Cavigelli, M. and G.P. Robertson
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
Controls on in situ N2O production are very poorly understood in most ecosystems, and the global N2O budget is far from balanced. Environmental factors that affect N2O fluxes are well-studied, but are poor predictors of measured rates. An untested contributor to systematic variation in N2O production is denitrifier population diversity. Pure culture studies have shown that relative rates of N2O production (N2O/(N2O+N2)) among denitrifier populations grown under identical conditions can differ substantially (1-3), which suggests that the composition of the denitrifying community in soils may be an important control on N2O production in situ. We are testing whether denitrifier population diversity is important to potential rates of N2O production in geomorphically identical soils that differ in C, NO3-, pH, and moisture.We sampled soil from a corn field and a never-tilled successional field (KBS LTER treatments 1 and 8). Soil organic C is different between these two ecosystems due to tillage and long-term differences in plant communities. The composition of the denitrifying community, as characterized using fatty acid profiles of isolated bacteria, was also different between the two ecosystems.We tested whether these differences in populations are functionally significant using a soil slurry technique in which all factors that control relative rates of N2O production by denitrifiers — other than denitrifier community composition - are controlled (pH, O2, denitrifier enzyme status) or provided in non-limiting quantities (C, NO3). Denitrifier enzymes were first induced by preincubating soil slurries anaerobically. An incubation was then initiated by adding 0.1 ml of a C and NO3- solution to final concentrations of 1 mM, and oxygen to various final concentrations between 0 and 1% in the headspace. Acetylene, which blocks the reduction of N2O to N2, was added to half the vials and N2O production was monitored in all vials during a 40 min incubation. As expected, N2O production rates were highest when acetylene was added to vials and generally decreased with increasing oxygen concentration, regardless of the presence of acetylene. At native pH, there was a significant difference in the relative rate of N2O production for these two soils at all O2 concentrations tested. These differences disappeared when CaCO3 was added to soils to adjust pH to 7.0. These results indicate either that different populations are active in each site or that the same community is present in both soils and their N2O response is pH sensitive. Current experiments are designed to separate possible pH effects from population effects.