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Title: Soil-atmosphere exchange of CH4, CO2, NOx, and N2O in the Colorado shortgrass steppe under elevated CO2

Author
item Mosier, Arvin
item Morgan, Jack
item KING, JENNIFER - UNIVERSITY OF MINNESOTA
item Lecain, Daniel
item MILCHUNAS, DANIEL - CO STATE UNIV RETIRED

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2001
Publication Date: 5/20/2002
Citation: Mosier, A.R., Morgan, J.A., King, J.Y., Lecain, D.R., Milchunas, D.G. 2002. Soil-atmosphere exchange of CH4, CO2, NOx, and N2O in the Colorado shortgrass steppe under elevated CO2. Plant and Soil Journal 240:201-211.

Interpretive Summary: While the accumulation of greenhouse gases in Earth’s atmosphere is attributed mostly to fossil fuel burning, there is a keen interest in understanding the role of terrestrial ecosystems and agriculture in this phenomenon since they also participate in the exchange of greenhouse gases with the atmosphere. Research was conducted on native shortgrass steppe vegetation in northeastern Colorado to determine whether rising levels of atmospheric CO2 might eventually feed-back and affect land-atmosphere exchange of several common greenhouse gases. Large open-top chambers were placed over the vegetation for five years, and CO2 was increased in some of the chambers from present-day concentrations of 380 parts per million (ppm) to 720 parts ppm, concentrations expected by the end of the century. None of the trace gases evaluated showed altered flux rates due to the CO2 enrichment. The results suggests that present-day evaluations of grassland fluxes do not need to be adjusted for possible feed-backs involving future higher atmospheric CO2 concentrations in projecting future contributions of grasslands to global exchanges of greenhouse gases.

Technical Abstract: In late March 1997, an open-top-chamber (OTC) CO2 enrichment study was begun in the Colorado shortgrass steppe. The main objectives of the study were to determine the effect of elevated CO2 ('720 µmol mol'1) on plant production, photosynthesis, and water use of this mixed C3/C4 plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by CO2 on trace gas exchange. From this study, we report here our weekly measurements of CO2, CH4, NOx and N2O fluxes within control (unchambered), ambient CO2 and elevated CO2 OTCs. Soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2000. Even though both C3 and C4 plant biomass increased under elevated CO2 and soil moisture content was typically higher than under ambient CO2 conditions, none of the trace gas fluxes were significantly altered by CO2 enrichment. Over the 43 month period of observation NOx and N2O flux averaged 4.3 and 1.7 in ambient and 4.1 and 1.7 µg N m'2 hr '1 in elevated CO2 OTCs, respectively. NOx flux was negatively correlated to plant biomass production. Methane oxidation rates averaged '31 and '34 µg C m'2 hr'1 and ecosystem respiration averaged 43 and 44 mg C m'2 hr'1 under ambient and elevated CO2, respectively, over the same time period.