Effects of pH thresholds on carbon, nitrogen, and phosphorus dynamics in Chinese terrestrial ecosystems: Differentiation of drivers in vegetation-soil-microorganisms.

Carbon, nitrogen, and phosphorus (CNP) are the essential elements of terrestrial ecosystems, and their cycling processes directly determine ecological productivity, C sequestration capacity, and nutrient use efficiency. Soil pH regulates microbial communities and activity, enzyme function and morphology, plant growth and productivity conversion, and is a key regulatory factor in the CNP cycle. Although the pathway through which pH affects the CNP cycle via microorganisms, vegetation interactions has been revealed, explicitly contrast known linear mechanisms vs. unknown nonlinear dynamics. Based on integrated data on CNP concentrations in vegetation, soil, and microorganisms from 1700 sampling sites across China, combined with environmental variables, this study aims to (1) identify pH thresholds for CNP in terrestrial ecosystems, and (2) determine the key drivers of CNP variations under different pH levels. Nonlinear ecosystem responses to aridity were identified via model selection; thresholds were quantified using segmented regressions. Threshold significance was confirmed by statistical tests of slope/intercept discontinuities (p < 0.05) and spatial robustness checks. We found the pH thresholds for CNP in vegetation, soil, and microorganisms were ~7.0, ~5.5, and ~6.5, respectively. When pH < 5.5, the main factor in regulating CNP in soil was leaf area index. When pH > 5.5, soil texture (sand) was the main factor in regulating the stability and decomposition of soil CNP. When pH < 6.5, the main factor in regulating microbial CNP was mean annual precipitation. When pH > 6.5, the main factor in regulating microbial CNP was soil moisture. When pH < 7.0, the main factor in regulating CNP in vegetation was pH; when pH > 7.0, the main factor in regulating CNP in vegetation was normalized difference vegetation index. Our study provided key pH threshold indicators for soil acidification and saline-alkali land management across different regions, offering a critical scientific basis for developing region-specific ecosystem management strategies.