Accounting for Time‐Variable Soil Porosity Improves the Accuracy of the Gradient Method for Estimating Soil Carbon Dioxide Production

Soil porosity is usually taken as a constant over time for a given field, although in reality it decreases with time after tillage. For the gradient method, estimating soil CO 2 production with a fixed porosity may lead to large errors when soil porosity varies over time. In this study, we compared soil air‐filled porosity, gas diffusivity, and CO 2 production based on a temporally variable soil porosity (ϕ V ) with those based on a constant porosity, either initial porosity just after soil tillage (ϕ i ) or final porosity at harvest after a tilled soil has settled (ϕ f ). Soil porosity was measured seven times during a maize ( Zea mays L.) growing season, and an exponential relationship of soil porosity with time was developed to describe ϕ V for the 0‐ to 5‐cm soil layer. Soil CO 2 production was estimated from the gradient method and the mass conservation law. Soil‐surface CO 2 efflux was measured with a dynamic chamber throughout the growing season. The ϕ i value was 0.49 m 3 m −3 and the ϕ f value was 0.43 m 3 m −3 . Compared with results obtained from ϕ V , soil air‐filled porosity, gas diffusivity, and CO 2 production values obtained from ϕ f were 6, 11, and 22% lower, whereas values obtained from ϕ i were 17, 36, and 70% larger. The soil‐surface CO 2 effluxes estimated with ϕ V better matched the chamber values than did the estimates with ϕ i or ϕ f . We conclude that use of variable soil porosity improves estimations of soil‐surface CO 2 effluxes and soil CO 2 production with the gradient method.