quickly absorbed by plants, which is an important reason for the decrease in SOC and TN contents [39,40]. Most studies have proved that straw returning can promote crop yield [21,37]. Straw returning can increase the cation exchange capacity of the soil, improve soil fertilizer retention capacity, and further increase the utilization rate of chemical fertilizers. In our study, straw returning significantly increased rice yield, and the yield was further increased after applying straw returning combined with a decomposition agent (Table 2). Straw returning combined with a decomposition agent can accelerate the decomposition of straw, increase the activity of microorganisms, and promote the release of nutrients, thereby improving soil fertility and crop yield [21,37]. Moreover, the decomposition agent can reduce and prevent the possible adverse effects of returning of large amounts of straw, such as N competition [41].
Under suitable moisture conditions, microbial activity is enhanced, accelerating the decomposition of straw [42,43]. Our results revealed that the SOC and TN contents in the RD treatment were significantly higher under the A condition than under the L condition (Table 1). Previous studies revealed that WC is an important factor affecting the straw decomposition rate [42]. Generally, the suitable WC for straw decomposition is 60–70% of the field capacity [42,44]. WC fluctuates during alternate flooding and drought, which significantly increases the straw decomposition rate and thus accelerates the release of nutrients under alternate wetting and drying in paddy fields [10,44]. Straw returning combined with a decomposition agent will lead to an anaerobic condition on paddy fields under long-term flooding conditions, which is not conducive to straw decomposition by microorganisms, resulting in lower SOC and TN contents than those under alternate wetting and drying.

4.2. Effects of Returning Straw on Soil N2O Emissions

Straw returning significantly promoted N2O emissions from paddy fields (Figure 2). Straw returning can increase the content of soil moisture-soluble organic C, provide energy for microbial activities, stimulate the metabolic activity of microorganisms, accelerate the consumption of oxygen, and form an anaerobic microenvironment, thereby promoting the activity of denitrifying bacteria and further increasing soil N2O emissions [23,45]. In addition, straw returning can provide sufficient substrate for soil microorganisms and improve soil N availability, thereby increasing soil N2O emissions [22]. Previous studies have revealed that straw returning can increase the abundance of nitrification-related genes in ammonia-oxidizing bacteria (AOB) and that of the denitrification-related gene nitrite reductase (nirK), while reducing the abundance of the nitrate reductase gene (nosZ), which ultimately promotes soil N2O emissions [23,45]. Our results revealed that the genes related to soil N transformation were the important factors affecting soil N2O emissions in tropical paddy fields (Figure 2 and Figure 3). Straw returning and straw returning with a decomposition agent significantly increased the abundance of genes related to nitrification (AOA and AOB) and denitrification (nirK, nirS, and fungi-nirK) (Figure 4), which significantly promoted soil N2O emissions from tropical paddy fields (Figure 5 and Figure 6).
The relationship between WC and N2O emissions is relatively complex. Our results revealed that soil N2O emissions from paddy fields were significantly higher under alternate wetting and drying than under long-term flooding (Figure 2). The specific mechanism for the increase in N2O emissions under alternate wetting and drying mainly involves the alternation of soil nitrification and denitrification processes [11,16]. The diffusion of oxygen into the soil increases due to the reduction of soil moisture in the drying stage, promoting the nitrification process, and leading to NO3− accumulation [30]. When the soil is rewetted, denitrification is enhanced. However, the reduction rate of NO3− may not be sufficient to completely reduce NO3− to N2, resulting in the increase of soil N2O emissions [46]. The low-oxygen environment inhibits nitrification under long-term flooding condition and instead helps capture N2O and reduce its release to the atmosphere [30,46]. In addition, the destruction of soil aggregate structure during the drying process will cause organic matter to lose physical protection and release a large amount of active organic C and N, making it easier for microorganisms to meet substrate needs and further promoting soil N2O emissions [6,15]. Under alternate wetting and drying, the N2O emission rate usually reaches the maximum after each drying period and is significantly reduced during the wetting period [11,47]. This periodic emission pattern indicates that soil N2O emissions under alternate wetting and drying are closely related to the alternating cycle of soil nitrification and denitrification processes.

