Quantifying wind and pressure effects on trace gas fluxes across the soil–atmosphere interface

K R Redeker,A J Baird,Y A Teh

doi:10.5194/bg-12-7423-2015

Abstract

Abstract. Large uncertainties persist in estimates of soil–atmosphere exchange of important trace gases. One significant source of uncertainty is the combined effect of wind and pressure on these fluxes. Wind and pressure effects are mediated by surface topography: few surfaces are uniform and over scales of tenths of a metre to tens of metres, air pressure and wind speed at the ground surface may be very variable. In this paper we consider how such spatial variability in air pressure and wind speed affects fluxes of trace gases. We used a novel nested wind tunnel design comprising a toroidial wind tunnel, in which wind speed and pressure may be controlled, set within a larger, linear wind tunnel. The effects of both wind speed and pressure differentials on fluxes of CO2 and CH4 within three different ecosystems (forest, grassland, peat bog) were quantified. We find that trace gas fluxes are positively correlated with both wind speed and pressure differential near the surface boundary. We argue that wind speed is the better proxy for trace gas fluxes because of its stronger correlation and because wind speed is more easily measured and wind speed measurement methodology more easily standardized. Trace gas fluxes, whether into or out of the soil, increase with wind speed within the toroidal tunnel (+55 % flux per m s−1), while faster, localized surface winds that are external to the toroidal wind tunnel reduce trace gas fluxes (−13 % flux per m s−1). These results are consistent for both trace gases over all ecosystem soil types studied. Our findings support the need for a revised conceptualization of soil–atmosphere gas exchange. We propose a conceptual model of the soil profile that has a "mixed layer", with fluxes controlled by wind speed, wind duration, porosity, water table, and gas production and consumption.

Highlights

Soils play a key role in the production, sequestration, consumption, and release of all climatically important trace gases
Our data show that wind speed was better at predicting trace gas fluxes than pressure differentials (Figs. 2, 3, and 4)
While the physical relationship between pressure and wind is well established, wind speed is not strongly correlated with pressure differences measured between the toroidal and straight-line wind tunnels (Fig. 2; r2 = 0.63)

Summary

Introduction

Soils play a key role in the production, sequestration, consumption, and release of all climatically important trace gases. The movement of gases within soils has been reviewed by, inter alios, Hillel (1998), Scanlon et al (2000), Rolston and Moldrup (2012), and Monson and Baldocchi (2014). Gas movement may occur via diffusion and/or advection. Different types of diffusion can occur in a soil, the most important is “ordinary” or molecular diffusion. Ordinary diffusion involves the transport of a gas along a gas concentration or mole fraction gradient. Ordinary diffusion of a mixture of two gases is usually modelled using Fick’s second law, while, for mixtures of three or more gases, the Stefan–Maxwell equations may be used (Rolston and Moldrup, 2012). Advective fluxes are typically modelled with Darcy’s law which is usually used in combination with the continuity equation

Methods

Results

Conclusion