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

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 and temporal scales, the influence of riparian land cover on stream DOM composition has received little attention. Furthermore, the combined effects of riparian land cover and streamflow rates on DOM composition require investigation. 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 riparian land cover and hydrology, and shifted towards recently produced, low molecular weight DOM with low aromaticity as the percentage of agricultural land within riparian wetlands increased. During periods of high discharge and high baseflow, the DOM pool was dominated by recalcitrant and terrestrial-derived material but shifted towards protein-like and microbial-derived with increasing cropland in the riparian area. 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.