Natural 15N abundances of inorganic nitrogen in soil treated with fertilizer and compost under changing soil moisture regimes

This study was conducted to examine whether the applications of N-inputs (compost and fertilizer) having different N isotopic compositions (delta(15)N) produce isotopically different inorganic-N and to investigate the effect of soil moisture regimes on the temporal variations in the delta(15)N of inorganic-N in soils. To do so, the temporal variations in the concentrations and the delta(15)N of NH4+ and NO3- in soils treated with two levels (0 and 150 mg N kg(-1)) of ammonium sulfate (delta(15)N = - 2.3parts per thousand) and compost (+ 13.9parts per thousand) during a 10-week incubation were compared by changing soil moisture regime after 6 weeks either from saturated to unsaturated conditions or vice versa. Another incubation study using N-15-labeled ammonium sulfate (3.05 N-15 atom%) was conducted to estimate the rates of nitrification and denitrification with a numerical model FLUAZ. The delta N-15 values of NH4+ and NO3- were greatly affected by the availability of substrate for each of the nitrification and denitrification processes and the soil moisture status that affects the relative predominance between the two processes. Under saturated conditions for 6 weeks, the delta N-15 of NH4+ in soils treated with fertilizer progressively increased from + 2.9parts per thousand at 0.5 week to + 18.9parts per thousand at 6 weeks due to nitrification. During the same period, NO3- concentrations were consistently low and the corresponding delta N-15 increased from + 16.3 to + 39.2parts per thousand through denitrification. Under subsequent water-unsaturated conditions, the NO3- concentrations increased through nitrification, which resulted in the decrease in the delta(15)N of NO3-. In soils, which were unsaturated for the first 6-weeks incubation, the delta N-15 of NH4+ increased sharply at 0.5 week due to fast nitrification. On the other hand, the delta N-15 of NO3- showed the lowest value at 0.5 week due to incomplete nitrification, but after a subsequence increase, they remained stable while nitrification and denitrification were negligible between I and 6 weeks. Changing to saturated conditions after the initial 6-weeks incubation, however, increased the delta N-15 of NO3- progressively with a concurrent decrease in NO3- concentration through denitrification. The differences in delta N-15 of NO3- between compost and fertilizer treatments were consistent throughout the incubation period. The delta N-15 of NO3- increased with the addition of compost (range: + 13.0 to + 35.4parts per thousand), but decreased with the addition of fertilizer (- 10.8 to + 11.4parts per thousand), thus resulting in intermediate values in soils receiving both fertilizer and compost (- 3.5 to + 20.3parts per thousand). Therefore, such differences in delta(15)N of NO3- observed in this study suggest a possibility that the delta N-15 of upland-grown plants receiving compost would be higher than those treated with fertilizer because N03 is the most abundant N for plant uptake in upland soils. (C) 2003 Elsevier Ltd. All rights reserved.