Soil microbial biomass and activity in soil from different grassland management treatments stored under controlled conditions

Soil taken during summer from permanent pasture on a clay loam in S.W. England which had been under four different N management treatments for at least 3 yr was stored under aerobic conditions at 18°C. Total soil microbial biomass (SMB), measured by fumigation-extraction, was higher in the unfertilized grass and grass–clover treatments than in the fertilized grass and clover treatments and these differences persisted for 30 weeks. Over this period total SMB decreased by 25% in all the treatments, soil respiration also decreased, but more rapidly and to a greater extent (50%). The respiration response of the biomass to glucose, estimating “active” biomass, was measured by a substrate induced respiration (SIR) procedure; it showed no differences between the four treatments but was reduced by 66% over 30 weeks. Concentrations of NO3−–N increased progressively in each treatment by more than 15-fold. At the end of the 30 weeks addition of available C in the form of glucose resulted in a small increase in total SMB and respiratory activity and a large decrease in NO3−–N in all treatments: the SIR response of the biomass was completely restored. It is suggested that the differences in total SMB in the management treatments throughout the incubation were attributable to the “inactive” fraction of the biomass.