Associations of soil Fe oxides and organic carbon vary in different aggregate fractions under warming

Purpose Soil warming is predicted to increase the degradation of soil organic carbon. Aggregates of different sizes vary in their ability to retain or transfer iron oxide-bound organic carbon (Fe-bound OC) in soils. However, direct observation of iron oxides and OC distribution in soil aggregates and determination of their interactions in response to climate warming remain challenging. Methods To determine the response of soil aggregates to warming, a 180-day incubation experiment was conducted with Ultisol mixed with rice straw at different temperatures (4 °C, 25 °C, and 45 °C). Changes in concentrations of total organic carbon (TOC) and Fe-bound OC and distributions of OC species and Fe oxides were determined. Results Warming significantly increased the content of TOC in silt + clay and micro-aggregates but decreased it in macro-aggregates. Increasing temperatures decreased the contents of Fe-bound OC from 3.22 to 1.46 g kg −1 in silt + clay and from 5.20 to 1.83 g kg −1 in macro-aggregates. However, Fe-bound OC concentration decreased from 2.54 to 0.85 g kg −1 and then increased to 1.99 g kg −1 in micro-aggregates with increasing temperature. Nuclear magnetic resonance results suggested that increasing temperatures increased the proportions of aromatic C and carboxyl C in silt + clay and macro-aggregates but decreased those proportions in micro-aggregates. Synchrotron radiation-based Fourier-transform infrared spectroscopy indicated that warming promoted associations between Fe oxides and OC in silt + clay but decreased associations in macro-aggregates. Conclusions Our study indicates that soil warming increased the sensitivity of OC decomposition to a greater extent in micro-aggregates than in silt + clay and macro-aggregate fractions. Thus, although organo-Fe interactions are essential for OC stability, carbon forms associated with Fe oxide phases are not entirely resistant to degradation, especially for associations in different aggregate sizes. Determination of how warming affects Fe-bound OC distributions in size-dependent soil aggregates will improve understanding of the mechanisms that influence SOC stability and sequestration. Graphical abstract