Ecosystem-level evapotranspiration and water-use efficiency in the desert biome of Biosphere 2

We estimate whole-system water and carbon fluxes for the desert biome of Bi osphere 2 under two different daily-mean CO2 concentrations: 450 ppmv and 8 50 ppmv. The desert mesocosm occupies an area of approximately 1500 m(2), h as a total atmospheric Volume of about 25000 m(3) and contains a heterogene ous distribution of plants and soils. Atmospheric water content and CO2 con centrations were measured continuously using a variety of sensors, includin g a Li-cor 6262 for CO2 deployed within the experimental area. Daily carbon and water budgets were calculated in the desert biome, isolated from the r est of Biosphere 2 by deploying isolation curtains for 24-h periods. Data c ollected for six closure periods suggest that elevated CO2 concentration in creased whole-system carbon uptake, while evapotranspiration remained const ant. As a result, whole-system water-use efficiency (WUE, defined as net ec osystem carbon uptake per unit water transpired) in the Biosphere 2 desert increased by more than 40%. Our measurements investigate soil-plant process es at a medium scale, ideally bridging the gap between traditional controll ed-environment growth chambers and open-held studies. (C) 1999 Elsevier Sci ence B.V. All rights reserved.