N2O emission rates in a California meadow soil are influenced by fertilizer level, soil moisture and the community structure of ammonia-oxidizing bacteria

The response of nitrous oxide (N2O) emission rates and beta-proteobacterial ammonia-oxidizing (AOB) communities to manipulations of temperature, soil moisture and nitrogenous fertilizer concentration were studied for 16-20 weeks in a multifactorial laboratory experiment using a California meadow soil. Interactions among these three environmental factors influenced the N2O emission rates, and two patterns of N2O emission rates due to nitrification (NitN(2)O) were observed. First, in soils receiving low or moderate amounts of fertilizer, the rates decreased sharply in response to increasing soil moisture and temperature. Second, in soils receiving high amounts of fertilizer, the rates were influenced by an interaction between soil moisture and temperature, such that at 20 degrees C increasing soil moisture resulted in an increase in the rates, and at 30 degrees C the highest rate was observed at moderate soil moisture. We used path analysis to identify the interrelationships that best explain these two patterns. Path analysis revealed that in the high fertilizer (HF) treatment, the major path by which ammonia influenced NitN(2)O rates was indirect through an influence on the abundance of one particular phylogenetic group (AOB 'cluster 10'). In contrast, in the low and moderate fertilizer treatments soil moisture influenced the rates both directly (the major path) and indirectly through AOB community structure. Although terminal restriction fragment length polymorphism (T-RFLP) analysis revealed shifts in the community structure of AOB in all treatments, the shifts at HF concentrations were particularly striking, with dominance by three different phylogenetic groups under different combinations of the three environmental factors. The high emission rates observed at the lowest soil moistures suggest that bacterial nitrifiers may use denitrification as a stress response.