Xiaoqing Cui

Grassland ecosystems, a major component of the global carbon (C) and nitrogen (N) cycles, are increasingly impacted by anthropogenic N addition. However, a comprehensive, integrated assessment of all three major greenhouse gas (GHG) responses in grasslands is lacking. Here, we present the first global meta-analysis to evaluate the effects of N addition on all three major GHGs (i.e., nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂) fluxes) in grasslands. Our results show that N addition significantly and consistently stimulates N₂O emissions, a response primarily modulated by key drivers such as grassland type, management, N addition rate and forms, humidity index (HI), and soil pH, clay, and total nitrogen (TN) content. In contrast, N addition has a minimal and non-significant overall effect on soil CO₂ fluxes. For CH₄, N addition causes a context-dependent reduction in uptake, an effect that is exacerbated by high mean annual precipitation (MAP) and soil bulk density (BD) but alleviated by high soil organic carbon (SOC) content. Notably, both CO₂ and N₂O showed a dose-dependent effect, while soil CO₂ fluxes were unexpectedly suppressed by nitrate nitrogen (NO₃⁻) addition. Our findings indicate that the pronounced and consistent increase in N₂O emissions is the dominant factor in GHG-related impacts in grasslands, implying a net positive climate forcing in grasslands from N enrichment, even if there is insufficient data to calculate net climate forcing directly. Our study highlights the heterogeneous nature of grassland GHG responses and provides critical insights for developing sustainable N management strategies to mitigate climate change.

Croplands account for ~ one‐third of global anthropogenic nitrous oxide (N₂O) emissions. A number of recent field experiments found substantial fallow‐period N₂O emissions, which have been neglected for decades. However, the global contribution of the fallow‐period emissions and the associated drivers remain unclear. Based on 360 observations across global agroecosystems, we simulated the ratio of the fallow to the whole‐year N₂O emissions (R_fallow) by developing a mixed‐effect model and compiling cropping‐system‐specific input data. Our results revealed that the mean global gridded R_fallow was 44% (15%–75%, 95% confidence interval), with hotspots mainly in the northern high latitudes. For most cropping systems, soil pH was the dominant driver of global variation in R_fallow. Global cropland emission factors (i.e., the percentage of fertilizer N emitted as N₂O, EFs) in EF‐based models doubled to 1.9% when the fallow‐period N₂O emissions were included in our simulation, similar to EFs estimated by process‐based and atmospheric inversion models (1.8%–2.3%). Overall, our study highlights the importance of fallow‐period N₂O emissions in annual totals, especially for single cropping systems and croplands in acidic areas. To accurately estimate N₂O emissions for national greenhouse gas inventories, it is crucial to update current EFs with full consideration of the fallow‐period N₂O emissions in the Intergovernmental Panel on Climate Change (IPCC) Tier 1 method.

Crop types have significant impacts (p < 0.05) on the background nitrous oxide emissions (BNE) from soils. The annual BNE for China when considering the proportion of planting area is 0.69 ± 1.52 kg N–N₂O ha⁻¹.