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Article

Meta-Analysis of Factors Affecting C-N Fractions and Yield of Paddy Soils by Total Straw Return and N Fertilizer Application

by
Liqiang Zhang
1,
Yunlong Wang
1,
Zixi Lou
1,
Lefei Hsu
1,
Di Chen
1,
Renzhe Piao
1,
Hongyan Zhao
1,* and
Zongjun Cui
2
1
College of Agronomy, Yanbian University, Yanji 133002, China
2
College of Agronomy and Biotechnology, China Agricultural University, Beijing 100193, China
*
Author to whom correspondence should be addressed.
Agronomy 2022, 12(12), 3168; https://doi.org/10.3390/agronomy12123168
Submission received: 15 October 2022 / Revised: 27 November 2022 / Accepted: 9 December 2022 / Published: 14 December 2022
(This article belongs to the Section Soil and Plant Nutrition)
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Abstract

:
The effective use of nutrient-rich crop straw is an important way to use resources efficiently and to sustain agricultural development. This meta-analysis study collected and analyzed the data of 6788 observations published in 238 peer-reviewed papers to investigate differences in soil C-N fractions and yields of paddy soils under different straw-return amounts. This large dataset was also used to quantify the degree of influence of factors such as climate characteristics, soil properties, N fertilizer application rates, straw-rotting agent addition, rice varieties, and straw return methods. The results showed that straw return amounts improved soil alkaline-hydrolysable N (7%), total N (10%), organic C (11%), the C:N ratio (8%), rice N accumulation (12%), and overall yield (18%). The most significant effect was in northeast China fields for total soil nitrogen (TN) content and yield with increases of 13% and 22%, respectively. We also found more effective N utilization and a greater rice yield when 220–260 kg ha−1 N fertilizer was applied with 20–30 kg ha−1 straw-rotting agent with the total amount of straw return. These findings have important implications for choosing appropriate conditions and field management practices and to improve rice yield in China.

1. Introduction

China is both a large grain-producing country and a large straw-resource country. Wheat and corn straw, for example, are especially abundant in China’s northern region, and rice and wheat straw are abundant in the southern region [1]. Studies from 2015 show that for all of China’s rice fields, the total nutrient resources that can be produced from straw return are about 6.256 million tons N, 1.979 million tons P2O5, and 11.595 million tons K2O [2]. Additional studies have shown that straw return to the field is significantly beneficial [3,4,5]. When straw return is fully implemented, it can improve soil structural properties, enhance soil nutrients, retain water and moisture, promote soil nutrient cycling, and benefit crop cultivation [6]. Most studies have shown that straw return to the field can increase the nutrients available for crop growth in the soil [7]. Additionally, numerous research scholars have suggested that when straw return is used with reduced use of nitrogen fertilizer, the increase in nutrient levels in the soil can eventually increase the actual yield of rice. Therefore, straw return can reduce the use of chemical fertilizers and, at the same time, ensure no large fluctuations in rice yield. Furthermore, proper nitrogen fertilizer-transport facilitates the decomposition of straw and more easily releases the nutrients from the straw to promote the growth of rice, thus increasing the yield [8,9,10]. Based on these promising findings, it is important to conduct a comprehensive study of how effective straw return is at boosting C-N fractions in paddy soil, rice growth and development, and rice yield. Rice straw has a high C:N ratio of 60–80:1 according to Qiu et al. (2012) [11]. The results of related studies show that rice straw return to the field will have a certain inhibitory effect on rice growth in the early stages, but a significant promotion of growth in the middle and late stages [12,13,14]. One study found that the N accumulation in rice when a straw return treatment was used was not significantly increased in the early stage of rice fertility, but it was significantly increased during the stages of rice growth and development [15,16]. In another study that compared treatments of straw return and no straw return, straw return was correlated with an average increase of 25% in N uptake for the rice straw, seeds, and the whole plant [17,18]. Several researchers have suggested that when straw rewetting is used with straw-decay bacteria and in conjunction with the reduced use of N fertilizer, it can increase the nutrients in the soil and thus ultimately increase the actual yield of rice [19,20]. Although previous authors have confirmed through different experiments that straw return with N fertilizer application can effectively increase rice yield and improve soil C-N fractions in paddy fields [21,22,23,24,25,26], their experimental methods vary, and the results depend on specific regional environments and soil factors. Additionally, in general, these studies have not been conducted at regional or even national scales. In order to understand the extent of the effect of straw return on soil C-N fractions and the yield of paddy soils as a whole in China, it is necessary to collect and synthesize all relevant information in the literature related to soil C-N fractions, yield, and the influencing factors on paddy soils under straw return conditions. Our goal was to evaluate the datasets of these relatively independent studies and make comprehensive and unbiased conclusions.

2. Materials and Methods

Meta-analysis is a statistical method to integrate and analyze the results of multiple studies of the same kind in order to draw general conclusions that would be otherwise hard to reach by looking at the studies one-by-one. In recent years, the application of meta-analysis in the field of agronomy in China has grown, and it is continuously playing a unique and important role as a statistical tool suitable for larger temporal and spatial scales. It is proving to be an effective method for analyzing the impact of key factors and changes in these factors [27]. In this meta-analysis study, we analyzed relevant published data from 1991 to 2021 and discussed the effects of different influencing factors, such as the application of N fertilizer and straw-rotting agent on soil C-N fractions and yield under straw return conditions in different rice-growing regions of China. We also analyzed changes in important influencing factors such as annual average temperature and precipitation. We also investigated the effect of N fertilizer application on the growth, development, and yield of rice when there are differences in climate, soil type, and the amount of straw returned to the field. An additional purpose of our study was to provide a scientific basis for straw return and N fertilizer use, and management in rice production.

