Watershed biogeochemistry: Source, composition, processing and export of terrestrial organic matter from an agricultural watershed

Riverine controls on terrestrial organic matter (TOM) dynamics play an important role in the global carbon cycle and much attention has been given to riverine export of TOM to the world\u27s oceans. Also important to carbon cycling is the role that land management and drainage have in changing soil organic carbon pools. What remain unknown and understudied however, are the origin and dynamics of flood-mobilized TOM in agricultural landscapes. Molecular and isotopic techniques have been successfully applied to study riverine controls on TOM export; however, the majority of these studies were conducted on large rivers and similar studies of smaller streams are underrepresented in the literature. In order to determine the role that hydrology and upland processes play in TOM export, a multiproxy approach was applied to organic matter samples from an agricultural watershed (Big Pine Creek). Dissolved organic carbon and stream discharge data showed that brief streamflow events contribute a large portion of the total annual organic carbon load. For Big Pine Creek, up to 82% of the annual organic carbon load occurred during only 20% of the time. Molecular and stable carbon isotope data showed that both quantity and source of TOM changed during increased flow conditions. Relative contributions from C4 plants increased during flood conditions by up to 22, 31 and 38% for particulate, colloids, and dissolved organic matter, respectively. Samples collected from multiple locations throughout the watershed showed that, during baseflow conditions, sample location was not an important factor in determining TOM characteristics. During flood conditions, molecular proxies showed that colloids changed in relative degradation state with distance downstream; however, stable carbon isotope values suggested that colloid source material was no different than that of dissolved organic matter. I interpreted this as selective partitioning of relatively undegraded dissolved organic matter onto clay minerals during flood conditions. Results from this study shed light on the importance of hydrology and land management practices on TOM dynamics. Furthermore, focus of studies on riverine TOM should be shifted to smaller watersheds over a wider range of hydrologic conditions to capture variability that is likely to be muted in larger rivers