Straw management influences the stabilization of organic carbon by Fe (oxyhydr)oxides in soil aggregates

The association of soil organic carbon (SOC) with Fe (oxyhydr)oxides is a major SOC stabilization process. However, the effects of straw incorporation on the role of Fe (oxyhydr)oxides in stabilization of SOC composition in aggregates have not been fully explored. The objective of this study was to explore the effect of straw incorporation on the relationship between iron oxides and SOC in soil aggregates in the rice-rape cropping system. In this study, SOC concentration, soil iron fractions and SOC composition in soil aggregates were analyzed. Soil samples were collected under two treatments, conventional tillage without straw incorporation (T) and conventional tillage with straw incorporation (TS), from surface 0-20 cm layer in 10-year and 6-year experiments in Wuxue City (WX) and Jingzhou City (JZ) in China. Compared with the T treatment, the TS treatment increased the proportion of > 2 mm aggregates and decreased the proportion of aggregates of other sizes. Except for the 0.25-2 mm aggregates at WX, TS treatment increased the SOC concentration in the aggregates by 36-39% and 20-40% at WX and JZ, respectively, compared with the T treatment. Straw incorporation increased the abundance of alkyl C and the alkyl C/O-alkyl C ratio (an SOC decomposition index), but it slightly decreased the abundance of O-alkyl C at WX, which was opposite to the trend at JZ. Meanwhile, TS treatment increased the abundance of aromatic C in the silt + clay fraction (< 0.053 mm) at WX and in aggregates ( > 0.25 and 0.053-0.25 mm) at JZ. Additionally, straw incorporation decreased the aliphatic C/aromatic C ratio (an SOC complexity index) in the < 0.25 mm fraction at WX and in the > 0.25 mm fraction at JZ. Furthermore, straw incorporation promoted the accumulation of poorly crystalline (Feo) and organically complexed (Fep) Fe (oxyhydr)oxides in aggregates. Feo was also significantly and positively correlated with the proportion of aromatic C (r = 0.617, P < 0.05) and carbonyl C (r = 0.801, P < 0.05) in the organic matter at WX experimental site. In conclusion, straw incorporation drives the formation of SOC-Fe (oxyhydr)oxides association in aggregates, thus contributing to soil organic carbon stabilization in paddy soils.