Manipulation of soil microbial community structure in bog and forest soils using chloroform fumigation

Chloroform fumigation was used to manipulate the composition of microbial communities as a means of investigating relationships between community structure and the functioning of soil processes. Soils from two upland sites, a coniferous forest and a peat bog were fumigated with chloroform for 2, 12, 24 or 72 h. Samples were then incubated at 20°C for 4 weeks to investigate whether they developed different microbial communities or whether there was convergence in community structure following a severe stress. The identity of individual samples was maintained throughout the analyses not only to avoid pseudo-replication, but also to reflect natural heterogeneity in the structure of microbial communities. Microbial community structure was characterised by phospholipid fatty acid (PLFA) profiles. Total PLFA concentrations in the bog soil were 10 times higher than in the forest soil and also contained higher concentrations of major PLFA groups (straight-chain, branched-chain, mono-unsaturated, poly-unsaturated, hydroxy and cyclic PLFAs). Chloroform fumigation for 24 h or more caused a large reduction in total PLFAs and poor regrowth of the residual community. Fumigating the bog soil for 12 h resulted in better recovery of PLFA concentrations after incubation than for soil fumigated for 2 h or 24. There were also differences in the susceptibility of specific PLFAs to fumigation. Relatively high concentrations of cyclic 19:0 were measured in the fumigated soils following incubation which may be indicative of the responses of microorganisms to physiological stress. Variation in PLFAs between core samples both before and after treatments reflected considerable heterogeneity in the composition of microbial communities in the two sites and differences in the responses of these microbial assemblages to fumigation. Diversity and equability of individual PLFA concentrations were reduced with the fumigation process. There was no convergence of community structure in response to fumigation followed by incubation under constant environmental conditions. The composition of the initial community was more important than treatment effects in determining the final structure of the manipulated community. The consequences of these changes in community structure for the functioning of soil processes are being investigated.