Land use influences dissolved organic matter composition in the U.S. Coastal Critical Zone Network

Abstract

Coastal watersheds are highly productive biogeochemical systems supporting robust coastal fisheries and contributing significantly to the marine gross primary production flux. These ecosystems are comprised of civil infrastructure, farms, forests, and marshes, and their diversity is reflected in the quantity and quality of dissolved organic matter (DOM). Here, we assess the role of farming in influencing DOM from surface waters of six coastal tidal creeks adjacent to forest and farm sites in the US Critical Zone Network. Molecular fingerprinting using electrospray ionization - Fourier transform - ion cyclotron resonance - mass spectrometry revealed that farm-influenced DOM was enriched in nitrogen-, sulfur-, and phosphorus-containing compounds. The concentration of ammonium [NH 4 + ] correlated significantly with nitrogen-containing formulas that could comprise NH 3 adducts, suggesting that NH 4 + (e.g., from fertilizer) can be incorporated into DOM through two-electron chemical reaction pathways such as the Michael addition, one-electron reactions such as Fenton oxidation, or biotic pathways. This suggests the use of [NH 4 + ] measurements as a potential approach to parameterize the impact of agriculture on coastal environments. Collectively, these results establish a foundation for future studies in the U.S. Critical Zone Network on how land and water interact. More broadly, our molecular fingerprinting of DOM corroborated with the global observations of elevated concentrations of nitrogen, sulfur, and phosphorus in farm-influenced freshwater DOM, showing that agricultural activities (e.g., annual fertilizer application) can also influence the DOM composition in coastal environments of high salinity and pH.

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