4.3. Effects of Returning Decomposed Straw on Soil N2O Emissions

Straw returning combined with a decomposition agent accelerates straw decomposition through microbial agents and quickly converts difficult-to-decompose straw into easily decomposed organic matter [48]. Our results revealed that straw returning combined with a decomposition agent significantly reduced soil N2O emissions under flooding and alternate wetting and drying compared with only straw returning (Figure 2). In straw returning with a decomposition agent, organic matter is decomposed and transformed by microorganisms; it becomes more stable and easier to be utilized by soil microorganisms [41]. This may reduce the unstable intermediate products produced during further decomposition in the soil, thereby reducing the potential for N2O production [49]. Our study indicated that the abundance of the soil nirS gene was significantly lower in the RD treatment than in the R treatment (Figure 3). This may be due to the fact that returning of decomposed straw usually has better structure and porosity, which can improve soil aeration and water retention capacity [30,50]. Appropriate aeration can reduce the formation of anaerobic environments, thereby inhibiting denitrification and reducing N2O emissions [51]. Moreover, during the decomposition process, some microorganisms may fix N in the soil, reducing the available N content for N transformation processes, thereby reducing N2O production [41,52]. The N2O emissions per unit yield was significantly lower in the RD treatment than in the CK treatment (Figure 2). This is mainly due to the increase in the soil C/N ratio after returning decomposed straw (Table 1), which can promote absorption and utilization of soil inorganic N by rice and ultimately increase rice yield.
Both the R and RD treatments significantly promoted soil N2O emissions under alternate wetting and drying. Although returning decomposed straw provides a C source, under long-term flooding conditions, other microorganisms in the soil will also use these C sources for growth and metabolism [11,47]. This may lead to C source competition between denitrifying bacteria and other microorganisms, thereby inhibiting N2O production [14,46]. The organic matter provided by returning decomposed straw will promote the abundance of soil micro-nitrification-related and denitrification-related genes during alternate wetting and drying, further increasing N2O emissions [8,23,38]. However, the impact of returning decomposed straw on soil N2O emissions under long-term flooding and alternate wetting and drying in paddy fields is complex and is affected by the interaction of multiple factors, such as soil types, climatic conditions, rice varieties, and agricultural management measures in different regions. Therefore, it is difficult to reach a generally applicable conclusion. According to the local climate and soil conditions, the water management strategy on paddy fields should be reasonably adjusted to reduce N2O emissions. Considering the factors of yield and soil N2O emissions, straw returning with a decomposition agent in the long-term flooding condition has high significance for practical production.

5. Conclusions

Straw returning significantly increased rice yield in tropical paddy fields, and the yield further improved after applying straw returning combined with a decomposition agent. The soil N2O emissions and yield-scaled N2O emissions were significantly higher under the straw returning treatment than under the control. Straw returning with a decomposition agent greatly reduced soil N2O emissions and yield-scaled N2O emissions under various water management strategies compared with only the straw returning treatment. Moreover, yield-scaled N2O emissions were lower in straw returning combined with a decomposition agent than in the control. Long-term flooding and straw returning combined with a decomposition agent during rice planting should be employed to improve rice yield while reducing soil N2O emissions in tropical areas.

Author Contributions

Conceptualization, L.M. (Longwei Meng), Q.Z., L.M. (Lei Meng), Q.L. and A.Y.; investigation, L.M. (Longwei Meng), T.L. and L.D.; formal analysis, Q.Z., L.M. (Lei Meng), X.W., Q.L. and Y.Z.; software, L.M. (Longwei Meng), L.M. (Lei Meng), Q.L. and A.Y.; supervision, L.D., T.L. and X.W.; writing—original draft preparation, L.M. (Longwei Meng), Y.Z., Q.Z., Q.L. and A.Y.; writing—review and editing, L.M. (Longwei Meng), Y.Z., L.D., T.L., Q.L. and X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (42067008), the High-level Talent Project of the Natural Science Foundation of Hainan Province (320RC493), the Special Foundation of Government Financial of Hainan Province, China (No. ZC2018-196).

