Biological sources and molecular composition of iron oxides bound organic carbon in agricultural soils

A strong link exists between iron oxides and soil organic carbon (SOC). However, the role of iron oxides in the preservation of SOC in agricultural soil remains poorly understood. In this study we comprehensively examined the concentration, molecular composition and biological sources of iron oxide-bound organic carbon (Fe-bound OC) in arable soils collected from 12 sites in central and east China. The effect of elevated temperatures on Fe-bound OC in two contrasting soils was also investigated. The results indicated that 6.2 ~ 31.2% of the SOC was bound to iron oxides in agricultural soil, and that the binding mechanisms varied from adsorption in most soils to coprecipitation in those with a large content of organic carbon. The distribution of Fe-bound OC showed no clear variation in relation to site, but Fe-bound OC reached a peak in soils with an annual mean temperature of 16.4°C. Correlation analysis demonstrated that TOC might be the main determinant for the amount of Fe-bound OC, and that the binding mechanism is influenced by both TOC and the active Fe ratio. Analysis of C/N, ¹³C isotope, and synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy showed that iron oxides selectively protected plant-derived aliphatic compounds and polysaccharides in agricultural soil. Warming decreased the content of Fe-bound OC from 3.46 g kg⁻¹ to 1.99 g kg⁻¹ in Ultisol, while enhanced that from 4.04 g kg⁻¹ to 5.12 g kg⁻¹ in Histosol. NMR results suggested that warming could alter the composition of soil organic matter by accelerating O-alkyl C degradation and increasing the sequestration of recalcitrant alkyl C and carboxyl C. It is supposed that warming promoted the association of iron oxides with microbial-derived polysaccharides and aliphatic compounds. This study revealed the quantitative characterization, biological sources and molecular composition of Fe-bound OC in arable soils, which provides useful information for evaluating and managing the global C cycle under the framework of climate change.