Linking soils and streams: Sources and chemistry of dissolved organic matter in a small coastal watershed

Abstract

To understand the hydrologic and biogeochemical controls on the age and recalcitrance of dissolved organic matter (DOM) found in stream waters, we combined hydrometric monitoring along a topographic gradient from ridge to channel with isotopic (13C and 14C) and spectroscopic (UV and 13C nuclear magnetic resonance) analyses of soil and stream water samples in a small coastal watershed in California. With increasing discharge, dissolved organic carbon concentrations increased from 2.2 to 10.9 mg C L−1, Δ14C values increased from −125 to +120‰, δ13C values decreased from −24 to −29‰, C:N ratios increased from 6.5 to 15.4, and specific UV adsorption increased from 1.4 to 3.8 L mg C−1 m−1. These changes in DOM composition are consistent with a shift in source from old and recalcitrant soil organic matter (OM) sources found in deep soil horizons to young and relatively fresh OM sources found in the surface horizons. Results from this study suggest upland soils of the watershed become DOM production limited as indicated by a seasonal depletion and chemical shift in soil DOM, whereas highly productive soils in the hollow act as a near‐infinite DOM source. Hydrologic connectivity of this DOM‐rich riparian source region to the stream ultimately constrains DOM export, and the stream DOM composition reflects the combined influence of soil biogeochemical cycling of OM and hydrologic routing of water through the landscape.