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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

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

Dep. of Agriculture and Rural Development, Agricultural and Environmental Science Division, Newforge Lane, Belfast BT9 5PX, UK

* Corresponding author (jim.stevens{at}dardni.gov.uk)

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 N₂ production in a grassland soil from Northern Ireland by combining the substrate-induced respiration inhibition method and the ¹⁵N 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 N₂ 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 N₂ was only detected in control and streptomycin treatments. The distribution of the ¹⁵N atoms in the labeled N₂ 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 ¹⁵N atom from labeled nitrogen dioxide (NO₂) combines with a ¹⁴N atom from a natural abundance source, was proposed as the process forming labeled N₂. About 92% of the labeled N₂ was estimated to be due to codenitrification and 8% due to denitrification. The flux of N₂O was always greater than the flux of N₂, the mole fraction of N₂O averaging 0.7. Fungal denitrification could be of ecological significance because N₂O is the dominant gaseous end product.

Abbreviations: IRMS, isotope-ratio mass spectrometry • NO₂, nitrogen dioxide • SIRIN, substrate-induced respiration inhibition

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