Overestimated nitrogen fertilization effects on global terrestrial carbon sinks due to neglect of long-term soil acidification

Nitrogen (N) addition from anthropogenic atmospheric deposition and fertilizer application is widely recognized to enhance terrestrial carbon (C) storage by alleviating ecosystem nutrient limitation. However, long-term N addition can also acidify soils and impair ecosystem functioning, an effect that is often overlooked in global assessments of terrestrial C cycling. Here, we performed a global meta-analysis to systematically quantify both the impacts of long-term N addition on soil pH and the responses of vegetation root growth and soil microbial respiration to soil pH changes. This data-driven understanding was then used to develop a parameterization for soil acidification and its impacts on vegetation and soil microbe within the C-N-coupled terrestrial biosphere model QUINCY. Model simulations show that present-day global N fertilization effects on terrestrial net ecosystem productivity (NEP) are around 240 Tg C yr^-1, which are approximately 20% lower than the estimate when neglecting the long-term N-induced soil acidification. In the meanwhile, inclusion of soil pH effects increases the simulated soil carbon storage, consistent with patterns emerging from the meta-analysis. By explicitly incorporating soil acidification into terrestrial C–N interactions, our results reveal critical gaps in current representations of long-term ecosystem responses to N enrichment, with important implications for future sustainable N management and climate change mitigation.