Nitrogen transformations in wetland soil cores measured by 15N isotope pairing and dilution at four infiltration rates

Abstract

The effect of water infiltration rate (IR) on nitrogen cycling in a saturated wetland soil was investigated by applying a 15N isotope dilution and pairing method. Water containing [15N]nitrate was infiltrated through 10-cm-long cores of sieved and homogenized soil at rates of 72, 168, 267, and 638 mm day-1. Then the frequencies of 30N2, 29N2, 15NO3-, and 15NH4+ in the outflow water were measured. This method allowed simultaneous determination of nitrification, coupled and uncoupled denitrification, and nitrate assimilation rates. From 3% (at the highest IR) to 95% (at the lowest IR) of nitrate was removed from the water, mainly by denitrification. The nitrate removal was compensated for by the net release of ammonium and dissolved organic nitrogen. Lower oxygen concentrations in the sail 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 decreasing organic-carbon-to-nitrogen ratio (from 12.8 to 5.1) and the increasing light A250/A365 ratio (from 4.5 to 5.2) indicated an increasing 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 was close to the outlet of the soil cores. N2O then constituted 8% of the total gaseous N lost from the soil.