Evidence for fungal dominance of denitrification and codenitrification in a grassland soil

Fungi are capable of nitrification and denitrification and often dominate the microbial biomass of temperate grassland soils. We determined the contributions of bacteria and fungi to N2O and N-2 production in a grassland soil from Northern Ireland by combining the substrate-induced respiration inhibition method and the N-15 gas-flux method. Streptomycin (C21H39N7O12) was used as the bacterial inhibitor and cycloheximide (C15H23NO4) as the fungal inhibitor. By labeling the NH4 and NO3 Pools, we tested the hypothesis that fungi produce N2O and N-2 solely by the reduction of NO3. Cycloheximide decreased the flux of N2O by 89% and streptomycin decreased the flux by 23%, indicating that fungi were responsible for most of the N2O production. All of the N2O was derived from NO3 reduction. Labeled N-2 was only detected in control and streptomycin treatments. The distribution of the N-15 atoms in the labeled N-2 indicated that the source of the labeling was predominantly the NO3 pool, but that the process of formation was not dominated by denitrification. Codenitrification, where a N-15 atom from labeled nitrogen dioxide (NO2) combines with a N-15 atom from a natural abundance source, was proposed as the process forming labeled N-2. About 92% of the labeled N-2 was estimated to be due to codenitrification and 8% due to denitrification. The flux of N2O was always greater than the flux of N-2, the mole fraction of N2O averaging 0.7. Fungal denitrification could be of ecological significance because N2O is the dominant gaseous end product.