Research Progress and Development Trends of Greenhouse Gas Emissions from Cereal–Legume Intercropping Systems

Abstract

High yields and low carbon emissions are new challenges for modern crop production. Balancing the crop yield and reducing greenhouse gas (GHG) emissions has become a new field of agronomic technology innovation. Cereal–legume intercropping is a typical diversification planting system, which has been expected to achieve the dual goals of high production and low GHG emissions. However, the synergistic effect of integrating various technologies in an intercropping system on GHG emissions and whether it will achieve the high yield and low emissions goal remains to be determined. Therefore, bibliometric analysis has investigated the worldwide development trend of cereal–legume intercropping designs. The literature on the GHG emissions of the cereal–legume intercropping system was summarized. Additionally, the effects and mechanisms of different agricultural management methods regarding soil nitrous oxide and carbon dioxide emissions in the cereal–legume intercropping system were summarized. The research on GHG emissions of cereal–legume intercropping systems in non-growing seasons must be revised. In situ observations of GHG emissions from intercropping systems in different regions should be strengthened. This work is valuable in supporting and evaluating the potential of GHG reduction in a cereal–legume intercropping system in various farming areas.

1. Introduction

Mitigating global warming caused by greenhouse gas (GHG) emissions is challenging for all of humanity. As emphasized by the International Panel on Climate Change (IPCC) Special Report on Global Warming of 1.5 °C (SR1.5) in 2018, drastic measures are urgently needed to reduce the risks and effects of climate change by limiting the increase in the global average temperature to 1.5 °C above the pre-industrial level [1]. Agricultural production has been considered one of the essential contributors to the anthropogenic source of non-carbon greenhouse gases (GHG_S) [2]. It is estimated that the food system was responsible for 25% to 30% of global emissions, around one-third if we include all agricultural products [3]. Meanwhile, agriculture also has considerable potential to mitigate climate change [4].

GHG emissions from farmland soil greatly depend on cultivation practices. Intercropping, as a classical multi-cropping system, has been widely demonstrated to enhance the crop yield and nutrient use efficiency by exploiting niche crop and seasonal differentiation and positive interactions between organisms, if appropriately managed. Therefore, intercropping is a crucial way to achieve the dual goals of increasing crop yields and reducing GHG emissions [5,6,7,8]. The primary consideration for intercropping is selecting compatible crops to minimize competitive inhibition, allow for the ease of field management, and increase the profit per land unit compared to monoculture [9]. Cereal–legume intercropping is the most widely used combination globally and is an essential component of climate-smart agriculture and sustainable intensification in some regions [10]. However, intercropping will affect the soil’s physical and chemical properties, influencing microbial-mediated GHG emissions [11]. Many studies have shown a reduction in soil carbon dioxide (CO₂) and N₂O emissions in a cereal–legume system compared to monoculture [11,12,13]. However, the reason behind this still needs to be clarified, with limited systematic studies on GHG emissions with cereal–legume intercropping.

Bibliometrics has been widely used to study the frontier progress of many disciplines or fields since the 21st century. Its theories and methods have been extensively used to evaluate and predict the historical process, current situation, development trend, and hotspots of various research fields. Researchers have analyzed the research trends of global greenhouse gas emissions [14] and intercropping (including agroforest, crop mixture, vegetable legume, cereal legume, etc.) [15]. Still, a comprehensive and objective statistical report on the achievements must be provided. The objectives of the study were (1) to analyze the development trend of cereal–legume intercropping through a bibliometric method, (2) to summarize the emission characteristics and mechanism of soil N₂O and CO₂ and elaborate the main factors affecting soil GHG emissions from intercropping systems, and (3) to propose three issues (compound effect of multi-technology, indirect GHG emissions and non-growing season GHG emissions) that should be strengthened in the study of GHG emissions from cereal–legume intercropping systems.

2. Analysis of the Development Trend of Cereal–Legume Intercropping

The article uses the information visualization software CiteSpace [16] to study the research related to cereal–legume intercropping between 2000 and 2021 in the Web of Science core collection (WoSCC) database. The query sets used for the literature search were TS = (legume OR soybean OR peanut OR pea OR bean) AND (interplant OR intercropping). Diversity in languages further restricted the search results and document types. Therefore, only articles written in English were retrieved. Two thousand and two hundred publications from 2005–2021 were obtained and saved as text files (see Supplementary materials for details) containing the “full record with citation data”. In the final step, 839 relevant papers were selected by manual screening one by one (removing the literature about legume intercropping with noncereal crop plants and ensuring all works of literature were research articles). At the same time, the CiteSpace version. 5.6.5 was used to remove duplications.

2.1. Annual Trend in Publications and the Top Contributing Institutions

The trend of the number of publications and institutions in the database was analyzed (Figure 1). As shown in Figure 1, the number of publications in this field was relatively small and steady before 2016, with an average of 33 per year. After 2017, it increased rapidly, and the emphasis on cereal–legume intercropping increased; for example, in 2021, there were 137 publications. Figure 2 shows the Institution Collaboration Network in cereal–legume intercropping from 2005 to 2021. The thicker connection line indicates the closer cooperation between institutions, and each link between two different institutions is represented by a spectrum of colors corresponding to the years of occurrence. The 17 colors, from light gray to red, correspond to 2005 to 2021. The size of the nodes represents the number of papers published by the author’s institution. The top five institutions were the China Agricultural University, Sichuan Agricultural University, the Ministry of Agriculture, the Chinese Academy of Agricultural Sciences, and Wageningen University in the Netherlands. Previous research results showed that the World Agroforestry Center ranked first in terms of publication volume in all intercropping fields, followed by the Chinese Academy of Science and INRA [15]. However, the distribution of research institutions in Figure 2 reflects the current high level of attention to cereal–legume intercropping research in China, especially by the China Agricultural University and Sichuan Agricultural University. However, the cooperation between these institutions needs to be more cohesive. Further exchanges and collaboration between different research institutions are required.

