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SOIL PHYSICS

Effects of Soil Physical Nonuniformity on Chamber-Based Gas Flux Estimates

USDA-ARS, Soil and Water Research Management Unit, 439 Borlaug Hall, 1991 Upper Buford Cir., St. Paul, MN 55108

^* Corresponding author (rod.venterea{at}ars.usda.gov).

Chamber methods for measuring trace gas fluxes are prone to errors resulting in large part from the alteration of near-surface concentration gradients. There is little information available, however, for quantifying these errors or determining how they vary with soil physical properties, chamber deployment methods, and flux calculation schemes. This study used numerical modeling to examine how these factors influence flux estimate errors in physically uniform and nonuniform soil profiles. Errors varied widely among profiles and flux calculation techniques. Soil profiles having identical predeployment fluxes but differing in water content and bulk density generated substantially different flux chamber data. A theoretical flux model that assumes physical uniformity performed relatively well in nonuniform soils but still generated substantial errors. For all flux models, errors were minimized with larger effective chamber heights (h) and shorter deployment times (DT). In light of these findings, recent studies that recommend minimizing h and extending DT to enhance nonlinearity of chamber data need to be reevaluated. It was also determined that random measurement error can result in skewed flux-estimate errors. Selection of chamber and flux calculation methods should consider the physical characteristics of the soil profile as well as measurement error. The techniques presented here can be used to develop soil- and method-specific error estimates.

Abbreviations: CE, comparative error • CS, crusted surface • DK, disk tillage • DT, deployment time • Exp, exponential model • HM, Hutchinson and Mosier • MB, moldboard plow • NDFE, non-steady-state diffusive flux estimator • NT, no-till • Quad, quadratic model • RE, relative error • TF, temperate forest