The Responses of Soil N₂O Emissions to Residue Returning Systems: A Meta-Analysis

Abstract

Background: Much attention has been focused on the influences of residue returning on N₂O emissions. However, comprehensive quantification of the effect size on N₂O emission following crop residue returning in subtropical, tropical and warm temperate conditions remains untested. Methods: To identify site-specific factors that influence N₂O emission (kg N₂O-N ha⁻¹) in residue returning systems, we performed a meta-analysis involving 260 comparisons from 72 studies. Results: The data indicated that significant promoting effects were observed under residue returning by rotary tillage, no-tillage and mulch, whereas N₂O release was significantly inhibited by 8% under residue returning by plough. For other contributors, the stimulatory and significant effects occurred in upland fields, under short- and medium-term residue returning durations, acidic/neutral soils, medium organic C and clay content. Nitrogen fertilizer application significantly stimulated N₂O emission, even though application rate at 100–150 kg N ha⁻¹ was inhibitory. Although a negative correlation between residue C/N ratio and N₂O emission has been shown, residue returning could not reduce N₂O emission with a higher C/N ratio and amount. Conclusions: Some options, such as converting residue returning methods, decreasing N fertilizer application rate, and regulating soil C/N ratio could be adopted to mitigate soil N₂O emission following residue returning.

1. Introduction

Anthropogenic greenhouse gas emission (e.g., CO₂, CH₄, and N₂O) is a major contributor to global warming [1], and agricultural activities account for 10–12% of total anthropogenic greenhouse gas emission [2]. Agriculture was responsible for more than 61% of N₂O emissions, resulting from agricultural inputs and direct soil-derived N₂O [3,4]. There are continuous concerns regarding N₂O emission due to its contribution to both global warming and ozone layer depletion [5]. In fact, N₂O has been identified as the most significant anthropogenic ozone-depleting compound [6]. In a time horizon of 100 years, it has been shown to have 265 and 28 times greater global warming potential than CO₂ and CH₄, respectively [1].

Soil N₂O is naturally produced through the processes of microbial nitrification and/or denitrification [7]. Under aerobic soil conditions, N₂O is mainly derived from nitrification process, in which autotrophic nitrifiers convert NH₄⁺ to NO₃⁻. However, under anaerobic soil conditions, denitrification is the main pathway for N₂O production. During this process, heterotrophic denitrifiers transform NO₃⁻ to N₂O and N₂ [8]. Generally, denitrification is believed to be the main source of N₂O production in soils, as N₂O yield potential of denitrification is much higher (1–100%) than nitrification (0.1–1%) [9,10]). The ratio between the gaseous products of denitrification depends on NO₃⁻ availability, oxygen (O₂) availability in the soil and/or microsites, amount of easily decomposable C as an energy source, soil pH, and microbial community structure [11,12]. Chen et al. [13] have reported that nitrification and denitrification dominate soil N₂O production when soil water-filled pore spaces were at 30–60% and 50–90%, respectively. Furthermore, N₂O emission dynamics are controlled by other factors, including soil temperature and agricultural management practices [14,15,16]. For example, a positive correlation between N₂O fluxes and soil temperatures was found, and no tillage can improve soil structure and lower its temperature, which in turn could reduce N₂O emission [17,18,19].

Crop residue returning, as an important management practice, is widely popular due to its benefits of increasing soil fertility and grain yields [20,21,22]. Meanwhile, this process also brings soil N₂O emission via regulating soil C and N availability and microbial activities, etc. [23,24,25]. Mosier et al. [26] calculated that crop residue could produce 0.4 million metric tons of N₂O-N year⁻¹ globally. However, contradictory views exist in the literature regarding the influence of residue returning on N₂O production and release. Several studies have reported that residue returning stimulates N₂O emission [27,28,29], while others have reported inhibitory effects [30,31]. These results indicate that crop residues may play multiple roles, such as an organic N fertilizer, organic C substrate, and energy provider in regulating soil N₂O production and emission [13]. Crop residue could further modify soil aeration by improving soil aggregation and microbial O₂ demand, a major factor mediating soil nitrification and denitrification processes for N₂O production. In addition, the effects of residue application on soil N₂O emission depend on soil properties [4,32,33]. For example, soil pH can affect the decomposition rate of crop residues, as well as C and N sources for microbial denitrification [34]; soil texture could impact soil permeability and water condition, and thus residue decomposition rate and N transformation process [35]. Hence, taking soil properties into consideration is necessary when estimating N₂O emission associated with crop residues.