2.2. Research Hotspots and Trend Analysis

The keywords in the paper reflect the main direction and core point of view of the research content. The keywords with the most substantial citations were used to illustrate the development and evolution of research directions [17]. We found that 93 keywords appeared more than 10 times through the data integration of CiteSpace software. The top 20 keywords with the most robust citation bursts between 2005 and 2021 are shown in Table 1. From the perspective of burst strength, the top five keywords were “cereal (S = 8.18)”, “inorganic N (S = 6.21)”, “tillage (S = 4.85)”, “weed (S = 4.15)”, and “light (S = 4.13)”. This indicates that tillage practices, weed control, light resource utilization, and inorganic nitrogen dynamics in intercropping systems are receiving more attention. In terms of development time, “cereal”, “weed”, “density”, “nitrogen fixation”, “inorganic nitrogen”, and “crude protein” appeared earlier, and these were the main areas of early research in the field of cereal–legume intercropping. “Greenhouse gas emissions” was a research hotspot between 2013 and 2015. “Weed” had the most prolonged duration and was the research hotspot between 2005 and 2015. “Intensification”, “soybean”, “light”, “crop productivity”, and “climate change” are more recent burst words, which are still research hotspots from 2019 until now.

3. Effects of Intercropping on Soil Greenhouse Gas Emissions

3.1. Effects of Intercropping on Soil N₂O Emissions

N₂O emissions from the soil are an intermediate product in the nitrification–denitrification reaction [18,19]. Soil physicochemical properties and microbial community diversity are changed with increased crop diversification, resulting in changes in soil N₂O emissions [20]. Differences in N₂O emissions can also result from intercropping treatments that use different legume species and cultivars [21]. In contrast to conventional maize cultivation based on monocropping, the nitrogen fixation of legume crops and the transfer of nitrogen between maize and legume crops significantly influenced the nitrogen cycle in intercropping systems [22]. It was found that maize–peanut intercropping could reduce soil N₂O emissions by 13% compared with maize monoculture [13]. Soil N₂O emissions in maize–soybean intercropping were 32.0% and 47.8% lower than those in maize and soybean, respectively [23]. It is assumed that the soil water content was the main reason for the difference in soil N₂O emissions in different intercropping systems. There was no significant difference in the N₂O emission flux between monocultures and intercropping systems when the soil water content was <15% [24]. Under aerobic conditions, other environmental and soil physio-chemical factors might be more important than soil moisture. Excessive soil moisture may lead to loss of dissolved nitrogen, leading to increased indirect emissions of N₂O [10]. Most studies showed that, under the same nitrogen application level, cereal–legume intercropping significantly reduced soil N₂O emissions in the whole growth period of crops compared with cereal monoculture [10,11,12,13,25]. This may be related to the improving nitrogen use efficiency in cereal–legume intercropping [11,26].

3.2. Effects of Intercropping on Soil CO₂ Emissions

The heterotrophic respiration of microorganisms and autotrophic respiration from roots are the primary sources of CO₂ emissions in soil. In addition to soil physicochemical properties, fertilization, irrigation, and the planting system also affect soil CO₂ emissions [27]. However, there are still many uncertainties about the effects of cereal–legume intercropping on soil CO₂ emissions. Dyer et al. [28] reported that, compared with maize monoculture and soybean monoculture, maize–soybean intercropping significantly reduced the total soil CO₂ emissions in the crop growing season by 18.1–20.4%. Ma et al. [13] concluded that maize–peanut intercropping reduced soil CO₂ emissions by 23.4%. However, there is still some variance in the results. It was found that there was no significant difference in soil CO₂ emissions between intercropping and monoculture treatments despite the cumulative CO₂ emissions in maize intercropping treatments being 4.0–8.9% lower than those in maize monoculture treatments during the whole growth period [29]. Sweet corn and soybean intercropping with crop row ratios of 2:3 and 2:4 increased the CO₂ emission compared with sweet corn monocropping. Additionally, CO₂ emissions were shown to increase with increased nitrogen application rates [30]. Rainfall variability, differing soil types across test sites, and variable sampling methods may have added to the inconsistency in the results [29].

4. Effects of Diversified Intercropping on GHG Emissions

Based on the diversification and refinement of basic research on cereal–legume intercropping systems in recent years, we summarized the effects of different management measures such as the intercrop row ratio and in-crop strip width, crop residue retention, and no-tillage on GHG (CO₂ and N₂O) emissions from intercropping systems. Methane emissions were not considered, as emissions from rain-fed farming systems are usually negligible, and few studies exist.

4.1. Effects of Spatial Arrangements on GHG Emissions in Intercropping Systems

Crop yields in the intercropping systems are closely related to spatial arrangements (mainly border–row proportion, bandwidth (crop rows per strip), and planting density) [31,32,33]. The field configuration can influence the efficiency of resource utilization and GHG emissions. Wang et al. [30] reported that both the GHG emissions intensity and global warming potential of sweet corn and soybean intercropping with different crop row ratios did not differ significantly. Dyer et al. [28] observed that different intercropping ratios (1:2 and 2:3) did not substantially affect soil CO₂ and N₂O emissions. These results were confirmed by other studies on sweet corn–soybean intercropping [30,34]. In addition, the 2:2 equal row spacing and narrow row width ratio had no significant effect on soil GHG emissions [28]. According to the literature we have collected [28,29,30,34], the border–row proportion and bandwidth do not significantly affect the GHG emissions of a cereal–legume intercropping system.

Plant density is one of the leading management practices to affect the yield in cropping systems [33], especially for intercropping [35]. Therefore, producers often improve the intercropping yield by increasing the plant density of component crops [36]. Luo et al. [37] showed that plant density slightly impacted N₂O emissions. However, the result obtained by Yang [38] through a three-year field experiment showed that increasing the maize density increased the carbon emissions. Other studies on plant density’s effect on GHG emissions under the intercropping system exist. However, whether increasing plant density can reduce soil GHG emissions while ensuring viable intercropping production is still being determined. Further investigations are required to clarify its emission reduction potential and mechanisms.

