Agricultural production largely depends on resource inputs to increase crop yields and improve crop quality to feed the world’s growing population (Foley et al., 2011; Mueller et al., 2012; Fan et al., 2020). Although land, energy, and water have been identified as the most important agricultural resource inputs, they are faced with a number of challenges due to excess inputs and unsustainable management practices, which have resulted in adverse effects, such as those pertaining to land and marine ecosystems, biodiversity, and air and water quality (United Nations, 2019). Indeed, approximately 25% of global greenhouse gas (GHG) emissions are released through agricultural systems (IPCC, 2014). It has been reported that GHG emissions from agriculture will continue to increase at a rate of 1% annually due to increasing food demands (Frank et al., 2019). Accordingly, there is a strong need to mitigate increasing GHG concentrations. How to effectively manage agricultural resources and improve their utilization are crucial to achieving GHG mitigation goals.
Furthermore, the land, energy, and water that is used in agricultural production are highly interdependent, interconnected, and interactive (Skaggs et al., 2012; Wang et al., 2012; Fan et al., 2020). For example, land preparation, crop planting, and harvesting directly consume energy, while energy can also be indirectly consumed during the production of other associated land inputs, such as fertilizers and pesticides. Similarly, irrigation, namely, the groundwater pumping practices used in agriculture, accounts for approximately 70% of the water withdrawn from groundwater supplies globally, which is in itself a major energy consumer (Wang et al., 2012). Owing to their inextricable interlinkage, some previous studies have proposed the nexus concept to explain energy-land-water interactions involved in agricultural production (Liu et al., 2018; Zhao et al., 2018). Linkages among the components of this nexus vary broadly. Consequently, previous studies have assessed the relationships among these interactive land-energy-water components in various ways, such as the land-energy nexus, the water-energy nexus, the land-water nexus, and the energy-land-water. For example, Wang et al. (2012) estimated GHG emissions from groundwater irrigation based on the water-energy nexus (Wang et al., 2012); Fan et al. (2020) reported regional agricultural GHG emissions in terms of the water-land-energy nexus (Fan et al., 2020). These studies highlight the importance of considering interactions among the energy-land-water as a comprehensive system to guide agricultural production, avoiding inadvertent outcomes during managerial and policymaking planning stages of agricultural production.
Interactive processes associated with the energy-land-water nexus are the main factors that impact agricultural GHG emissions. Although certain studies have focused their attention on one or two components of the land-energy-water nexus, a comprehensive perspective remains elusive. For example, many studies have focused on GHG emissions from land-use patterns (Zheng et al., 2013) and land management practices, including nutrient inputs (Xia et al., 2016a and 2019), no-tillage and tillage practices (Powlson et al., 2014), and land consolidation initiatives (Tan et al., 2011). Liu et al. (2019) demonstrated that GHG emissions from a double-cropping system generally exceeded that of a single-cropping system (Liu et al., 2019). Zhang et al. (2015) reported that optimal soil tillage and straw management practices decreased GHG emissions in a rice-wheat cropping system (Zhang et al., 2015). However, these investigations neglected to consider the effects of water and energy inputs on GHG emissions. Moreover, owing to the energy consumption required, the extraction of water resources for purposes of irrigation can also induce GHG emissions. For example, Wang et al. (2012) reported that GHG emissions from groundwater irrigation in China reached 33.1 MtCO2e based on the water-energy nexus, accounting for 3% of its agricultural GHG emissions (Wang et al., 2012). Additionally, other agricultural activities, such as the manufacture and transport of fertilizers and pesticides as well as machine operations, consume energy and further result in GHG emissions (Grassini et al., 2012). Therefore, exploring mechanisms of the energy-land-water nexus in agricultural production is crucial to accurately estimate GHG emissions, improve resource use efficiency, and support appropriate GHG mitigation policies.
In China, agricultural production plays a crucial role in meeting the food demand of its growing population alongside the rapid socioeconomic growth and expanding urbanization that has occurred over the past several decades. Agricultural production in China consumes 12.5% of its total land area and 19% of its total water resource (Wang et al., 2012). These agricultural resource inputs convert to approximately 820 Mt CO2-eq., accounting for 11.64% of China’s total net GHG emissions (NDRCC, 2005). Among these inputs, GHG emissions from agricultural energy consumption contribute 66.7 Mt CO2-eq. in China (NDRCC, 2005), and half of these emissions derive from irrigation alone. Continuous GHG emissions into the atmosphere from progressively increasing inputs from the land-water-energy nexus have become a global concern. From the perspective of the land-water-energy nexus, Fan et al. (2020) demonstrated that four crop types in the Sanjiang Plain, China, consumed 3.0 million ha of arable land, 12.1 billion m3 of water, and 100.4 PJ of energy, releasing a CO2eq. of 10.9 million tonnes (Fan et al., 2020). Although previous studies have shown that resource utilization and associated GHG emissions vary among the different crop types grown in China, GHG emissions from the same crop type also exhibit significant differences on a provincial scale (Cheng et al., 2015; Xia et al., 2016b; Zhang et al., 2018). Such differences are attributed to dissimilarities in environmental and edaphic conditions (e.g., climate, soil, water, and energy), crop types and cropping systems, land-use types, and the agricultural management practices used among China’s different provinces (Yu et al., 2016; Liu et al., 2019). Moreover, at present, different water and land resource use efficiencies and applications among China’s provinces (Deng et al., 2006) have resulted in significant differences in agricultural GHG emissions among provinces (Zhao et al., 2018). Ignoring such provincial scale differences may lead to considerable deviations when planning sustainable objectives and supporting GHG mitigation practices in China (Zhao et al., 2018). Given these inter-provincial differences in resource, environmental, and edaphic conditions, food trade practices among China’s provinces can provide insight into resolving the stress, scarcity, and imbalance of resources, which can subsequently be used to maintain the balance between food supplies and demands. For example, the virtual-water flow from water-rich to water-scarce regions through the food trade can alleviate stress in water deficit regions (e.g., North China) on a regional scale (Dalin et al., 2014). These abovementioned studies highlight the need and importance of implementing regional-based agricultural policies, accounting for specific climate and agricultural management practices. Moreover, it is essential to comprehensively explore the processes and mechanisms associated with the regional (i.e., inter-provincial and intra-provincial) land-water-energy nexus to ensure both appropriate regional resource allocation and GHG emission mitigation.
For this study, we selected the three major cereal crops grown in China (rice, wheat, and maize) to explore interactions among land-water-energy systems during crop production and to analyze impacts of the land-water-energy nexus on GHG emissions. Furthermore, the spatial distribution of GHG emissions from these three cereal crops was investigated on a provincial scale, after which the provinces that were determined to be GHG emission hotspots were identified based on our analysis. Finally, we propose GHG mitigation practices based on region-specific conditions and inter-province linkages, and finally discuss the limitations of this study.