Chamber Measurements of Soil Nitrous Oxide Flux: Are Absolute Values Reliable?

The vast majority of soil N 2 O flux data reported in the literature was obtained using non‐flow‐through non‐steady‐state (NFT‐NSS) chambers. Considerable variation in chamber methodology may influence N 2 O flux measurements, however, raising concerns about the reliability and accuracy of these measurements. The objectives of this study were to determine criteria for assessing the quality of soil N 2 O flux measurements made using NFT‐NSS chambers, to evaluate NFT‐NSS chamber methodologies used in the scientific literature, and to propose a minimum set of criteria for NFT‐NSS chamber design and deployment methodology. We identified 16 characteristics of chamber methodology and developed four factors contributing to the quality of N 2 O flux measurements made using NFT‐NSS chambers. We compiled a data set of 356 studies and evaluated the quality of each study against the set of characteristics and factors to determine the confidence in the reported N 2 O flux. Confidence in the absolute flux values reported in about 60% of the studies was estimated to be very low or low due to poor methodologies or incomplete reporting. The confidence in flux measurements improved with time; however, there were still about 50% of recent studies (2005–2007) with low or very low confidence levels. This study has shown that the quality of soil N 2 O flux measurements reported in the literature is often poor. While the flux data obtained may be valid for comparisons between situations (e.g., treatments) within a given study, they are often biased estimates of actual fluxes. We propose a minimum set of criteria for reliable soil N 2 O flux measurements using NFT‐NSS chambers.