2.1. Data Acquisition

The online databases used in this study were CNKI (https://www.cnki.net (accessed on 14 October 2022)), Wanfang (https://www.wanfangdata.com.cn (accessed on 14 October 2022)), VIP (http://qikan.cqvip.com (accessed on 14 October 2022)), and Web of Science (https://www.webofscience.com (accessed on 14 October 2022)). Relevant papers published from 1991 to 2021 were gathered that met certain criteria; these criteria are explained in Section 2.2. Ultimately, 238 published papers were collected and studied. The search terms used were straw return, N fertilizer, rice, paddy field, soil nutrients, and yield.

2.2. Inclusion Criteria

To ensure the validity of the conclusions of this study, a large database was assembled. Relevant papers were screened according to the following inclusion criteria:
(1) the field trials were conducted in China; (2) no rice straw return and no N fertilizer application were used as the control treatments; (3) all trial data were presented with means and variance based on at least three replications or more than two years of data collection; (4) planting location, year, and fertilizer application were clearly identified; and (5) studies that duplicated work of another study already included in our group were excluded. Note, joint effects of treatments such as planting density were ignored in all papers collected.

2.3. Literature Screening and Data Extraction

We screened potentially relevant titles and abstracts from the literature and excluded those that did not meet our inclusion criteria. Once the collection of papers was selected, data and information were extracted. The extracted information included the described methodology, interventions, and outcome measures.

2.4. Statistical Analysis

MetaWin2.1 was used to conduct meta-analysis [28]. The natural logarithm of the response ratio (R) was used as the effect value (lnR), and this was calculated for each pair of data using Equations (1) and (2) [29]:
R = Xt/Xc
lnR = ln (Xt/Xc) = lnXt − lnXc
where Xt, total straw return treatment, kg ha−1, for various conditions including rice yield, soil organic carbon (SOC), total N (TN), and alkaline-hydrolysable N (AN); and Xc, the control treatment (no straw return) for the various conditions.
The corresponding weights for each effect value were obtained using Equation (3):
ω = (nt × nc)/(nt + nc)
where ω, weights; nt, the number of trial replications under the total straw return treatment; and nc, the number of trial replications under the control treatment. The 95% confidence interval of the effect value was calculated using the resampling method, and the effect value was considered significant if the 95% confidence interval of the effect value did not overlap with 0. If all confidence intervals were >0, then total straw return with N fertilizer application significantly increased rice yield (p < 0.05). If all confidence intervals were <0, then total straw return with N fertilizer application significantly reduced rice yield (p < 0.05). If the confidence interval contained 0, then it meant that total straw return and N fertilizer application had no significant effect on rice yield [30,31,32].
For the purpose of description, the percent change in rice yield was calculated using Equation (4):
E = (exp (lnR) − 1) × 100%

2.5. Subgroup Analysis

Due to the wide variation in climate conditions, tillage practices, and soil types in different agricultural production regions of China, and the fact that many factors such as different fertilizer application rates and rice varieties all have an effect on rice growth and development, the available data were grouped in various ways to test the effect of a particular factor on soil C-N fractions and yield in paddy fields using subgroup meta-analysis. The different groupings were based on:
(1)
dividing the study area into six major rice-growing regions: eastern; southwestern; southern; northern; northeastern; and northwestern China;
(2)
classifying rice variety into three categories: early (≤125 d), medium (125–150 d), and late (≥150 d) maturity;
(3)
using four mean annual temperature intervals (<10 °C, 10–15 °C, 15–20 °C, and >20 °C); these were used for each test area;
(4)
classifying average annual precipitation in rice fields as very low (400–600 mm), low (600–800 mm), medium (800–1,500 mm), and high (>1500 mm);
(5)
including three types of paddy return methods: zero tillage (the straw is crushed or the whole straw is directly and evenly covered on the surface); rotary tillage (the straw is crushed and evenly scattered on the surface while the rice is harvested mechanically, then the rotary tiller is used to shallowly turn it into the plow layer); and deep tillage (the straw is crushed and evenly scattered on the surface during the mechanical harvesting of rice, then the straw is turned and buried under the soil plow layer);
(6)
identifying four types of paddy soils: saline-alkali soil, gleyed paddy soil, submerged paddy soil, and waterlogged genic paddy soil;
(7)
categorizing C-N fractions of the different soil layers into three categories: 0–5 cm, 5–10 cm, and 10–20 cm;
(8)
identifying N accumulation in both the plants and the seeds;
(9)
classifying N fertilizer application rates into six categories: 0–140, 140–180, 180–220, 220–260, 260–300, and 300–450 kg ha−1. The application amount of straw-decayed bacterium (a complex flora, mostly composed of Bacillus subtilis) was divided into four categories: 0–10; 10–20; 20–30; and 30–40 kg ha−1 (straw-decayed bacterium were from: Jiangsu, Tianxiang, Beijing Century, Chengdu Hualong, Nanjing Ningliang, Shanghai Lianye, and Beijing Zhongnong).
Figure 1 shows the geographic distribution of the rice-field trial sites identified in the papers that met the inclusion criteria.

