Shifts in microbial biomass and community composition in subtropical paddy soils under a gradient of manure amendment

To study the responses of microbial community composition to excessive manure amendment and their relationships with soil nutrients, a pot experiment was conducted using soils of different fertilities under a gradient (0-44.8 Mg ha(-1)) of manure amendment. After four rice-growing seasons, the soil nutrients were enriched by intense manure amendment, especially the phosphorus (P) content, and the P-related nutrient stoichiometry was altered. The total P content increased by 1.51 and 1.09 g kg(-1) with the highest manure input in fertile and infertile soils, respectively. For the two soils, both the carbon (C)-to-P and nitrogen (N)-to-P ratios decreased up to around 55 and 4, respectively, by increasing amendment rates from 0 to 44.8 Mg ha(-1). The microbial biomass C (MBC) in the infertile soil increased steadily from 330.73 to 445.93 mg kg(-1) under amendment rates increasing from 2.8 to 44.8 Mg ha(-1). In fertile soil, MBC increased significantly from 590.46 to 784.54 mg kg(-1) and slowly to 875.56 mg kg(-1) before remaining constant. The metabolic quotient (qCO(2)) increased about 2.5-fold when the amendment rate increased from 11.2 to 44.8 Mg ha(-1). The fungal-to-bacterial ratio doubled with the highest manure amendment rate at the expense of actinomycetes, and the community composition shifted at amendment rates a parts per thousand yen22.4 Mg ha(-1). Changes in the composition of microbial communities were associated with the increase in the soil P content and reduced C-to-P and N-to-P ratios. In conclusion, an imbalance in P accumulation with excessive manure amendment may affect the functions of the soil ecosystem by reshaping the soil microbial communities.