Meta-analysis is a systematic process applied to research data from a variety of experiments to analyze and summarize the estimated quantitative average effects [36]. In recent years, this method has been used to assess the effects of agricultural management practice’s driving factors on N₂O emissions. For example, Liu et al. [37] conducted a meta-analysis to assess the impacts of fertilizer application on soil N₂O emissions. Shan and Yan [38] applied a meta-analysis and found that crop residue application methods could affect N₂O release, but the effect sizes were treated as unknown due to limited datasets. Therefore, more information is required regarding the effect size of residue returning attributers (e.g., climate condition, soil initial organic C content, residue returning methods and amount) on N₂O emissions. The objective of this study was to conduct a meta-analysis to comprehensively quantify the effect size of crop residue returning on N₂O emission, varying soil conditions, and residue returning attributes based on field experiments.

2. Materials and Methods

2.1. Data Collection

A detailed evaluation was performed of peer-reviewed journal articles from the Web of Science (1900–2017). “Straw”, “residue”, “tillage”, “N₂O”, “nitrous oxide”, and “greenhouse gas” were selected as search keywords. Data were collected from 72 publications, which included both no-residue returning (controls) and residue returning treatments. Only field scale studies were selected for this meta-analysis (Figure 1). Specific residue returning methods chosen for comparative analysis included residue returning with conventional tillage/plowing/plough/20 cm depth/25 cm depth (PT), rotary tillage/shallow-rotary tillage/10 cm depth/12 cm depth/15 cm depth (RT), no-tillage/reduced tillage/zero tillage (NT), and mulch (MC). No-residue returning condition was set as the control. When collecting the paired experiments, we considered the following criteria: (1) Field scale studies on N₂O emission were performed using the closed chamber-gas chromatography method in agroecosystem; (2) tillage and residue returning were clearly stated; (3) cumulative N₂O emission could be obtained from each study.

From the 72 studies, data listed in tables were obtained directly, whereas, data presented in. figures were extracted using the “Getdata Graph Digitizer, Version 2.26” software (Version 2.26: Getdata Graph Digitizer, Fedorov S, Krasnoyarsk, Russia). A total of 260 observations from 72 field studies were collected for this meta-analysis. Other related information including the location (longitude and latitude), mean annual precipitation, annual mean temperature, climate regimes, pH, cropping systems, crop species, soil types, N fertilizer application, and residue returning duration were obtained from the selected studies. More detailed information regarding these published papers is represented in Supplementary Table S1.

For all studies, the cumulative N₂O emission, replications, and standard deviation (SD) of both no-residue and residue returning treatments were obtained directly. If N₂O emission was shown as the value of global warming potential, the unit was converted. SD was estimated through the value of standard errors (SE) using Equation (1):

$$\mathrm{SD}=\mathrm{SE}\times \sqrt{\mathrm{n}},$$

(1)

where n is replications. The average SD value was estimated as 23% and 36% for controls and residue returning treatments, respectively. For absent SD and SE, values were the relative mean values of SD for N₂O emissions in the dataset [39].

2.2. Data Analysis

A random-effects meta-analysis was carried out to explore the impact of experimental conditions, initial soil properties, and agricultural management practices on N₂O emissions (kg N₂O-N ha⁻¹) associated with crop residues. The natural logarithm of response ratio (lnR) was adopted as the effect size of the comparing soil N₂O emissions between controls and residue returning treatments [40].

$$\ln R=\ln \frac{X_t}{X_c}=\ln (X_t)-\ln (X_c),$$

(2)

where X_c and X_t represented the average value of N₂O emission in controls and residue returning treatments, respectively.

