Nutrient stress reduces the extent but increases the intensity of root-driven destabilization of mineral-associated organic matter

Abstract Mineral-associated organic matter (MAOM) constitutes a vast terrestrial carbon reservoir, yet its vulnerability to plant root activity remains poorly constrained. Here, we show that MAOM destabilization by plant roots is governed by two distinct factors: the overall extent of destabilization, which scales with cumulative root growth, and the intensity of destabilization, which reflects the efficiency of individual roots. Using stable isotope tracing combined with metabolomics, we quantified MAOM destabilization in the rhizosphere of common oat ( Avena sativa ) across gradients of nitrogen and water availability. The overall extent of MAOM destabilization increased linearly with root biomass across all treatments. However, nitrogen-limited plants exhibited two-fold greater MAOM destabilization per unit root biomass—here termed MAOM destabilization intensity—than nitrogen-replete plants, coinciding with shifts in rhizodeposition indicative of enhanced nitrogen mining. Across treatments, this destabilization intensity increased nonlinearly as root biomass declined, highlighting a disproportionate increase in per-root destabilization efficiency at low root biomass. These findings demonstrate that environmental stressors cause opposing responses in the extent and intensity of root-driven MAOM destabilization, such that reduced root growth can coincide with increased per-root destabilization efficiency. This contrast reveals a dynamic plant–soil feedback with important implications for predicting soil carbon persistence under future environmental change.