Nitrous Oxide (N2O) production by Rhizosphere fungi from Nitrogen fertilized grasslands

The use of nitrogen (N) fertilizers has created international concern as denitrification of fertilizer-derived nitrate could result in higher rates of N₂O release to the atmosphere. The discovery that many eukaryotic fungi can transform N through nitrification and denitrification pathways is significant from both biochemical and ecological perspectives because fungi dominate microbial metabolism in many terrestrial ecosystems. Roots from semi-arid pastures are colonized by a diverse fungal community and suppression of such fungal biomass can cause a substantial reduction of N₂O production. N₂O production by 16 soil fungal species was estimated by analyzing gas samples from pure cultures isolated from the rhizosphere of the Blue grama grassland biome from the N-addition experiment at the Sevilleta Long-Term Ecological Research in Central New Mexico. Cultures were maintained in liquid medium containing nitrate as N source and incubated at 25°C for 10 days prior to sealing the flasks in order to limit aeration. CO₂ production was recorded for normalizing N₂O generation by fungal respiration. A repeated measures ANOVA showed significant differences in N₂O production relative to species and sampling dates (F_(21,44)=23.20; p<.00001 and F_(8,352)=36.94; p<0.0001), respectively. Average total N₂O production ranged from 6.3 to 12.7 nmol h^-1 ml^-1, and the highest values were observed in Monosporascus ibericus and in an unidentified group (best match Genbank DQ420786). N₂O increased abruptly after day five leveling off until day 10. Finally, a significant interaction between species and sampling days was observed (F_(168,352)=3.17; p<0.0001). Although previous studies emphasized the importance of fungi in N transformations and N₂O production in arid ecosystems, our results indicate that only a few species are responsible for most of the total N₂O emissions