For each study, the variance (υ) of lnR was estimated as follows:

$$\mathsf{\upsilon }=\frac{S{D}_t^2}{n_t{X}_t^2} +\frac{S{D}_c^2}{n_c{X}_c^2},$$

(3)

where SD_c and SD_t represented the standard deviations of all comparisons in controls and residue returning treatments, respectively; and n_t and n_c present repetition numbers for residue returning treatments and the controls, respectively. The weight effect sizes were calculated using Equation (4):

$$\ln R=\frac{\sum \left(ln R_i\times {\omega }_i\right)}{\sum {\omega }_i},$$

(4)

where lnR_i was the effect size, and ω_i was the weight of corresponding comparisons. ω_i was computed as follows:

$$\mathsf{\omega }=\frac{1}{\mathsf{\upsilon }},$$

(5)

where υ was the variance of lnR as stated above.

The aim of this meta-analysis was to explore how soil and residue returning attributes drive soil N₂O emissions. Therefore, to determine significant differences in effect size under residue and no-residue returning treatments, the attributes were classified into three different groups (experimental conditions, soil initial properties, and agricultural management practices), which included 11 categorical variables (climate zone, land use, residue returning duration, pH, soil organic C content, soil texture, clay content, N fertilizer input, crop residue C/N ratio, residue returning amount and methods). Every categorical variable was then divided into several levels. The detailed classification is shown in

Table 1. Categorical variables, numbers of studies and paired observations for crop residue returning relative to no-residue returning treatments, levels (L) of each variable, between-group heterogeneity (Q_b), and significant p values in the meta-analysis.

Groups	Variable	Studies	Observations	L1	L2	L3	L4	L5	L6	L7	Qb	P
Experimental conditions	Climate zone	72	260	Subtropical	Tropical	Warm temperate					16.12	0.002
	Land use	72	260	Upland	Paddy-upland	Paddy					19.45	0.001
	Residue returning duration (year)	72	260	≦2	2–5	>5					3.60	0.213
Soil initial properties	pH	65	216	≦6.5	6.5–7.3	>7.3					1.76	0.492
	Soil organic C content (g kg−1)	64	207	≦10	10–18	>18					88.37	0.049
	Soil texture	50	144	Clay	Clay loam	Loam	Sandy clay	Sandy clay loam	Sandy loam	Silt loam	34.59	0.001
	Clay content (%)	37	96	≦15	15–25	>25					4.05	0.202
Agricultural management practices	N fertilizer input (kg N ha−1)	66	231	≦100	100–150	150–250	>250				100.07	0.058
	Crop residue C: N	39	96	≦45	45–100	>100					20.01	0.001
	Residue returning amount (kg DM ha-1)	58	199	≦4000	4000–6000	>6000					29.97	0.39
	Residue returning method	72	260	PT	RT	MC	NT				126.8	0.016

Note: PT means residue returning with conventional tillage/plowing/plough/20 cm depth/25 cm depth; RT means residue returning with rotary tillage/shallow-rotary tillage/10 cm depth/12 cm depth; NT means residue returning with no-tillage/reduced tillage/zero tillage; MC means residue mulch on the fields.

. For each attribute, total heterogeneity (Q_t) was divided into within-group (Q_w) and between-group (Q_b) variations. The significance of Q_b represents mean effect sizes that are significantly different between various levels of the categorical group [41]. The Q statistic obeyed a chi-square distribution with k−1 degrees of freedom, where k was the number of paired observations for a categorical variable between residue returning treatments.

In order to determine mean effect size, METAWIN 2.1 software was performed and 95% bootstrapped confidence intervals (CIs) were generated [41]. Relative to control, the treatments were considered significantly positive or negative if the 95% CIs for cumulative seasonal N₂O emission changes did not overlap with zero [42]. p values for differences between categories of studies were calculated in METAWIN 2.1 software. For simplicity, lnR analysis results were back-transformed and reported as percentage change under residue returning relative to the controls, and the calculation equation was (($e^{\ln R}-1$) × 100%) [4].

