Elevated Carbon Dioxide and Irrigation Effects on Soil Nitrogen Gas Exchange in Irrigated Sorghum

The impacts of increasing atmospheric CO 2 , an important greenhouse gas, on soil microbial production and consumption of other greenhouse gases such as N 2 O are uncertain. This study was conducted during the 1998 and 1999 summer growing seasons at the Free‐Air CO 2 Enrichment (FACE) site in Maricopa, AZ. The objective was to measure N 2 O and denitrification emission rates in a C 4 sorghum [ Sorghum bicolor (L.) Moench] production system with ample and limited flood irrigation rates under FACE (seasonal mean = 579 μmol mol −1 ) and control (seasonal mean = 396 μmol mol −1 ) CO 2 Plots were sampled for N 2 O flux using both chamber and intact incubated soil core techniques. Nitrogen gas (N 2 O plus N 2 ) emissions were measured using intact incubated soil cores with C 2 H 2 inhibition. Nitrous oxide emissions measured with chambers increased markedly after irrigation and fertilization following prolonged periods without water under both elevated and control CO 2 conditions. Within 5 d of fertilization and irrigation, N 2 O emissions measured with chambers were <250 g N 2 O‐N ha −1 d −1 until subsequent irrigations. Emissions measured from cores ranged from −0.11 to >250 g N 2 O‐N ha −1 d −1 Seasonal cumulative N 2 O‐N emissions measured using chambers were <1.5 kg N ha −1 Seasonal N‐gas losses measured during 1999 were as high as 3.7 kg N ha −1 , and were highest with elevated CO 2 and the high irrigation treatment. During periods when significant emissions were recorded, the primary end product of denitrification was N 2 rather than N 2 O. Water‐filled pore space (WFPS) was the most important single factor controlling N‐gas emissions, with the largest emissions (>500 g N 2 O‐N ha −1 d −1 ) coming with >55% WFPS. Neither soil NO 3 − nor soil organic C alone limited N gas emissions. Elevated CO 2 did not result in increased N 2 O or N‐gas emissions with either ample or limited irrigation.