The joint estimation of soil trace gas fluxes

Soil gas flux is commonly measured by monitoring the change in headspace gas concentration over time Within a sealed compartment at the soil surface. Often, more than one trace gas is monitored at a time (e.g., CO2 and CH4), but the data fit separately. Flux estimates for CO2 and CH4 were obtained simultaneously by minimizing a weighted sum-of-squares error. The approximation of one model parameter for CH4, through theoretical relationship to the respective CO2 parameter, reduced the total parameter count by one and allowed for the joint estimation of one parameter using the combined CO2 and CH4 datasets. The method of joint optimization was compared with separate optimization for two nonlinear models, using both real and simulated data. The datasets were best fit with the jointly optimized models. Furthermore, the jointly optimized models more accurately estimated initial soil-air fluxes (simulated data only). The method of joint optimization is recommended as a means to apply better-fitting nonlinear models to typically small gas sample sets. This method is applicable to any number of trace gases monitored simultaneously.