Short-term nitrogen transformation rates in riparian wetland soil determined with nitrogen-15

Abstract

N transformation rates in soil from a riparian wetland that receives runoff from adjacent pastoral land were investigated in a short-term (250 min), anaerobic laboratory incubation (20°C). A joint ¹⁵N tracing-isotope dilution technique was employed that used paired incubations of labelled (99 atom % ¹⁵N) NO₃ ^–-unlabelled NH₄ ⁺ and unlabelled NO₃ ^–-labelled (99 atom % ¹⁵N) NH₄ ⁺ at three N input levels (0.4, 4 and 24 μg N g^–1 soil). At each N input level NO₃ ^– and NH₄ ⁺ were added in equal proportions (0.2, 2 and 12 μg N g^–1 soil). Soil and gas samples were analysed after 10, 70 and 250 min, and the fate of ¹⁵N and N transformation rates were determined for each time period; 0–10 min (phase 1), 10–70 min (phase 2) and 70–250 min (phase 3). N transformation rates for all processes except gross NH₄ ⁺ mineralisation were very high during phase 1. Processes favoured by aerobic conditions, NO₃ ^– immobilisation (0–17% NO₃ ^– removal, 0–8.2 μg N g^–1 soil h^–1), autotrophic nitrification (~2% NH₄ ⁺ removal, 0.58–0.88 μg N g^–1 soil h^–1) and heterotrophic nitrification (11–35 μg N g^–1 soil h^–1) increased with increased N input while the anaerobic dissimilatory NO₃ ^– reduction to NH₄ ⁺ process (1–6% NO₃ ^– removal, 0.48–0.62 μg N g^–1 soil h^–1) decreased, presumably due to the oxidising effect of higher NO₃ ^– inputs. Denitrification (8–78% NO₃ ^– removal, 3.8–9.6 μg N g^–1 soil h^–1) exhibited no clear trend related to N input levels. NH₄ ⁺ immobilisation (39–72% NH₄ ⁺ removal, 15–19 μg N g^–1 soil h^–1) was higher than NO₃ ^– immobilisation. Gross NH₄ ⁺ mineralisation (0.27–0.80 μg N g^–1 soil h^–1) was the only process not detected in phase 1 and one of few processes measurable in phases 2 or 3.