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 N 2 O and N 2 production in a grassland soil from Northern Ireland by combining the substrate-induced respiration inhibition method and the 15 N gas-flux method. Streptomycin (C 21 H 39 N 7 O 12 ) was used as the bacterial inhibitor and cycloheximide (C 15 H 23 NO 4 ) as the fungal inhibitor. By labeling the NH 4 and NO 3 pools, we tested the hypothesis that fungi produce N 2 O and N 2 solely by the reduction of NO 3 . Cycloheximide decreased the flux of N 2 O by 89% and streptomycin decreased the flux by 23%, indicating that fungi were responsible for most of the N 2 O production. All of the N 2 O was derived from NO 3 reduction. Labeled N 2 was only detected in control and streptomycin treatments. The distribution of the 15 N atoms in the labeled N 2 indicated that the source of the labeling was predominantly the NO 3 pool, but that the process of formation was not dominated by denitrification. Codenitrification, where a 15 N atom from labeled nitrogen dioxide (NO 2 ) combines with a N 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 N 2 O was always greater than the flux of N 2 , the mole fraction of N 2 O averaging 0.7. Fungal denitrification could be of ecological significance because N 2 O is the dominant gaseous end product.