Asymmetric Response of Grassland Greenhouse Gases to Nitrogen Addition: A Global Meta-Analysis

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.