Effects of free-air carbon dioxide enrichment (FACE) and water stress on the energy balance and evapotranspiration of sorghum

Abstract

PROBLEM: Atmospheric carbon dioxide (CO₂) levels have increased from 280 µmol mol^-1 in the eighteenth century to over 360 µmol mol^-1 today, and they are expected to double in the next hundred years. Concomitant with an increase in CO₂ are predicted changes to lower atmospheric temperature and precipitation. Such variations in CO₂ and climate could alter the hydrologic cycle and future water resources.

One significant component of the hydrologic cycle that could be affected is transpiration (T), shown to decrease with increasing CO₂ due to decreased stomatal conductance to water vapor. This may be partially offset by a demonstrated increase in growth, which could lead to an increase in evapotranspiration (ET) per unit land area. This investigation sought to determine this effect by measuring ET and plant growth for sorghum grown at two CO₂ and two water treatments.

HYPOTHESIS: We hypothesize that for sorghum exposed to higher CO₂, there is a reduction in ET. While sorghum is expected to exhibit a decrease in stomatal conductance to water vapor, we do not expect a large increase in biomass. Therefore the net water use per unit area will be decreased.

METHOD: Field-grown grain sorghum [Sorghum bicolor (L.) DK54] was exposed to CO₂ concentrations enriched to 200 µmol mol^-1 above ambient (ca. ~370 µmol mol^-1) using free-air CO₂ enrichment (FACE) apparatus. Half of each main CO₂ plot received ample irrigation (Wet), while the other half received a deficit amount thereby exposing the plants to severe drought (Dry). Measurements were made of net radiation, canopy temperature, air temperature, soil heat flux, and wind speed. Energy balance components, including evapotranspiration, will be calculated from the data and comparisons made among the plots.

RESULTS: While the energy balance data analyses for the sorghum are ongoing, in a prior FACE experiment cotton experienced a large growth response (40% increase) to the elevated CO₂; however, no effect on ET was detectable. In contrast, FACE wheat had a moderate growth response (about 20%), and ET was reduced by nearly 7% for well-watered, high-nitrogen conditions.

INTERPRETATION: If ET does decrease as we hypothesize under CO₂ enriched conditions, water requirements for sorghum may decrease in the future, provided any global warming is small.