The effect of soil moisture on the response by fungi and bacteria to nitrogen additions for N2O production

In addition to bacteria, the contribution of fungi to nitrous oxide (N2O) production has been recognized but the responses of these two broad and unrelated groups of microorganisms to global environmental changes, atmospheric nitrogen (N) deposition, and precipitation in terms of N2O production are unclear. We studied how these two microbial-mediated N2O production pathways responded to soil moisture conditions and to N addition in an N-limited temperate forest. Soils from a long-term N addition experiment in Changbai Mountain, northeastern China were incubated. Varied concentrations of cycloheximide and streptomycin, both inhibitors of fungal and bacterial activity, were used to determine the contributions of both to N2O production in 66%, 98% and 130% water-filled pore spaces (WFPS). The results showed that N2O production decreased significantly with increasing cycloheximide concentration whereas streptomycin was only inhibiting N2O emissions at 98% and 130% WFPS. The bacterial pathway of N2O production in N-addition (Nadd) soil was significantly more dominant than that in untreated (Namb) soil. The difference in the fungal pathway of N2O production between the soil with nitrogen addition and the untreated soil was not significant. Net N2O emissions increased with increasing soil moisture, especially at 130% WFPS, a completely flooded condition. Bacteria dominated carbon dioxide (CO2) and N2O emissions in Nadd soil and at 130% WFPS regardless of N status, while fungi dominated CO2 and N2O emissions in soil without N addition at 66% and 98% WFPS. The results suggest that flooded soil is an important source of N2O emissions and that bacteria might be better adapted to compete in fertile soils under anoxic conditions.