Redox-driven changes in organic C stabilization and Fe mineral transformations in temperate hydromorphic soils

Paddy soils experience long-term redox alternations affecting the interactions between the biogeochemical cycling of iron (Fe) and carbon (C). Differences in particle aggregation and soil organic matter (SOM) turnover are likely to both affect and be affected by the trajectory of Fe mineral evolution/crystallinity with redox fluctuations. We hypothesized that the legacy effects of redox cycling under paddy management affects particle aggregation, the distribution and mineralogy of Fe (hydr)oxides between particle-size fractions, and the interaction with SOM stabilization. Moreover, we expected underlying processes to be different in paddy eluvial and illuvial horizons, particularly due to the different inputs and redox conditions these horizons experience. To test these hypotheses, we evaluated the distribution of Fe species and organic C between different aggregate and particle-size fractions in topsoil (eluvial) and subsoil (illuvial) horizons of soils under long-term paddy and nonpaddy management in NW Italy, as well as mineralogical changes in Fe phases by Fe K-edge Extended X-ray Adsorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopy. Our findings indicate that although paddy topsoils are depleted in hydrous Fe oxides with respect to non-paddy soils, they can stabilize important amounts of C through mineral associations, particularly with finer particle-size fractions rich in less crystalline Fe phases. We also show that redox cycling can influence microaggregate stability and consequently the distribution of Fe phases and OC between intra and inter-micmaggregate fractions. On the other hand, illuvial horizons under paddy management were enriched in short-range ordered hydrous Fe oxides and this contributed to enhanced microaggregate formation and C stabilization with respect to non-paddy subsoils.

Automated summary

Paddy soils undergo long-term redox alternations affecting the interaction between iron (Fe) and carbon (C) biogeochemical cycling. Different particle aggregation and soil organic matter (SOM) turnover could be influenced by the trajectory of Fe mineral evolution/crystallinity with redox fluctuations. Our study found that paddy topsoils, although depleted in hydrous Fe oxides, can stabilize significant amounts of carbon through mineral associations, especially with finer particle-size fractions containing less crystalline Fe phases. Moreover, redox cycling can impact microaggregate stability and the distribution of Fe phases and organic carbon between intra and inter-microaggregate fractions. Illuvial horizons under paddy management were enriched in short-range ordered hydrous Fe oxides, leading to enhanced microaggregate formation and carbon stabilization compared to non-paddy subsoils.