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

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Temporal variations in δ 15N of NH 4 + and NO 3 − in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate ( δ 15N=−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 NH 4 +-N concentration decreased from 164.8 to 34.4 mg kg −1, and the δ 15N of NH 4 + increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO 3 −-N concentration and lower δ 15N of NO 3 − (product) than that of NH 4 + (substrate) at each sampling time. In saturated soil, the concentration of NH 4 +-N decreased gradually from 162.4 to 24.2 mg kg −1, and the δ 15N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO 3 − concentration was not observed due to loss of NO 3 − through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors ( α s/p) associated with decrease in NH 4 + 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 NH 4 + were relatively more dominant than nitrification under the anaerobic conditions.