Vertical Patterns of Nitrogen Transformations during Infiltration in Two Wetland Soils

Abstract

The (sup15)N isotope dilution and pairing methods were applied to investigate the vertical distribution of nitrogen transformations during infiltration in one peaty soil and one sandy soil. Water containing (sup15)N-nitrate (99.9%; 200 (mu)M) as the only nitrogen fraction was infiltrated through cores containing homogenized soil, with lengths varying from 5.5 to 38 cm. Oxygen and nitrogen dynamics were investigated by measuring inflowing and outflowing water. The experimental design allowed determinations of vertical profiles of aerobic respiration, nitrification, and coupled and uncoupled denitrification and ammonification. In the sandy soil, all oxygen was consumed in the upper 14 cm and nitrate was subsequently consumed and removed, up to a maximum of 70% in the longest core (28 cm). In the peaty soil, oxygen was consumed in the upper 7.5 cm and all nitrate was denitrified in the top 20 cm. In both soils, nitrogen removal by denitrification was counteracted by the release of ammonium and dissolved organic nitrogen. In the sandy soil, net nitrogen removal occurred in cores of 14 cm and longer; in the longest core, 40% was removed. In the peaty soil, release was equal to removal in the top 14 cm but release exceeded removal in the deeper layers, leading to a 100% increase of total nitrogen in the effluent water from the longest core (38 cm).