2.6. Literature Screening

A total of 18,542 relevant literature articles were evaluated according to the inclusion criteria, and 2201 randomized controlled trials on rice straw returning to the field were collected. This study focused only on randomized controlled trials with different application rates of straw return, N fertilizer, and straw rotary tillage. Eventually, we identified 238 articles that met our criteria and these were included in our study.

2.7. Study Indicators and Data Grouping

The sample size of the observation data for AN was 534. AN includes both inorganic nitrogen and organic nitrogen, which has a simple structure and can be directly absorbed and used by crops. The AN content has a better correlation with plant nitrogen nutrition than inorganic nitrogen and plays an important role in plant growth and development. The level of AN content depends on the level and quality of organic matter content and the amount of nitrogen fertilizer put into the soil, which is consistent with the main idea of this study, so it is used as an indicator of soil nitrogen effectiveness [33]. The sample size of the observation data for TN was 581. The TN content of the soil represents the total soil nitrogen storage and nitrogen supply potential. Therefore, TN content is one of the main indicators of soil fertility [34]. The sample size of the observation data of SOC was 749. SOC is closely related to crop yield and is an important characterization of soil fertility [35]. The sample size of the observation data for the C:N ratio was 444. The C:N ratio can directly influence the rate of conversion of humus in organic matter and the speed of mineralization processes in the soil [36]. The sample size of the observation data of yield was 1047, and the sample size of the observation data of total N accumulation was 468. The line of best fit equation is y = y0 + (A/(w × sqrt(pi/2))) × exp(−2 × ((x − xc)/w)2) (Figure 2). A small number of the data were too off-center, so we removed them and subsequent analyses of these data were no longer used.

3. Results

3.1. Different Regions

Nationwide, total straw return with N fertilizer application significantly increased rice yield by 18% (p < 0.05), and total straw return with N fertilizer application promoted rice yield increase in different regions, but the degree of impact varied (Figure 3). In northeast China, the effect of total straw return with N fertilizer application on rice yield was the most significant at 23% (p < 0.05). East China’s rice yield was also significantly increased at 20% (p < 0.05), which was higher than the national average. In north, central, southwest, and northwest China, the effect of total straw return in combination with N fertilizer application on rice yield was lower than the national average. North, central, and northwest China had increased yields of 11.2%, 11%, and 6%, respectively. In southwest China, the total amount of straw returned to the field with N fertilizer application significantly increased rice yield by 2%, the smallest effect among all regions.
On a national scale, total straw return with N fertilizer application significantly increased AN, TN, TNA, and SOC content, and the C:N ratio in rice paddy soils by 7%, 10%, 5%, 11%, and 8% (p < 0.05), respectively. For example, in southwest China, AN content increased the most—18%. In east, central, northwest, and north China, the increases of AN were 15%, 12%, 9%, and 7%, respectively. However, the total amount of straw returned to the field with N fertilizer application in northeast China did not affect the AN content in paddy soils (p < 0.05). This may be due to the unique natural environment and climate factors in the Northeast, which affect soil N transformation. In northeast China, TN and TNA in paddy fields was the most significantly increased at 13% and 19%, respectively (p < 0.05). In north and southwest China, TN increases were higher than the national average, 12.3% and 11.5%, respectively. TN was lower than the national average in three regions—northwest, east, and central China—with significant increases of 10%, 7%, and 4% (p < 0.05), respectively. The magnitude of the increase in SOC content with the total straw return with N fertilizer application treatment in different regions was southwest (12%) > northwest (10%) > northeast (9%) > north China (6.3%) > central (6%) > east China (5%). In northwest, southwest, northeast, and north China, total straw return with N fertilizer application significantly increased the soil C:N ratio in paddy fields by 15%. In east and central China, total straw return with N fertilizer application significantly decreased the soil C:N ratio in paddy fields by 3% (p < 0.05) in each region.
Under the condition of straw return with N fertilizer application, total straw return had the most significant effect on increasing rice yield and soil TN content in northeast China, 23% and 13% (p < 0.05), respectively. The effect was highest on soil AN and SOC content in paddy fields of southwest China, at 18% and 12.05%, respectively.

3.2. Temperature

As shown in Figure 4a, the effect of total straw return with N fertilizer application on soil AN content in paddy fields generally tended to increase (<20 °C) and then decrease (>20 °C) with increasing mean annual temperature. Under high temperature (>20 °C) conditions, the soil AN content of paddy field increased by 8% (p < 0.05), which was higher than that under low temperature (0–10 °C) conditions by 2%. Soil AN was increased by 14.99%, under 15–20 °C conditions. At 10–15 °C conditions, the increase was 8%. The effect of total straw return with N fertilizer application on soil TN content in rice fields was generally not significantly different but did significantly increase by 9% to 10% (p < 0.05) in different temperature zones (0–20 °C). The largest increases of total straw return with N fertilizer application on SOC and yield were 28% and 22%, respectively, in the temperature range of 10–15 °C. Additionally, there were significant increases of 4–9% (p < 0.05) at these temperature levels: 0–10 °C, 15–20 °C, and >20 °C. The soil C:N ratio significantly increased by 14% at both the low (0–10 °C) and medium (10–15 °C) temperatures but significantly decreased at high temperatures, 15–20 °C and >20 °C, by 1–4%. The increasing effect of total N accumulation in rice was 19% at both low temperature (0–10 °C) and high temperature (>20 °C), and 2% and 12% at 10–15 °C and 15–20 °C, respectively.

