Application of the sup15/supN gas-flux method for measuring in situ Nsub2/sub and Nsub2/subO fluxes due to denitrification in natural and semi-natural terrestrial ecosystems and comparison with the acetylene inhibition technique

摘要

Soil denitrification is considered the most un-constrained process in the global N cycle due to uncertain in situ Nsub2/sub flux measurements, particularly in natural and semi-natural terrestrial ecosystems. sup15/supN tracer approaches can provide in situ measurements of both Nsub2/sub and Nsub2/subO simultaneously, but their use has been limited to fertilized agro-ecosystems due to the need for large sup15/supN additions in order to detect sup15/supNsub2/sub production against the high atmospheric Nsub2/sub. For sup15/supN–Nsub2/sub analyses, we have used an “in-house” laboratory designed and manufactured Nsub2/sub preparation instrument which can be interfaced to any commercial continuous flow isotope ratio mass spectrometer (CF-IRMS). The Nsub2/sub prep unit has gas purification steps and a copper-based reduction furnace, and allows the analysis of small gas injection volumes (4?μL) for sup15/supN–Nsub2/sub analysis. For the analysis of Nsub2/subO, an automated Tracegas Preconcentrator (Isoprime Ltd) coupled to an IRMS was used to measure the sup15/supN–Nsub2/subO (4?mL gas injection volume). Consequently, the coefficient of variation for the determination of isotope ratios for Nsub2/sub in air and in standard Nsub2/subO (0.5?ppm) was better than 0.5?%. The sup15/supN gas-flux method was adapted for application in natural and semi-natural land use types (peatlands, forests, and grasslands) by lowering the sup15/supN tracer application rate to 0.04–0.5?kg?sup15/supN?hasup?1/sup. The minimum detectable flux rates were 4?μg?N?msup?2/sup?hsup?1/sup and 0.2?ng?N?msup?2/sup?hsup?1/sup for the Nsub2/sub and Nsub2/subO fluxes respectively. Total denitrification rates measured by the acetylene inhibition technique in the same land use types correlated (ir/i?=??0.58) with the denitrification rates measured under the sup15/supN gas-flux method, but were underestimated by a factor of 4, and this was partially attributed to the incomplete inhibition of Nsub2/subO reduction to Nsub2/sub, under a relatively high soil moisture content, and/or the catalytic NO decomposition in the presence of acetylene. Even though relatively robust for in situ denitrification measurements, methodological uncertainties still exist in the estimation of Nsub2/sub and Nsub2/subO fluxes with the sup15/supN gas-flux method due to issues related to non-homogenous distribution of the added tracer and subsoil gas diffusion using open-bottom chambers, particularly during longer incubation duration. Despite these uncertainties, the sup15/supN gas-flux method constitutes a more reliable field technique for large-scale quantification of Nsub2/sub and Nsub2/subO fluxes in natural terrestrial ecosystems, thus significantly improving our ability to constrain ecosystem N budgets.