Microbially-Mediated Soil Carbon-Nitrogen Dynamics in Response to Future Soil Moisture Change

The interactions between soil carbon and nitrogen (C-N) processes with environmental factors, particularly soil moisture, are critical to maintaining soil ecosystem functions. However, the lagged effects of future change in soil moisture on soil C-N dynamics remain poorly understood. Here, we employed the Microbial-ENzyme Decomposition model to simulate the long-term impacts of future soil moisture variation on soil C-N dynamics using the standardized soil moisture index (SSI) across four Shared Socioeconomic Pathways (SSPs). Our results demonstrated that soil C-N dynamics exhibited both lagged and cumulative responses to moisture fluctuations over extended periods. Active microbes were closely associated with short-term (3-month) change in soil moisture, whereas soil organic C (SOC) and total N (TN) exhibited stronger correlations over extended periods (72 months). Under the SSP5-8.5 scenario, SOC and TN decreased in wet conditions but increased during droughts, with increases of 28.9% and 13.1%, respectively, under extreme drought conditions. We found that the active microbial biomass was significantly more sensitive to soil moisture variation than total microbial biomass, especially under extreme drought conditions. Furthermore, microbes and enzymes were key drivers of soil C-N transformations, with soil enzymes displaying the highest correlation with SSI (nonlinear correlation coefficient based on mutual information = 0.81). This study establishes a foundational relationship between soil C-N variables and soil moisture, accounting for lag effects, to enhance our understanding of the complex responses of these variables under future climate change scenarios.