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SOIL FERTILITY & PLANT NUTRITION

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

^a Dep. of Soil, Water and Environmental Science, Univ. of Arizona, 1177E. 4th St., 429 Shantz Building, Tucson, AZ 85721
^b Dep. of Plant and Soil Science, Texas Tech Univ., 15th and Detroit, Lubbock, TX 79409-3121

^* Corresponding author (thomas.thompson{at}ttu.edu).

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.

Abbreviations: DEA, denitrifying enzyme activity • FACE, Free-Air Carbon Dioxide Enrichment • GC, gas chromatography • SOC, soluble organic carbon • WFPS, water-filled pore space