Dissolved Organic Matter Sources and Dynamics in an Agricultural Watershed
Author | : Sandrine Journet Matiasek |
Publisher | : |
Total Pages | : |
Release | : 2014 |
Genre | : |
ISBN | : 9781321019490 |
Download Dissolved Organic Matter Sources and Dynamics in an Agricultural Watershed Book in PDF, Epub and Kindle
Agriculture is a dominant land use on Earth, but its impacts on dissolved organic matter (DOM) sources, processing, and fate remain relatively unclear. The goal of this research was to study DOM sources and dynamics in an irrigated agricultural watershed of Central Valley, California by 1) assessing the quantitative and qualitative impacts of organic matter (OM) desorption from sediments and soils, and by 2) generating an amino acids time series to link seasonal processes (natural and anthropogenic) to changes in stream DOM chemistry and reactivity. Abiotic desorption experiments revealed that mineral-bound organic carbon (OC) desorption is a predictable and finite process in agricultural surface waters, contributing 5 - 7% of the annual dissolved OC (DOC) export, with local maxima up to 50% of typical field runoff concentrations and 20% of winter storm DOC concentrations. The relevance of OM desorption was even more evident at the molecular level. Relative to mineral-bound OM, desorbed DOM was nitrogen-poor and depleted in amino acids and lignin. Distinct fractionation patterns of amino acids and lignin phenols led to a more acidic DOM pool that appeared substantially more degraded than particulate OM, with decreased mole fractions of basic amino acids, increased molar contributions of non-protein amino acids, decreased degradation index values, and increased proportions of acidic lignin phenols. If unaccounted for, the profound alteration of DOM composition incurred by desorption could lead to misinterpretations of indicators commonly used to assess the diagenetic state of OM. At the watershed scale, the role of land use and hydrologic controls on labile DOM dynamics was evidenced by elevated total hydrolyzable amino acid (THAA) concentrations throughout the low-discharge irrigation season and by seasonal compositional differences. DOM during winter baseflow was the most degraded, with the lowest THAA content and the lowest degradation index (DI), while winter storms and summer irrigation were two critical hydrologic regimes for DOM cycling with the largest amino acid contents, the largest proportions of basic amino acids, and the largest DI values, indicative of a relatively fresh DOM pool. The biogeochemical relevance of irrigation practices was therefore demonstrated by the mobilization of DOM similar in concentration and reactivity to DOM during winter storms.