Can nitrogen isotope fractionation reveal ammonia oxidation responses to varying soil moisture?

To interpret the response of ammonia oxidation to changing soil moisture, we conducted a batch aerobic incubation with a loam soil at different soil water potentials (SWP) from −1100 to −11 kPa, and calculated net nitrification rates and apparent isotope fractionation factors (αs/p). With increasing SWP, net nitrification rates increased from 2.3 to 9.8 mg N kg−1 d−1, while αs/p increased from 1.025 to 1.031 at field-capacity (SWP of −33 kPa) but decreased with increasing SWP above field capacity. The increased αs/p at field-capacity indicated that intracellular NH4+ concentration increased as a result of NH4+ supply exceeding NH3 oxidation, while NH3 oxidation exceeding NH4+ supply above field-capacity resulted in both decreased intracellular NH4+ concentration and αs/p. Our results suggest that NH4+ diffusion contributes more sensitively to increasing SWP than NH3 oxidation below field-capacity, while the reverse is the case above field-capacity.