Straw type and returning amount affects SOC fractions and Fe/Al oxides in a rice-wheat rotation system

Straw returning is a vital agronomic practice for soil organic carbon (SOC) sequestration and stabilization by changing the storage and fraction composition. Iron and aluminum (Fe/Al) oxides have a protective effect on SOC through adsorption and co-precipitation, which depends on the fraction composition of SOC itself, especially originated from externally added organic materials. However, the changes and interactions of SOC components and Fe/Al oxides in straw returning soil are not clearly understood. To evaluate the effect of straw returning on SOC composition and Fe/Al oxides, we have conducted an in situ field experiment for 10 yrs., which including five treatments: no rice and wheat straw returning (NRW, as control), total amount wheat straw returning (W), total amount rice straw returning (R), half amount rice and wheat straw returning (HRW), and total amount rice and wheat straw returning (TRW). The results showed that SOC and heavy fraction organic carbon (HFOC) increased under all five treatments, especially for R, HRW, and TRW treatments (P < 0.05). Compared with wheat straw returning, rice straw returning was more conducive to increasing the contents of SOC and its fractions, free Fe/Al oxides, complex aluminum oxides and the carbon utilization efficiency. Collectively, these properties were strongly affected by both the type and amount of straw returning. Soil organic carbon was significantly correlated with HFOC, water-soluble organic carbon, alkali-hydrolyzed nitrogen, available phos-phorous, available potassium, and total nitrogen (P < 0.05). Of these, HFOC had the most significant effect on SOC and was vitally important for improving SOC sequestration and stability (P < 0.001). The increase of labile carbon fractions was more conducive to the absorption and utilization of nutrients by crops. Straw returning promoted the interaction between increasing organic matter and complex iron oxide (Fec). This served to strengthen the protective effect of SOC, which increased the content of Fec and stability of SOC, resulting in decreased soil pH. Taken together, these results highlight the importance of straw returning in promoting the storage and stability of SOC pool. Critically, rice straw returning can reduce soil pH and Fec content, therefore could be considered as an ideal model for a rice-wheat rotation system.