Fungal and bacterial N2O production regulated by soil amendments of simple and complex substrates

Abstract Fungal N 2 O production results from a respiratory denitrification that reduces NO 3 − /NO 2 − in response to the oxidation of an electron donor, often organic C. Despite similar heterotrophic nature, fungal denitrifiers may differ from bacterial ones in exploiting diverse resources. We hypothesized that complex C compounds and substances could favor the growth of fungi over bacteria, and thereby leading to fungal dominance for soil N 2 O emissions. Effects of substrate quality on fungal and bacterial N 2 O production were, therefore, examined in a 44-d incubation after soils were amended with four different substrates, i.e., glucose, cellulose, winter pea, and switchgrass at 2 mg C g −1 soil. During periodic measurements of soil N 2 O fluxes at 80% soil water-filled pore space and with the supply of KNO 3 , substrate treatments were further subjected to four antibiotic treatments, i.e., no antibiotics or soil addition of streptomycin, cycloheximide or both so that fungal and bacterial N 2 O production could be separated. Up to d 8 when antibiotic inhibition on substrate-induced microbial activity and/or growth was still detectable, bacterial N 2 O production was generally greater in glucose- than in cellulose-amended soils and also in winter pea- than in switchgrass-amended soils. In contrast, fungal N 2 O production was more enhanced in soils amended with cellulose than with glucose. Therefore, fungal-to-bacterial contribution ratios were greater in complex than in simple C substrates. These ratios were positively correlated with fungal-to-bacterial activity ratios, i.e., CO 2 production ratios, suggesting that substrate-associated fungal or bacterial preferential activity and/or growth might be the cause. Considering substrate depletion over time and thereby becoming limited for microbial N 2 O production, measurements of soil N 2 O fluxes were also carried out with additional supply of glucose, irrespective of different substrate treatments. This measurement condition might lead to potentially high rates of fungal and bacterial N 2 O production. As expected, bacterial N 2 O production was greater with added glucose than with added cellulose on d 4 and d 8. However, this pattern was broken on d 28, with bacterial N 2 O production lower with added glucose than with added cellulose. In contrast, plant residue impacts on soil N 2 O fluxes were consistent over 44-d, with greater bacterial contribution, lower fungal contribution, and thus lower fungal-to-bacterial contribution ratios in winter pea- than in switchgrass-amended soils. Real-time PCR analysis also demonstrated that the ratios of 16S rDNA to ITS and the copy numbers of bacterial denitrifying genes were greater in winter pea- than in switchgrass-amended soils. Despite some inconsistency found on the impacts of cellulose versus glucose on fungal and bacterial leading roles for N 2 O production, the results generally supported the working hypothesis that complex substrates promoted fungal dominance for soil N 2 O emissions.