Can soil fungi affect bacterial functional groups that mediate greenhouse gas emission in forests?

Background/Question/Methods

Whether northern hardwood forests can act as sources or sinks for greenhouse gases will depend on the activity of microbial functional groups in soil, which can be affected by plant community composition, seasonal patterns of precipitation, edaphic factors, and the presence of other soil microbes.  One of the most important groups of organisms in forest soil are the mycorrhizal fungi, that form mutually beneficial relationships with plant roots, and which can interact with soil bacteria.  Although arbuscular mycorrhizas (AM) and ectomycorrhizas (ECM) affect can affect forest N cycling differently, their differential effects on bacterial functional groups has not been examined.  We hypothesized that 1) fungal biomass would affect communities of methane oxidizing (MOB) and denitrifying (DNB) bacteria in forest soils and 2) that both groups would be affected by changes in microsite soil chemistry.  To examine these hypotheses, we measured fungal biomass using PLFA and examined the community structure of MOB and DNB using molecular techniques in soil cores (5 cm depth) collected in September of 2006 from a mature beech-maple forest in Northeastern Ohio.

Results/Conclusions

We detected 42 TRFs for DNB and 19 TRFs for MOB in our soil samples using terminal restriction fragment length polymorphism (TRFLP) screening.  We found that only 27% of soil cores contained both MOB and DNB.  40% of soil cores contained only MOB and 32% of soil cores contained only DNB suggesting spatial separation among these bacterial functional groups.  Non-metric multidimensional scaling of TRFLP profiles found that DNB and MOB largely separated within ordination space, again suggesting little overlap of these bacteria in soil cores.  In addition, DNB were significantly positively correlated with fungal biomass, specifically the 18:1w9c biomarker, and with chitinase activity.  MOB were negatively correlated with 18:1w9c biomass.  Biomass of the AMF marker 16:1w5c was not significantly correlated with either DNB or MOB.  Soil chemistry including soil C, N and bicarbonate P were also not significantly correlated with either functional group.  Our results suggest that soil fungi might influence the spatial distribution of important bacterial functional groups, including some groups that mediate the production and consumption of important greenhouse gases.