Theoretical Comparison Of Advanced Methods For Calculating Nitrous Oxide Fluxes Using Non-Steady State Chambers.

Non-steady state (NSS) chambers provide much of the data used in bottom-up assessments of global nitrous oxide (N₂O) emissions.  Due to inherent limitations in NSS chambers and flux calculation (FC) methods, it is likely that these assessments are negatively biased.  Potentially more accurate FC schemes have been developed recently.  However, there is little consensus regarding optimum FC methods and they are often selected without critical evaluation of their theoretical basis.  This study used diffusion modeling to assess the accuracy of several advanced and conventional FC methods under conditions that both adhered to and violated theoretical assumptions upon which the methods are based.  Two methods (NDFE and CBC) having the same theoretical basis but differing computational approaches displayed contrasting behavior.  NDFE tended to overestimate the actual pre-deployment flux (f₀) to an increasing degree as assumptions were increasingly violated.  In contrast, CBC was most accurate over the broadest range of conditions and relatively insensitive to assumption violation, except when unaccounted-for processes increased above certain levels.  HMR underestimated f₀ under all conditions but became more accurate relative to NDFE and CBC as lateral gas diffusion and soil biological N₂O uptake increased.  This analysis offers new insight into the behavior of FC schemes under varying conditions and provides additional evidence that the most commonly used schemes tend to substantially underestimate f₀.  Improved understanding of the theoretical basis and limitations of FC schemes should promote more prudent use of chambers, development of improved methods, and more accurate N₂O flux estimates at the site- and global-scale.