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

Abstract

The impacts of increasing atmospheric CO2, an important greenhouse gas, on soil microbial production and consumption of other greenhouse gases such as N2O are uncertain. This study was conducted during the 1998 and 1999 summer growing seasons at the Free‐Air CO2 Enrichment (FACE) site in Maricopa, AZ. The objective was to measure N2O and denitrification emission rates in a C4 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) CO2 Plots were sampled for N2O flux using both chamber and intact incubated soil core techniques. Nitrogen gas (N2O plus N2) emissions were measured using intact incubated soil cores with C2H2 inhibition. Nitrous oxide emissions measured with chambers increased markedly after irrigation and fertilization following prolonged periods without water under both elevated and control CO2 conditions. Within 5 d of fertilization and irrigation, N2O emissions measured with chambers were <250 g N2O‐N ha−1 d−1 until subsequent irrigations. Emissions measured from cores ranged from −0.11 to >250 g N2O‐N ha−1 d−1 Seasonal cumulative N2O‐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 CO2 and the high irrigation treatment. During periods when significant emissions were recorded, the primary end product of denitrification was N2 rather than N2O. Water‐filled pore space (WFPS) was the most important single factor controlling N‐gas emissions, with the largest emissions (>500 g N2O‐N ha−1 d−1) coming with >55% WFPS. Neither soil NO3− nor soil organic C alone limited N gas emissions. Elevated CO2 did not result in increased N2O or N‐gas emissions with either ample or limited irrigation.