Reconciling multiple impacts of nitrogen enrichment on soil carbon: plant, microbial and geochemical controls

Abstract

Impacts of reactive nitrogen (N) inputs on ecosystem carbon (C) dynamics are highly variable, and the underlying mechanisms remain unclear. Here, we proposed a new conceptual framework that integrates plant, microbial and geochemical mechanisms to reconcile diverse and contrasting impacts of N on soil C. This framework was tested using long‐term N enrichment and acid addition experiments in a Mongolian steppe grassland. Distinct mechanisms could explain effects of N on particulate and mineral‐associated soil C pools, potentially explaining discrepancies among previous N addition studies. While plant production predominated particulate C changes, N‐induced soil acidification strongly affected mineral‐associated C through decreased microbial growth and pH‐sensitive associations between iron and aluminium minerals and C. Our findings suggest that effects of N‐induced acidification on microbial respiration and geochemical properties should be included in Earth system models that predict ecosystem C budgets under future N deposition/input scenarios.

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