3. Results

3.1. Residue Returning Methods, Amounts, and C/N Ratio

Overall, residue returning had significant, positive effects on soil N₂O emission compared to no-residue returning, with an increase by 8% (Figure 2a). RT, MC, and NT significantly increased N₂O emission by 11%, 16%, and 35%, respectively, compared to the controls (Figure 2a), while PT resulted in a significant decrease by 8%. There were also significant differences in N₂O emissions between different residue returning methods. Soil N₂O emissions were significantly higher in NT than MC and RT, which were also significantly higher than PT treatments, while no significant effects were founded between RT and MC (Figure 2a).

The effect size of residue returning on N₂O release was dependent on the amount of residue returning, but no tendency with amount of residue application was observed (Figure 2b). N₂O emissions significantly increased for studies with higher (>6000 kg DM ha⁻¹; mean: 16%) and lower (≦4000 kg DM ha⁻¹; mean: 15%) amounts of residue returning. In addition, no significant effect was found for studies with intermediate (4000–6000 kg DM ha⁻¹; mean: −2%) amounts of residue returning (Figure 2b).

The effect size of residue returning on soil N₂O emission was strongly dependent on the residue C/N ratio, and the effect size generally decreased following residue C/N ratio increase (Figure 2c). Significantly elevated N₂O emission were noted at residue C/N ratios at ≦45 (mean: 50%) and 45–100 (mean: 21%), whereas, no significant effect was observed when the residue C/N ratio exceeded 100 (mean: −31%; Figure 2c).

3.2. Climate Zone, Land Use, and Residue Returning Duration

Regarding climate zone, residue returning significantly increased soil N₂O emissions relative to no-residue returning in both warm temperate (mean: 31%) and tropical (mean: 54%) climate zones (Figure 3a). In contrast, the observed inhibitory effect was not significant in the subtropical (mean: −3%) climate zone (Figure 3a).

The effect size of residue returning on N₂O emission varied with land use types (Figure 3b). Soil N₂O release increased significantly in upland fields (mean: 24%), decreased significantly on paddy fields (mean: −19%), and no effect was observed on upland-paddy fields (mean: −2%; Figure 3b). Meanwhile, there was a significant difference of N₂O release between upland and paddy fields.

The effect of residue returning on N₂O release relied on the residue returning duration (Figure 3c). Mean effect size from short studies (2 year) was significantly enhanced by 15% relative to no-residue returning. However, there were no significant effects on N₂O emissions for medium term (2–5 year) and long term (>5 year) duration studies following residue application (Figure 3c). In addition, a negative correlation was found between N₂O emission and residue returning duration.

3.3. Soil pH, Soil Organic C Content, Soil Texture, and Clay Content

Soil pH significantly affected soil N₂O release after adopting residue returning compared to controls, and stimulatory effects were observed for acidic soils (pH ≦ 6.5) and neutral soils (6.5–7.3) (Figure 4a). However, in alkaline soils (>7.3), there were no significant changes in soil N₂O release. On average, different values of soil organic C resulted in varying effects of residue returning, where N₂O emission were significantly enhanced by 21% at a 10–18 g kg⁻¹ range, significantly reduced by 11% at values exceeding 18 g kg⁻¹ (Figure 4b), and no significant effect was observed below 10 g kg⁻¹ (Figure 4b).

According to the United State Department of Agriculture (USDA) soil texture triangle, in this study we segregated soils into seven different textural classes. As shown in Figure 5a, the effects of residue returning on soil N₂O release were discrepant depending on soil types. In the soils of clay loam and sandy clay loam, residue returning significantly stimulated soil N₂O emissions. While in other soils types—except silt loam soils—residue returning significantly suppressed N₂O release. A significant negative effect on soil N₂O emissions was shown at clay content ≦15% (mean: −27%), whereas, N₂O emission was significantly stimulated by 22% at 15–25% clay content (Figure 5b). No significant effect was observed when clay content was over 25% (Figure 5b).

