Climate change has become a global concern because of increased greenhouse gas (GHG) emissions in recent decades, especially from agricultural soil, which is the largest anthropogenic source of GHG emissions with more than 10% of the total emissions (Zhai et al., 2015; Cheng et al., 2020), mainly including methane (CH₄) (25%) and nitrous oxide (N₂O) (52%) (Lin et al., 2022). However, whether agricultural soil was a pool or a source for GHG remains debatable because agricultural soil was so complicated and the GHG emission was affected by climate, soil properties, and management practices (Jiang et al., 2017). No-tillage (NT) is considered to be an environment-friendly way to increase soil carbon storage, improve soil aggregation and stability, and reduce production costs compared to conventional tillage (CT) (Zhang and Wang, 2020). However, the effect of NT on GHG emissions remains inconsistent from literal reports. Necpalova et al. (2018) reported that reduced tillage (RT) and NT reduced net soil GHG emissions by 31% and 58% in long-term NT practice, respectively. While NT could reduce CH₄ emissions significantly, but had no effect on N₂O emissions from paddy soil (Zhang et al., 2016). NT decreased the cumulation of N₂O emissions, but increased CH₄ emissions from deep soil caused by higher temperature and less precipitation (Tu and Li, 2017). Accordingly, the effect of NT on GHG emissions was not consistent due to different NT practice duration, soil types, environmental factors, as well as planting patterns (Hoffman et al., 2018), and so on. Therefore, it is vital and urgent to synthetically evaluate the effect of NT on GHG emissions from farmland and its key factors.

Meta-analysis, a statistical method for synthesizing relevant case studies of the same topic, was more and more introduced to quantitatively evaluate the effect of natural and anthropogenic activities in ecology research (Zhou et al., 2019). It could improve the statistical power of the original results and resolve inconsistencies among the results to improve the effect estimates. However, the application of meta-analysis is not well developed in the macro-scale of agricultural ecology topic yet (Yin et al., 2018). Previous studies on GHG emissions were increasingly published in recent dozens of years, which have possibilities for meta-analysis on the overall effect on GHG emissions of anthropogenic practice. For example, Huang et al. (2018) evaluated the effects of NT on crop yield, GHG emissions, and GWP of major cereal cropping systems by meta-analysis, which suggested that NT has the potential to mitigate GHG emissions and increase crop yield. While Shakoor et al. (2021) used meta-analysis to conclude that NT has decreased GWP but increased GHG emissions compared to CT for cultured agroecosystem. However, most studies on GHG emissions from cropland have focused on one crop annual agroecosystem, which is inconsistent with the crop rotation strategy of China. In China, winter wheat-based cropping rotation is the main grain agricultural production, with a 66.09% percentage rate of the total cropping area (NBSC, 2021), and GHG emission has extremely varied due to the great differences in climatic conditions and soil properties. Previous results of meta-analyses may conflict with wheat-based cropping rotation system. In addition, the GHG emissions from agricultural soil contribute more than 15% of the total GHG emissions, including 90% N₂O emissions and 60% CH₄ emissions (Gao et al., 2022). Therefore, we conducted a meta-analysis to quantify the overall effect on GHG emissions from wheat-based cropping fields in China, to disentangle the effects of driving factors (including soil properties, climate conditions, and field management practices) on GHG emissions, which could have significant implications for the mitigation strategy of GHG emissions from agricultural soil.

Since other databases such as the web of science only have five studies on the winter wheat-based cropping rotation, we selected the papers published by the keywords of ‘tillage methods’ ‘no-tillage’ ‘greenhouse gases’, ‘N₂O’, and ‘CH₄’, through the China National Knowledge Internet (CNKI) database (www.cnki.net) before June 30th 2019. The criteria of data picked up were as follows.

(1) The experiments were in experimental fields in China (including Hong Kong, Macao and Taiwan), excluding incubation experiments.

(2) The fertilizers used in field experiments were synthetic nitrogen fertilizers, excluding organic fertilizers and green manure.

