Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2008
Publication Date: 9/1/2008
Citation: Welp, L.R., Lee, X., Kim, K., Griffis, T.J., Billmark, K., Baker, J.M. 2008. Delta 18O of Evapotranspiration and the Sites of Leaf Evaporation in a Soybean Canopy. Plant Cell and Environment. 31(9):1214-1228. Interpretive Summary: Climate change could potentially have significant impacts on the earth’s hydrologic cycle so it is important to develop methods to track global water movement. Stable isotopes of water are a particularly promising tool, but instruments capable of measuring them are just now becoming available, and have not yet been tested under field conditions. We used one of these new methods, tunable dioded laser spectroscopy to measure the ratio of 18-O to 16-O of evapotranspiration above a soybean canopy through an entire growing season. In addition, the isotope ratios of leaf water, soil water, and dew were also measured. The data were used to test models of water transport in plants, and to estimate isotopic enrichment at the site of evaporation in leaves. We found that steady-state models predicted leaf isotopic composition reasonably well during midday, but underestimated enrichment at evaporative sites in the evening. Results during dewfall showed unexpected isotopic gradients that suggested simultaneous dew formation in the upper canopy and evaporation in the lower canopy and at the soil surface. These data will be useful in refining models of water transport in plant canopies that will help to constrain regional atmospheric models.
Technical Abstract: Stable isotopes in water have the potential to diagnose changes in the Earth’s hydrological budget in response to climate change and land use change. However, there have been few measurements of the isotopic composition of water in the vapor phase (delta v). Here we present high frequency measurements of the oxygen isotopic compositions of water vapor and evapotranspiration (delta ET) above a soybean canopy using the tunable diode laser (TDL) technique for the entire 2006 growing season. We observed large variability in surface delta v from the daily to seasonal timescales that can be largely explained by Rayleigh processes but was also influenced by local evapotranspiration (ET). We used delta ET measurements to calculate the isotopic composition at the sites of evaporative enrichment in leaves (delta L,e) and compared that to the commonly used steady state prediction (delta L,s). During mid-day there was good agreement. In the evening, non-steady state conditions caused delta L,s to underestimate delta L,e. We also found that vertical humidity and temperature variability associated with canopy structure must be addressed in canopy-scale leaf water models. Finally, we explored this dataset for direct evidence of the Péclet effect and found that the existing theory did not fully explain the observed variability in delta L,e and bulk leaf water (delta L,b).