Is Nitrous Oxide Reduction Primarily Regulated by the Fungi-to-Bacteria Abundance Ratio in Fertilized Soils?

Abstract

The production of nitrous oxide (N2O) is a widespread trait in fungi and is of interest because denitrifying fungi lack the N2O reductase gene (nosZ) that regulates N2O reduction to nitrogen gas (N2). The adaptive ability of soil fungi is better than that of bacteria in acidic soils. We investigated the N2O reduction potential, described by the N2O product ratio (RN2O), N2O/(N2O+N2), in soils of different types of fields under crop cultivation with different fertilizer inputs and a bare fallow field with no fertilization as a control. The fungi-to-bacteria abundance ratio (RF/B) was negatively correlated (P < 0.01) with the natural pH of the soil; however, the high value of RF/B measured in vineyards was due to the large inputs of manure. When the denitrification potential was measured at natural pH values of soils, RN2O was negatively correlated (P < 0.01) with soil pH. When the denitrification potential was measured after short-term modifications of soil pH, however, no significant correlation was found between RN2O and the modified pH. Based on stepwise multiple regression analysis, soil pH and residual nitrate (NO3−) were the key factors regulating N2O reduction in soils at natural pH values (R2 = 0.88, P < 0.001), whereas the key factor was the soil residual NO3− alone (R2 = 0.83, P < 0.001) when the soil pH was modified. When the effect of the soil chemical properties was weakened, a high RF/B value had the potential (P < 0.01) to affect N2O reduction; however, the role of fungi was offset by the presence of denitrifying bacteria. These results provide evidence that compared to the indirect effects of RF/B, the direct effects of the soil chemical properties have a greater effect on N2O reduction in fertilized soils.