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Soil nitrous oxide (N2O) emissions in paddy field under different treatments.
Figure 1. Soil nitrous oxide (N2O) emissions in paddy field under different treatments.
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Figure 2. Cumulative soil N2O emissions in paddy field under different treatments. (a) represents soil N2O cumulative emission; (b) represents soil N2O emission per unit of yield production. The same lowercase letters suggest no significant difference between different treatments at 0.05 level.
Figure 2. Cumulative soil N2O emissions in paddy field under different treatments. (a) represents soil N2O cumulative emission; (b) represents soil N2O emission per unit of yield production. The same lowercase letters suggest no significant difference between different treatments at 0.05 level.
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Figure 3. The NH4+ and NO3− contents in paddy field under different treatments.
Figure 3. The NH4+ and NO3− contents in paddy field under different treatments.
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Figure 4. Copy numbers of nirK (a), nirS (b), AOA (c) AOB (d) Fungi-nirK (e) nosZ, (f) gene (mean ± standard errors) under different treatments in the paddy field. The same lowercase letters suggest no significant difference between different treatments at 0.05 level.
Figure 4. Copy numbers of nirK (a), nirS (b), AOA (c) AOB (d) Fungi-nirK (e) nosZ, (f) gene (mean ± standard errors) under different treatments in the paddy field. The same lowercase letters suggest no significant difference between different treatments at 0.05 level.
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Figure 5. Partial correlations between the soil N2O emissions (a), rice yield (b), and four types of contributing factors. * p < 0.05; ** p < 0.01.
Figure 5. Partial correlations between the soil N2O emissions (a), rice yield (b), and four types of contributing factors. * p < 0.05; ** p < 0.01.
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Figure 6. Pearson’s correlation among soil properties, plant nutrients, yield, and soil N2O emissions. * p < 0.05; ** p < 0.01.
Figure 6. Pearson’s correlation among soil properties, plant nutrients, yield, and soil N2O emissions. * p < 0.05; ** p < 0.01.
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Table 1. Chemical properties of soil under different treatments.
Table 1. Chemical properties of soil under different treatments.
TreatmentSOC
(g·kg−1)		Total N (g·kg−1)C/N ratiopHAP
(mg·kg−1)		AK
(mg·kg−1)		AN
(mg·kg−1)
CKL12.1 ± 0.04 e0.87 ± 0.02 d14.0 ± 0.35 b5.60 ± 0.03 d28.5 ± 1.51 e44.7 ± 1.62 b110 ± 2.36 bc
RL18.6 ± 0.59 b1.17 ± 0.02 b15.9 ± 0.38 a5.70 ± 0.05 cd56.6 ± 2.56 b73.9 ± 2.49 a114 ± 3.31 bc
RDL16.8 ± 0.38 c1.05 ± 0.03 c16.1 ± 0.46 a5.95 ± 0.04 a45.1 ± 1.30 c60.9 ± 2.62 ab109 ± 3.07 c
CKA13.0 ± 0.44 d0.88 ± 0.01 d14.7 ± 0.61 b5.76 ± 0.09 bc41.0 ± 0.71 d53.4 ± 1.71 b123 ± 1.91 a