3.3. Precipitation

As shown in Figure 4b, the effect of the total amount of straw return with N fertilizer application on soil AN generally showed an increasing (0–1200 mm) and then a decreasing (>1200 mm) trend for precipitation. The increasing effect was largest under the conditions of 600–1200 mm precipitation; the soil AN content of the paddy field increased by 15% (p < 0.05) under this condition. Under the conditions of 1200–1800 mm precipitation, the increase was 11%, and 8% under the conditions of >1800 mm precipitation. The smallest effect on AN was an increase of 2% under the conditions of 0–600 mm. With increasing mean annual precipitation, the effect of total straw return with N fertilizer application on TN, SOC, and the C:N ratio generally showed a decreasing trend. The increases for TN, SOC, and the C:N ratio were highest at 0–600 mm precipitation ranging between 11 and 17% (p < 0.05). The positive effect on yield was largest under the conditions of 0–600 mm precipitation. For the precipitation conditions of 600–1200 mm, 1200–1800 mm, and >1800 mm, the increase in TN content ranged between 5 and 6% (p < 0.05). For these same precipitation conditions, SOC content increased significantly by 4–7% (p < 0.05). Total straw return with N fertilizer application for 600–1200 mm precipitation had no significant effect on the soil C:N ratio in rice fields. There was a significantly reduced soil C:N ratio of 2% and 4% (p < 0.05) for the 1200–1800 mm and >1800 mm precipitation conditions, respectively. The effect of total N accumulation in the rice plants tended to increase with the increase in average annual precipitation. The most significant increase effect was observed at >1800 mm, with an 18% increase. Within the precipitation areas 0–600 mm, 600–1200 mm, and 1200–1800 mm, the total N accumulation increase ranged between 8 and 17%.

3.4. Soil Layer

Compared to the control treatment (no return and no fertilizer application), the total straw return with N fertilizer application significantly increased the content of AN, TN, and SOC in the 0–5 cm soil layer of the paddy field by 9%, 12%, and 7%, respectively (p < 0.05) (Figure 5). In the 10–20 cm soil layer, the total amount of straw return with N fertilizer application significantly increased AN and the C:N ratio by 10% and 4% (p < 0.05), respectively; however, it significantly decreased the TN content by 11% (p < 0.05). There was no significant effect on SOC content of this soil layer under these conditions. Total straw return with N fertilizer application significantly increased AN, TN, and SOC in the 0–20 cm soil layer of the rice field by 6–10% (p < 0.05). Taken together, total straw return with N fertilizer application effectively increased C-N fractions at 0–5 cm and 5–10 cm but had no significant effect on C-N fractions in the 10–20 cm soil layer.

3.5. Varieties

Different rice varieties also affected C-N fractions and the yields of rice fields (Figure 6). Under early maturing rice varieties, total straw return with N fertilizer application significantly increased AN, TN, SOC, TNA, the C:N ratio, and yield by 7–15% (p < 0.05). Under the conditions of medium maturity rice varieties, total straw return with N fertilizer application significantly increased AN and SOC by 4–5% (p < 0.05), and TN and the C:N ratio by 9–10% (p < 0.05). Under the conditions of late-maturing rice varieties, the total straw return with N fertilizer application significantly increased AN by 12% (p < 0.05) and TN and SOC by 6–7% (p < 0.05). However, total straw return reduced the C:N ratio in the paddy soil. Overall, straw return with N fertilizer application had the best effect on the C-N fractions and yield increase in paddy soil of early maturing rice varieties, with a significant yield increase of 23% (p < 0.05).

3.6. Tillage

As shown in Figure 7, under zero tillage conditions, soil TN, SOC content, and rice N accumulation increased by 5–7%. The soil C:N ratio was significantly reduced by 6% (p < 0.05). There was no significant effect on soil AN. Under deep-tillage straw return conditions, total straw return with N fertilizer application provided the largest results among the three return methods for soil AN, SOC content, the C:N ratio, TNA, and yield with percent increases of 13%, 19%, 6%, 11%, and 19%, respectively. Additionally, soil TN content increased by 10%. Under rotary tillage, the effect of total straw return with N fertilizer application on the increase in soil TN content was the best among the three return methods with a 10% increase, and there were 11%, 18%, 1%, and 6% increases in soil AN, SOC content, the C:N ratio, and rice N accumulation, respectively. Overall, the positive effect of straw return was largest on C-N fraction content and rice N accumulation in paddy fields under deep-tillage straw return conditions. For the return method, straw mixed return was more effective than straw mulch return. The positive effect of straw return on the C and N content and nitrogen accumulation in rice was obvious.

