Influence of long-term fertilisation and crop rotation on changes in fungal and bacterial residues in a tropical rice-field soil

Amino sugars, as a microbial residue biomarker, are highly involved in microbial-mediated soil organic matter formation. However, accumulation of microbial biomass and responses of bacterial and fungal residues to the management practices are different and poorly characterized in rice soils. The objectives of this study were to evaluate the effects of mineral fertiliser (MIN), farmyard manure (FYM) and groundnut oil cake (GOC) on crop yield and co-accumulation of microbial residues and microbial biomass under rice-monoculture (RRR) and rice–legume–rice (RLR) systems. In the organic fertiliser treatments and RLR, rice grain yield and stocks of soil and microbial nutrients were significantly higher than those of the MIN treatment and RRR, respectively. The increased presence of saprotrophic fungi in the organic fertiliser treatments and RRR was indicated by significantly increased ergosterol/C_mic ratio and extractable sulphur. In both crop rotation systems, the long-term application of FYM and GOC led to increased bacterial residues as indicated by greater accumulation of muramic acid. In contrast, the higher fungal C/bacterial C ratio and lower ergosterol/C_mic ratio in the MIN treatment, is likely caused by a shift within the fungal community structure towards ergosterol-free arbuscular mycorrhizal fungi (AMF). The organic fertiliser treatments contributed 22 % more microbial residual C to soil organic C compared to the MIN treatment. Our results suggest that the negative relationship between the ratios ergosterol/C_mic and fungal C/bacterial C encourages studying responses of both saprotrophic fungi and AMF when assessing management effects on the soil microbial community.