3.4. Nitrogen Fertilizer Application

On average, relative to the controls, N fertilizer application significantly increased soil N₂O emissions following residue returning. First, soil N₂O emission decreased significantly, and then increased significantly with an increasing N input following residue returning (Figure 6). Significant stimulatory effects were observed at N input rates of ≦100 (mean: 36%), 150–250 (mean: 14%), and >250 kg N ha⁻¹ (mean: 22%), whereas an N fertilizer application rate of 100–150 kg N ha⁻¹ (mean: −12%) resulted in a significant decrease following residue returning (Figure 6).

4. Discussion

4.1. Effects of Residue Returning Methods on N₂O Emission

Our meta-analysis showed that residue returning significantly increased soil N₂O emissions, relative to no-residue returning, which indicating that changes in quantity and quality of soil C and N substrate and environmental conditions from residue application may favor N₂O production and emission because N₂O emissions are moderated by multiple factors such as soil inorganic N and organic C availability, soil O₂ condition, soil temperature and soil moisture [10]. Residue returning methods are considered major influencers of soil N₂O emissions, due to their effects on soil microbial activities and ventilation [30,43]. Our meta-analysis showed that RT and NT both significantly stimulated N₂O release compared to the controls. Soil compaction in RT and NT can moderate soil aeration condition and promote N₂O production through denitrification [44]. On the other hand, RT and NT can provide organic C as the energy source for heterotrophic microbial communities [45], which leading to more rapid O₂ consumption and increasing the anaerobic environment, thus stimulating the denitrification process [46,47]. These results were consistent with the analysis by Mei et al. [48] and Zhao et al. [49]. However, Shan and Yan [38] reported that although residue returning enhanced N₂O release, no significant difference was found between residue returning and controls. This could be related to the different comparison numbers (only 68 comparisons were collected in the previously published paper [38]). MC significantly increased soil N₂O emissions compared to the controls. Residue mulch on the surface of soils is generally found in upland areas, and MC possibly results in favorable soil moisture conditions, high soil temperatures, and excess N availability for microbial activities, thus promoting N₂O production [50,51,52]. Additionally, higher O₂ demands following residue application promote microbial denitrification for N₂O emission [53].

In this study, PT significantly inhibited N₂O release compared to controls, which might be attributed to a more effective reduction of N₂O to N₂ during denitrification. Although residue-derived soil dissolved C was available in PT compared to no-residue returning [54], PT created more pores and improved soil aeration condition, which promoting the conversion of labile organic matter to CO₂, consequently, leading to a lower rates of denitrification [55]. In addition, our results show that N₂O emission in PT was significantly lower than that in RT and NT. On one hand, PT may decrease bulk density and increase gas-diffusion capacity, leading to a decrease in surface-water-retention capability. As a result, the soil anaerobic condition and topsoil denitrification potential decreased [56]. On the other hand, PT had lower bacterial biomass [57] and CO₂ emission than RT and NT [58]. Chen et al. [13] also found a positive correlation between soil N₂O and CO₂ emissions following residue returning.

4.2. Effects of Residue Returning Amount, and Residue C/N Ratio on N₂O Emission

Residue returning could considerably alter soil availability of NH₄⁺ and NO₃⁻, the major factors controlling nitrification and denitrification processes, respectively. N₂O emission significantly increased when residue returning amounts were at high and low levels, while no significant effects were observed at medium levels. The reason was mainly related to soil C/N ratio. With increasing residue application amounts, the C/N ratio increased, leading to net N immobilization and reducing N₂O production [59]. This result was identified by the correction between N₂O emission and residue C/N ratio (Figure 7). Higher quantities of residue returning resulted in elevated inputs of C and N into soils. This promoted the soil’s heterotrophic microbial respiration, activities, and accelerated soil oxygen consumption to build an anaerobic environment that promotes denitrification [60], thus stimulating soil N₂O production. Previous studies [61,62] have also reported similar increase in N₂O emissions with increasing residue amounts.