3.7. Soil Type Conditions

Under different soil type conditions, the total amount of straw return with N fertilizer application had different effects on C-N fractions and rice N accumulation in paddy soils. In this study’s meta-analysis, the soil types are divided into gley paddy soil, submerged paddy soil, waterlogged paddy soil, and saline–alkali soil (Figure 8). Under saline–alkali soil conditions, the effect of total straw return with N fertilizer application on the increase in SOC, TN content, and the C:N ratio was the largest increase among the four soil types with 20%, 9%, and 14% increases, respectively (p < 0.05). There was no significant effect on soil AN content and rice total N accumulation. Under the gley paddy soil conditions, there was the largest increase in soil AN (18%) among the four soil types. Additionally, soil TN, SOC content, the C:N ratio, TNA, and yield increased as well—5%, 8%, 9%, 10%, and 8%, respectively. Under the submerged paddy soil conditions, total straw return with N fertilizer application had the largest increase in rice TNA and yield among the four soil types with 21% and 23%, respectively. There was an 8%, 3%, and 6% increase in soil AN, TN, and SOC content, respectively. There was no significant effect on the C:N ratio. Under the waterlogged paddy soil conditions, soil AN, TN, SOC content, and rice N accumulation increased by 8%, 8%, 4%, and 10%, respectively. There was no significant effect on the C:N ratio. Overall, total straw return with N fertilizer application had the best effect on the increase in soil C-N fractions in saline–alkali soil, but had no significant effect on the uptake and utilization of rice N. Therefore, straw return can effectively improve the soil physicochemical properties of saline rice soils.

3.8. N Application Rates

Increasing amounts of N fertilizer application led to significant differences in the effects of total straw return on rice yield, the N accumulation, and soil C-N fractions (Figure 9). For example, 0–140 kg ha−1 application had the largest increase of rice N accumulation (22%) among all N fertilizer applications. AN, TN, and SOC were all affected, but there was no effect on the soil C:N ratio. For the 140–180 kg ha−1 fertilizer application, AN, TN, SOC content, rice N accumulation, and yield increased by 12%, 4%, 7%, 18%, and 13%, respectively. There was no effect on the soil C:N ratio. For the 180–220 kg ha−1 fertilizer application, the AN, TN, SOC content, rice N accumulation, and yield increased by 12%, 4%, 7%, 18%, and 12.8%, respectively. There was no significant effect on the soil C:N ratio. The 220 kg ha−1 fertilizer condition had the most significant effect on soil AN, TN, SOC content, and the C:N ratio among all N fertilizer applications, causing an increase of 16%, 16%, 14%, and 14% (p < 0.05), respectively; the yield increased by 21% (p < 0.05). There was no significant effect on the N accumulation in rice. The 220–260 kg ha−1 fertilizer application had the most significant effect on yield among all the N fertilizer applications, increasing by 28.0% (p < 0.05). The most significant effect on yield increase was 28% (p < 0.05), and the increase of AN, TN, SOC content, and rice N accumulation was 16%, 9%, 6%, and 13%, respectively. This condition significantly reduced the soil C:N ratio by 4%. For the 260–300 kg ha−1 fertilizer application, the most significant effect on the soil C:N ratio among all N fertilizer applications was a reduction of this ratio by 5% (p < 0.05). The TN content and yield increased by 3% and 13% under the 300–450 kg ha−1 fertilizer application, respectively, and the soil C:N ratio decreased by 2%. There was no significant effect on soil AN, SOC content, and rice N accumulation. Under the total straw return to the field, with the increased amount of N fertilizer application, the net profit also increased correspondingly, up to 220–260 kg ha−1 (Table 1). This level of applied N resulted in the highest net profit with the best yield because of the decreased production costs and increased profits at higher levels of added N.

3.9. Decomposing Bacteria

With the increase in straw-rotting agent application, there were significant differences in the effects of the total amount of straw return with N fertilizer application on rice yield, the N accumulation, and soil C-N fractions (Figure 10). A 17% increase (p < 0.05) in yield was observed under the 0–10 kg ha−1 conditions, but there was no significant effect on soil AN, TN, SOC content, and rice N accumulation. The effect of the 10–20 kg ha−1 conditions was the most significant among all straw-rotting agent applications on rice N accumulation and yield increase, with a 15.0% and 19% increase, respectively, and a 7.4% and 6% increase in AN and organic C content, respectively. There was no significant effect on soil TN content. The 20–30 kg ha−1 conditions were the most effective among all straw-rotting agent applications on increasing soil AN, TN, and SOC content. The most significant effect on soil AN, TN, and SOC content was an increase of 11%, 5%, and 15% (p < 0.05), respectively; rice N accumulation and yield increased by 8% and 16%, respectively. Yield increased by 4% and soil TN content decreased by 1.9% under the conditions of >30 kg ha−1, which had no significant effect on soil AN, SOC content, and rice N accumulation. In conclusion, the application of 220–260 kg ha−1 N fertilizer with 20–30 kg ha−1 straw-rotting agent and straw rotary tillage with total straw return was the most effective in improving soil C-N fractions, rice yield increase, and the N accumulation in paddy fields.

