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

Authors: FRANKS, MATTHEW - Bowling Green State University; DUNCAN, EMILY - Los Angeles Regional Water Control Board; King, Kevin; VASQUEZ-ORTEGA, ANGELICA - Bowling Green State University

Submitted to: Chemical Geology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2020
Publication Date: 12/20/2020
Citation: Franks, M., Duncan, E., King, K.W., Vasquez-Ortega, A. 2020. Role of Fe- and Mn-(oxy)hydroxides on carbon and nutrient dynamics in agricultural soils: A chemical sequential extraction approach. Chemical Geology. 561. Article 120035. https://doi.org/10.1016/j.chemgeo.2020.120035.
DOI: https://doi.org/10.1016/j.chemgeo.2020.120035

Interpretive Summary: Soil organic carbon and soil health continue to be promoted as a means to address water quality concerns. Understanding the different mineral phases responsible for organic carbon as well as nitrogen and phosphorus stabilization and sequestration is important in interpreting findings from different conservation practice studies. The findings from this study showed that iron and manganese oxy(hydroxide) minerals were primarily responsible for stabilizing soil organic carbon and adsorbing phosphorus. Understanding the relationship between soil chemical properties and nutrient adsorption will provide important information to researchers, policy makers, and practitioners concerned with promoting conservation practices that are aimed at addressing water quality concerns but that may affect soil chemistry.

Technical 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, e.g., birnessite (Step 2), reductive dissolution of amorphous Fe-(oxy)hydroxide, e.g., ferrihydrite (Step 3), and reductive dissolution of crystalline Fe-(oxy)hydroxide, e.g., goethite (Step 4). 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, such as drainage water management systems, in order to fully understand their effect on soil chemistry and health.