The negative correlation between residue C/N ratio and soil N₂O emission has been previously reported [38,63]. In our meta-analysis, the effect size generally decreased with increasing crop residue C/N ratio (Figure 7), and these values were significant at ratios below 100. With low residue C/N ratio, crop residue returning could supply enough N to meet crop growth, and improve soil microbial communities, further promoting N₂O production and emission [64]. In contrast, crop residue with high C/N ratio increased N consumption and caused more N immobilization during residue degradation, thus, leading to lower N₂O yields. In addition, other characters of crop residue, including lignin, polyphenol, and dissolved C may also play important roles in soil N₂O production and emission [65]. However, relevant studies are lacking, and more are needed regarding the effects of returning residue characters on N₂O emissions.

4.3. Effects of Climate Zone, Land Use, and Residue Returning Duration on N₂O Emission

Previous studies have identified that climate regimes can be considered as an impact factor of soil microbial nitrification/ denitrification processes, and N₂O emissions through regulating soil moisture and temperature [66]. Our study indicated that relative to no-residue returning, residue returning significantly increased N₂O emission by an average of 54% and 31% in warm temperate and tropical climate zones, respectively. In general, nitrification is favored at optimal soil temperature of 25–40 °C [17]. Tropical climate with high temperatures may enhance nitrification activities, thus leading to N₂O production increase [67]. In warm temperate, coefficients of N turnover rate and microbial decomposition of residues were faster, which can promote heterotrophic microbial growth and increase soil respiration, thus resulting in an anoxic condition [68]. Subtropical zones with frequent droughts in the dry season and extreme rainfall events in the wet season [69] can affect soil N transformations by disturbing soil moisture, temperature, microbial activities, etc. [70]. Our meta-analysis showed a negative effect on N₂O emission in subtropical zones associated with residue returning but with no significant difference. The possible explanation might be that precipitation reduction in dry season could decrease net nitrification and N mineralization rates, while wet season with large precipitation events causes substantial NO₃- losses via leaching [71], thus leading to small changes in average N₂O emission.

In this study, soil N₂O emission was closely related to land use types, where upland significantly increased but paddy fields significantly decreased N₂O emissions. Previous studies reported similar results [37,72]. In upland fields, residue returning created higher soil temperatures and water content, which enhanced microbial activities of nitrifier and denitrifier. This resulted in a more anaerobic environment, promoting denitrification and stimulating N₂O production [73]. In paddy fields, N₂O release derived from both nitrification and denitrification processes, which mainly occurs during the stage of alternate wetting and drying [74].

Residue returning significantly enhanced N₂O release in short- and medium-term, especially in the 2–5 year interval relative to controls. Microbial denitrification is believed to be a primary source of N₂O [75], and C availability is one of the most important factors controlling denitrification rates [76]. Hence, residues with higher decomposition rates in short-term periods can provide C and N sources for denitrifiers [25,44] to stimulate microbial activities and N₂O emissions. With the increasing of residue returning duration, more organic matter content input could improve soil structure and aeration conditions due to increased faunal and microbial activity, and thus reduce the favorable tendency for N₂O formation [44,77].

4.4. Effects of Initial Properties on N₂O Emission

Soil pH has been identified as a key regulator for N₂O production [10]. Our meta-analysis indicated that acidic soils (pH ≤ 6.5) and neutral soils (pH: 6.5–7.3) both had stimulatory effects on N₂O production. In acidic soils, stepwise denitrification processes might be inhibited by a reduction of reductase (N₂OR) activities, which could prevent N₂O conversion to N₂, leading to an increase in N₂O yield [78]. In contrast, autotrophic nitrifiers generally prefer neutral soils [79], and heterotrophic denitrifiers also function better at neutral than acidic soils [80], hence, the meta-analysis indicated that significant effect of neutral soil were the greatest at neutral conditions. Similar results were reported by Chen et al. [13]. Soil organic C had positive effects on N₂O production when the content was 10–18 g kg⁻¹, but with the content >18 g kg⁻¹, an inhibitory effect was observed. Soil organic C could be a sufficient C source for heterotrophic denitrifiers, thus improving soil microbial activities [81], but active microorganisms will absorb native soil N into their biomass, leading to net N immobilization with higher soil organic C [82]. Notably, this N consumption will weaken nitrification and/or denitrification processes, and thus decrease soil N₂O production.

