Formation of mineral-associated organic matter in soils: role of land use and mineral type

Mineral-associated organic matter (MAOM) represents the largest and most stable fraction of soil organic carbon (OC), making its formation and stabilization critical for climate change mitigation. While mineral type, land use, and management intensity are known controls of these processes, their relative importance across large spatial scales and on subdecadal timescales remains unresolved. To address this research gap, I leveraged a large-scale field experiment where permeable containers filled with pristine minerals—goethite (iron oxide) or illite (phyllosilicate clay)—were exposed at different soil depths in differently managed forest and grassland sites across the three study regions of Germany’s Biodiversity Exploratories. Mineral type emerged as the principal control of MAOM formation, with goethite accumulating about four times more OC than illite. Land use further modified MAOM formation, with OC accumulation being higher under coniferous forests than under deciduous forests and grasslands. Formation of MAOM also responded to management intensity, further demonstrating its sensitivity to environmental change on subdecadal timescales. I also observed consistently lower decomposition of goethite- than illite-associated MAOM, indicating that goethite’s higher OC retention was not exclusively due to its greater sorption capacity. Across both minerals, decomposition of MAOM was always lower for forests than grasslands. Overall, 34.6% and 23.2% of the variance in MAOM decomposition was explained by mineral type and land use. Results further show that while MAOM formation declined consistently with depth, changes in its decomposition were mineral-specific. Specifically, MAOM decomposition declined consistently with depth for illite but not for goethite. Overall, these findings show that while mineral type defines the potential for MAOM formation and stabilization, land use and management practices determine the extent to which this potential is realized.