Riparian land cover and hydrology influence stream dissolved organic matter composition in an agricultural watershed

Authors: Pisani, Oliva; Bosch, David - Dave; Coffin, Alisa; Endale, Dinku; Liebert, Daniel - Dan; Strickland, Timothy - Tim

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2020
Publication Date: 2/6/2020
Citation: Pisani, O., Bosch, D.D., Coffin, A.W., Endale, D.M., Liebert, D.P., Strickland, T.C. 2020. Riparian land cover and hydrology influence stream dissolved organic matter composition in an agricultural watershed. Science of the Total Environment. 717. https://doi.org/10.1016/j.scitotenv.2020.137165.
DOI: https://doi.org/10.1016/j.scitotenv.2020.137165

Interpretive Summary: Dissolved organic matter (DOM) represents a large component of the carbon pool in aquatic systems and agricultural practices can influence its amount, composition and bioavailability. A detailed understanding of DOM dynamics in agricultural watersheds is lacking and may provide insight into relationships between land use and DOM quality. For this purpose, water samples were collected in the Little River Experimental Watershed (LREW) near Tifton, Georgia, USA and DOM characteristics were assessed through the analysis of optical properties. DOM in the LREW was dominated by material of terrestrial (plants and soil), microbial, and human origin and was influenced by land cover and hydrology. As the percent agricultural land adjacent to a stream increased, DOM composition shifted towards recently produced, low molecular weight material. Periods of low stream flow were dominated by low molecular weight DOM of microbial origin. The results of this two-year study indicate that assessment of DOM optical properties and composition can help infer land cover and changes during varying hydrological phases in the LREW and in other agricultural watersheds. Such information may be helpful in predicting the source of organic matter in streams.

Technical Abstract: Dissolved organic matter (DOM) represents an essential component of the carbon cycle and controls biogeochemical and ecological processes in aquatic systems. The composition and reactivity of DOM are determined by the spatial distribution of its sources and its residence time in a watershed. While the effects of agricultural land cover on DOM quality have been reported across spatial scales, little is known about how this relationship can change over time. Furthermore, the influence of riparian land cover on stream DOM composition has received little attention. To this end, a multi-year (2016-2018) DOM characterization study was conducted using bi-weekly water samples collected from seven sub-watersheds nested within the Little River Experimental Watershed (LREW) near Tifton, Georgia, USA. DOM optical properties were determined to assess compositional variations using UV-Vis and excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor (PARAFAC) analysis. PARAFAC analysis indicated that DOM in the LREW was dominated by three humic-like fluorescing components of terrestrial, microbial, and anthropogenic origin and a protein-like component. DOM composition was influenced by land cover, and shifted towards recently produced, low molecular weight DOM with low aromaticity as the percentage of agricultural land within riparian wetlands increased. The optical properties of DOM were dominated by recently produced, microbial-derived material during low discharge and low baseflow periods. The results of this two-year study indicate that the replacement of forested riparian buffers with agricultural land can result in altered DOM composition which may affect carbon cycling and downstream water quality in agricultural watersheds.