RA20.7 ± 0.34 a1.30 ± 0.01 a15.9 ± 0.15 a5.83 ± 0.02 b45.1 ± 0.55 c47.1 ± 11.1 b115 ± 0.41 b
RDA19.1 ± 0.25 b1.19 ± 0.01 b16.0 ± 0.24 a5.73 ± 0.02 bc74.5 ± 1.06 a60.6 ± 14.5 ab127 ± 3.28 a
SOC, soil organic carbon; total N, total nitrogen; AP, available phosphorus; AK, available potassium; AN, available nitrogen. CKL, RL, RDL represent no straw returning, straw returning, and straw returning with a decomposition agent under long-term flooding condition, and CKA, RA, RDA represent no straw returning, straw returning, and straw returning with a decomposition agent under alternating dry and wet conditions. The same lowercase letters suggest no significant difference between different treatments at 0.05 level. This footer also works for other tables and figures.
Table 2. Rice yield and nutrient content under different treatments.
Table 2. Rice yield and nutrient content under different treatments.
TreatmentYield
(t·ha−1)		Straw N (g·kg−1)Grain N (g·kg−1)Straw P (g·kg−1)Grain P (g·kg−1)Straw K (g·kg−1)Grain K (g·kg−1)
CKL5.55 ± 0.28 c3.31 ± 0.58 c6.21 ± 0.38 d3.25 ± 0.85 c3.04 ± 0.48 c20.27 ± 1.48 c2.31 ± 0.18 c
RL7.08 ± 0.13 b4.41 ± 0.69 b6.76 ± 0.54 a3.43 ± 0.86 b3.09 ± 0.33 b21.20 ± 2.33 b2.56 ± 0.33 b
RDL7.41 ± 0.35 a5.18 ± 0.83 a6.37 ± 0.87 c3.93 ± 0.67 a3.26 ± 0.28 a22.25 ± 2.28 a2.76 ± 0.28 a
CKA5.89 ± 0.32 c3.36 ± 0.54 c6.31 ± 0.62 d3.12 ± 0.19 d2.62 ± 0.201 d19.11 ± 1.21 d2.19 ± 0.21 d
RA6.94 ± 0.13 b4.61 ± 0.26 ab6.25 ± 0.58 c3.22 ± 0.22 c2.82 ± 0.87 c20.35 ± 2.87 c2.32 ± 0.17 c
RDA7.85 ± 0.28 a4.86 ± 0.41 a6.41 ± 1.38 b3.35 ± 0.55 b3.06 ± 0.28 b22.47 ± 1.28 a2.47 ± 0.28 b
The same lowercase letters suggest no significant difference between different treatments at 0.05 level.
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Meng, L.; Lu, Q.; Lin, T.; Dong, L.; Wu, X.; Zhuo, Y.; Yang, A.; Zhu, Q.; Meng, L. Straw Returning with Decomposition Agent Enhanced Rice Yield and Decreased Yield-Scaled N2O Emissions in Tropical Paddy Fields. Agronomy 2024, 14, 3060. https://doi.org/10.3390/agronomy14123060

AMA Style

Meng L, Lu Q, Lin T, Dong L, Wu X, Zhuo Y, Yang A, Zhu Q, Meng L. Straw Returning with Decomposition Agent Enhanced Rice Yield and Decreased Yield-Scaled N2O Emissions in Tropical Paddy Fields. Agronomy. 2024; 14(12):3060. https://doi.org/10.3390/agronomy14123060

Chicago/Turabian Style

Meng, Longwei, Qiqian Lu, Tian Lin, Lu Dong, Xiaochen Wu, Yixiu Zhuo, Anfu Yang, Qilin Zhu, and Lei Meng. 2024. "Straw Returning with Decomposition Agent Enhanced Rice Yield and Decreased Yield-Scaled N2O Emissions in Tropical Paddy Fields" Agronomy 14, no. 12: 3060. https://doi.org/10.3390/agronomy14123060

APA Style

Meng, L., Lu, Q., Lin, T., Dong, L., Wu, X., Zhuo, Y., Yang, A., Zhu, Q., & Meng, L. (2024). Straw Returning with Decomposition Agent Enhanced Rice Yield and Decreased Yield-Scaled N2O Emissions in Tropical Paddy Fields. Agronomy, 14(12), 3060. https://doi.org/10.3390/agronomy14123060

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