Tillage-induced changes in fungal and bacterial biomass associated with soil aggregates: A long-term field study in a subtropical rice soil in China

The composition and function of the soil microbial community can be strongly influenced by soil structure and tillage. Soil total microbial biomass C (MBC), microbial biomass N (MBN), and fungal and bacterial biomass associated with different soil aggregate-size fractions were measured for a long-term no-till subtropical rice soil ecosystem to determine how tillage shifts microbial community structure and to detect the spatial scale at which microorganisms are the most sensitive to disturbance. Surface soil (0–20cm) was fractionated into aggregate-sizes (>4.76mm, 4.76–2.0mm, 2.0–1.0mm, 1.0–0.25mm, 0.25–0.053mm, <0.053mm) under three tillage regimes. Soil MBC, MBN, fungal biomass and bacterial biomass were significantly higher under no tillage (NT) than conventional tillage (CT) and CT flooded paddy field (FPF) in whole soil (p<0.05). Microbial biomass C, N and fungal biomass were significantly higher under NT than CT and FPF for all aggregate sizes. No significant tillage effects on microbial biomass C:N ratio were observed, but analysis of soil aggregates revealed significant differences due to tillage. Microbial biomass C:N ratio averaged 8.8, 8.5, and 8.4 for CT, NT, and FPF, respectively, while the ratio was significantly higher under CT than NT for macroaggregates >1.0mm, indicating a tillage-induced N limitation in macroaggregates. The fungal:bacterial biomass ratio ranged from 0.5 to 4.5, and was highest in 4.76–2mm aggregates for all tillage regimes. Fungal:bacterial ratios were significantly higher for aggregate sizes >1.0mm (2.4) than for <1.0mm aggregates (0.7). Tillage effects on fungal over bacterial dominance were not significant for whole soil and most aggregate sizes. The NT regime increased microbial biomass, but this increase was proportional for both bacteria and fungi. The soil microbial biomass and community structure was likely controlled by particle size at the aggregate-scale (0.05–5.0mm), while tillage played a role in regulating the microbial community structure.