Biophysical and temporal drivers outweigh management in tropical agroforestry soil carbon sequestration

Abstract

Agroforestry is a cornerstone of Natural Climate Solutions, yet the hierarchical importance of its soil organic carbon (SOC) drivers remains poorly resolved across heterogeneous tropical landscapes. Current global assessments predominantly rely on categorical system typologies that mask the continuous influence of biophysical drivers, leaving the reliability of mitigation estimates unclear. Here, we synthesize 643 observations from 54 field studies in Latin America and the Caribbean to decouple the determinants of SOC sequestration using a machine-learning framework. We show that baseline soil carbon stocks and temporal kinetics override management design, collectively explaining [~]85% of sequestration variability, whereas system typology and species richness contribute marginally (R2<0.10). While the median SOC storage rate was 0.26 Mg C ha{superscript 1} yr{superscript 1}, accumulation followed a distinct non-linear trajectory: sequestration intensity peaked early before decelerating sharply after a critical inflection at year 7. Critically, sequestration is governed by a robust negative feedback from initial SOC stocks, which cross a zero-net-gain threshold at [~]80 Mg C ha{superscript 1}. Depth-resolved analyses reveal that subsoil layers (up to 55-75 cm) exhibit a cumulative relative response up to fourfold greater than surface horizons, indicating that conventional shallow monitoring could systematically underestimates long-term stabilization potential. Our findings demonstrate that current carbon accounting frameworks, rooted in generic system averages (IPCC Tier 1), are structurally limited by their inability to account for baseline-dependent saturation feedbacks and non-linear effects. Transitioning toward Tier 3 context-aware, depth-explicit modeling is therefore essential to transform agroforestry from a broad practice into a precision-based, high-integrity Natural Climate Solution. HighlightsO_LISoil carbon sequestration in tropical agroforestry is primarily controlled by baseline soil conditions and temporal dynamics rather than system typology. C_LIO_LIDepth-resolved analyses reveal long-term carbon stabilization processes overlooked by surface-based assessments. C_LIO_LICarbon accumulation is strongly front-loaded, declining sharply after system establishment. C_LIO_LIContext-dependent responses challenge generic carbon accounting frameworks and highlight the need for predictive, site-specific deployment of agroforestry. C_LI