(3) The experiment must include both NT and CT treatments, with clear information about the site, the year, climatic conditions, cropping system, and field management measures.

(4) Either N₂O or CH₄ emissions mean value and standard deviation in the wheat growing season were provided or could be extracted from graphs by Engauge Digitizer.

We collected 33 papers about GHG emissions in wheat field of China, containing 150 data pairs with 92 pairs for CH₄ emission and 121 pairs for N₂O emission. These studies came from eight provinces in northern China (including Hebei, Henan, Shandong, Shanxi, Shaanxi, Gansu, Anhui and Jiangsu), where the cropping systems were all wheat-based cropping rotation per year. The experimental period was between 2005 and 2016. The dataset contains CH₄ average flux, N₂O average flux, GWP, the variables of soil properties (soil types, pH values, organic matter, total nitrogen, and so on), climate conditions (annual average temperature, average annual precipitation) and field management practices (cropping system, whether straw returned, and so on). Data and paper information refer to supporting material ( Table S1 in the supplement file (http://egeoscien.neigae.ac.cn/index.htm)).

Generally, meta-analysis could reveal the effect of treatment on N₂O and CH₄ emission compared to control through overall effect size (Zhang et al., 2015). We used Review Manager 5.3 (https://training.cochrane.org/online-learning/core-software/revman/revman-5-download, cited 30 Sep. 2022) for meta-analysis with NT as treatment and CT as control. The effect size ($ \mathrm{ln}R $) was calculated by the natural logarithm of the response ratio, which was calculated by the ratio of GHG emissions from wheat fields under NT ($ {X}_{{\rm{NT}}} $) to CT ($ {X}_{{\rm{CT}}} $). The details are shown in Eq. (1):

where R is the response ratio, lnR is the effect size, and X is the CH₄ or N₂O average emission flux under the NT or the CT of each study (μg/(m²·h)). If the effect size is larger than 0, the factor has a positive effect on the result. Otherwise, the factor has a negative effect on the result.

The percentage change of GHG emissions was calculated by Eq. (2):

where m is the change of GHG emissions under NT compared to CT (percentage of increase or decrease).

CH₄ and N₂O global warming potential (GWP) of 100-yr scale were calculated based on The Intergovernmental Panel on Climate Change (IPCC) GHG inventory methodology (IPCC, 2006). The detail was in Eq. (3)

where CE is the cumulative emissions of variables (CH₄ or N₂O), 25 and 297 is CO₂-eq value in 100-yr scale, respectively.

Due to great spatial variation in the climate conditions, soil properties, and management practices of wheat cropping in China, we considered eight moderators, including annual temperature, annual precipitation, pH, C: N ratio, tillage duration, nitrogen fertilizer input, cropping system, and residue return, to identify the main driving factors of GHG emissions in the winter wheat-based cropping rotation under NT. These moderators were divided into 2–4 subgroups. The detailed information on subgroups refers in Table 1.

The overall effect size of NT on CH₄ uptake is 0.70 (95% Confidence Interval (CI): 0.21–1.19), and the overall effect size of NT on N₂O emission is −0.27 (95%CI: −0.72–0.18), in addition, the overall effect size ofNT on GWP is −0.39 (95%CI: −1.01–0.23). The details are in Fig. 1. Previous studies on meta-analysis have shown that NT practice could decrease CH₄ emission and increase N₂O emission by 15.5% and 10.4%, respectively (Huang et al., 2018), which is not consistent with the results in this study. It is possible that the crop types, and climatic zone attribute to this discrepancy.

Figure 1.  The overall effect of no-tillage (NT) relative to conventional tillage on methane (CH₄) uptake, nitrous oxide (N₂O) emissions, and global warming potential (GWP) in wheat field of China. The error bars indicate the 95% confidence interval for the percentage change

The subgroup of 10–15°C showed positive effects on CH₄ uptake under NT, of which the effect size is 0.99 (95%CI: 0.38–1.60) (Fig. 2), but showed negative effects on N₂O emission under NT, of which the effect size is –0.49 (95%CI: −0.93 to −0.05). In addition, both the subgroup of 10–15°C and higher than 15°C showed negative effects on GWP under NT, of which the effect size is −3.12 (95%CI: −5.28 to −0.96) and −2.23 (95%CI: −4.65 to −0.48), respectively. Other subgroups had no significant contribution to the overall effect size.

