Co-Occurring Anammox, Denitrification, and Codenitrification in Agricultural Soils

Abstract

ABSTRACTAnammox and denitrification mediated by bacteria are known to be the major microbial processes converting fixed N to N2gas in various ecosystems. Codenitrification and denitrification by fungi are additional pathways producing N2in soils. However, fungal codenitrification and denitrification have not been well investigated in agricultural soils. To evaluate bacterial and fungal processes contributing to N2production, molecular and15N isotope analyses were conducted with soil samples collected at six different agricultural fields in the United States. Denitrifying and anammox bacterial abundances were measured based on quantitative PCR (qPCR) of nitrous oxide reductase (nosZ) and hydrazine oxidase (hzo) genes, respectively, while the internal transcribed spacer (ITS) ofFusarium oxysporumwas quantified to estimate the abundance of codenitrifying and denitrifying fungi.15N tracer incubation experiments with15NO3−or15NH4+addition were conducted to measure the N2production rates from anammox, denitrification, and codenitrification. Soil incubation experiments with antibiotic treatments were also used to differentiate between fungal and bacterial N2production rates in soil samples. Denitrifying bacteria were found to be the most abundant, followed byF. oxysporumbased on the qPCR assays. The potential denitrification rates by bacteria and fungi ranged from 4.118 to 42.121 nmol N2-N g−1day−1, while the combined potential rates of anammox and codenitrification ranged from 2.796 to 147.711 nmol N2-N g−1day−1. Soil incubation experiments with antibiotics indicated that fungal codenitrification was the primary process contributing to N2production in the North Carolina soil. This study clearly demonstrates the importance of fungal processes in the agricultural N cycle.