Differences in isotopic fractionation of nitrogen in water-saturated and unsaturated soils

Temporal variations in δ15N of NH4+ and NO3− in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate (δ15N=−2.6‰) was added to 25 g samples of soil at concentrations of 160 mg N kg−1. Soils were then incubated under unsaturated (50% of water holding capacity at saturation, WHC) or saturated (100% of WHC) water conditions for 7 and 36 d, respectively. During 7 d incubation of unsaturated soil, the NH4+-N concentration decreased from 164.8 to 34.4 mg kg−1, and the δ15N of NH4+ increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO3−-N concentration and lower δ15N of NO3− (product) than that of NH4+ (substrate) at each sampling time. In saturated soil, the concentration of NH4+-N decreased gradually from 162.4 to 24.2 mg kg−1, and the δ15N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO3− concentration was not observed due to loss of NO3− through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors (αs/p) associated with decrease in NH4+ concentration were 1.04 and 1.01 in unsaturated and saturated soils, respectively. Since nitrification is likely to introduce greater isotope fractionation than microbial immobilization, the higher value for unsaturated soil probably reflected faster nitrification under aerobic conditions. The lower value for saturated soil suggests that immobilization and subsequent remineralization of NH4+ were relatively more dominant than nitrification under the anaerobic conditions.