Evidence for the Functional Significance of Microbial Diversity Among Free-Living Diazotrophs in Soils of a Long Term Agricultural Site.

To evaluate whether microbial community composition has functional significance in an agronomic context, we have examined the effects of tillage and biomass management on diazotroph community structure, N-fixation, and soil characteristics in a long-term (> 30 years) experimental site. Results indicate that tillage was the primary driver of most soil characteristics but that biomass management had the largest impact on diazotroph community composition and N-fixation rates. Biomass retention was associated with both a reduction in the evenness of the diazotroph community and a reduction in N-fixation rates. We assessed the functional diversity of the diazotrophic community through both cultivation dependent and independent techniques. In the cultivation dependent approach, selective growth conditions with N-free media were used to enumerate different functional classes of diazotrophs. Growth conditions included differences in carbon source (malate, cellobiose, sucrose, mannitol, vanillin, and acetate), pH (7.5, 5.5 and 3.5), oxygen levels (20%, 5% and 0%), trace metals (Mo, V and Fe), and temperatures (4°C, 22°C, 30°C, and 42°C). The cultivation experiment showed that more diverse diazotrophic communities could respond to a greater range of growth conditions. Isolates obtained from the cultivation experiment were identified and their nifH genes compared to those recovered directly from soil to evaluate the fraction of the diazotrophic community that was recovered through cultivation. In the cultivation independent approach, soils that differed in diazotroph diversity were incubated under a range of different conditions including temperature (22°C, 42°C, and freeze-thaw), moisture (field moisture or 50% of WHC), pH (field pH or pH 4.5) and oxygen (oxic and anoxic) to determine whether the diversity of the community could impact the resilience of soil N-fixation to changing environmental conditions. These experiments provide insight on the functional significance of microbial diversity in soils.