Bromide tracer study to investigate depth of water uptake by switchgrass underneath vs. outside rainout shelters

In 2021 we conducted a bromide tracer experiment at the GLBRC Bioenergy Land Experiment (formerly known as the Marginal Land Experiment) at Lux Arbor Reserve to investigate whether switchgrass taps into deeper soil water under rainout shelters as the surface soil water dries out. Tapping deeper water could explain how switchgrass survives weeks without rainfall under the shelters and during natural dry periods. Bromide is useful as a conservative solute tracer because it is present at very low concentrations in soil solutions, is generally unreactive and remains in solution in soils, and can be readily measured using ion chromatography. Previous studies at KBS employing bromide as a tracer of soil water movement include Jin et al. (2008), Hess et al. (2018), and Hamilton (2020). Various studies in the literature have shown that bromide is taken up by plants, but this is mostly described in relation to how plant uptake may interfere with its use as a hydrologic tracer in wetlands. A prior study of bromide as a tracer by Bowman et al. (1997) documented plant uptake and considered toxicity, but not in switchgrass. Prior to the experiment with switchgrass under rainout shelters, we conducted a test for bromide uptake and toxicity in switchgrass to ensure that bromide was taken up into foliage in proportion to concentrations in the soil, and that the growth of the grass was not affected by high bromide in soil solutions. That study is summarized below.Support for this research was provided by the Great Lakes Bioenergy Research Center, U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research (Award DE-SC0018409), by the National Science Foundation Long-term Ecological Research Program (DEB 1832042) at the Kellogg Biological Station, and by Michigan State University AgBioResearch