4.2. Effects of Nitrogen Fertilizer Management on GHG Emissions from Intercropping Systems

Optimized nitrogen fertilizer management is essential for crop yields and carbon sequestration [4,30]. Nitrogen fertilizer application significantly contributes to N₂O emissions from agriculture, and mitigating soil N₂O emissions is vital for staying below a 1.5 °C warming threshold [39]. Studies have shown that intercropping with legume crops and an appropriate pre-plant or basal nitrogen fertilizer split application could significantly mitigate GHG emissions while ensuring the crop yield. Tang et al. [40] concluded that, under two rows of sweet corn and four rows of soybean intercropping patterns, a reduction in nitrogen fertilizer application could significantly reduce the soil mean cumulative N₂O emission in six seasons but had no significant effect under an intercropping pattern of two rows of sweet corn and three rows of soybean. Fu et al. [41], through a long-term location experiment, showed that maize–soybean intercropping with reduced nitrogen application (180 kg/hm²) could reduce GHG emissions compared to traditional nitrogen application (240 kg/hm²). However, a recent study concluded that the sweet corn–soybean 2:4 intercropping patterns with 300 kg N ha⁻¹ significantly affected crop yields and reduced net GHG emissions compared to conventional sweet corn farming systems [30].

4.3. Effects of No-Tillage on GHG Emissions from Intercropping Systems

More sustainable cultivation systems, such as no-tillage, are increasingly used worldwide because of their environmental advantages. Studies on traditional monoculture have shown that no-tillage management reduced soil disturbance, reduced the loss of soil organic carbon, and facilitated the utilization of activated carbon by soil microorganisms. Therefore, soil CO₂ emissions were suppressed [42]. A meta-analysis showed that no-tillage decreases GHG emissions with no crop yield tradeoff at the global scale [43]. To evaluate the crop yield and carbon mitigation of intercropping systems, field experiments of maize–pea intercropping with no-tillage and conventional tillage were carried out by Yang et al. [38]. It was found that, over the growing season of maize and pea, no-tillage reduced carbon emissions by an average of 10% compared to conventional tillage, and no-tillage reduced carbon emissions from the maize strips. The effect of no-tillage on reducing GHG emissions was also demonstrated in other cereal–legume intercropping systems [44,45,46]. However, some scholars believe that the biophysical soil conditions of different no-tillage soil determined the GHG emissions. A reliable evaluation can only be made based on the local situation [47]. Therefore, more research is required in the future.

4.4. Effects of Crop Residue Retention on GHG Emissions in Intercropping Systems

Crop residue resources are ubiquitous in crop production industries and play a decisive role in conservation and regenerative agriculture for sustainable development and food security. Crop residue retention in the field is currently the economic utilization of the residue. It is also essential to improving farmland soil health and carbon sink capacity. Crop residue retention and the introduction of legume crops will inevitably increase soil biodiversity, microbial activities, soil carbon, and nitrogen processes, resulting in changes in soil GHG emissions and nitrogen availability [9,48]. Long-term field experiments showed that crop residue retention or the intercropping of legume crops could significantly improve biophysical soil properties and increase the soil organic matter content and crop yields [49,50]. The crop residue nitrogen content and C/N influence soil GHG emissions [51,52]. Therefore, crop residue retention is usually combined with applying nitrogen fertilizer to regulate soil C/N, reducing GHG emissions and improving nitrogen use efficiency [53].

Crop residue retention promotes soil respiration and increases CO₂ emissions [53]. The amount of residue retained also significantly affects soil CO₂ emissions [54]. However, the mechanisms of soil N₂O emission are complex, there are different views on the effect of residue retention on upland N₂O emission, and the influencing mechanisms need to be clarified [55]. Studies have shown that residue retention can promote soil N₂O emissions, but other studies have concluded that crop residue retention can reduce or not affect soil N₂O emissions [45,56]. In addition, the effects of residue retention on soil N₂O emissions were significantly correlated with the residue carbon input and crop residue ground cover depth [57,58]. Therefore, the relationship between the quantity and quality of crop residue and biophysical soil properties should be considered when studying soil N₂O emissions. There may be an interaction between residue retention and intercropping regarding soil GHG emissions, but there need to be more relevant studies. More field observations are still required to accurately evaluate the contribution of residue retention to GHG emissions and analyze its influencing factors.

5. Problems and Prospects

Recent studies have shown that countries with higher effective crop populations and species diversity have greater temporal stability in terms of total national agricultural output [59]. Agriculture diversity has been proposed to achieve win–win choices among ecosystem services in agriculture production [60,61]. There have been many studies on the effects of cereal–legume intercropping on crop yields. With the intensification of global climate change, more attention has been given to GHG emissions from intercropping systems [9,62,63,64]. However, the three following aspects still need to be studied:

(1) Compound effect of multi-technology. Through a field experiment conducted for 12 consecutive years (2006–2017), Chai et al. [65] found that multi-technology (e.g., no-tillage or less tillage, crop residue mulching, crop rotation, Nitrogen fertilizer timing, and other agronomic measures, akin to “conservation agriculture”) in the intercropping system could significantly increase the yield and optimize ecological benefits. Compared with traditional monoculture, the integrated intercropping system increased the annual profit by 15.6–49.9% and the net agricultural income by 39.2% and reduced the environmental footprint by 17.3%. However, most studies on the characteristics of soil GHG emissions from intercropping are single-factor experiments, and the research on the synergistic effects of cereal–legume intercropping and agronomic conservation practices on GHG emissions is still minimal, which is not conducive to the evaluation and understanding of the multi-technology synergistic impacts of conservation and regenerative cropping systems. Crop residue retention effectively increases the soil organic matter content, especially in areas where soil fertility is declining (e.g., Northeast China). However, both experimental and model studies showed that residue retention could stimulate the emission of N₂O in dry farmland and become the primary “leakage” factor in the carbon sequestration process [47,55,66]. However, the mechanism causing the increase in N₂O emissions needs to be clarified [55]. Whether multi-technology could achieve the synergy of crop productivity and environmental benefits has yet to be well determined.

