Substrate Quality Regulates Fungal and Bacterial Contribution to Soil Nitrous Oxide Emissions.

This study aimed to test the hypothesis that complex organic compounds or materials could promote fungal dominance in soil N₂O emissions. Two pairs of organics (glucose vs. cellulose and winter pea vs. switchgrass) were used for evaluating the impacts of simple and complex substrates on fungal and bacterial contribution to soil N₂O production. During six-week incubation, fungal and bacterial N₂O fluxes were measured periodically with the aid of antibiotics and under the conditions of 80% soil water-filled pore space and sufficient nitrate. Pairwise comparisons showed that up to day 8 when antibiotics still exerted inhibitory effects on substrate-induced microbial growth, bacterial N₂O production was generally greater in soil amended with simple substrates (glucose and winter pea) than with complex substrates (cellulose and switchgrass). In contrast, complex substrates promoted fungal dominance in N₂O emissions compared to respective simple substrates. Therefore, relative fungal-to-bacterial contribution ratios were greater in complex than in simple C substrates. These ratios were also positively correlated with fungal-to-bacterial CO₂ respiration ratios, suggesting that substrate-induced divergence between fungal and bacterial growth and activity might be the cause. Due to substrate limitation on microbial N₂O production after day 8, bacterial and fungal N₂O fluxes were also measured with an additional supply of glucose. On day 4 and 8, bacterial N₂O production was greater in glucose- than in cellulose-amended soils and vice versa for fungal N₂O production. However, this pattern was reversed on day 28. The relative impacts of winter pea vs. switchgrass on soil N₂O fluxes were consistent over 44 days, 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 confirmed that the ratios of 16S to ITS rDNA and the abundance of bacterial denitrifying genes were greater in winter pea- than in switchgrass-amended soils. Despite some inconsistency about the effects of cellulose vs. glucose on fungal and bacterial dominance in N₂O emissions, our results generally supported the working hypothesis that complex substrates promoted fungal dominance for soil N₂O production.