Aridity regulates the vital drivers of soil organic carbon content in the Northeast China

Abstract

The increasing prevalence of aridity events profoundly influences the terrestrial soil organic carbon (SOC) dynamics. Varying aridity conditions may induce differential effects of changing environments on SOC content. Current knowledge regarding how aridity intervenes with the key drivers of SOC remains limited, especially in complex ecosystems along aridity gradients. Here, we utilized nearly 1,000 soil samples (0–20 cm and 20–100 cm) from the Second National Soil Survey in 1980 s as the foundation. Employing machine learning methods, generalized additive model (GAM), and structural equation model (SEM), we estimated the spatial patterns of SOC in the Northeastern and elucidated the intervention of aridity on the key drivers of SOC. Our findings revealed that SOC content gradually decreased along the aridity gradient from the northeast to the southwest in both soil layers. Aridity and exchangeable Ca²⁺ were identified as the key factors controlling this pattern. Results from nonlinear analysis demonstrated a non-stationary response of SOC content to aridity. As the intensity of aridity increased, SOC content shifted from a slow decline to a rapid decrease, with numerical thresholds identified in the value space (0–20 cm, 0.48; 20–100 cm, 0.45). The range of aridity thresholds in the numerical space delineated the type boundaries of phaeozems and luvisols in geographical space. Results from SEM indicated that aridity and its critical thresholds significantly coordinated the key driving forces of SOC content. In the aridity environment beyond the threshold, with the contraction of other drivers, the controlling role of exchangeable Ca²⁺ was further amplified and consistently mediated SOC content. We emphasized the contribution of aridity to understanding of SOC driving patterns, with other drivers exhibiting different behaviors in their contributions to SOC, accompanied by differences (even thresholds). This provides an important prerequisite for further understanding the response of soil carbon pool to ecosystem environmental changes.