Responses of the bacterial and fungal biomass in a grassland soil to multi-year applications of dairy manure slurry and fertilizer

Increases in bacterial abundance, during the one to three week intervals immediately following individual applications of manure, were inconsistent and other factors, such as soil moisture, temperature and perhaps crop phenology appear to have had strong effects on the timing of these microbial responses. Annual means for bacterial abundance and total microbial biomass in the continuous manure treatment were similar for all 3 years. This suggested that the manure-induced increase in microbial biomass probably reached a plateau between one and 3 years after applications commenced. The large bacterial populations along with abundant carbon substrates in manured soil, relative to fertilized soil, were probably capable of immobilizing influxes of mineral N, explaining the observations that less leaching occured from manured than from fertilized soils.