Figure 2.  The effect of no-tillage relative to conventional tillage on methane (CH₄) uptake, nitrous oxide (N₂O) emissions and global warming potential (GWP) with annual average temperature and average annual precipitation in wheat field of China. The error bars indicate the 95% confidence interval for the percentage change

The subgroup of 500–800 mm showed positive effects on CH₄ uptake under NT, of which the effect size is 1.05 (95%CI: 0.50–1.60), but showed negative effects on N₂O emission under NT, of which the effect size is −0.86 (95%CI: −1.68 to −0.04). Both subgroups of 500–800 mm and higher than 800 mm showed negative effects on GWP under NT, of which the effect size is −3.03 (95%CI: −4.93 to −1.14) and −2.56 (95%CI: −4.65 to −0.48), respectively. Other subgroups had no significant contribution to the overall effect size (Fig. 2).

The subgroup of pH in 7–8 showed positive effects on CH₄ uptake under NT (Fig. 3), of which the effect size is 0.93 (95%CI: 0.29–1.58), while both the subgroup of pH < 7 and pH in 7–8 showed negative effects on GWP under NT, of which the effect size is −2.56 (95%CI: −4.65 to −0.48) and −3.02 (95%CI: −5.40 to −0.64), respectively. Other subgroups had no significant contribution to the overall effect size.

Figure 3.  The effect of no-tillage relative to conventional tillage on methane (CH₄) uptake, nitrous oxide (N₂O) emissions and global warming potential (GWP) with pH and C ∶ N in wheat field of China. The error bars indicate the 95% confidence interval for the percentage change

The subgroup of C∶N ratio > 20 showed positive effects on CH₄ uptake under NT, of which the effect size is 2.68 (95%CI: 0.94–4.42), while both the subgroup of 10–20 and C∶N ratio > 20 showed negative effects on GWP under NT, of which the effect size is −2.56 (95%CI: −4.65 to −0.48) and −4.48 (95%CI: −6.04to −2.93), respectively. Other subgroups had no significant contribution to the overall effect size (Fig. 3).

The subgroup of 120–240 kg/ha showed positive effects on CH₄ uptake under NT, of which the effect size is 1.22 (95%CI: 0.42–2.02) (Fig. 4), but showed negative effects on N₂O emission and GWP under NT, of which the effect size is −1.20 (95%CI: −2.18 to −0.22) and −2.94 (95%CI: −4.74 to −1.15), respectively. Other subgroups had no significant contribution to the overall effect size.

Figure 4.  The effect of no-tillage relative to conventional tillage on methane (CH₄) uptake, nitrous oxide (N₂O) emissions and global warming potential (GWP) with tillage duration and nitrogen fertilizer in wheat field of China. The error bars indicate the 95% confidence interval for the percentage change

The subgroup of 5–10 yr showed positive effects on CH₄ uptake under NT, of which the effect size is 0.92 (95%CI: 0.14–1.70), but showed negative effects on N₂O emission under NT, of which the effect size is −1.08(95%CI: −2.11 to −0.04). Both subgroups of duration le-ss than 5 yr and 5–10 yr showed negative effects on GWPunder NT, of which the effect size is −1.87 (95%CI: −3.10 to −0.64) and −4.48 (95%CI: −6.04 to −2.93), respectively. Other subgroups had no significant contribution to the overall effect size (Fig. 4).

The subgroup of wheat-maize and wheat showed positive effects on CH₄ uptake under NT, of which the effect size is 0.77 (95%CI: 0.24–1.29) and 0.99 (95%CI: 0.22–1.75), respectively (Fig. 5). While the subgroup of wheat-maize showed negative effects on N₂O emission under NT, of which the effect size is −1.85 (95%CI: −2.96to −0.74). In addition, the subgroup of wheat-rice and wheat maize showed negative effects on GWP under NT, of which the effect size is −2.98 (95%CI: −5.56to −0.40) and −2.61 (95%CI: −4.45 to −0.76). Other subgroups had no significant contribution to the overall effect size.