3.10. Time

As shown in Figure 11, with the increase in years, the increasing effects of full straw return with N fertilizer application on soil AN, TN, SOC, the C:N ratio, and yield of paddy fields generally showed an increasing (0–6 years) and then a decreasing (>6 years) trend. The increasing effects of soil AN and TN content were most significant (p < 0.05) for 3–6 years, with a 14% and 8% increase in the soil AN and TN content of paddy fields, respectively; 10% and 7% for 0–3 years, respectively; and 10% and 8% for 6–9 years, respectively. The lowest increasing effects for soil AN and TN were 10% and 7%, respectively, for >9 years. SOC, the C:N ratio, and yield had the most significant increasing effect at 6–9 years with 12%, 12%, and 14%, respectively, and at 3–6 years with 11%, 4%, and 10%, respectively; and at >9 years with 11%, 8%, and 13%, respectively. For the period 0–3 years, there was no significant effect on the soil C:N ratio and SOC and yield increased 3% and 6%, respectively. In conclusion, the best effect on the increase in soil N content was observed when the return years were 3–6 years. The time period of 6–9 years had the best effect on the increase in soil SOC content, the C:N ratio, and yield.

3.11. The Relative Importance of Variables

Figure 12a–d shows that after straw return, soil AN and SOC content were highly significant (p < 0.01), and soil TN content and the C:N ratio were significant (p < 0.05); additionally, soil AN, SOC content, soil TN content, and the C:N ratio were all positively correlated with yield. Overall, 77% of the variation in lnR of rice yield could be explained by the first six factors: soil SOC, N fertilizer application amounts, straw return method, soil AN, and climate factors (mean annual temperature and mean annual precipitation) (Figure 13). In particular, soil SOC and N fertilizer application amounts were the best predictors and explained 42% of the variation in lnR of rice yield. Soil SOC was the dominant factor driving variation in rice yield (RI = 24%). Regarding categorical factors, edaphic variables (RI = 38%) were the most important determinants controlling lnR variance in rice yield for total straw return in rice planting across China, followed by management practices (RI = 32%), climate factors (RI = 16%), and other factors (RI = 14%).

4. Discussion

In this study, meta-analysis was applied to quantify the effects of total straw return with N fertilizer application on soil C-N fractions, rice N accumulation, yield, and their influencing factors, which helped to reveal the mechanisms of total straw return with N fertilizer application on rice growth and development. The ability of total straw return with N fertilizer application to improve soil physicochemical properties and increase yield and quality in paddy fields was related to field management and the rice growth environment [37,38].
Soil AN content measures the capacity of the soil supply N to the growing crop [39]. In this study, we found that straw return with N fertilizer application increased soil AN content of rice fields by 7%, which was consistent with the findings of Wang et al. (2020) [40] and Yang et al. (2017) [41]. In the middle and late stages of rice growth, the soil AN content of paddy fields with reduced N fertilizer application was significantly increased compared with that of the treatment with regular N fertilizer application. Zhao et al. (2014) [42] found that the continuous return of wheat straw to the field for 32 years could increase the TN content in the soil. Soil TN usually includes various forms of N, and these are always being converted from one form to another. Additionally, the level of TN content is influenced by the accumulation and depletion of N [43,44]. In this study, we found that straw return with N fertilizer application could increase soil AN content of a rice field by 9.8%, which is consistent with the results of Zhou et al. (2018) [45]. SOC content can reflect the level of soil fertility, and the increase in SOC content is helpful to overcome the challenge of soil nutrient deficiency in agricultural production. In this study, it was found that the SOC content of paddy soils increased when N application was appropriately reduced (0–40 kg ha−1). However, the potential impact of changes in SOC on crop yield and input requirements in farming systems is unclear, as the relationship between soil organic matter and crop yield remains highly uncertain [46]. Straw return with N fertilizer application increased soil AN content of paddy fields by 10.8%, which is consistent with the results of Zhou et al. (2022) [47].
Efficient N fertilizer management is essential to ensure maximum economic yield and improve N use efficiency [48,49,50]. For rice, improving N fertilizer utilization is an effective measure to obtain the desired yield [51]. Studies have shown that straw return promotes rice growth, increases N uptake and accumulation in rice, and improves the agronomic utilization of N fertilizer. TN uptake in rice treated with straw return increased by 13.9%. Deep tillage of straw return increased TN uptake and utilization and reduced the N deficit (12.68%). Straw return also increased the N harvest index [52]. The N harvest index refers to the ratio of grain nitrogen accumulation to plant total nitrogen accumulation, which is an important indicator reflecting the transfer of plant nitrogen from vegetative organs to grains. The results of the present paper show that straw return significantly increases the TN accumulation in rice plants and leaf N accumulation in all growth periods, thus increasing rice grain and rice straw yields. Zhang et al. (2016) [53] concluded that the utilization of N fertilizer varied greatly with the amount of N fertilizer applied. Zhao et al. 2011 [54] showed that N fertilizer utilization could be significantly improved by increasing the amount of N fertilizer applied at a later stage or supplementing N fertilizer with straw in the field. Our study showed that the appropriate reduction in N fertilizer application (0–150 kg ha−1) under the total amount of straw return to the field was beneficial to the improvement of N fertilizer agronomic-utilization, N fertilizer bias productivity, N fertilizer contribution, and N fertilizer uptake-utilization (13.67%), while the N fertilizer application mode had little effect on N fertilizer utilization, but it could be seen that the N fertilizer uptake-utilization was slightly higher under the total straw return with the N fertilizer application-rate of 220–260 kg ha−1.
Crop yield is the result of a combination of intrinsic characteristics, climate conditions, and management strategies including fertilization, irrigation, and tillage [55,56,57,58]. Sha et al. (2018) [59] concluded that after total return of straw to the field, N fertilizer application affects the yield components by influencing the reservoir capacity of the plant, which ultimately positively affects the yield of rice. Numerous studies [60,61,62] found that low N fertilizer dosage slowed down rice seedlings and was detrimental to their growth and development. However, the results of the current study showed that the reduced N fertilizer application did not reduce yield. This was possibly due to two reasons. First, the reduced N fertilizer application (220–260 kg ha−1) was within a reasonable range. Secondly, as the rice straw decomposed, released nutrients improved rice growth. These findings are similar to those found by Zhang et al. (2019) [63]. The results of this study showed that the yield was maintained and even increased by improving the N application pattern, while the N fertilizer application was reduced. The yields of straw zero tillage-return, straw deep tillage-return, and straw rotary tillage-return were all higher than the yields of fields where straw was not returned to the field. This result was because the straw released nutrients as it decomposed, and these nutrients could be absorbed by the soil faster in the rotary tillage fields. Additionally, the degree and rate of decomposition were directly related to the growth of rice [64].

