Nitrous oxide producing activity of diverse fungi from distinct agroecosystems

Fungi represent a significant component of the soil microbial community and play critical ecological roles in carbon and nitrogen mediated processes. Therefore, fungi capable of nitrous oxide (N2O) production may have great implications to soil N2O emission. The primary objective of this research was to identify and characterize N2O-producing fungi in agricultural soil systems and determine their relative physiological responses to inorganic N, pH and oxygen availability. Soil samples were collected from five agricultural-based systems: conventional farming, organic farming, integrated crop and livestock, plantation forestry, and an abandoned agriculture field subjected to natural succession. Fungi were isolated from soil and examined for N2O production in a nitrate-containing liquid Czapek medium amended with or without cycloheximide or streptomycin. Fungal population levels were similar among the five systems, ranging from 1.1 to 3.7 × 105 colony-forming units per gram of soil. One hundred-fifty one fungal colonies were selected based on colony morphology and tested for N2O production. About half (i.e., 45%) of tested isolates representing at least 16 genera and 30 species of filamentous fungi were capable of producing N2O. Neocosmospora vasinfecta exhibited the highest production of N2O in laboratory based assays, followed by Aspergillus versicolor, A. oryzae, A. terreus, Fusarium oxysporum and Penicillium pinophilum. Ten selected N2O-producing fungus isolates were subsequently evaluated to determine the influence of nitrogen species, pH and O2 on N2O production. Seven of the 10 selected isolates had 65% or greater N2O production in a nitrite than a nitrate medium. Ninety and 60%, of isolates showed greatest N2O production at neutral pH 7.0 and ≤5% headspace O2 conditions, respectively. Our results demonstrate that N2O-producing fungi were prevalent in the five soil systems and production of N2O varied among isolates examined under different imposed abiotic conditions in the laboratory.