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Title: Warming and carbon dioxide enrichment alter plant production and ecosystem gas exchange in a semi-arid grassland through direct responses to global change factors and indirect effects on water relations

Author
item Morgan, Jack
item PENDALL, ELISE - UNIVRSITY OF WYOMING
item WILLIAMS, DAVID - UNIVERSITY OF WYOMING
item BACHMAN, SARAH - UNIVERSITY OF WYOMING
item Dijkstra, Feike
item Lecain, Daniel
item Follett, Ronald

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 10/15/2007
Publication Date: 12/10/2007
Citation: Morgan, J.A., Pendall, E., Williams, D.G., Bachman, S., Dijkstra, F.A., Lecain, D.R., Follett, R.F. 2007. Warming and carbon dioxide enrichment alter plant production and ecosystem gas exchange in a semi-arid grasslands through direct responses to global change factors and indirect effects on water relations. American Geophysical Union Meeting Abstract.

Interpretive Summary:

Technical Abstract: The Prairie Heating and CO2 Enrichment (PHACE) experiment was initiated in Spring 2007 to evaluate the combined effects of warming and elevated CO2 on a northern mixed-grass prairie (NMP). Thirty 3-m diameter circular experimental plots were installed in Spring, 2006 at the USDA-ARS High Plains Grasslands Research Station, just west of Cheyenne, WY, USA. Twenty plots were assigned to a two level factorial combination of two CO2 concentrations (present ambient [380 ppm], and elevated [600 ppm] CO2, and two levels of temperature (present ambient, and elevated temperature [1.5/3.0 C warmer day/night]), with five replications for each treatment. Five of the ten remaining plots were subjected to either frequent, small water additions throughout the growing season, and the other five to a deep watering once or twice during the growing season. The watering treatments were imposed to simulate hypothesized water saving in the CO2-enriched plots, and to contrast the influence of variable water dynamics on ecosystem processes. Carbon dioxide enrichment of the 10 CO2-enriched plots is accomplished with Free Air CO2 Enrichment (FACE) technology and occurs during daylight hours of the mid-April – October growing season. Warming is done year-round with circularly-arranged ceramic heater arrays positioned above the ring perimeters, and with temperature feed-backs to control day/night canopy surface temperatures. Carbon dioxide enrichment began in Spring, 2006, and warming was added in Spring, 2007. Results from the first year of CO2 enrichment experiment (2006) confirmed earlier reports that CO2 increases productivity in semi-arid grasslands (21% increase in peak seasonal above ground biomass for plants grown under elevated CO2 compared to non-enriched controls), and that the response was related to CO2-induced water savings. Growth at elevated CO2 reduced leaf carbon isotope discrimination and N concentrations in plants compared to results obtained in control plots, but the magnitude of changes were highly species specific. Ecosystem-level gas exchange measurements indicated that interactions between watering and CO2 enrichment increased C cycling over a range of soil moisture conditions, although watering had a greater relative impact on C fluxes than CO2 enrichment. Results from the combined warming and CO2 enrichment experiment in 2007 indicate soil fluxes of CO2 increased with elevated CO2 and warming, but decreased with warming later in the year compared to un-heated controls. Soil CH4 uptake was enhanced by elevated CO2 but reduced by warming, particularly later in the year. Soil fluxes of N2O were unaffected by treatment. These preliminary results indicate potentially strong feedbacks between carbon cycling and warming are mediated by ecosystem processes in this semiarid rangeland.