Resilience of soil organic carbon under precipitation variability: Insights from carbon-nitrogen dynamics in semi-arid grasslands

Abstract

Soil organic carbon (SOC) stability and sequestration are crucial for ecosystem resilience, especially under shifting global precipitation patterns that impact the carbon (C) and nitrogen (N) cycles in semi-arid grasslands. This research primarily addresses the gap in understanding the dynamic responses of SOC to varying precipitation regimes and their effects on C and N balance. We employed a long-term simulated rainfall experiment across 21 plots with precipitation levels adjusted by −60 %, −40 %, −20 %, + 20 %, + 40 %, + 60 %, and a control group with natural rainfall, collecting soil samples in the fifth and seventh years to investigate the stability and resilience of SOC. Through this experiment, we assessed soil samples at multiple temporal scales, focusing on the soil C/N ratio, particulate organic matter (POM), and mineral-associated organic matter (MAOM). Our findings reveal that in 2019 and 2021, equivalent increases or decreases in precipitation reduced SOC content by 11 %. Extreme rainfall (±60 %) in 2021 decreased NH₄⁺ content by 40 % compared to control. A 40 % precipitation reduction in 2019 decreased SOC and MAOC by 29 % and 25 %, respectively, while a 20 % reduction in 2021 reduced POC by 24 % but increased SOC and MAOC by 18 % and 13 %, revealing amplitude- and duration-dependent nonlinear responses of soil C forms to precipitation changes. Linear regression confirmed that SOC significantly increased with rising C/N ratios of particulate organic matter (C/N_POM) and mineral-associated organic matter (C/N_MAOM) under drought, indicating dual water-N limitation promotes high-C/N POM accumulation. Increased precipitation enhanced microbial biomass nitrogen (MBN) and MAOC sequestration but reduced SOC resilience via pH-driven mineral protection loss, whereas drought maintained C resilience through high-C/N POM despite MAOC suppression. This stability-resilience paradox highlights inherent trade-offs: MAOC accumulation under wet conditions strengthens long-term C storage at the expense of system recovery capacity, while drought sustains POC resilience through recalcitrant C/N ratios. Understanding these mechanisms is critical for predicting climate change impacts and guiding adaptive soil management.