Seasonal Flooding Disrupts Expected Depth‐Dependent Patterns of Mineral Associated Carbon, Nitrogen, and Phosphorus Across Land Uses

Abstract

Approximately 25%–70% of soil organic matter (OM) is stored below 30 cm, rendering subsoil OM cycling an important control on OM persistence. With climate change, soils are experiencing new pedoclimatic disturbances like seasonal flooding that can destabilize mineral associated organic matter (MAOM) and undermine carbon (C), nitrogen (N), and phosphorus (P) persistence. Our current understanding of how OM cycles down the soil profile is largely based on well‐drained soils. Thus, it remains uncertain if expected depth‐dependent trends and subsoil OM persistence differ in soils subjected to seasonal flooding across land uses. We collected 1‐m‐deep soil cores within a seasonal floodplain, representing a land‐use gradient: forest, pasture, and cropland. We compared MAOM concentrations (C, N, and P), stoichiometry, isotopic, and molecular chemical composition, in combination with microbial biomass and carbon use efficiency at four soil depths to understand the transformation and persistence of OM with depth. We found that the composition of OM by depth in our soils diverges from common trends in well‐drained soils. For example, in well‐drained soils, we expect a decrease in mineral associated C:N, and an increase in polar compounds with depth, as these compounds are thought to be more persistent. Instead, we found an increase in MAOM C:N in the forest and cropland, and an increase in non‐polar C rich compounds in all land uses with depth. The pasture varied from the forest and cropland, with OM composition largely unchanged with depth. Our results suggest that the extensive root system in the pasture causes desorption of older compounds resulting in younger, plant derived compounds at all depths. We propose that the controls on persistence differ in a seasonal floodplain, where MAOM persistence is shaped more by compound resistance to hydrological stress than by formation pathways.