Fungal control of nitrous oxide production in semiarid grassland

Fungi are capable of both nitrification and denitrification and dominate the microbial biomass in many soils. Recent work suggests that fungal rather than bacterial pathways dominate N transformation in desert soils. We evaluated this hypothesis by comparing the contributions of bacteria and fungi to N2O production at control and N fertilized sites within a semiarid grassland in central New Mexico (USA). Soil samples were taken from the rhizosphere of blue grama (B. gracilus) and the microbiotic crusts that grow in open areas between the bunch grasses. Soils incubated at 30% or 70% water holding capacity, were exposed to one of three biocide treatments (control, cycloheximide or streptomycin). After 48 h, N2O and CO2 production were quantified along with the activities of several extracellular enzymes. N2O production from N fertilized soils was higher than that of control soils (165 vs. 41 pmol h(-1) g(-1)), was higher for crust soil than for rhizosphere soil (108 vs. 97 pmol h(-1) g(-1)), and increased with soil water content (146 vs. 60 pmol h(-1) g(-1)). On average, fungicide (cycloheximide) addition reduced N2O production by 85% while increasing CO2 production by 69%; bactericide (streptomycin) reduced N2O by 53% with mixed effects on CO2 production. N2O production was significantly correlated with C and N mineralization potential as measured by assays for glycosidic and proteolytic enzymes, and with extractable nitrate and ammonium. Our data indicate that fungal nitrifier denitrification and bacterial autotrophic nitrification dominate N transformation in this ecosystem and that N2O production is highly sensitive to soil cover, N deposition and moisture.