Global Research Alliance N₂O chamber methodology guidelines: Statistical considerations, emission factor calculation, and data reporting

Abstract

Static chambers are often used for measuring nitrous oxide (N 2 O) fluxes from soils, but statistical analysis of chamber data is challenged by the inherently heterogeneous nature of N 2 O fluxes. Because N 2 O chamber measurements are commonly used to assess N 2 O mitigation strategies or to determine country-specific emission factors (EFs) for calculating national greenhouse gas inventories, it is important that statistical analysis of the data is sound and that EFs are robustly estimated. This paper is one of a series of articles that provide guidance on different aspects of N 2 O chamber methodologies. Here, we discuss the challenges associated with statistical analysis of heterogeneous data, by summarizing statistical approaches used in recent publications and providing guidance on assessing normality and options for transforming data that follow a non-normal distribution. We also recommend minimum requirements for reporting of experimental and metadata of N 2 O studies to ensure that the robustness of the results can be reliably evaluated. This includes detailed information on the experimental site, methodology and measurement procedures, gas analysis, data and statistical analyses, and approaches to generate EFs, as well as results of ancillary measurements. The reliability, robustness, and comparability of soil N 2 O emissions data will be improved through (a) application, and reporting, of more rigorous methodological standards by researchers and (b) greater vigilance by reviewers and scientific editors to ensure that all necessary information is reported in scientific publications.

Global Research Alliance N2O chamber methodology guidelines: Recommendations for deployment and accounting for sources of variability

“…Fully replicated untreated control plots are recommended to assess background emissions, which will vary spatially and temporally and are required for the calculation of EFs (de Klein, Alfaro, et al., 2020). In addition, pretreatment N 2 O flux measurements for treated plots provides information on preexisting spatial patterns of emissions, which can be used as covariates in statistical analyses.…”

“…Furthermore, inhibitors often add further N to treatment plots (e.g., dicyandiamide [DCD] contains 67% N), and not all studies account for this in EF calculations. Greater standardization in experimental protocols and EF calculations are required to facilitate the use of EFs as comparable indicators (de Klein, Alfaro, et al., 2020).…”

“…To minimize the uncertainty in N 2 O estimates and EF calculations due to treatment application, researchers should consider (a) the treatment application method; (b) appropriate application rates for the treatment being investigated, but ensuring sufficient treatment or N to induce a discernible effect; (c) the treatment area (and potential diffusive area) and distribution within the plot (and the necessary chamber size; Section 3.3. ); (d) the delivery method of treatments with additives; (e) how repeated or overlapping treatments will be accounted for; and (f) the EF calculation (de Klein, Alfaro, et al., 2020), to ensure the chosen approach is appropriate to the study aim(s) and site(s). Table 1 summarizes our recommendations for reducing uncertainty from the spatial variability of N 2 O fluxes.…”

“…Accordingly, journals are increasingly requiring more than one site year of N 2 O flux data. Researchers should consider this in grant applications, experiment planning, and overall use of the resulting emissions data, as single‐year measurements are still useful for model validation and in future meta‐analyses (especially if appropriate meta‐data are included in the study; de Klein, Alfaro, et al., 2020).…”

Global Research Alliance N2O chamber methodology guidelines: Introduction, with health and safety considerations

“…The latter paper also provides supplemental information in the form of spreadsheets that perform site‐specific error analyses and example calculations. Lastly, the “data reporting” paper (de Klein et al., 2020) has been expanded to include a review of recent approaches to statistical and meta‐analysis of N 2 O flux data and emission factors, and associated requirements for data reporting.…”