The University of California prides itself on conducting research on an inter- national scale. That said, when I arrived at UC Merced in 2013, the new, small campus had only two other ecologists and very few earth scientists. There were very few high-tech instruments on campus, even in physics and chemistry. While UC Merced lacked the accoutrements I was accustomed to at the Geophysical Laboratory, it was a very upbeat environment and was improving its infrastruc- ture fast. I built my first isotope lab there in a building that struggled with envi- ronmental controls, but we managed to make things work. While pursuing my interests in compound specific isotope analyses of amino acids from various experiments and organisms, the majority of my colleagues were interested in various aspects of soil ecology and biogeochemistry. Adjacent to the campus is the Merced Vernal Pools and Grassland Reserve, a 65,000 acre preserve created in 2013 and available to UC students and faculty for study. My husband served as Director of the Reserve, so we took advantage of his access and its proximity to introduce the students and faculty on campus to stable isotope biogeochemistry.
Asmeret Berhe, Stephen Hart, and Teamrat Ghezzehei, faculty at UCM, were all interested in various aspects of soil isotope biogeochemistry. As the director of the isotope facility, I became involved in many of their studies. At first glance, the work did not fully capture my interest, particularly after studying exotic meteorites and amino acids from owls. California is almost
exclusively a C3-plant based ecosystem, so the δ13C of soils typically ranges
from -24 to -27 ‰. Berhe and her students developed methods for separating bulk soils into density fractions, which added to the quantitative assessment of
how carbon, nitrogen, and even hydrogen is cycled in soils. With δ13C, % total
organic carbon, % total nitrogen, as well as the mass balance of the density
fractions, the research became more engaging (e.g., McCorkle et al., 2016; Abney
et al., 2017).
Postdoctoral researcher Elizabeth Williams pioneered work on hydrogen
isotopes in soils based on the work by Ruppenthal et al. (2015). One of the major
questions in soil ecology today is whether the organic matter in soil comes from root exudates, litter deposition from the surface, microbial biomass, or a combination of these three sources. Ruppenthal took advantage of the differ-
ence in the δ2H of leaf water (more positive) relative to δ2H in root tissue. He
argued that if decomposed leaf litter were the source of soil organic matter, then
its δ2H would be more positive than if it came from root exudates. He devel-
oped a protocol for measuring the δ2H of exchangeable and non-exchangeable
hydrogen in soils. His conclusion was that soil organic matter in his study site, a grassland ecosystem, was derived primarily from root exudates.
Liz Williams set up a series of experiments using soils from an altitudinal gradient in the nearby Sierra Nevada mountains. Density fractionated soils were subjected to isotopic exchange with water, then quickly measured on the
TC/EA-IRMS system. She calculated that only a small portion (5-15 %) of the hydrogen in soils was exchangeable, probably because the majority was water that was tightly adsorbed to high density minerals. Even the low density plant
fragments had little hydrogen in exchangeable positions. The δ2H of bulk soil
and the heavy, mineral fraction were related to the δ2H of local precipitation.
It remains to be seen whether the δ2H of deeper soils will carry a palaeoen-
vironmental signal. Because of complexities in relating δ2H of leaf waxes to
environmental parameters, Williams’ approach might provide an independent and more holistic assessment than the measurement of a single biomarker.
At UC Riverside, my lab group joined efforts with Seth Newsome’s lab
group at the University of New Mexico to measure the δ2H of amino acids in
primary producers. When Newsome and I started our research on δ2H in amino
acids, we were thinking about microbes, birds and animals, but not plants (e.g.
Rodriguez Curras et al., 2018). Knowledge about plants and hydrogen isotope
fractionation was lacking. Newsome’s graduate students, Alexi Besser and Emma Elliot-Smith, came to Riverside armed with collections of plants from terrestrial and marine habitats. Joined by my lab group, Bobby Nakamoto, Jon
Nye, and Kaycee Morra, we’ve measured the δ2H of amino acids from 50 plant
samples to understand primary isotope fractionations during photosynthesis. In terrestrial and freshwater plants, for the amino acids that can be made by any
organism, serine has the most negative δ2H value, whereas in marine plants
aspartate has the most negative δ2H. In general, glycine, serine, aspartate,
proline, and glutamate, are more closely involved in an organism’s central
metabolism. Variation in the δ2H of these amino acids will be influenced by
fluxes of energy in plants.
Those amino acids with more complicated biosynthetic pathways, thre- onine, valine, leucine, isoleucine, phenylalanine, and lysine, typically have more
negative δ2H values, probably owing to the additional enzymatic steps needed
for their synthesis. The ultimate goal of this work is to determine whether
there are isotope fingerprints in δ2H of amino acids in primary producers,
similar to those measured with carbon isotopes (Scott et al., 2006; Larsen et al.,