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.