Contrasting Effects of Nitrogen Input on Soil Carbon Pools in Arid and Humid Climates

Nitrogen (N) input affects soil organic carbon (SOC) dynamics and storage by specifically influencing the light (LFOC) and heavy fractions of organic C (HFOC). While the effects of N input on SOC pools are known, its specific influence in arid and humid regions remains unresolved, although it is critically important to predict carbon-climate feedback and SOC accumulation. We conducted a meta-analysis of 881 paired observations from 60 studies to quantify the climate-specific responses of SOC, LFOC, and HFOC contents to N input, thereby revealing distinct effects in arid versus humid regions. N input increased the LFOC content by 25% in arid regions, whereas the HFOC and SOC contents remained unchanged. This LFOC accumulation may reflect increased plant C inputs, suppressed microbial decomposition, and strong physical protection compared with soil without N input. Conversely, in humid regions, N input increased the HFOC and SOC contents by 10%. These increases were associated with the following four mechanisms: (1) increases in plant and microbial necromass production after N input, (2) microbially-mediated formation of the HFOC from the LFOC, (3) accelerated lignin-derived C accumulation due to suppressed saprotrophic fungal decomposition, and (4) raised physico-chemical stabilization by various biotic and abiotic mechanisms. Specifically, increases in LFOC content in arid regions occurred under medium to high N input rates (≥ 50 kg N ha^-1 year^-1), whereas increases in HFOC and SOC contents in humid regions occurred only under medium N input (50-100 kg N ha-1 year^-1). These increases occurred primarily after periods of N input longer than 5 years and mainly in topsoil (< 30 cm). Overall, these climate-specific effects demonstrated that the moisture regime governs N-induced organic matter transformations, with aridity raising light fraction organic matter accumulation and humidity increasing stabilization of heavy fraction.