Weakened growth of cropland‐N₂O emissions in China associated with nationwide policy interventions

Abstract

China has experienced rapid agricultural development over recent decades, accompanied by increased fertilizer consumption in croplands; yet, the trend and drivers of the associated nitrous oxide (N2O) emissions remain uncertain. The primary sources of this uncertainty are the coarse spatial variation of activity data and the incomplete model representation of N2O emissions in response to agricultural management. Here, we provide new data‐driven estimates of cropland‐N2O emissions across China in 1990–2014, compiled using a global cropland‐N2O flux observation dataset, nationwide survey‐based reconstruction of N‐fertilization and irrigation, and an updated nonlinear model. In addition, we have evaluated the drivers behind changing cropland‐N2O patterns using an index decomposition analysis approach. We find that China's annual cropland‐N2O emissions increased on average by 11.2 Gg N/year2 (p < .001) from 1990 to 2003, after which emissions plateaued until 2014 (2.8 Gg N/year2, p = .02), consistent with the output from an ensemble of process‐based terrestrial biosphere models. The slowdown of the increase in cropland‐N2O emissions after 2003 was pervasive across two thirds of China's sowing areas. This change was mainly driven by the nationwide reduction in N‐fertilizer applied per area, partially due to the prevalence of nationwide technological adoptions. This reduction has almost offset the N2O emissions induced by policy‐driven expansion of sowing areas, particularly in the Northeast Plain and the lower Yangtze River Basin. Our results underline the importance of high‐resolution activity data and adoption of nonlinear model of N2O emission for capturing cropland‐N2O emission changes. Improving the representation of policy interventions is also recommended for future projections.

Can N2O emissions offset the benefits from soil organic carbon storage?

“…Various models may be used to account for scale or leakage effects such as indirect land-use changes (Qin et al, 2016). The methods currently available include datadriven approaches based on worldwide measurement networks (Shang et al, 2019), statistical or empirical flux-upscaling models (Shang et al, 2019;Wang et al, 2019), process-based models and, lastly, integrated assessment models (IAM;Zomer et al, 2016).…”

“…Various models may be used to account for scale or leakage effects such as indirect land‐use changes (Qin et al, 2016). The methods currently available include data‐driven approaches based on worldwide measurement networks (Shang et al, 2019), statistical or empirical flux‐upscaling models (Shang et al., 2019; Wang et al., 2019), process‐based models and, lastly, integrated assessment models (IAM; Zomer et al., 2016). Process‐based models include a representation of N cycling processes, which are an essential tool in assessing and predicting the terrestrial N cycle and N 2 O fluxes in response to multi‐factor global changes.…”

Greenhouse gas emissions from synthetic nitrogen manufacture and fertilization for main upland crops in China

“…N fertilizer-induced direct N 2 O emissions from agricultural fields are critically affected by many factors, including N application rate, climate variables, crop types and soil characteristics [21,[23][24][25][26]. Several models have been developed and used to estimate N fertilizerinduced N 2 O emissions from the soil [25,27,28]. Due to limited availability of enough relevant information, the determination of direct N 2 O emissions from synthetic N fertilization for wheat and maize in China in the current work was done using the IPCC emission factor method combined with region-and crop-specific direct N 2 O emission factors.…”

“…China is now on the right track to implement improved N management techniques in croplands, which would help to identify options for lowering GHG emissions from agricultural production. In recent years, N 2 O emissions from Chinese croplands have grown slowly, owing to nationwide policy interventions and prevalence of technological adoptions [28,42,43]. Moreover, technological innovation in China's N fertilizer industry could make a significant contribution to synthetic N manufacture-related GHG emissions reduction [10].…”

Soil nitrogen substrates determine global N2O emission more than climate and other soil properties

“…A recent global assessment using ensemble of terrestrial biosphere models also found that global N fertilizer application contributes 2.0 ± 0.8 Tg N 2 O-N yr -1 during 2007-2016, manure application contributes 0.6 ± 0.4 Tg N 2 O-N yr -1 , and N deposition contributes 26% of global soil N 2 O emission (Tianet al 2019). Furthermore, the deceases in cropland-N 2 O emissions after 2003 are mainly ascribed to the reduction in usage of N fertilizer in China (Shang et al 2019). Therefore, we hypothesized that soil N contents might play an important role on regulating soil N 2 O emissions at the global scale.…”