A New Method of Evaluating Gas Fluxes in a Closed Chamber System with Theoretical Consideration for Dynamic Characteristics of a Concentration Sensor

For various physio-ecological investigations of soilplant-atmosphere systems, the accurate determination of the fluxes of gases such as CO2, H2O, and other greenhouse gases is required. The quantification of CO2 fluxes by photosynthesis and respiration of a plant canopy, for example, is crucial not only for understanding crop productivity but also for analyzing a global carbon balance. The quantification of H2O fluxes by evapotranspiration is required to estimate irrigation requirement and water use efficiency in crop fields. The fluxes of CH4 and N2O from agricultural lands have been widely investigated for evaluating their effects on global warming. To measure these gas fluxes, several methods can be used, each with advantages and limitations. Micrometeorological techniques such as the eddy covariance or Bowen ratio methods have a clear advantage in continuous measurements without disturbing the microenvironment of a measuring field (Müller et al., 2009). These methods, however, are not applicable to experiments in small-scale fields where certain assumptions of these methods, such as sufficient fetch length or stable conditions, are not satisfied (Stannard,1997; Baldocchi, 2003). Conversely, chamber methods, where a transparent chamber is placed over vegetation or soil and gas fluxes are estimated from the concentration changes of the gases in the chamber, have remained the sole method in smallscale fields (Steduto et al., 2002). Typically, chamber methods are classified into two categories: (i) closed chamber method and (ii) open chamber method (Livingston and Hutchinson, 1995). The closed chamber method can be advantageous over the open chamber method in terms of system simplicity and has been widely used in numerous studies related to gas flux measurements of different subjects and scales (Wheeler, 1992; Kitano et al., 1997; Scott et al., 1999; Hoffmann et al., 2015; Lesmeister and Koschorreck, 2017). The closed-chamber method estimates gas fluxes by measuring the rate of change in gas concentrations in the chamber air in a short time during chamber closure. The rates of change in gas concentrations are frequently assumed to be constant, and the linear regression function (LR) has been fit to the measured changes of gas concentrations to estimate gas fluxes (Reicosky et al., 1990; Nykanen et al., 2003; Juszczak et al., 2012). However, the nonlinear nature of changes in gas concentration due to diminishing concentration differences of gases between the measured subject and chamber air during chamber closure has been recognized, and there are several studies arguing that the use of LR can lead to underestimation of gas