Biochar promoted soil organic carbon accumulation and aggregate stability by increasing the content of organic complex metal oxides in paddy soil

The physical protection of soil organic carbon (SOC) by soil aggregates is one of the important mechanisms on SOC accumulation. Mineral-organic complexes can immobilize organic carbon. It is also a key factor affecting aggregate stability. Biochar can not only promote the accumulation of SOC, but also improve the stability of aggregates. Under frequent fluctuations of soil redox potential in paddy soil, the precipitation-dissolution equilibrium of reactive metal oxides demonstrates heightened sensitivity to biochar addition. These metal oxides play a critical role in stabilizing organic carbon through their regulatory effects. So far, the effect of biochar on the stability of SOC in soil aggregates and mineral-organic complexes in paddy soils has rarely been reported. A field experiment using gradient application rates of biochar was conducted to verify whether mineral-organic complexes play a key role in aggregate stability under biochar application. The results showed that biochar application improved the contents of SOC and total nitrogen. The contents of the soil complexed iron (Fep) and aluminum (Alp), exchangeable manganese (Mnexc) and organic complex manganese (Mnorg) oxides increased with the increase of the biochar application rate. Biochar application resulted in the increased proportion of large macroaggregates and macroaggregates, while the proportion of microaggregates and silt-clay aggregates was decreased. At the same time, biochar application resulted in higher SOC content in four aggregate size fractions with the lower effect in the smaller size fractions compared to the larger size fractions. Structural equation model showed that application of biochar enhanced SOC accumulation by increasing the contents of organic matter and forming Fep, Alp and Mnorg in macroaggregate. The result of gradient application rates of biochar showed that the treatment of 45 t·hm−2 was the best. In summary, the application of biochar promoted soil organic carbon accumulation and aggregate stability by increasing the content of organic complex metal oxides in paddy soil. This finding could provide practical implications for carbon sequestration, soil health and sustainable agriculture in paddy fields.