Influence of air porosity on distribution of gases in soil under assay for denitrification

There has been concern that the measurement of gas emissions from a soil surface may not accurately reflect gas production within the soil profile. But, there have been few direct assessments of the error associated with the use of surface emissions for estimating gas production within soil profiles at different water contents. To determine the influence of air porosity on the distribution of gases within soil profiles, denitrification assays were performed using soil columns incubated with different water contents to provide air porosities of 18%, 13%, and 0% (equivalent to 62%, 73%, and 100% water-filled pore space, respectively). The soil columns were formed by packing sieved soil into cylinders which could be sealed at the top to form a headspace for the measurement of surface emissions of soil gases. Gas-permeable silicone tubing was placed at three depths (4.5, 9, and 13.5 cm) within each soil core to permit the measurement of gas concentration gradients within the soil core. Assays for denitrification were initiated by the addition of acetylene (5 kPa) to the soil column, and gas samples were taken from both the headspace and gas-permeable tubing at various times during a 46-h incubation. The results showed that at 18% air porosity, the headspace gases were well equilibrated with pore-space gases, and that gas emissions from the soil could provide good estimates of N2O and CO2 production. At air porosities of 13% and 0%, however, substantial storage of these gases occurred within the soil profiles, and measurements of surface emissions of gas from the soils greatly underestimated gas production. For example, the sole use of N2O emission measurements caused three to five fold underestimates of N2O production in soil maintained at 13% air porosity. It was concluded that the confounding influence of soil moisture on gas production and transport in soil greatly limits the use of surface emissions as a reliable indicator of gas production. This is particularly pertinent when assessing processes such as denitrification in which N gas production is greatly promoted by the conditions that limit O-2 influx and concurrently limit N gas efflux.