NITROGEN TRANSFORMATIONS IN WETLAND SOIL CORES MEASURED BY N-15 ISOTOPE PAIRING AND DILUTION AT 4 INFILTRATION RATES

The effect of water infiltration rate (IR) on nitrogen cycling in a sa turated wetland soil was investigated by applying a N-15 isotope dilut ion and pairing method. Water containing [N-15]nitrate was infiltrated through 10-cm-long cores of sieved and homogenized soil at rates of 7 2, 168, 267, and 638 mm day(-1). Then the frequencies of N-30(2), N-29 (2), (NO3-)-N-15, and (NH4+)-N-15 in the outflow water were measured, This method allowed simultaneous determination of nitrification, coupl ed and uncoupled denitrification, and nitrate assimilation rates. From 3% (at the highest IR) to 95% (at the lowest IR) of nitrate was remov ed from the water, mainly by denitrification, The nitrate removal was compensated for by the net release of ammonium and dissolved organic n itrogen, Lower oxygen concentrations in the soil at lower IRs led to a sharper decrease in the nitrification rate than in the ammonification rate, and, consequently, more ammonium leaked from the soil. The decr easing organic-carbon-to-nitrogen ratio (from 12.8 to 5.1) and the inc reasing light A(250)/A(365) ratio (from 4.5 to 5.2) indicated an incre asing bioavailability of the outflowing dissolved organic matter with increasing IR, The efflux of nitrous oxide was also very sensitive to IR and increased severalfold when a zone of low oxygen concentration a as close to the outlet of the soil cores. N2O then constituted 8% of t he total gaseous N lost from the soil.