Natural Nitrogen Isotope Ratios as a Potential Indicator of N<sub>2</sub>O Production Pathways in a Floodplain Fen
Mohit Masta | Holar Sepp | Jaan Pärn | Kalle Kirsimäe | Ülo Mander
Nitrous oxide (N<sub>2</sub>O), 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 (O<sub>2</sub>) content on N<sub>2</sub>O 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 O<sub>2</sub>/L) N<sub>2</sub>O emissions peaked at 6 mg O<sub>2</sub>/L and afterwards decreased with decreasing O<sub>2</sub>. From the anoxic and oxic peat (0 and >6 mg O<sub>2</sub>/L, respectively) N<sub>2</sub>O emissions were low. Under anoxic conditions the δ<sup>15</sup>N/δ<sup>14</sup>N ratio of the top 10 cm peat layer was low, gradually decreasing to 30 cm. In the suboxic peat, δ<sup>15</sup>N/δ<sup>14</sup>N ratios increased with depth. In samples of peat fluctuating between suboxic and anoxic, the elevated <sup>15</sup>N/<sup>14</sup>N ratios (δ<sup>15</sup>N = 7−9‱ ambient N<sub>2</sub>) indicated intensive microbial processing of nitrogen. Low values of site preference (SP; difference between the central and peripheral <sup>15</sup>N atoms) and δ<sup>18</sup>O-N<sub>2</sub>O in the captured gas samples indicate nitrifier denitrification in the floodplain fen.
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