The use of stable isotopes to partition evapotranspiration fluxes into evaporation and transpiration

Variations in microbial community on different materials in Sanya Marine Environment Experimental Station, China

Marine biofouling occurs through the colonization of undesired microorganisms on the surfaces of structures. In this study, four immersion cycles (2, 5, 15, and 25 days) of total immersion in seawater were carried out at the Sanya Marine Environmental Test Station using three materials: industrial pure titanium (Ti), hot-dip zinc (Zn), and glass slide (GS). Three phyla, four classes, and nine bacterial genera were identified. The dominant genera were Pseudomonas, Alteromonas, and Pseudoalteromonas. The number of bacteria increased with soaking time. Sixty-one species of diatoms belonging to 30 genera, 24 families, and 16 orders were detected, among which the dominant genera were Amphora, Nitzschia, and Navicula. Four genera of ciliates belonged to two classes, three orders, and four families, among which the dominant species were Euplotes sp. and Uronema marinum. Tubular polychaetes was the dominant metazoans. Species diversity increased over time. The highest biofilm diversity was observed on the GS surface. The diversity of biofilms on the Ti surface was higher than that on the Zn surface. This study provides basic data for marine material research, marine corrosion, and national defence construction.

High-Frequency Stable-Isotope Measurements of Evapotranspiration Partitioning in a Maize Field

Knowledge of the evaporation (E) and transpiration (T) components of evapotranspiration (ET) is important for ecohydrological modeling and agricultural productivity. The stable-isotope method offers the possibility to partition E and T due to the distinct differences in the isotopic signals of the sources. In this study, the concentration and isotopic ratios for oxygen-18 (18O) of water vapor in the ecosystem boundary layer of a growing maize field at the Hydrological Open Air Laboratory (HOAL) catchment in Austria were measured using a high-frequency field-sampling device. In conjunction with isotope samples from the soil and maize plants, these data were used to partition ET using the Keeling plot technique. Eddy covariance and sap flow measurements were used to provide a comparison to test the stable-isotope method. The fraction of transpiration (Ft) calculated with the stable-isotope method showed good agreement with the sap flow method. Overall daily average values of Ft were in a range from 43.0 to 88.5% with T accounting for an average value of 67.5% of the evapotranspiration over the nine days of the experimental period. Following a precipitation event of 9.7 mm, Ft increased from 63.4 to 88.5% over the next four days as the upper layer of the soil dried out while the plants accessed deeper soil water.

Using High-Frequency Water Vapor Isotopic Measurements as a Novel Method to Partition Daily Evapotranspiration in an Oak Woodland

Partitioning evapotranspiration (ET) into its constituent fluxes (transpiration (T) and evaporation (E)) is important for understanding water use efficiency in forests and other ecosystems. Recent advancements in cavity ringdown spectrometers (CRDS) have made collecting high-resolution water isotope data possible in remote locations, but this technology has rarely been utilized for partitioning ET in forests and other natural systems. To understand how the CRDS can be integrated with more traditional techniques, we combined stable isotope, eddy covariance, and sap flux techniques to partition ET in an oak woodland using continuous water vapor CRDS measurements and monthly soil and twig samples processed using isotope ratio mass spectrometry (IRMS). Furthermore, we wanted to compare the efficacy of δ2H versus δ18O within the stable isotope method for partitioning ET. We determined that average daytime vapor pressure deficit and soil moisture could successfully predict the relative isotopic compositions of soil (δe) and xylem (δt) water, respectively. Contrary to past studies, δ2H and δ18O performed similarly, indicating CRDS can increase the utility of δ18O in stable isotope studies. However, we found a 41–49% overestimation of the contribution of T to ET (fT) when utilizing the stable isotope technique compared to traditional techniques (reduced to 4–12% when corrected for bias), suggesting there may be a systematic bias to the Craig-Gordon Model in natural systems.

The application of δ¹⁸O and δD for understanding water pools and fluxes in a Typha marsh

The δ¹⁸O and δD composition of water pools (leaf, root, standing water and soil water) and fluxes [transpiration (T), evaporation (E)] were used to understand ecohydrological processes in a managed Typha latifolia L. freshwater marsh. We observed isotopic steady-state T and deep rooting in Typha. The isotopic mass balance of marsh standing water showed that E accounted for 3% of the total water loss, T accounted for 17% and subsurface drainage (D) accounted for the majority (80%). There was a vertical gradient in water vapour content and isotopic composition within and above the canopy sufficient for constructing an isotopic mass balance of water vapour during some sampling periods. During these periods, the proportion of T in evapotranspiration (T/ET) was between 56 ± 17% and 96 ± 67%, and the estimated error was relatively high (>37%) because of non-local, background sources in vapour. Independent estimates of T/ET using eddy covariance measurements yielded similar mean values during the Typha growing season. The various T/ET estimates agreed that T was the dominant source of marsh vapour loss in the growing season. The isotopic mass balance of water vapour yielded reasonable results, but the mass balance of standing water provided more definitive estimates of water losses.