Linking the chemical nature of soil organic carbon and biological binding agent in aggregates to soil aggregate stability following biochar amendment in a rice paddy

Changes in soil aggregation with biochar amendment have been investigated extensively, but how biochar affects the chemical composition of organic carbon (C) and biological binding agents in aggregates and their linkage with soil aggregate stability remains unclear. Soil samples were collected in a rice paddy treated with 0 (C0, control), 10 t ha−1 (C10), 20 t ha−1 (C20) and 40 t ha−1 (C40) biochar for twenty months. The amount and chemical composition of soil organic C (SOC), microbial abundances and glomalin-related soil protein (GRSP) were determined in bulk soil and four fractions: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm), and silt + clay (<53 μm). Our results showed that the proportion of >250 μm water-stable aggregates and mean weight diameter were gradually increased with increasing biochar addition rate. The concentrations of SOC, readily oxidizable C and microbial biomass C increased most in the small macroaggregates, followed by microaggregates under biochar amendment. Increasing biochar addition rate gradually decreased the relative contents of alkyl C, O-alkyl C and carbonyl C, but increased those of aromatic C across the aggregates, resulting in a higher aromaticity and hydrophobicity of SOC with respect to the control. The abundances of bacteria, fungi and archaea and the content of GRSP were significantly enhanced in the large and small macroaggregates under the C40 treatment. The proportion of >250 μm aggregates was significantly correlated with the contents of soil organic C fractions, GRSP and microbial abundance. Structural equation modeling further revealed that changes in SOC hydrophobicity and GRSP content under biochar amendment had significant and direct effects on the soil aggregate size distribution. In summary, our findings suggest that biochar amendment in rice paddy could improve soil aggregation through altering the chemical composition of soil organic C and the abundance of biological binding agents.