Soil Moisture and pH Control Relative Contributions of Fungi and Bacteria to N2O Production

Fungal N2O production has been progressively recognized, but its controlling factors remain unclear. This study examined the impacts of soil moisture and pH on fungal and bacterial N2O production in two ecosystems, conventional farming and plantation forestry. Four treatments, antibiotic-free soil and soil amended with streptomycin, cycloheximide, or both were used to determine N2O production of fungi versus bacteria. Soil moisture and pH effects were assessed under 65-90 % water-filled pore space (WFPS) and pH 4.0-9.0, respectively. Irrespective of antibiotic treatments, soil N2O fluxes peaked at 85-90 % WFPS and pH 7.0 or 8.0, indicating that both fungi and bacteria preferred more anoxic and neutral or slightly alkaline conditions in producing N2O. However, compared with bacteria, fungi contributed more to N2O production under sub-anoxic and acidic conditions. Real-time polymerase chain reaction of 16S, ITS rDNA, and denitrifying genes for quantifications of bacteria, fungi, and denitrifying bacteria, respectively, showed that fungi were more abundant at acidic pH, whereas total and denitrifying bacteria favored neutral conditions. Such variations in the abundance appeared to be related to the pH effects on the relative fungal and bacterial contribution to N2O production.