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.