Integrating soil carbon and nitrogen dynamics into a tipping point framework: Weak land-use signals, strong climate forcing in southwestern Amazonia

Deforestation and intensifying extreme climatic events are reshaping ecosystem functioning globally and increasing the risk of crossing tipping points in social-ecological systems. To understand the sensitivity of key soil properties to these stressors, we assessed soil organic carbon (SOC) and total nitrogen (TN) along a west-to-east precipitation gradient in southwestern Amazonia (1800–3980 mm; mean annual precipitation: MAP). Our study included 23 pastures (2 to 50+ years since conversion), 26 sondary forests (2 to 34 years of regrowth), and 43 old-growth forests. We differentiated topsoil (0–40 cm) from subsoil (40–100 cm) to disentangle surface-driven responses from deeper processes. According to linear mixed-effects models MAP is the primary driver of topsoil SOC and TN, with clay content and bulk density as secondary factors, jointly explaining 67.2% of SOC and 73.2% of TN, yet land-use history had minimal effects. Given topsoil sensitivity to precipitation regimes, self-intensifying feedback loops linking regional-scale deforestation to reduced rainfall could deplete SOC stocks, potentially triggering tipping cascades. In subsoils, soil water content and texture explained 81.3% of TN variance, while these factors and pH explained 69.2% of SOC variance; both remained largely independent of MAP and land-use history. The apparent steadiness of bulk SOC and TN stocks to decades of land-use change masks underlying shifts in soil organic matter functionality and microbial communities, limiting their utility as early indicators of ecosystem degradation. Our findings underscore that assessments of ecosystem vulnerability must integrate climatic drivers and soil biogeochemistry to better predict and prevent critical transitions in tropical social-ecological systems.