5. Conclusions

Total straw return with N fertilizer application showed significant increases in soil C-N fractions, yield, and N fertilizer utilization in paddy fields. However, there are specific conditions that contribute to supporting the beneficial effects of the total amount of straw return with N fertilizer. These are appropriate precipitation (0–1,200 mm), rice variety (early maturing variety), average annual temperature (10–15 °C), soil type (submerged rice soil) as well as a suitable field return method (deep tillage), a N fertilizer application rate of 220–260 kg ha−1, and a straw-rotting agent application rate of 20–30 kg ha−1. Ensuring these conditions are met combined with the total straw return with N fertilizer application is an effective strategy for achieving improved yields and quality in rice production. The nitrogen nutrients released after the decomposition of straw returning can effectively replace some chemical fertilizer used in the production process. The method of “reducing fertilizer application by volume + straw returning” was adopted to find the “maximum bearing capacity” of sections of farmland for fertilizer reduction so as to achieve the goal of using less nitrogen fertilizer without reducing yield.

Author Contributions

H.Z.: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing—original draft, Funding acquisition; L.Z.: Investigation, Data curation, Methodology, Formal analysis, Writing—original draft, Visualization, Supervision; Y.W.: Formal analysis, Investigation, Methodology, Visualization; Z.L.: Formal analysis, Investigation, Methodology, Visualization; L.H.: Investigation, Validation, Writing—review and editing; R.P.: Conceptualization, Methodology, Project administration, Resources; D.C.: Validation, Funding acquisition, Writing—review and editing; Z.C.: Conceptualization, Supervision, Funding acquisition, Project administration, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Special Fund for Agro-scientific Research in the Public Interest (No. 201503137) and the Department of Natural Science Fund Project of Jilin Province (No. YDZJ202201ZYTS578) and the Department of Key R&D Project of Jilin Province Science and Technology (No.20200402040N C).

