Global Research Alliance N2O chamber methodology guidelines: Flux calculations

A critical step in determining soil-to-atmosphere nitrous oxide (N2O) exchange using non-steady state chambers is converting collected gas concentration versus time data to flux values using a flux-calculation (FC) scheme. It is well-documented that different FC schemes can produce different flux estimates for a given set of data, and that scheme selection can be a major source of uncertainty. Available schemes differ in their theoretical basis, computational requirements and performance in terms of both accuracy and precision. Non-linear schemes tend to increase accuracy compared to linear regression, but also can decrease precision. The chamber bias correction method can be used if soil physical data are available, but this introduces additional sources of error. Here, the essential theoretical and practical aspects of the most commonly used FC schemes are described as a basis for their selection and use. A gold standard approach for application and selection of FC schemes is presented as well as alternative approaches based on availability of soil physical property data and frequency of sample collection during each chamber deployment. Additional criteria for scheme selection are provided in the form of an error analysis tool that quantifies statistical performance metrics based on chamber dimensions and sampling duration, soil properties and measurement precision. Example error analyses are presented for hypothetical conditions illustrating how the analysis can be used to guide FC scheme selection, estimate bias and inform design of chambers and sampling regimes.