(2) Indirect GHG emissions. Agricultural carbon footprint theory can systematically evaluate the direct and indirect carbon emissions caused by human factors in agricultural production, which is essential for sustainable agriculture [67]. Life cycle assessment (LCA)-based carbon footprint assessment proved to be an efficient method for quantifying greenhouse gas emissions [68]. Only some studies have considered carbon sequestration in cropping ecosystems and indirect GHG emissions from inputs such as fertilizers and pesticides in cereal and legume production [29,69]. Although some studies have shown that maize intercropping has a higher production efficiency and less of an environmental footprint in arid northwest China [70,71], most intercropping systems still face a trade-off between improving productivity and reducing the carbon footprint [72]. This aspect needs to be more conducive to a comprehensive evaluation of the contribution of various production and market activities in crop production to GHG emissions and a thorough evaluation of the future development potential of the cereal–legume intercropping system as economically and environmentally sustainable.

(3) Non-growing season GHG emissions. According to studies, GHG emissions from farmland are mainly concentrated in the growing season, but the GHG emissions from cropland in the non-growing season must be addressed [33]. Our previous study showed that the N₂O emission contribution of farmland soil in the “black soil” region of northeast China [73] in the non-growing season could reach 15–46% [72]. In years with high snowfall, N₂O emissions during the spring thaw accounted for 54–76% of the annual total, leading the yearly emissions [32]. However, understanding the drivers and processes underlying N₂O emissions related to freeze–thaw events in residue retention no-till farming systems still needs to be vastly improved. Moreover, there are few studies on GHG emissions under these conditions, which may lead to a miscalculation of the GHG emission reduction effect of managing the entire intercropping system, especially in seasonal freeze–thaw zones.

Sustainable food production is a fundamental challenge in a global warming scenario. An intercropping approach to sustainable food production is crucial to overcoming this global issue. Intercropping has been practiced worldwide in traditional and sustainable agriculture to feed the growing population. However, compared with conventional cereal planting systems, the research on the GHG emissions of a cereal–legume intercropping system in non-growing seasons needs to be more extensive. In situ observations of GHG emissions from intercropping systems in different regions and research on the synergistic mechanism of soil carbon sequestration and GHG emission reduction [74,75] should be strengthened in the future to improve the cereal–legume intercropping technology system according to local conditions and further explore the GHG emission reduction potential of cereal crop intercropping systems (Figure 3).

Author Contributions

Conceptualization, F.Y. and Y.W. (Yongjun Wang); methodology, F.Y. and Y.W. (Yang Wu); software, Y.C. and X.L.; validation, F.Y., Y.W. (Yang Wu), X.L., Y.C., Y.L., W.W., W.X., Z.L., J.L. and Y.W. (Yongjun Wang); formal analysis, Y.L., W.W., W.X., Z.L. and J.L.; investigation, X.L. and Z.L.; resources, Y.W. (Yang Wu) and X.L.; data curation, Y.L. and J.L.; writing—original draft preparation, F.Y.; writing—review and editing, Y.W. (Yang Wu) and Y.W. (Yongjun Wang); visualization, Y.C. and Y.W. (Yang Wu); supervision, Y.W. (Yongjun Wang); project administration, Y.W. (Yongjun Wang); funding acquisition, F.Y. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are available on request from the first author.

Acknowledgments

We thank the three anonymous reviewers and the editor for their helpful comments and suggestions that have improved this manuscript. We also thank Allen David McHugh, University of Southern Queensland Australia, for checking the manuscript and for the English improvement.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Figure 1. Annual trend in publications on research about cereal–legume intercropping based on data between 2005 and 2021 from WoSCC.

Figure 2. Distribution of research institutions in cereal–legume intercropping from 2005 to 2021.

Figure 3. Schematic diagram of crop residue retention, intercropping, and freeze–thaw effects on soil organic carbon sequestration and GHGS emissions.

Table 1

Top 20 keywords with the most substantial citation bursts between 2005 and 2021.

Keywords	Strength	Begin	End	2005–2021
cereal	8.18	2005	2011	▃▃▃▃▃▃▃ ▂▂▂▂▂▂▂▂▂▂
weed	4.15	2005	2015	▃▃▃▃▃▃▃▃▃▃▃ ▂▂▂▂▂▂
density	3.97	2006	2008	▂ ▃▃▃ ▂▂▂▂▂▂▂▂▂▂▂▂▂
nitrogen fixation	2.79	2006	2010	▂ ▃▃▃▃▃ ▂▂▂▂▂▂▂▂▂▂▂
inorganic nitrogen	6.21	2007	2011	▂▂ ▃▃▃▃▃ ▂▂▂▂▂▂▂▂▂▂
crude protein	2.83	2007	2011	▂▂ ▃▃▃▃▃ ▂▂▂▂▂▂▂▂▂▂
forage	2.89	2008	2010	▂▂▂ ▃▃▃ ▂▂▂▂▂▂▂▂▂▂▂
N2 fixation	3.60	2009	2010	▂▂▂▂ ▃▃ ▂▂▂▂▂▂▂▂▂▂▂
nutrient acquisition	3.21	2011	2014	▂▂▂▂▂▂ ▃▃▃▃ ▂▂▂▂▂▂▂
nitrogen fertilization	2.86	2011	2013	▂▂▂▂▂▂ ▃▃▃ ▂▂▂▂▂▂▂▂
tillage	4.85	2012	2015	▂▂▂▂▂▂▂ ▃▃▃▃ ▂▂▂▂▂▂
greenhouse gas emission	3.21	2013	2015	▂▂▂▂▂▂▂▂ ▃▃▃ ▂▂▂▂▂▂
rhizosphere	2.93	2014	2018	▂▂▂▂▂▂▂▂▂ ▃▃▃▃▃ ▂▂▂
yield advantage	3.50	2015	2019	▂▂▂▂▂▂▂▂▂▂ ▃▃▃▃▃ ▂▂
interspecific interaction	3.69	2017	2019	▂▂▂▂▂▂▂▂▂▂▂▂ ▃▃▃ ▂▂
intensification	2.58	2017	2021	▂▂▂▂▂▂▂▂▂▂▂▂ ▃▃▃▃▃
soybean	3.27	2018	2021	▂▂▂▂▂▂▂▂▂▂▂▂▂ ▃▃▃▃
light	4.13	2019	2021	▂▂▂▂▂▂▂▂▂▂▂▂▂▂ ▃▃▃
crop productivity	3.94	2019	2021	▂▂▂▂▂▂▂▂▂▂▂▂▂▂ ▃▃▃
climate change	3.13	2019	2021	▂▂▂▂▂▂▂▂▂▂▂▂▂▂ ▃▃▃

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy13041085/s1, File S1: 839 literatures.

