Release of nitrous oxide and dinitrogen from a transition bog under drained and rewetted conditions due to denitrification: results from a [¹⁵N]nitrate–bromide double-tracer study

Abstract

Denitrification is well known being the most important nitrate-consuming process in water-logged peat soils, whereby the intermediate compound nitrous oxide (N(2)O) and the end product dinitrogen (N(2)) are ultimately released. The present study was aimed at evaluating the release of these gases (due to denitrification) from a nutrient-poor transition bog ecosystem under drained and three differently rewetted conditions at the field scale using a (15)N-tracer approach ([(15)N]nitrate application, 30 kg N ha(-1)) and a common closed-chamber technique. The drained site is characterized by a constant water table (WT) of -30 cm (here referred to as D30), while rewetted sites represent a constant WT of -15 cm, a constant WT of 0 cm (i.e. waterlogged), and an initial WT of 0 cm (which decreased slightly during the experiment), respectively, (here referred to as R15, R0, and R0(d), respectively). The highest N(2)O emissions were observed at D30 (291 µg N(2)O-N m(-2) h(-1)) as well as at R0d (665 µg N(2)O-N m(-2) h(-1)). At the rewetted peat sites with a constant WT (i.e. R15 and R0), considerably lower N2O emissions were observed (maximal 37 µg N(2)O-N m(-2) h(-1)). Concerning N(2) only at the initially water-logged peat site R0d considerable release rates (up to 3110 µg N(2)-N m(-2) h(-1)) were observed, while under drained conditions (D30) no N(2) emission and under rewetted conditions with a constant WT (R15 and R0) significantly lower N(2) release rates (maximal 668 µg N(2)-N m(-2) h(-1)) could be detected. In addition, it has been found that natural WT fluctuations at rewetted peat sites, in particular a rapid drop down of the WT, can induce high emission rates for both N(2)O and N(2).

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

“…For instance, Rückauf et al (2004) [29] performed 15 N tracer experiments in drained and reflooded microcosms filled with fen peat and found that denitrification was the main N transformation process, whereas N 2 O emission from reflooded (anoxic) conditions was significantly lower than that from drained microcosms. Similar results were gained in field studies by Tauchnitz et al (2015) [30] with 15 N tracer studies on nitrogen gases released from a transition bog and found high N 2 O and low N 2 from drained conditions and the reversed situation in rewetted cases. Likewise, Yang et al (2011) [31] found significantly higher emission from drained soils with oxic conditions, using field-based 15 N-N 2 O pool dilution technique to measure gross N 2 O production in soil.…”

“…We also observed temporal variation of emissions from each position. High spatial and temporal variability of fluxes seems to be typical for N2O and has been observed in both mineral and organic soils [7][8][9]27,30,37,38,40]. Characteristically large temporal variations of N2O emissions have also reported in a review paper by Henault et al (2012) [45].…”

Application of the <sup>15</sup>N-Gas Flux method for measuring in situ N<sub>2</sub> and N<sub>2</sub>O fluxes due to denitrification in natural and semi-natural terrestrial ecosystems and comparison with the acetylene inhibition technique

“…The average 15 N tracer application rate (0.04-0.5 kg 15 N ha −1 or 0.4-1.2 mg 15 N kg −1 dry soil) across land use types was 1 to 2 orders of magnitude lower than previous applications of the 15 N gas-flux method in highly fertilized agricultural systems (Baily et al, 2012;Bergsma et al, 2001;Cuhel et al, 2010;Graham et al, 2013) and in restored peatland soils (Tauchnitz et al, 2015). The estimated enrichment of the total soil NO − 3 pool was variable (2-40 15 N at %, Table S2) and this wide range was due to the fact that the tracer concentration was calculated based on the previous campaign's soil nitrate data, which in some cases did not reflect the soil nitrate content on the day of the tracer application a month later.…”

“…Most studies using 15 N tracers and static chambers in highly fertilized systems typically deploy their chambers between 1 and 2 h (Baily et al, 2012;Cuhel et al, 2010;Tauchnitz et al, 2015), but it has been shown that longer incubation periods (up to 24 or 48 h) may be needed in case of low 15 N enrichment applications in intact soil cores (Morse and Bernhardt, 2013) and laboratory incubations (Yang et al, 2014) for a more precise and accurate detectable 15 N-N 2 signal. However, it should be noted that in these cases the soil cores or slurries were incubated in fully enclosed systems and were thus not affected by potential bias from diffusion of evolved N 2 and N 2 O to the subsoil (Clough et al, 2005).…”

“…Assuming that both N 2 and N 2 O originate from the same uniformly labelled soil NO − 3 pool (Stevens and Laughlin, 2001), the true denitrification product ratio can be more accurately estimated as opposed to the direct flux approaches (Bergsma et al, 2001). Field applications of the 15 N gas-flux method so far have been limited to fertilized agro-ecosystems (Baily et al, 2012;Cuhel et al, 2010;Graham et al, 2013) and more recently restored peatland soils (Tauchnitz et al, 2015) with high 15 N tracer application rates (between 10 and 200 kg N ha −1 ), with the exception of Kulkarni et al (2014), who have measured denitrification rates in northern hardwood forests of the USA by adding tracer amounts of 15 Nlabelled nitrate, and Morse and Bernhardt (2013), who applied the same technique in intact soil cores collected from mature and restored forested wetlands in North Carolina, USA. These recent studies hold much promise that the 15 N gas-flux method can be applied to a range of natural and semi-natural terrestrial ecosystems, allowing the quantification of the relative magnitude of N 2 and N 2 O fluxes due to denitrification from these under-represented ecosystems.…”