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

摘要

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.