Abstract
Abstract. Soils are highly complex physical and biological systems, and hence measuring soil gas exchange fluxes with high accuracy and adequate spatial representativity remains a challenge. A technique which has become increasingly popular is the eddy covariance (EC) method. This method takes advantage of the fact that surface fluxes are mixed into the near-surface atmosphere via turbulence. As a consequence, measurements with an EC system can be done at some distance above the surface, providing accurate and spatially integrated flux density estimates. In this paper we provide a basic overview targeting scientists who are not familiar with the EC method. This review gives examples of successful deployments from a wide variety of ecosystems. The primary focus is on the three major greenhouse gases: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Several limitations to the application of EC systems exist, requiring a careful experimental design, which we discuss in detail. Thereby we group these experiments into two main classes: (1) manipulative experiments, and (2) survey-type experiments. Recommendations and examples of successful studies using various approaches are given, including the combination of EC flux measurements with online measurements of stable isotopes. We conclude that EC should not be considered a substitute to traditional (e.g., chamber based) flux measurements but instead an addition to them. The greatest strength of EC measurements in soil science are (1) their uninterrupted continuous measurement of gas concentrations and fluxes that can also capture short-term bursts of fluxes that easily could be missed by other methods and (2) the spatial integration covering the ecosystem scale (several square meters to hectares), thereby integrating over small-scale heterogeneity in the soil.
Highlights
Soils are highly complex physical and biological systems which have long been, and still are, a challenge for scientists who study soil processes in situ
The eddy covariance method is a micrometeorological technique to quantify surface flux densities of many trace gases produced by soil organisms or taken up by soils and the vegetation
The eddy covariance (EC) method’s main advantage compared to other methods is its spatial scale of integration, ranging from several square meters to a hectare and more, depending on measurement height, and its temporal coverage that includes all turbulent timescales that are relevant for trace gas mixing in the near-surface atmosphere from fractions of seconds to a typical 30 min averaging interval and longer
Summary
Soils are highly complex physical and biological systems which have long been, and still are, a challenge for scientists who study soil processes in situ. Even simple soils that purely consist of quartz sand exhibit a heterogeneity that leaves experimental scientists with an impressively large uncertainty when measuring gas fluxes from the soil to the atmosphere under controlled conditions (Gao et al, 1998a, b; Pumpanen et al, 2004; Pihlatie et al, 2013). In this review we will provide the basic background as to why atmospheric flux measurements can provide sensitive, accurate, and timely quantitative information on soil processes that are otherwise difficult to measure with traditional techniques. At the same time we will discuss the limitations of the eddy covariance technique in soil process research studies and provide suggestions, how to design field experiments when using the eddy covariance technique, and when to rather deploy traditional enclosures or chambers for flux measurements.
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