Data Availability Statement

Data used for the analysis are available from the corresponding authors on request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Geographical regions of rice cultivation in China and the distribution of rice-field trial sites identified in the papers that met the inclusion criteria.
Figure 1. Geographical regions of rice cultivation in China and the distribution of rice-field trial sites identified in the papers that met the inclusion criteria.
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Figure 2. Frequency distribution of effect size, lnR, on alkaline-hydrolysable N (a), total N (b), soil organic C (c), the C:N ratio (d), yield (e), and the N accumulation (f) in paddy soils under total straw return conditions.
Figure 2. Frequency distribution of effect size, lnR, on alkaline-hydrolysable N (a), total N (b), soil organic C (c), the C:N ratio (d), yield (e), and the N accumulation (f) in paddy soils under total straw return conditions.
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Figure 3. Relative change rates for different rice planting regions in China. Description of symbols: AN, soil alkaline−hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, soil carbon–nitrogen ratio. Numbers in parentheses represent the effect value (lnR).
Figure 3. Relative change rates for different rice planting regions in China. Description of symbols: AN, soil alkaline−hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, soil carbon–nitrogen ratio. Numbers in parentheses represent the effect value (lnR).
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Figure 4. Relative change rates for different temperature (a) and precipitation (b) conditions. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, the soil carbon to nitrogen ratio; and TNA, total N accumulation by rice. Numbers in parentheses represent the effect value (lnR).
Figure 4. Relative change rates for different temperature (a) and precipitation (b) conditions. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, the soil carbon to nitrogen ratio; and TNA, total N accumulation by rice. Numbers in parentheses represent the effect value (lnR).
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Figure 5. Relative change rates for different soil layers of paddy fields. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter. Numbers in parentheses represent the effect value (lnR).
Figure 5. Relative change rates for different soil layers of paddy fields. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter. Numbers in parentheses represent the effect value (lnR).
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Figure 6. Relative change rates for different rice varieties. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, the soil carbon–nitrogen ratio. Numbers in parentheses represent the effect value (lnR).
Figure 6. Relative change rates for different rice varieties. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, the soil carbon–nitrogen ratio. Numbers in parentheses represent the effect value (lnR).
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Figure 7. Relative change rate for different straw return methods. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, soil carbon to nitrogen ratio; and TNA, total N accumulation. Numbers in parentheses represent the effect value (lnR).
Figure 7. Relative change rate for different straw return methods. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, soil carbon to nitrogen ratio; and TNA, total N accumulation. Numbers in parentheses represent the effect value (lnR).
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Figure 8. Relative change rates for different soil types. Description of symbols: AN, soil available nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, soil carbon-nitrogen ratio; and TNA, total N accumulation. Numbers in parentheses represent the effect value (lnR).
Figure 8. Relative change rates for different soil types. Description of symbols: AN, soil available nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, soil carbon-nitrogen ratio; and TNA, total N accumulation. Numbers in parentheses represent the effect value (lnR).
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Figure 9. Relative change rates for different N fertilizer application amounts. Description of symbols: AN, soil alkaline-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, the soil carbon to nitrogen ratio; and TNA, total N accumulation. Numbers in parentheses represent the effect value (lnR).
Figure 9. Relative change rates for different N fertilizer application amounts. Description of symbols: AN, soil alkaline-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, the soil carbon to nitrogen ratio; and TNA, total N accumulation. Numbers in parentheses represent the effect value (lnR).
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Figure 10. Relative change rates for different bacterial dosages. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; and TNA, total N accumulation. Numbers in parentheses represent the effect value (lnR).
Figure 10. Relative change rates for different bacterial dosages. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; and TNA, total N accumulation. Numbers in parentheses represent the effect value (lnR).
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Figure 11. Relative change rates for different years. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, soil carbon–nitrogen ratio. Numbers in parentheses represent the effect value (lnR).
Figure 11. Relative change rates for different years. Description of symbols: AN, soil alkali-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, soil carbon–nitrogen ratio. Numbers in parentheses represent the effect value (lnR).
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Figure 12. Correlation between yield effect value and (a) soil AN, (b) soil TN, (c) SOC, and (d) the C:N ratio effect values under total straw return. Description of symbols: AN, soil alkaline-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, the soil carbon-nitrogen ratio.
Figure 12. Correlation between yield effect value and (a) soil AN, (b) soil TN, (c) SOC, and (d) the C:N ratio effect values under total straw return. Description of symbols: AN, soil alkaline-hydrolysable nitrogen; TN, soil total nitrogen; SOC, soil organic matter; C:N, the soil carbon-nitrogen ratio.
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Figure 13. The relative influence (RI, %) of climate factors (CF, mean annual temperature and precipitation), edaphic variables (EV: SOC, AN, TN, and the C:N ratio), management practices (MP: N fertilizer application amounts, straw return methods, and bacterial dosage), and other factors (Other: soil depth, soil type, and variety) on rice yield in the aggregated boosted tree model analysis.
Figure 13. The relative influence (RI, %) of climate factors (CF, mean annual temperature and precipitation), edaphic variables (EV: SOC, AN, TN, and the C:N ratio), management practices (MP: N fertilizer application amounts, straw return methods, and bacterial dosage), and other factors (Other: soil depth, soil type, and variety) on rice yield in the aggregated boosted tree model analysis.
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Table
Table 1. Output value, total cost, and net income of rice under different N fertilizer application rates.
Table 1. Output value, total cost, and net income of rice under different N fertilizer application rates.
1 N (kg ha−1)2 Total Cost (USD ha−1) 3 Production Increase (%)Net Profit (USD ha−1)
0~140<15748.5<1110
140~1801574–160212.81110–1121
180~2201602–163120.851121–1700
220~2601631–166728.241700–2081
260~3001667–167921.131700–1724
300~4501679–172712.71028–1700
1 N, different nitrogen fertilizer application rates; 2 Total cost (includes fertilizer cost, straw return cost, seed cost, and labor cost, etc.); 3 Production increase (the data are from Figure 9)
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Zhang, L.; Wang, Y.; Lou, Z.; Hsu, L.; Chen, D.; Piao, R.; Zhao, H.; Cui, Z. Meta-Analysis of Factors Affecting C-N Fractions and Yield of Paddy Soils by Total Straw Return and N Fertilizer Application. Agronomy 2022, 12, 3168. https://doi.org/10.3390/agronomy12123168

AMA Style

Zhang L, Wang Y, Lou Z, Hsu L, Chen D, Piao R, Zhao H, Cui Z. Meta-Analysis of Factors Affecting C-N Fractions and Yield of Paddy Soils by Total Straw Return and N Fertilizer Application. Agronomy. 2022; 12(12):3168. https://doi.org/10.3390/agronomy12123168

Chicago/Turabian Style

Zhang, Liqiang, Yunlong Wang, Zixi Lou, Lefei Hsu, Di Chen, Renzhe Piao, Hongyan Zhao, and Zongjun Cui. 2022. "Meta-Analysis of Factors Affecting C-N Fractions and Yield of Paddy Soils by Total Straw Return and N Fertilizer Application" Agronomy 12, no. 12: 3168. https://doi.org/10.3390/agronomy12123168

APA Style

Zhang, L., Wang, Y., Lou, Z., Hsu, L., Chen, D., Piao, R., Zhao, H., & Cui, Z. (2022). Meta-Analysis of Factors Affecting C-N Fractions and Yield of Paddy Soils by Total Straw Return and N Fertilizer Application. Agronomy, 12(12), 3168. https://doi.org/10.3390/agronomy12123168

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