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

Abstract

Abstract. Soil denitrification is considered the most un-constrained process in the global N cycle due to uncertain in situ N2 flux measurements, particularly in natural and semi-natural terrestrial ecosystems. 15N tracer approaches can provide in situ measurements of both N2 and N2O simultaneously, but their use has been limited to fertilised agro-ecosystems due to the need for large 15N additions in order to detect 15N2 production against the high atmospheric N2. For 15N-N2 analyses, we have used an "in house" laboratory designed and manufactured N2 preparation instrument which can be interfaced to any commercial continuous flow isotope ratio mass spectrometer (CF-IRMS). The N2 prep unit has gas purification steps, a copper based reduction furnace, and allows the analysis of small gas injection volumes (4 μL) for 15N-N2 analysis. For the analysis of N2O, an automated Tracegas Pre-concentrator (Isoprime Ltd) coupled to an IRMS was used to measure the 15N-N2O (4 mL gas injection volume). Consequently, the coefficient of variation for the determination of isotope ratios for N2 in air and in standard N2O (0.5 ppm) was better than 0.5 %. The 15N Gas-Flux method was adapted for application in natural and semi-natural land use types (peatlands, forests and grasslands) by lowering the 15N tracer application rate to 0.04–0.5 kg 15N ha−1. For our chamber design (volume / surface = 8:1) and a 20 h incubation period, the minimum detectable flux rates were 4 μg N m−2 h−1 and 0.2 ng N m−2 h−1 for the N2 and N2O fluxes respectively. The N2 flux ranged between 2.4 and 416.6 μg N m−2 h−1, and the grassland soils showed on average 3 and 14 times higher denitrification rates than the woodland and organic soils respectively. The N2O flux was on average 20 to 200 times lower than the N2 flux, while the denitrification product ratio (N2O/N2 + N2O) was low, ranging between 0.03 and 13 %. Total denitrification rates measured by the acetylene inhibition technique under the same field conditions correlated (r = 0.58) with the denitrification rates measured under the 15N Gas-Flux method but were underestimated by a factor of 4 and this was attributed to the incomplete inhibition of N2O reduction to N2 under relatively high soil moisture content. The results show that the 15N Gas-Flux method can be used for quantifying N2 and N2O production rates in natural terrestrial ecosystems, thus significantly improving our ability to constrain ecosystem N budgets.

Improvement of the 15N gas flux method for in situ measurement of soil denitrification and its product stoichiometry

“…Using the results of the field experiment it was possible to compare 15 NGF+ and conventional 15 NGF because the denitrification rates were high enough due to litter amendment and NO 3 − fertilisation, thus inducing detectable fluxes with both methods. In the past, comparison of different methods to determine denitrification processes had always been complicated by the difficulty in keeping the soil conditions for the compared methods identical . This also applied to our study because we did not have the resources to conduct 15 NGF+ and conventional 15 NGF at exactly the same time.…”

“…When applying the 15 NGF in the field, a closed chamber method is used to accumulate 15 N‐labelled denitrification products emitted from soil in an enclosed air volume over time. Underestimation of denitrification rates can occur because a certain fraction of the 15 N‐labelled denitrification products is not emitted from the soil surface, but diffuses to the subsoil . When applying 15 NGF in the laboratory, the sensitivity can be augmented by incubation under an N 2 ‐depleted atmosphere in a closed system or in an open system where the soil headspace is continuously flushed at a constant rate with an artificial N 2 ‐depleted atmosphere .…”

Soil Greenhouse Gas Fluxes, Environmental Controls, and the Partitioning of N2O Sources in UK Natural and Seminatural Land Use Types

“…Greenhouse gas fluxes were measured monthly using static chambers between April 2013 and October 2014 (total 17 months) with the exception of November 2013 and January 2014, which were not sampled. The measurement of greenhouse gas fluxes was part of a broader study aimed at quantifying in situ N 2 and N 2 O fluxes due to denitrification from the same land use types (Sgouridis & Ullah, ) using an adapted 15 N gas flux method (Mosier & Klemedtsson, ) for low 15 N tracer application, appropriate for natural and seminatural terrestrial ecosystems, which is described in detail in Sgouridis et al (). Briefly, five plots were randomly established in each study site within each catchment amounting to 40 plots per monthly sampling campaign.…”

“…For the natural land use types, the average tracer application rate reflected the current daily estimates of atmospheric N deposition in the UK (0.05 kg N ha −1 d −1 ) (Payne, ), while for the grassland soils the tracer application mimicked a daily fertilizer application rate of 0.5 kg N ha −1 d −1 . The application of the 15 N tracer at these low rates should not be expected to enrich the soil nitrate pool significantly and potentially affect greenhouse gas emissions, in excess of the amount of nitrogen normally deposited via natural processes and common management practices in these land use types (Sgouridis et al, ). Following the 15 N tracer application, the collars were covered with the acrylic chamber fitted with a rubber septum for gas sampling.…”

“…Stable isotope enrichment approaches that involve the addition of 15 N‐NH 4 + and/or 15 N‐NO 3 − to soil have been the most reliable quantification method of N 2 O source partitioning so far, but the need for large amounts of 15 N tracer application has restricted their use to fertilized agro‐ecosystems (Baggs et al, ; Bateman & Baggs, ; Li et al, ; Mathieu et al, ) and forests receiving high rates of atmospheric N deposition (Ambus, Zechmeister‐Boltenstern, & Butterbach‐Bahl, ; Zhu et al, ). However, it was recently shown that low 15 N tracer application methods can be used in situ to quantify N 2 and N 2 O fluxes from natural and seminatural ecosystems (Sgouridis, Stott, & Ullah, ; Sgouridis & Ullah, ) and this approach could be further expanded to account for N 2 O source partitioning (Buchen et al, ; Morse & Bernhardt, ).…”