Evidence for Fungal Dominance of Denitrification and Codenitrification in a Grassland Soil

<jats:p> 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 <jats:sub>2</jats:sub> O and N <jats:sub>2</jats:sub> production in a grassland soil from Northern Ireland by combining the substrate‐induced respiration inhibition method and the <jats:sup>15</jats:sup> N gas‐flux method. Streptomycin (C <jats:sub>21</jats:sub> H <jats:sub>39</jats:sub> N <jats:sub>7</jats:sub> O <jats:sub>12</jats:sub> ) was used as the bacterial inhibitor and cycloheximide (C <jats:sub>15</jats:sub> H <jats:sub>23</jats:sub> NO <jats:sub>4</jats:sub> ) as the fungal inhibitor. By labeling the NH <jats:sub>4</jats:sub> and NO <jats:sub>3</jats:sub> pools, we tested the hypothesis that fungi produce N <jats:sub>2</jats:sub> O and N <jats:sub>2</jats:sub> solely by the reduction of NO <jats:sub>3</jats:sub> Cycloheximide decreased the flux of N <jats:sub>2</jats:sub> O by 89% and streptomycin decreased the flux by 23%, indicating that fungi were responsible for most of the N <jats:sub>2</jats:sub> O production. All of the N <jats:sub>2</jats:sub> O was derived from NO <jats:sub>3</jats:sub> reduction. Labeled N <jats:sub>2</jats:sub> was only detected in control and streptomycin treatments. The distribution of the <jats:sup>15</jats:sup> N atoms in the labeled N <jats:sub>2</jats:sub> indicated that the source of the labeling was predominantly the NO <jats:sub>3</jats:sub> pool, but that the process of formation was not dominated by denitrification. Codenitrification, where a <jats:sup>15</jats:sup> N atom from labeled nitrogen dioxide (NO <jats:sub>2</jats:sub> ) combines with a <jats:sup>14</jats:sup> N atom from a natural abundance source, was proposed as the process forming labeled N <jats:sub>2</jats:sub> About 92% of the labeled N <jats:sub>2</jats:sub> was estimated to be due to codenitrification and 8% due to denitrification. The flux of N <jats:sub>2</jats:sub> O was always greater than the flux of N <jats:sub>2</jats:sub> , the mole fraction of N <jats:sub>2</jats:sub> O averaging 0.7. Fungal denitrification could be of ecological significance because N <jats:sub>2</jats:sub> O is the dominant gaseous end product. </jats:p>