Denitrification is an important step in the nitrogen cycle where nitrate (NO3) is removed by denitrifying microbes under anaerobic conditions. The denitrification enzyme activity assay (DEA) is the most widely used method for measuring denitrification potential (DNP).  As it is conducted under nonlimiting conditions for denitrification, it is assumed that microbes in the soils collected from the same site will perform at their peak activity during the incubation and therefore differences between sampling dates within the same growing seaon should be minimal.  Rates of DNP have been shown to respond to longer-term variation in factors such as soil moisture and NO3 that control denitrification.  The two goals of this study were to 1) examine the level of variability in DNP at different points within the growing season, 2) determine whether changes in soil variables and/or plant functional diversity explain seasonal DNP variability. The study site is a restored wetland in the NC Piedmont with native wetland herbs planted in monocultures or mixes of four or eight species. 

Results/Conclusions

We found that denitrification potentials for soils collected in July 2006 were significantly greater than soils collected in May and late August 2006 (p<0.0001). Among the other soil variables measured—soil moisture, organic matter, total inorganic nitrogen, and microbial biomass—none appeared to explain the pattern observed in DNP, further emphasizing DNP’s complex and dynamic interaction between microbes, soil, plants, and season.  There was no significant relationship between DNP and species richness or plot plant functional diversity.  However, when we normalized DNP by dividing it by microbial biomass C (MBC), there was a significant inverse relationship between DEA/MBC variance and plot species functional diversity (p<0.01).  These findings suggest that the functional diversity of the plots is supporting a more constant level of DNP through time, buffering the system from changes in season and soil conditions.