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 (qCO2) 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 ≥22.4 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.