Effects of Moisture on Soil CO₂ Efflux in a Cotton Field in Northwestern China

In order to explore the effect of moisture on soil CO₂ effluxes in a Cotton field in Northwestern China, soil profile CO₂ concentration, soil profile temperature and moisture at depths 5, 10 and 15 cm were measured simultaneously. Based on the models of Fick first law of diffusion, Moldrup model, gas transport equation, van’t Hoffequation, exponential equation and quadratic function equation, the effects of soil moisture and temperature on soil CO₂ efflux (F₍₀₎) was determined. The results were as following: (1) F₍₀₎ has an optimal soil moisture, and the optimal moisture of F₍₀₎ is close to the field capacity (FC). In this study, the field capacity (FC) was close to 0.278 m³/m³. When the values of soil moisture were higher than the optimal soil moisture values of F₍₀₎, soil moisture is the main limiting factor for F₍₀₎, and F₍₀₎ is negatively correlated with soil moisture at soil moisture value >0.278 m³/m³ (FC). However, the values of F₍₀₎ increased with the increase of soil moisture when the values of soil moisture were lower than the optimal soil moisture values of F₍₀₎. (2) Soil moisture affected the temperature responses of F₍₀₎ by a clear threshold inflection point, which significantly influence the temperature sensitivity of F₍₀₎. We concluded that the scale of changes in soil moisture is significant to F₍₀₎. These factors should be incorporated into models to improve the prediction of carbon-climate feedbacks.