Soil microbial communities play a pivotal role in providing ecosystem services, given that they are key drivers of biogeochemical processes such as carbon and nitrogen cycling. As species-rich communities, made-up of populations with short generation times, it is commonly assumed that there is a high degree of functional redundancy within soil communities with respect to broad-physiological processes, such as organic carbon decomposition. This assumption underlies the majority of terrestrial ecosystem models, where relationships between processes and controlling factors are parameterized using statistical relationships generated from measurements across space.  However, microbial communities display biogeographic patterns, even at fine scales.  New work shows that these biogeographic patterns extend to microbial community function, with functioning influenced by differences in resource histories. We examined whether a common resource history might cause functionally dissimilar communities to converge functionally. Next, we tested whether functional convergence (partial or complete) is associated with a reduction in function in alternate environments (a functional ‘trade-off’).  We used a 6 x 2 (soil community inoculum x litter environment) full-factorial design under controlled, laboratory conditions.  Microcosm CO₂ efflux was measured over three, successive 100-day periods, each representing a fresh inoculum step. Inocula were ‘back-crossed’ at the third step to explore trade-offs.

Results/Conclusions

Cumulative CO₂-C values in the initial 100-day run, for inocula in both grass and hardwood litter environments, were widely divergent. This indicated that the initial inocula were functionally distinct.  Soil inocula sourced locally to field litter sources exhibited ‘home-field advantage’, where inocula which had a history of the litter species used as the microcosm environment had the greatest cumulative CO₂-C values. In the second 100-day run, differences in cumulative CO₂-C values among inocula were less, with the cumulative values increasing toward the highest values observed in the initial run. These differences lessened further in the third run. The increasing similarity in cumulative CO₂-C values among inocula, observed in the successive 100-day runs, is consistent with functional convergence of communities exposed to a common environment. In the third 100-day run inocula were crossed to an alternate environment (e.g. those communities maintained in a grass environment were inoculated into a hardwood litter environment). Cumulative CO₂-C values in the alternate environments were approximately half those observed for communities inoculated into the same environment. Our results suggest that distinct microbial communities can converge functionally when exposed to a common environment, but that convergence is associated with loss of function in alternate environments.