Role of Fe- and Mn-(oxy)hydroxides on carbon and nutrient dynamics in agricultural soils: A chemical sequential extraction approach

Abstract

Soil organic carbon (SOC) is a key component of maintaining favorable soil physical, biological and chemical health and ensures the sustainability of agricultural practices. Fe- and Mn-(oxy)hydroxide minerals play an important role in SOC stabilization and sequestration, as well as nutrient adsorption. To better understand the mineral phases responsible for the stabilization and sequestration of SOC, as well as PO4−3 and NO3−; a four-step chemical sequential extraction (SE) was applied to soils from eight agricultural fields. Our SE scheme targeted operationally defined mineral phases namely, water extractable (Step 1), reductive dissolution of Mn-(oxy)hydroxide (Step 2), reductive dissolution of amorphous Fe-(oxy)hydroxide (Step 3), reductive dissolution of crystalline Fe-(oxy)hydroxide (Step 4) and residual minerals. Results showed that SOC was stabilized in the following order: crystalline Fe-(oxy)hydroxide > amorphous Fe-(oxy)hydroxide > Mn-(oxy)hydroxide. Fe- and Mn-(oxy)hydroxide minerals can promote the stabilization and long-term sequestration of SOC via the formation of inner sphere complexes (e.g., ligand-exchange) within the mineral surfaces contact zone. Fe- and Mn-(oxy)hydroxide minerals adsorbed PO4−3 species to a large extent; however, NO3− was adsorbed marginally. Results indicated that PO4−3 adsorption is largely mediated by Fe- and Mn-(oxy)hydroxide minerals, and NO3− by bulk soil organic matter. The coupling interaction between SOC, nutrients, and mineral phases in agricultural soils can better inform the application of conservation management practices to fully understand their effect on soil chemistry and health.