Influence of air porosity on distribution of gases in soil under assay fordenitrification

There has been concern that the measurement of gas emissions from a soil su rface may not accurately reflect gas production within the soil profile. Bu t, 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 wer e performed using soil columns incubated with different water contents to p rovide air porosities of 18%, 13%, and 0% (equivalent to 62%, 73%, and 100% water-filled pore space, respectively). The soil columns were formed by pa cking 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-perm eable silicone tubing was placed at three depths (4.5, 9, and 13.5 cm) with in each soil core to permit the measurement of gas concentration gradients within the soil core. Assays for denitrification were initiated by the addi tion of acetylene (5 kPa) to the soil column, and gas samples were taken fr om 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 g ases were well equilibrated with pore-space gases, and that gas emissions f rom the soil could provide good estimates of N2O and CO2 production. At air porosities of 13% and 0%, however, substantial storage of these gases occu rred within the soil profiles, and measurements of surface emissions of gas from the soils greatly underestimated gas production. For example, the sol e 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 tran sport in soil greatly limits the use of surface emissions as a reliable ind icator of gas production. This is particularly pertinent when assessing pro cesses such as denitrification in which N gas production is greatly promote d by the conditions that limit O-2 influx and concurrently limit N gas effl ux.