Figure 5.  The effect of no-tillage relative to conventional tillage on methane (CH₄) uptake, nitrous oxide (N₂O) emissions and global warming potential (GWP) with cropping system and residue return in wheat field of China. The error bars indicate the 95% confidence interval for the percentage change

The subgroup of straw mulch showed positive effects on CH₄ uptake under NT, of which the effect size is 0.82 (95%CI: 0.18–1.46), but showed negative effects on N₂O emission under NT, of which the effect sizeis –0.80 (95%CI: −1.58 to −0.02). Other subgroups had no significant contribution to the overall effect size(Fig. 5).

Precipitation and temperature could influence GHG emissions mainly by changes in the soil water content and soil temperature. Previous studies have shown that the ability of CH₄ to sink in soil decreased as the precipitation increasing (Liu, 2015), and N₂O emission would increase with soil water content increasing. Accordingly, soil water content under NT practice is 3.4% higher than CT (Shu and Qi, 2019), subsequently, both the CH₄ uptakes and N₂O emissions will increase with precipitation increasing under NT. Similar results were found in this study, the GWP of agricultural soil is the lowest under NT when the precipitation is 500–800 mm.

Generally, more precipitation responds to high temperature, which indicated the effect of increased temperature on GHG emissions may be similar to the effect of increased precipitation. In this study, the rice field was in the areas where the temperature is over 15°C, but the upland field was in the areas where the temperature is below 15°C. In the upland field, higher temperature will improve the evaporation of soil water, which decreases the soil water content, consequently repress the N₂O emission and promote the CH₄ uptake. While in the rice field, the temperature will influence the activity of methane-oxidizing bacteria and microbial denitrification, and then decrease the CH₄ uptake and promote the N₂O emission (Yao et al., 2021). Accordingly, the effect size of GWP showed a decreasing tendency as temperature increased. Therefore, NT practice would be suitable in the area where the temperature is 10–15°C and precipitation is 500–800 mm for staple crop production.

Soil properties play an important role in GHG emissions. Soil pH could affect the CH₄ emission by changing the activity of methanogenic bacteria. According to previous studies, the optimum pH for methanogenic bacteria was 6.90–7.50, and when pH was over 8.75, the microbial activity would decrease significantly, and the ability of CH₄ emission almost disappeared (Li et al., 2007; Sun et al., 2018). Besides, pH could affect N₂O emission by regulating the process of nitrification and denitrification (Bahram et al., 2022). It has been reported that N₂O emission would be decreased with the soil pH increasing, especially when the soil was neutral or alkaline, the activity of the N₂O reducing enzyme would be improved, which converts N₂O to N₂, to decrease the emission of N₂O (Zhu et al., 2022). In summary, NT practice would be available in the area where soil pH is 7.00–8.00.

C∶N ratio is an important soil fertility index close to the activity of soil microbes. Previous studies have indicated that the increasing C: N ratio could improve the microbial nitrogen use efficiency, and then reduce the product of N₂O, an intermediate of nitrification and denitrification processes (Li et al., 2012). Generally, the suitable C∶N ratio for soil microbes is 25–30 (Wang et al., 2021). Meanwhile, NT could increase the protection of nitrogen by soil aggregates (Li et al., 2021), and reduce the microbial accessibility of nitrogen, to reduce N₂O emission. Similarly, the effect size of GWP is decreasing significantly as C∶N ratio increases, especially when C∶N ratio is higher than 20 under NT.

Field management is a vital factor for the GHG emissions from the upland field. In this study, the effect size of N₂O and GWP were negative when the nitrogen application was 120–240 kg/ha. This indicated that when the nitrogen application was equivalent to crop demand, NT practice has higher nitrogen use efficiency through increasing crop yield, which has the possibility to decrease the N₂O emission. While, with the nitrogen application exceeding crop demand, the effect size of N₂O was close to zero even the effect size of GWP became positive, which most likely hides the mitigation potential of N₂O in NT practice by surplus nitrogen.

