Soil nitrification is the process that converts ammonia (NH3) to nitrite (NO2-) and nitrate (NO3-) with N2O released as a byproduct. Soil nitrification has direct environmental impacts: N2O is a potent greenhouse gas and NO3- can be easily leached out by precipitation and thus results in water pollution. Both ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) oxidize NH3 via ammonia monooxygenase (AMO), and understanding their population dynamics and functional responses to N fertilization seems critical for improved soil N management. Almost all studies of AOA and AOB have reported positive responses of AOB amoA gene copies and nitrification rates to N fertilization in agricultural systems using short term incubation experiments. But little is known about the effects of long-term N additions on nitrifier communities under unmanaged vegetation subjected to increased N deposition inputs. In this study we measured nitrification kinetics, amoA gene abundance and other environmental covariates under a management gradient ranging from intensively managed row crops to successional systems of different stages including an unmanaged late successional deciduous forest at the Kellogg Biological Station (KBS) LTER site in southwest Michigan.
Results/Conclusions
Soil nitrification exhibited Michaelis-Menten kinetics in conventional and biologically based row crop systems with a similar AOB-derived maximum nitrification rate (Vmax) of 4.75 mg N kg-1 day-1. In comparison, soil nitrification was inhibited with NH4+ addition in the poplar soils and exhibited Haldane kinetics with a Vmax of 1.48 mg N kg-1 day-1 for AOB. In the early successional system, Vmax of total and AOB-derived nitrification in subplots receiving N fertilizer for the past 30 years were twice higher than unfertilized main plots, but not in the deciduous forest soils, where Vmax for total and AOB-derived nitrification in fertilized subplots were only 30-40% of unfertilized control. The reduced nitrification rates in fertilized deciduous forest soils cannot be explained by a smaller nitrifier population size as the number of copies of amoA AOA was unresponsive to fertilization while the number of copies of amoA AOB was significantly enhanced by N fertilization (P < 0.05). It is possible that the low soil pH in fertilized deciduous forest inhibited nitrification. However, both amoA AOA and amoA AOB gene abundance were negatively related to soil pH (P < 0.05). Results reveal the complexity of interactions between environmental and biological factors controlling soil nitrification.