References

1. Bongaarts, J. Intergovernmental panel on cimate change special report on global warming of 1.5 °C Switzerland: IPCC, 2018. Popul. Dev. Rev.; 2019; 45, pp. 251-252. [DOI: https://dx.doi.org/10.1111/padr.12234]

2. Lawrence, N.C.; Tenesaca, C.G.; VanLoocke, A.; Hall, S.J. Nitrous oxide emissions from agricultural soils challenge climate sustainability in the US Corn Belt. Proc. Natl. Acad. Sci. USA; 2021; 118, e2112108118. [DOI: https://dx.doi.org/10.1073/pnas.2112108118] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/34750266]

3. Ritchie, H.; Rosado, P.; Roser, M. Environmental Impacts of Food Production; Our World in Data: Oxford, UK, 2022; Available online: https://ourworldindata.org/environmental-impacts-of-food (accessed on 6 April 2023).

4. Snyder, C.S.; Bruulsema, T.W.; Jensen, T.L.; Fixen, P.E. Review of Greenhouse gas emissions from crop production systems and fertilizer management effects. Agric. Ecosyst. Environ.; 2009; 133, pp. 247-266. [DOI: https://dx.doi.org/10.1016/j.agee.2009.04.021]

5. Cong, W.; Zhang, C.; Li, C.; Wang, G.; Zhang, F. Designing diversified cropping systems in china: Theory, approaches, and implementation. Front. Agric. Sci. Eng.; 2021; 8, pp. 362-372.

6. Feng, C.; Sun, Z.; Zhang, L.; Feng, L.; Zheng, J.; Bai, W.; Gu, C.; Wang, Q.; Xu, Z.; Werf, W.V.D. Maize/peanut intercropping increases land productivity: A meta-analysis. Field Crop. Res.; 2021; 270, 108208. [DOI: https://dx.doi.org/10.1016/j.fcr.2021.108208]

7. Kermah, M.; Franke, A.C.; Adjei-Nsiah, S.; Ahiabor, B.D.K.; Abaidoo, R.C.; Giller, K.E. Maize-grain legume intercropping for enhanced resource use efficiency and crop productivity in the Guinea savanna of northern Ghana. Field Crop. Res.; 2017; 213, pp. 38-50. [DOI: https://dx.doi.org/10.1016/j.fcr.2017.07.008]

8. Glaze-Corcoran, S.; Hashemi, M.; Sadeghpour, A.; Jahanzad, E.; Afshar, R.K.; Liu, X.B.; Herbert, S.J. Understanding intercropping to improve agricultural resiliency and environmental sustainability—ScienceDirect. Adv. Agron.; 2020; 162, pp. 199-256.

9. Xu, Z.; Li, C.; Zhang, C.; Yu, Y.; Werf, W.V.D.; Zhang, F. Intercropping maize and soybean increases efficiency of land and fertilizer nitrogen use; A meta-analysis. Field Crop. Res.; 2020; 246, 107661. [DOI: https://dx.doi.org/10.1016/j.fcr.2019.107661]

10. Raji, S.G.; Dörsch, P. Effect of legume intercropping on N₂O emissions and CH₄ uptake during maize production in the Great Rift Valley, Ethiopia. Biogeosciences; 2020; 17, pp. 345-359. [DOI: https://dx.doi.org/10.5194/bg-17-345-2020]

11. Shen, Y.; Peng, S.; Huang, J.; Dong, W.; Chen, Y. Greenhouse gas emissions from soil under maize–soybean intercrop in the North China Plain. Nutr. Cycl. Agroecosys.; 2018; 110, pp. 451-465. [DOI: https://dx.doi.org/10.1007/s10705-018-9908-8]

12. Hu, F.; Chai, Q.; Yu, A.; Yin, W.; Cui, H.; Gan, Y. Less carbon emissions of wheat–maize intercropping under reduced tillage in arid areas. Agron. Sustain. Dev.; 2015; 35, pp. 701-711. [DOI: https://dx.doi.org/10.1007/s13593-014-0257-y]

13. Ma, X.; Wu, H.; Yang, G.; Zhang, X.; Chai, Y.; Yang, H.; Sun, Q.; Zhang, Q.; Shen, W. Effects of cultivation patterns on greenhouse gases emissions and yield in corn field. Environ. Sci. Technol.; 2020; 43, pp. 71-77.

14. Liu, Y.; Zhang, Y.; Li, Z.; Guo, H. Research status analysis of global greenhouse gas emission based on bibliometrics. Acta Sci. Circumst.; 2021; 41, pp. 4740-4751.

15. Lv, W.; Zhao, X.; Wu, P.; Lv, J.; He, H. A Scientometric analysis of worldwide intercropping research based on web of science database between 1992 and 2020. Sustainability; 2021; 13, 2430. [DOI: https://dx.doi.org/10.3390/su13052430]

16. Chen, C. CiteSpace II: Detecting and visualizing emerging trends and transient patterns in scientific literature. J. Am. Soc. Inf. Sci. Technol.; 2006; 57, pp. 359-377. [DOI: https://dx.doi.org/10.1002/asi.20317]

17. Li, J.; Chen, C. CiteSpace: Text Mining and Visualization in Scientific Literature; 3rd ed. The Capital University of Economics and Business Press: Beijing, China, 2022; pp. 226-228.

