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Dissertation defense - Anna Hermes

Friday, October 22, 2021, 9:00AM - 11:00AM


Anna Hermes


SEEC S228 (Sievers Room)


Revealing modern sulfur cycle change: The biogeochemical fingerprint of agricultural sulfur from field-to-watershed scales


The past several decades have witnessed a fascinating evolution in the sulfur (S) cycle, resulting in an unprecedented increase in agricultural S use. However, little research has explored how intensive agricultural S applications alter S biogeochemistry across a range of agricultural settings and scales— essential for constraining the cascade of environmental effects of using S in agriculture. In this dissertation, I identify and trace the biogeochemical “fingerprint” of agricultural S from field-to- watershed scales.

Asking first how patterns of S chemistry change across a watershed with intensive agricultural S inputs, I contrasted S stable isotope (δ34S) and sulfate (SO42-) concentration measurements collected within agricultural areas and surrounding forests and grasslands with background (atmospheric and geologic) S sources. Stable S isotope results showed that agricultural S has a robust and distinctive biogeochemical fingerprint that is traceable beyond fields. I then delved deeply into processes affecting organic S composition—the largest pool of S within soils. I developed a novel method to directly measure δ34S of dissolved organic matter (DOM) and combined this approach with techniques to measure organic S speciation and molecular composition. Agricultural S applications increased DOM S-content by two-fold compared to forests and grasslands, and I found that a suite of molecules unique to agricultural areas have the potential to be used as agricultural S tracers. Finally, I investigated how wildfire disturbance affects agricultural S and its interactions with other elemental cycles. Though not appearing to strongly affect the agricultural S fingerprint, wildfire did enhance organic carbon leaching, producing a potentially potent cocktail for stimulating toxin production in downstream aquatic ecosystems. These results reinforce the importance of considering integrative studies at watershed scales to evaluate the transport and fates of agricultural S and point to the increasing role of climate change as an additional control on agricultural S biogeochemistry.

Combined, this research (1) reveals agricultural changes to the modern S cycle at multiple scales, (2) establishes tools and techniques to trace agricultural S through watersheds, and (3) provides a critical first step towards fully constraining the environmental fates and unintended consequences of S inputs to agricultural systems.


All are welcome - contact for the Zoom link for this talk if you can't attend in person.