Co-Occurring Anammox Denitrification and Codenitrification in Agricultural Soils

摘要

Anammox and denitrification mediated by bacteria are known to be the major microbial processes converting fixed N to N2 gas in various ecosystems. Codenitrification and denitrification by fungi are additional pathways producing N2 in soils. However, fungal codenitrification and denitrification have not been well investigated in agricultural soils. To evaluate bacterial and fungal processes contributing to N2 production, molecular and ¹⁵N 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) of Fusarium oxysporum was quantified to estimate the abundance of codenitrifying and denitrifying fungi. ¹⁵N tracer incubation experiments with ¹⁵NO3⁻ or ¹⁵NH4⁺ addition were conducted to measure the N2 production rates from anammox, denitrification, and codenitrification. Soil incubation experiments with antibiotic treatments were also used to differentiate between fungal and bacterial N2 production rates in soil samples. Denitrifying bacteria were found to be the most abundant, followed by F. oxysporum based on the qPCR assays. The potential denitrification rates by bacteria and fungi ranged from 4.118 to 42.121 nmol N2-N g⁻¹ day⁻¹, while the combined potential rates of anammox and codenitrification ranged from 2.796 to 147.711 nmol N2-N g⁻¹ day⁻¹. Soil incubation experiments with antibiotics indicated that fungal codenitrification was the primary process contributing to N2 production in the North Carolina soil. This study clearly demonstrates the importance of fungal processes in the agricultural N cycle.