Effects of degree of peat decomposition, loading rate and temperature on dissolved nitrogen turnover in rewetted fens

Rewetting of drained fens with N O 3 − enriched water from agricultural watersheds has been proposed as a valid strategy to reduce N O 3 − load of water courses although their role as dissolved nitrogen (DN) sinks remain unclear because the export of reduced nitrogen forms ( N H 4 + , DON) may exceed N O 3 − removal. A laboratory experiment was conducted to investigate the importance of temperature, nitrogen load and the degree of peat decomposition on the role of rewetted fens as DN sinks. Different peat substrates of one degraded drained fen in NE Germany were incubated under stagnant water conditions. Two degrees of peat decomposition (Highly decomposed: HD; Moderately decomposed: MD), two nitrogen loads (Low nitrogen, LN, 40 kg N ha−1 yr−1; High nitrogen, HN, 140 kg N ha−1 yr−1) and two incubation temperatures (20 °C and 5 °C) were examined. Moreover, net N mineralisation and N microbial immobilisation were also estimated to gain useful insights on the role of the considered factors over processes involved on DN turnover. Under all scenarios considered in this study, fen rewetting was shown to be a valid strategy to recover the function of fens as DN sinks, although large variability on the retention efficiency was observed (∼15–75%). N load increased N O 3 − (LN: 7.9 ± 1.0 mg N m−2 d−1; HN: 19.3 ± 3.1 mg N m−2 d−1) and DN (LN: 5.1 ± 0.8 mg N m−2 d−1; HN: 16.5 ± 2.4 mg N m−2 d−1) removal, even though the depletion of the added N O 3 − by organic matter decomposition took place only at the peat surface. Peat decomposition strongly influenced DN turnover due to differences in the size of the mobile organic carbon (C) (HD: 132 ± 7 mg C g−1 dry matter (DM); MD: 68 ± 2 mg C g−1 DM) and N (HD: 9.3 ± 0.5 mg N g−1 DM; MD: 2.8 ± 0.1 mg N g−1 DM) pools. As a result, N O 3 − removal, net N mineralisation, N microbial immobilisation and N H 4 + export were higher for highly decomposed peat. In addition, a higher incubation temperature increased N O 3 − removal rates (20 °C: 18.6 ± 3.0 mg N m−2 d−1; 5 °C: 8.5 ± 1.7 mg N m−2 d−1). Moreover, DN removal was much higher at 20 °C (20 °C: 14.9 ± 2.6 mg N m−2 d−1; 5 °C: 6.7 ± 1.6 mg N m−2 d−1), but lower than N O 3 − removal rates underpinning the importance of N mineralisation. The results from our study should be considered prior to restore degraded fens. Removing the highly decomposed peat layer, which has been proposed as a method to reduce nutrient release in rewetted fens, can hamper the N removal potential to some extent, especially at high N loading rates.