Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2003
Publication Date: 7/1/2003
Citation: Ferretti, D.F., Pendall, E., Morgan, J.A., Nelson, J.A., Lecain, D.R., Mosier, A.R. 2003. Partitioning evapotranspiration fluxes from a colorado grassland using stable isotopes: seasonal variations and implications for elevated atmospheric co2. Plant and Soil Journal 254:291-303. Interpretive Summary: Understanding the dynamics of evapotranspiration (e.g., water loss) from rangelands and how it is affected by management and climate is important since soil water availability is the most limiting resource in rangelands around the world. The partitioning of evapotranspiration between evaporation, water loss from the soil surface, and transpiration, water lost from the plants, is an important part of understanding evapotranspiration dynamics, yet it is a very difficult to measure. A new technique using variation in stable isotopes of oxygen which naturally occur in water was used to partition evapotranspiration into evaporation and transpiration in a Colorado shortgrass prairie. The experiment was conducted at present ambient CO2 concentrations and also at double present ambient CO2 concentrations to study the effect rising levels of atmospheric CO2 will have on water relations in this semi-arid grassland. The results showed that the efficiency with which plants produce new biomass per unit water transpired increases at elevated CO2 levels, which suggests that plant water relations may be improved by rising atmospheric CO2 concentrations.
Technical Abstract: The stable isotopic composition of soil water is controlled by precipitation inputs, antecedent conditions, and evaporative losses. Because transpiration does not fractionate soil water isotopes, the relative proportions of evaporation and transpiration can be estimated using a simple isotopic mass balance approach. At our site in the shortgrass steppe in semi-arid northeastern Colorado, delta oxygen isotope values of soil water were almost always more enriched than those of precipitation inputs, owing to evaporative losses. The proportion of water lost by evaporation (E/ET) during the growing season ranged from nil to about 40% (to >90% in the dormant season), and was related to the timing of precipitation inputs. The sum of transpiration plus evaporation losses estimated by isotopic mass balance were similar to actual evapotranspiration measured from a nearby Bowen ratio system. We also investigated the evapotranspiration response of this mixed C3/C4 grassland to doubled atmospheric [CO2] using Open-Top Chambers (OTC). Elevated atmospheric [CO2] led to increased soil-water conservation via reduced stomatal conductance, despite greater biomass growth. We used a non-invasive method to measure the delta oxygen isotope of soil CO2 as a proxy for soil water, after establishing a strong relationship between delta oxygen isotope of soil CO2 from non-chambered control (NC) plots and delta oxygen isotope of soil-water from an adjacent area of native grassland. Soil-CO2 delta oxygen isotope values showed significant treatment effects, particularly during a dry summer: values in ambient chambers (AC) were more enriched than in NC and elevated chamber (EC) plots. During the dry growing season of 2000, transpiration from the EC treatment was higher than from AC and lower than from NC treatments, but during 2001, transpiration was similar on all three treatments. Slightly higher evaporation rates from AC than either EC or NC treatments in 2000 may have resulted from increased convection across the soil surface from the OTC blowers, combined with lower biomass and litter cover on the AC treatment. Transpiration-use efficiency, or the amount of above-ground biomass produced per mm water transpired, was always greatest on EC and lowest on NC treatments.