It is reported that the organic carbon and nitrogen content of surface soil was increased under long-term NT practice due to the improvement of soil structure (Prajapati and Jacinthe, 2014). In this study, NT had no effect on the emission of CH₄ and N₂O when the NT duration was larger than 10 years. The reason is most likely attributed to increase CH₄ and N₂O emissions from enhanced carbon and nitrogen stock under long-term NT practice. The extra emission relative to C and N stock is possibly offset by the mitigation due to the lower emission factor under NT, which results in the zero effect size when NT duration is larger than 10 years. In consistent, the negative effect size of NT was observed when duration was less than 10 years.

The cropping system is a key factor to affect GHG emissions. It has been shown that NT practice could reduce CH₄ and N₂O emissions, and the emissions varied with different cropping systems (Sui, 2006). Root exudates and litterfall from previous crops may provide sufficient carbon and nitrogen substrates for soil microbes. The root exudates and litterfall are close related to the plant root system and varied with crop type, which plays an important role in the process of nitrification and denitrification (Cheng et al., 2022). The N₂O emission from the maize continuous and wheat continuous cropping is higher than soybean continuous cropping by 78% and 43%, respectively (Shi, 2013). However, in this study, NT practice seems to be more suitable for the wheat-maize rotation cropping system. The discrepancy is needed for further study in the future.

According to the previous studies on the rice field, compared with CT practice, NT combined with straw returning could improve microbial fixation of soil mineral nitrogen, which reduces substrate of nitrification and denitrification, thus reducing the N₂O emission (Li et al., 2016). Similarly, we found the same results in the upland field. In addition, the ability of CH₄ sink was enhanced by straw mulch under NT practice in the upland field, which is probably relevant to the increasing CH₄ oxidation through the straw mulch layer (Yang et al., 2022).

In summary, NT practice could influence GHG emissions through climatic conditions, soil properties, and field management, with a positive effect on CH₄ uptake and a negative effect on N₂O emissions. Wheat planting is mainly located in the North China Plain, Huang-Huai-hai region, and the middle and lower reaches of the Yangtze River in China (Bai et al., 2019), where the annual mean temperature is 8–10°C, and grain production contributes to 84.5% of the total national grain. According to our results, the NT practice is a priority applied in the area with a wheat-maize rotation cropping system.

In this study, we analyzed the GHG emissions from the upland field under NT practice in China and the key factors to the effect size by meta-analysis. The limitation is derived from the interaction of key factors and the information loss of key factors including irrigation amount, straw returning amount, and so on.

This study provides a meta-analysis of the effects of NT on GHG emissions of wheat-based cropping rotation in north China. The overall effect size of NT on CH₄ uptake, N₂O emission, and GWP is 0.70 (95%CI: 0.21–1.19), −0.27 (95%CI: −0.72–0.18), and −0.39 (95%CI: −1.01–0.23), respectively. The key factors including the temperature in 10–15°C, precipitation in 500–800 mm, and wheat-maize rotation cropping system significantly contribute to the overall effect size of CH₄ uptake. The key factors including the temperature in 10–15°C, precipitation in 500–800 mm, N fertilizer input, and wheat-maize rotation cropping system significantly contribute to the N₂O emission effect size. The mitigation effect on GWP of NT is most evident when the annual mean temperature is 10–15°C, the precipitation is 500–800 mm, the soil is weakly alkaline, the C: N ratio is higher than 20, the nitrogen application is 120–240 kg/ha and the duration is 5–10 yr. Our results indicated in temperate climate zones with alkaline soils, the nitrogen application rate of 120–240 kg/ha, NT could significantly reduce GHG emissions and GWP. However, the mitigation effect will be weakened along with NT duration, except for proper straw addition. Overall, NT has the potential to reduce GHG emissions from wheat-based rotation systems in China, but it is necessary to implement NT depending on local conditions, soil characteristics, and field management.