18. Baggs, E.M. Soil microbial sources of nitrous oxide: Recent advances in knowledge, emerging challenges, and future direction. Curr. Opin. Environ. Sustain.; 2011; 3, pp. 321-327. [DOI: https://dx.doi.org/10.1016/j.cosust.2011.08.011]

19. Thompson, K.A.; Bent, E.; Abalos, D.; Wagner-Riddle, C.; Dunfield, K.E. Soil microbial communities as potential regulators of in situ N₂O fluxes in annual and perennial cropping systems. Soil Biol. Biochem.; 2016; 103, pp. 262-273. [DOI: https://dx.doi.org/10.1016/j.soilbio.2016.08.030]

20. Linton, N.F.; Machado, P.; Deen, B.; Wagner-Riddle, C.; Dunfield, K.E. Long-term diverse rotation alters nitrogen cycling bacterial groups and nitrous oxide emissions after nitrogen fertilization. Soil Biol. Biochem.; 2020; 149, 107917. [DOI: https://dx.doi.org/10.1016/j.soilbio.2020.107917]

21. Pappa, V.A.; Rees, R.M.; Walker, R.L.; Baddeley, J.A.; Watson, C.A. Nitrous oxide emissions and nitrate leaching in an arable rotation resulting from the presence of an intercrop. Agric. Ecosyst. Environ.; 2011; 141, pp. 153-161. [DOI: https://dx.doi.org/10.1016/j.agee.2011.02.025]

22. Stern, W.R. Nitrogen fixation and transfer in intercrop systems. Field Crop. Res.; 1993; 34, pp. 335-356. [DOI: https://dx.doi.org/10.1016/0378-4290(93)90121-3]

23. Chen, J.; Amin, A.S.; Hamani, A.K.M.; Wang, G.; Zhang, Y.; Liu, K.; Gao, Y. Effects of maize (Zea mays L.) intercropping with legumes of nitrous oxide (N₂O) emissions. Appl. Ecol. Environ. Res.; 2021; 19, pp. 1589-1623. [DOI: https://dx.doi.org/10.15666/aeer/1905_33933407]

24. Huang, J.; Chen, Y.; Peng, S.; Nie, S.W.; Gao, W. Soil Nitrous oxide emissions under maize-legume intercropping system in the North China Plain. J. Integr. Agric.; 2014; 13, pp. 1363-1372. [DOI: https://dx.doi.org/10.1016/S2095-3119(13)60509-2]

25. Senbayram, M.; Wenthe, C.; Lingner, A.; Isselstein, J.; Steinmann, H.; Kaya, C.; Köbke, S. Legume-based mixed intercropping systems may lower agricultural born N₂O emissions. Energy Sustain. Soc.; 2015; 6, 2. [DOI: https://dx.doi.org/10.1186/s13705-015-0067-3]

26. Graf, D.R.H.; Saghaï, A.; Zhao, M.; Carlsson, G.; Jones, C.M.; Hallin, S. Lucerne (Medicago sativa) alters N₂O-reducing communities associated with cocksfoot (Dactylis glomerata) roots and promotes N₂O production in intercropping in a greenhouse experiment. Soil Biol. Biochem.; 2019; 137, 107547. [DOI: https://dx.doi.org/10.1016/j.soilbio.2019.107547]

27. He, J.; Qu, L.; Zhou, H.; Bo, L.; Du, P.; Wang, Z. Research progress on main factors of dry farmland cropping and carbon emission reduction measures. Modern Agri. Sci. Tech.; 2020; 9, pp. 184-186.

28. Dyer, L.; Oelbermann, M.; Echarte, L. Soil carbon dioxide and nitrous oxide emissions during the growing season from temperate maize-soybean intercrops. J. Plant Nutr. Soil Sc.; 2012; 175, pp. 394-400. [DOI: https://dx.doi.org/10.1002/jpln.201100167]

29. Huang, J.; Sui, P.; Gao, W.; Chen, Y. Effect of maize-soybean intercropping on greenhouse gas emission and the assessment of net greenhouse gas balance in North China Plain. J. China Agric. Univ.; 2015; 20, pp. 66-74.

30. Wang, X.; Chen, Y.; Yang, K.; Duan, F.; Liu, P.; Wang, Z.; Wang, J. Effects of legume intercropping and nitrogen input on net greenhouse gas balances, intensity, carbon footprint and crop productivity in sweet maize cropland in South China. J. Clean. Prod.; 2021; 314, 127997. [DOI: https://dx.doi.org/10.1016/j.jclepro.2021.127997]

31. Astiko, W.; Ernawati, N.; Silawibawa, I.P.J.I.C.S.E.; Science, E. The effect of row proportion of maize and soybean intercropping on growth and yield of component crops in sandy soil North Lombok, Indonesia. Earth Environ. Sci.; 2021; 637, 012005. [DOI: https://dx.doi.org/10.1088/1755-1315/637/1/012005]

32. Wang, R.; Sun, Z.; Zhang, L.; Yang, N.; Feng, L.; Bai, W.; Zhang, D.; Wang, Q.; Evers, J.; Liu, Y. et al. Border-row proportion determines strength of interspecific interactions and crop yields in maize/peanut strip intercropping. Field Crop. Res.; 2020; 253, 107819. [DOI: https://dx.doi.org/10.1016/j.fcr.2020.107819]

33. Zhang, N.; Miao, S.; Qiao, Y.; Chen, Z.; Ding, W. N₂O emissions from black soils in Northeast China. Acta Pedol. Sin.; 2022; 59, pp. 809-909.

34. Chen, Y.; Yang, K.; Duan, F.; Liu, P.; Wang, Z.; Wang, J. Effect of reduced nitrogen application on nodulation characteristics of soybean in sweet maize//soybean intercropping system. Ecol. Sci.; 2022; 41, pp. 1-8.

35. Zhang, D.; Sun, Z.; Feng, L.; Bai, W.; Yang, N.; Zhang, Z.; Du, G.; Feng, C.; Cai, Q.; Wang, Q. et al. Maize plant density affects yield, growth and source-sink relationship of crops in maize/peanut intercropping. Field Crop. Res.; 2020; 257, 107926. [DOI: https://dx.doi.org/10.1016/j.fcr.2020.107926]

36. Yang, C.; Fan, Z.; Chai, Q. Agronomic and economic benefits of pea/maize intercropping systems about N fertilizer and maize density. Agronomy; 2018; 8, 52. [DOI: https://dx.doi.org/10.3390/agronomy8040052]

37. Luo, S.; Yu, L.; Liu, Y.; Zhang, Y.; Yang, W.; Li, Z.; Wang, J. Effects of reduced nitrogen input on productivity and N₂O emissions in a sugarcane/soybean intercropping system. Eur. J. Agron.; 2016; 81, pp. 78-85. [DOI: https://dx.doi.org/10.1016/j.eja.2016.09.002]

