Coupled Biogeochemical Cycles of Iron and Organic Carbon: Investigation on Synthesized Complexes
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Soil organic carbon (SOC) is one of the largest carbon (C) reservoirs on the earth surface, containing about twice the amount of C in the atmosphere. Understanding SOC residence time is crucial for accurately modeling global C dynamics. Widely occurring iron (Fe) oxide minerals can strongly bind with SOC and affect its stability. Under aerobic condition, Fe-SOC association can potentially provide physicochemical protection to SOC and inhibit its degradation. However, transient or permanent anaerobic condition in soils promotes Fe redox reactions, which may compromise the stability of Fe-bound SOC. Similarly, the presence of cations is assumed to be important for the association between SOC and minerals, but there is limited direct analysis for the formation and reactivity of Fe oxide mineral-SOC-cation ternary complexes. Therefore, understanding the fate of Fe-bound SOC during anaerobic/aerobic incubation and redox reactions of Fe in binary and ternary systems is important for predicting SOC residence time in soil. In this study, we investigated the fate and transformation of SOC during the redox reactions of Fe oxide, by synthesizing and characterizing Fe oxide-SOC or Fe oxide-SOC-Ca ternary complexes as well as incubating complexes under anaerobic/aerobic conditions. During the microbial reduction of ferrihydrite (Fh)-SOC or Fh-SOC-Ca co-precipitates by Shewanella putrefaciens strain CN32, higher C/Fe ratios in the co-precipitates facilitated Fe reduction and subsequent reductive release of Fe-bound SOC, when the presence of SOC and Ca regulated the Fh mineral phase transformation during reduction. Phenolic SOC was completely removed while other aromatic SOC and carboxylic SOC was preferentially retained in the complex during the reduction. During the aerobic incubation with soil and indigenous microbes, the bioavailability of Fh-bound SOC was 55-86% lower than free SOC. Association with Fh also decreased the priming effect of SOC. Our results highlight that the fate and transformation of mineral-bound SOC under anaerobic and aerobic conditions are regulated by C/Fe ratio, mineral type, and cation content, with broad implications on the stability of SOC. The information will be valuable in the development of process-based models for C cycles and prediction of its response to climate change.