Abstract:

Enhancing soil organic carbon (SOC) sequestration capacity is a crucial nature−based solution for mitigating climate change. To elucidate the mechanisms underlying depth−resolved SOC responses to elevated atmospheric nitrogen (N) deposition and altered precipitation regimes, this study utilized a full−factorial experiment combining N addition (10g·m^–2·y^–1) and precipitation manipulation(±50% ambient precipitation) initiated in 2017 in a northeastern Qinghai−Tibet plateau alpine meadow. The dynamics of 0–40cm SOC, soil nutrients and plant biomass from 2022 to 2024 were quantified to reveal SOC responses to changes in nitrogen deposition and precipitation regimes. The results showed that aboveground biomass (AGB) increased significantly by approximately 30% under the N addition treatment compared to the control (CK, no N addition or precipitation change). Belowground biomass (BGB) showed depth−dependent variations and was more sensitive to precipitation change, where 30–40cm BGB was reduced by 35% under the decreased precipitation treatment. N addition decreased surface soil (0–10cm) pH by 7% but increased SOC and soil total nitrogen (STN) by 7%–9% in the same layer. Conversely, subsoil (30–40cm) SOC and STN declined by 2%–4%. N addition and precipitation change indirectly regulated 10–40cm STN and SOC through the soil pH pathway, rather than via changes in vegetation biomass. Specifically, precipitation change showed positive effects on 10–40cm SOC with a β of 0.08. These findings demonstrate a response of SOC with a "surface accumulation and subsurface depletion" pattern, which alters the vertical distribution of SOC and indicates a decoupling from plant−derived carbon inputs. This study provides valuable insights for assessing and projecting the soil carbon sink function of alpine meadows under future climate change scenarios. 

Key words: Alpine meadow, Vegetation biomass, Soil organic carbon, Soil total nitrogen, Soil pH