38. Yang, H.; Hu, F.; Yin, W.; Chai, Q.; Zhao, C.; Yu, A.; Fan, Z.; Fan, H.; Ren, X. Integration of tillage and planting density improves crop production and carbon mitigation of maize/pea intercropping in the oasis irrigation area of northwestern China. Field Crop. Res.; 2021; 272, 108281. [DOI: https://dx.doi.org/10.1016/j.fcr.2021.108281]

39. Cui, X.; Zhou, F.; Ciais, P.; Davidson, E.A.; Tubiello, F.N.; Ju, X.; Canadell, J.G.; Bouwman, A.F.; Jackson, R.B.; Mueller, N.D. et al. Global mapping of crop-specific emission factors highlights hotspots of nitrous oxide mitigation. Nat. Food; 2021; 2, pp. 886-893. [DOI: https://dx.doi.org/10.1038/s43016-021-00384-9]

40. Tang, Y.; Yu, L.; Guan, A.; Zhou, X.; Wang, Z.; Guo, Y.; Wang, J. Soil mineral nitrogen and yield-scaled soil N₂O emissions lowered by reducing nitrogen application and intercropping with soybean for sweet maize production in southern China. J. Integr. Agric.; 2017; 16, pp. 2586-2596. [DOI: https://dx.doi.org/10.1016/S2095-3119(17)61672-1]

41. Fu, Z.; Du, Q.; Chen, P.; Pang, T.; Ye, X.; Yang, W.; Yong, T. Effects of long-term located nitrogen application on greenhouse gas emissions and grain yield in a relay intercropped Soybean. J. Sichuan Agric. Univ.; 2017; 35, pp. 491-498.

42. Alskaf, K.; Mooney, S.J.; Sparkes, D.L.; Wilson, P.; Sjögersten, S. Short-term impacts of different tillage practices and plant residue retention on soil physical properties and greenhouse gas emissions. Soil Till. Res.; 2021; 206, 104803. [DOI: https://dx.doi.org/10.1016/j.still.2020.104803]

43. Yue, K.; Fornara, D.A.; Heděnec, P.; Wu, Q.; Peng, Y.; Peng, X.; Ni, X.; Wu, F.; Peñuelaset, J. No tillage decreases GHG emissions with no crop yield tradeoff at the global scale. Soil Till. Res.; 2023; 228, 105643. [DOI: https://dx.doi.org/10.1016/j.still.2023.105643]

44. Odhiambo, J.A.; Basweti, E.A. Soil Nitrogen and Greenhouse Gas Fluxes in Response to Reduced Tillage Practices under Maize-Bean Intercropping in Western Kenya. Ajausud; 2022; 1, pp. 46-59.

45. Fatumah, N.; Munishi, L.K.; Ndakidemi, P.A. The effect of land-use systems on greenhouse gas production and crop yields in Wakiso District, Uganda. Environ. Dev.; 2021; 37, 100607. [DOI: https://dx.doi.org/10.1016/j.envdev.2020.100607]

46. Bayer, C.; Gomes, J.; Zanatta, J.A.; Vieira, F.C.B.; Dieckow, J. Mitigating greenhouse gas emissions from a subtropical Ultisol by using long-term no-tillage in combination with legume cover crops. Soil Till. Res.; 2016; 161, pp. 86-94. [DOI: https://dx.doi.org/10.1016/j.still.2016.03.011]

47. Zhang, J.; Hao, Q.; Jiang, C. The emission and influencing factors of greenhouse gases in soil under the no-tillage cultivation: A review. Chin. J. Soil Sci.; 2011; 42, pp. 236-242.

48. Abalos, D.; Recous, S.; Butterbach-Bahl, K.; De Notaris, C.; Rittl, T.F.; Topp, C.F.E.; Petersen, S.O.; Hansen, S.; Bleken, M.A.; Rees, R.M. A review and meta-analysis of mitigation measures for nitrous oxide emissions from crop residues. Sci. Total. Environ.; 2022; 828, 154388. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2022.154388]

49. Wang, X.; Jia, Z.; Liang, L.; Zhao, Y.; Yang, B.; Ding, R.; Wang, J.; Nie, J. Changes in soil characteristics and maize yield under straw returning system in dryland farming. Field Crop. Res.; 2018; 218, pp. 11-17. [DOI: https://dx.doi.org/10.1016/j.fcr.2017.12.003]

50. Solanki, M.K.; Wang, F.; Wang, Z.; Li, C.; Lan, T.; Singh, R.K.; Singh, P.; Yang, L.; Li, Y. Rhizospheric and endospheric diazotrophs mediated soil fertility intensification in sugarcane-legume intercropping systems. J. Soil Sediment.; 2019; 19, pp. 1911-1927. [DOI: https://dx.doi.org/10.1007/s11368-018-2156-3]

51. Liu, S.; Zhang, X.; Liang, A.; Jia, S.; Zhang, S.; Sun, B.; Cheng, S.; Yang, X. Effects of corn and soybean straws returning on CO₂ efflux at initial stage in black soil. Chin. J. Ecol.; 2015; 26, pp. 2421-2427.

52. Li, B.; Wu, L. Soil greenhouse gases emission in response to the C/N. J. Agro-Environ. Sci.; 2015; 37, pp. 2067-2078.

53. He, J.; Li, H.; Fang, M.; Kong, W. Influence of straw application on agricultural greenhouse gas emissions in China. Chin. Agric. Sci. Bull.; 2011; 27, pp. 246-250.

54. Jiang, C.; Yu, W. Maize production and field CO₂ emission under different straw return rates in Northeast China. Plant Soil Environ.; 2019; 65, pp. 198-204. [DOI: https://dx.doi.org/10.17221/564/2018-PSE]

55. Chen, Z.; Tu, X.; Meng, H.; Chen, C.; Chen, Y.; Elrys, A.S.; Cheng, Y.; Zhang, J.B.; Cai, Z.C. Microbial process-oriented understanding of stimulation of soil N₂O emission following the input of organic materials. Environ. Pollut.; 2021; 284, 117176. [DOI: https://dx.doi.org/10.1016/j.envpol.2021.117176]

56. Zhang, R.; Zhao, X.; Pu, C.; Liu, S.; Xue, J.; Zhang, X.; Chen, F.; Zhang, H. Meta-analysis on effects of residue retention on soil N₂O emissions and influence factors in China. Trans. Chin. Soc. Agric. Eng.; 2015; 31, pp. 1-6.

