Long-term effects of biochar amendment on soil aggregate stability and biological binding agents in brown earth

The use of biochar as soil amendment might improve the soil structure and carbon sequestration. However, few studies have focused on the effects of biochar doses on soil aggregates in brown earth. A six-year field experiment was conducted from May 2013 to October 2018. Four biochar doses were tested: 0, 15.75, 31.5, and 47.25 t ha−1 (control, BC1, BC2, and BC3, respectively). The objective of this study was to explore the effects of different biochar doses on soil biological binding agents (soil organic carbon (SOC), glomalin-related soil protein (GRSP), and microbial biomass carbon (MBC)). The ignition method was used to analyze the biochar content within different aggregate fractions. Biochar was applied before sowing in the first year of this experiment, and mineral fertilizer was applied to all treatments every year before sowing. All biochar treatments increased small macroaggregates (0.25–2 mm) and the soil aggregate stability, as reflected by the MWD (mean weight diameter), GMD (geometric mean diameter) and R>0.25mm (proportions of macroaggregates). The aggregate stability increased with increasing biochar dose and then decreased in the BC3 treatment. All biochar treatments increased the SOC content within macroaggregates. Biochar contents increased with the biochar dosage in all soil aggregate fractions except the microaggregate fraction. Biochar increased the amount of biological binding agents and the soil pH; electrical conductivity (EC); cation exchange capacity (CEC); exchangeable K+, Na+ and Mg2+ levels. Structural equation modeling revealed that biochar enhanced SOC first; SOC influenced the small macroaggregate fraction and silt and clay fraction indirectly via MBC, easily extractable glomalin-related soil proteins (GRSPe) and exchangeable Mg2+. However, the reason why the MWD decreased in the BC3 treatment was not clear. Therefore, the use of biochar as a soil amendment might improve the soil structure under a suitable application dose, but the underlying mechanism still requires further research.