Soil texture and clay content are often implicated in regulating N₂O emission due to their impact on soil aeration and organic matter decomposition rates [83]. In our study, soil N₂O emission significantly increased in clay loam and sandy clay loam soils. Generally, N₂O formation and release were mainly attributed to the nitrification process in well-aerated and coarse-textured soils [84], whereas, denitrification is the primary process in fine-textured soils [59]. The clay loam and sandy clay loam soils belong to medium-textured soils, which could result in N₂O production from both nitrification and denitrification processes, thus leading to a higher N₂O emission. In other soil types, N₂O release was significantly inhibited, and a finding that was inconsistent with other research [13]. A possible explanation was related to different residue returning methods, which could alter soil aeration and water holding capacity [85]. Additionally, our study showed that significant increase in soil N₂O emission with medium clay content was consistent with results of soil texture estimation. However, a significant and negative effect was observed when clay content was ≦15%. These changes may be attributed to the associative influences of clay content on anaerobic microorganisms and soil denitrification process [86].

4.5. Effects of N Fertilizer Application Rate on N₂O Emission

It is generally accepted that N₂O emission is significantly enhanced with the increase of N fertilizer application rate [87]. Furthermore, our study shows that N fertilizer application significantly affects N₂O production. Similar results have been reported with synthetic N fertilizer application, which improves nitrification and denitrification processes, leading to increased N₂O production [88]. However, significant negative effects and lower N₂O release were observed at the N application rate of 100–150 kg ha⁻¹ regarding of residue returning. These data demonstrated that this N application level was the optimal application rate to achieve the crop yield demand level [89], and the finding has been reported by other researchers [90,91]. Kim et al. [92] proposed a conceptual model of the relationship between N₂O emission and N fertilizer application rate. They found that before crop growth demand, a linear increase exists between N₂O emissions and N application levels, then an exponential increase beyond N levels demanded by crops and soil microbes, finally a steady state due to soil organic C limits. Therefore, it is possible that reducing N fertilizer application rate to adjust crop growth will mitigate N₂O emissions [93].

Although these results provide clear information on N₂O emissions following residue application, there are still some shortcomings in our meta-analysis. Firstly, the collected experimental sites are mainly from Asia, especially China, other areas, such as Russia, Northern Europe, Africa, Australia, and New Zealand are not involved. Secondly, as widely known that soil temperatures and moisture levels have great effects on N₂O production and emission, however, our meta-analysis did not analyze this effect sizes due to the variability and absence of detailed information. Therefore, further studies will be helpful for understanding the effect of residue returning on N₂O emission from a global perspective regarding soil temperature and water-filled pore space.

5. Conclusions

In this meta-analysis, we found that residue returning increased N₂O emissions, and the effects were positively related to residue amount and N input, but negatively associated with residue C/N ratio. In warm temperate and tropical climate zones, residue returning treatments have significant stimulatory effects on N₂O emissions. For land use types, soil N₂O release significantly increased in upland fields, but significantly decreased in paddy fields following residue returning. The effects of crop residue returning on N₂O emissions relied on the soil’s initial properties, especially soil organic C content and texture. Significant stimulatory effects occurred when pH < 7.3, in soils with 15–25% clay content, or in soils with an organic C range of 10–18 g kg⁻¹. The effect of residue returning on soil N₂O emissions can be regulated by N fertilizer application rates due to crop yield demands, and negative effects were indicated at N fertilizer application rates at 100–150 kg N ha⁻¹ associated with crop residue returning. Therefore, appropriate strategies, such as converting residue returning methods, regulating soil C/N ratio and soil aeration conditions, and reducing N fertilizer application rate can be adopted to mitigate N₂O emission associated with crop residues.