Response of microbial biomass to air-drying and rewetting in soils and compost

The activity of microorganisms and the availability of composting substrates for decomposition are seriously affected by drying and rewetting cycles. We have measured microbial biomass C (BC) and ninhydrin reactive N (BNIN) of samples of composting material taken at different times from a pile of ligno-cellulosic wastes. Dynamics of these two parameters in control samples that were kept continuously moist were compared with those of samples of the same material that had been subjected to drying and rewetting. The study was also performed on soils to investigate analogies and differences of behaviour of microbial biomass in such different substrates. Moist samples from 10 soils with different organic C content (6.6-41.9 g kg-1 soil) were analyzed for their BC and BNIN content. The soils were then air-dried, rewetted and incubated at 25°C for 10 days. On days 1, 3, 6 and 10 of incubation, samples were analyzed for BC and BNIN content. Compost samples from different composting stages of a mixture of cotton carding and yard wastes were air-dried, rewetted and incubated at 25°C for 12 days, together with the corresponding moist samples (control). On days 1, 5 and 12 of incubation period, samples were analyzed for BC and BNIN content. The regression coefficient between BC and BNIN for all the compost data (continuously moist and rewetted) was 21.4 (r = 0.89 P < 0.001), very close to values normally found in soils, confirming the reliability of microbial biomass measurements performed on composting substrates. Soil drying caused on average a decrease in the size of microbial biomass with respect to moist samples of 13% for BC and 30% for BNIN. BC and BNIN of moist and rewetted soil samples were always significantly correlated. Microbial biomass contentof rewetted compost samples up to 19 days from the beginning of the process were significantly different, but highly correlated with moist controls. No significant differences between moist and rewetted samples (RW) were found in samples collected during the maturing and curing phases. The different response to drying of soils and compost could be related to the greater increase of new available substrate in compost with respect to soil following the drying-rewetting treatment. Dynamics of BC/BNIN ratio in compost was coherent with the normal trend observed in the composition of microbial community during the process, from prevailing bacteria and actinomycetes to prevailing fungi.