Elucidating the Shifting Controls on Organic Carbon Stabilization Across a Soil Moisture Gradient

Soil organic carbon (SOC) is stabilized principally through interactions with minerals, which provide protection from microbial enzymatic attack. Soil water content, a fundamental control on SOC cycling, dictates both plant C inputs and microbial C decomposition and affects mineral-organic stabilization mechanisms through its effect on mineral weathering, precipitation and dissolution. Soil water content may also influence organic matter composition. At the same time, soil water content is also affected by plot-scale topography and subsoil mineralogy.

To understand the effects of long-term soil moisture on SOC cycling we studied soils from upstate New York situated on a naturally occurring water content gradient, induced by plot topography and subsoil structure. Previously, it was shown that increasing long-term soil water content increased SOC accumulation and decreased mineralizibility (C-CO₂ per unit soil C). Mineralizibility negatively correlated with exchangeable Ca, Mg and pH. However, it was not clear whether Ca-mediated surface interactions or occlusion in microaggregates was more important and whether other mechanisms, such as association with Fe and Al, played a role in the more acidic, Ca-poorer soils.

To test which SOC stabilization mechanisms governed SOC stabilization we performed a sequential oxide extraction and determined the amount of C associated with each oxide phase. We also quantified C and N contents, and natural isotope abundances in shoots, roots, free and occluded particulate organic matter (fPOM and oPOM) and mineral heavy fraction. The vast majority of oxide-extracted C was in the form of Fe- and Al- organo-mineral complexes, and its contribution relative to total mineral-associated C increased with decreasing water content, indicating that the importance of polyvalent cations in SOC stabilization shifted from Al and Fe to Ca. Physical fractionation of SOC indicated an increasingly microbial nature with increasing soil water content. ¹³C NMR and NEXAFS analyses indicate that organic functionalities varied in composition and distribution under different moisture regimes, contributing differently to SOC stabilization. These results suggest that plot-scale topography and bedrock mineralogy influences SOC cycling through a combination of organic matter composition, microbial processing and mineralogy.