Natural Nitrogen Isotope Ratios as a Potential Indicator of N2O Production Pathways in a Floodplain Fen

Abstract

Nitrous oxide (N2O), a major greenhouse gas and ozone depleter, is emitted from drained organic soils typically developed in floodplains. We investigated the effect of the water table depth and soil oxygen (O2) content on N2O fluxes and their nitrogen isotope composition in a drained floodplain fen in Estonia. Measurements were done at natural water table depth, and we created a temporary anoxic environment by experimental flooding. From the suboxic peat (0.5–6 mg O2/L) N2O emissions peaked at 6 mg O2/L and afterwards decreased with decreasing O2. From the anoxic and oxic peat (0 and >6 mg O2/L, respectively) N2O emissions were low. Under anoxic conditions the δ15N/δ14N ratio of the top 10 cm peat layer was low, gradually decreasing to 30 cm. In the suboxic peat, δ15N/δ14N ratios increased with depth. In samples of peat fluctuating between suboxic and anoxic, the elevated 15N/14N ratios (δ15N = 7–9‰ ambient N2) indicated intensive microbial processing of nitrogen. Low values of site preference (SP; difference between the central and peripheral 15N atoms) and δ18O-N2O in the captured gas samples indicate nitrifier denitrification in the floodplain fen.

High denitrification potential but low nitrous oxide emission in a constructed wetland treating nitrate-polluted agricultural run-off

“…From previous studies, it is known that the fluctuating water table and pulsing water regime are enhancing N 2 O emissions from various wetlands (Jia et al, 2011;Mander et al, 2011;Kaushal et al, 2014). Most likely, this is due to the better aeration of temporarily inundated wetland sediments, which inhibits the full path of denitrification and enhances N 2 O release from other processes such as nitrification (Ji et al, 2011), DNRA (Jahangir et al, 2017;Wang et al, 2018) and nitrifier denitrification (Masta et al, 2020). Expectedly, one could see that phenomenon in Rampillon where the water table frequently fluctuates.…”