57. Chen, H.; Li, X.; Hu, F.; Shi, W. Soil nitrous oxide emissions following crop residue addition: A meta-analysis. Glob. Chang. Biol.; 2013; 19, pp. 2956-2964. [DOI: https://dx.doi.org/10.1111/gcb.12274] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23729165]

58. Zhu, X.; An, J.; Ma, L.; Chen, S.; Li, J.; Zou, H.; Zhang, Y. Effects of different straw returning depths on soil greenhouse gas emission and maize yield. Sci. Agri. Sin.; 2020; 53, pp. 977-989.

59. Li, X.; Wang, Z.; Bao, X.; Sun, J.; Yang, S.; Wang, P.; Wang, C.; Wu, J.; Liu, X.; Tian, X. et al. Long-term increased grain yield and soil fertility from intercropping. Nat. Sustain.; 2021; 4, pp. 943-950. [DOI: https://dx.doi.org/10.1038/s41893-021-00767-7]

60. Degani, E.; Leigh, S.G.; Barber, H.M.; Jones, H.E.; Lukac, M.; Sutton, P.; Potts, S.G. Crop rotations in a climate change scenario: Short-term effects of crop diversity on resilience and ecosystem service provision under drought. Agric. Ecosyst. Environ.; 2019; 285, 106625. [DOI: https://dx.doi.org/10.1016/j.agee.2019.106625]

61. Chai, Q.; Nemecek, T.; Liang, C.; Zhao, C.; Yu, A.; Coulter, J.A.; Wang, Y.; Hu, F.; Wang, L.; Kadambot, H.M. et al. Integrated farming with intercropping increases food production while reducing environmental footprint. Proc. Natl. Acad. Sci. USA; 2021; 118, e2106382118. [DOI: https://dx.doi.org/10.1073/pnas.2106382118]

62. Ananthi, T.; Amanullah, M.M.; Al-Tawaha, A.R.M.S. A review on maize-legume intercropping for enhancing the productivity and soil fertility for sustainable agriculture in India. Adv. Environ. Biol.; 2017; 11, pp. 49-64.

63. Maitra, S.; Shankar, T.; Banerjee, P. Potential and advantages of maize-legume intercropping system. Maize-Production and Use; Hossain, A. Intechopen: London, UK, 2020.

64. Ngwira, A.R.; Aune, J.B.; Mkwinda, S. On-farm evaluation of yield and economic benefit of short term maize legume intercropping systems under conservation agriculture in Malawi. Field Crop. Res.; 2012; 132, pp. 149-157. [DOI: https://dx.doi.org/10.1016/j.fcr.2011.12.014]

65. Yang, H.; Zhang, W.; Li, L. Intercropping: Feed more people and build more sustainable agroecosystems. Front. Agric. Sci. Eng.; 2021; 8, pp. 373-386.

66. Guenet, B.; Gabrielle, B.; Chenu, C.; Arrouays, D.; Balesdent, J.; Bernoux, M.; Bruni, E.; Caliman, J.P.; Cardinael, R.; Chen, S. Can N₂O emissions offset the benefits from soil organic carbon storage?. Global Chang. Biol.; 2021; 27, pp. 237-256. [DOI: https://dx.doi.org/10.1111/gcb.15342] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32894815]

67. Shi, L.; Chen, F.; Kong, F.; Fan, S. The carbon footprint of winter wheat-summer maize cropping pattern on North China Plain. China Popul. Resour. Environ.; 2011; 21, pp. 93-98.

68. Guinee, J.B. Handbook on Life Cycle Assessment Operational Guide to the ISO Standards; Springer: Berlin/Heidelberg, Germany, 2002.

69. Geng, C.; Ren, J.; Ma, X.; Long, G.; Lu, Y.; Tang, L. Yield improvement and greenhouse effect of different intercropping systems based on life cycle assessment. Chin. J. Eco-Agricul.; 2020; 28, pp. 159-167.

70. Qin, A.; Huang, G.; Chai, Q.; Yu, A.; Huang, P. Grain yield and soil respiratory response to intercropping systems on arid land. Field Crop. Res.; 2013; 144, pp. 1-10. [DOI: https://dx.doi.org/10.1016/j.fcr.2012.12.005]

71. Sun, T.; Zhao, C.; Feng, X.; Yin, W.; Gou, Z.; Lal, R.; Deng, A.; Chai, Q.; Song, Z.; Zhang, W. Maize-based intercropping systems achieve higher productivity and profitability with lesser environmental footprint in a water-scarce region of northwest China. Food Energy Secur.; 2020; 10, e260. [DOI: https://dx.doi.org/10.1002/fes3.260]

72. Yao, F.; Wang, L.; Duo, X.; Liu, Z.; Lv, Y.; Cao, Y.; Wei, W.; Wang, Y. Effects of different nitrogen fertilizers on annual emissions of greenhouse gas from maize field in Northeast China. Chin. J. Ecol.; 2019; 30, pp. 1303-1311.

73. Wen, D.; Liang, W. Soil fertility quality and agricultural sustainable development in the black soil region of northeast China. Environ. Dev. Sustain.; 2001; 3, pp. 31-43. [DOI: https://dx.doi.org/10.1023/A:1011480228613]

74. Zhang, Y.; Gao, Y.; Zhang, Y.; Huang, D.; Gregorich, E.; McLaughlin, N.; Zhang, X.; Chen, X.; Zhang, S.; Liang, A. et al. Effect of long-term tillage and cropping system on portion of fungal and bacterial necromass carbon in soil organic carbon. Soil Till. Res.; 2022; 218, 105307. [DOI: https://dx.doi.org/10.1016/j.still.2021.105307]

75. Yu, R.; Yang, H.; Xing, Y.; Zhang, W. Belowground processes and sustainability in agroecosystems with intercropping. Plant Soil; 2022; 476, pp. 63-288. [DOI: https://dx.doi.org/10.1007/s11104-022-05487-1]