Evolution of particulate organic matter (POM) along a headwater drainage: role of sources, particle size class, and storm magnitude Richard Rowland . Shreeram Inamdar . Thomas Parr Received: 14 October 2016 / Accepted: 18 March 2017 / Published online: 27 March 2017 Ó Springer International Publishing Switzerland 2017 Abstract Large storm events can not only increase the runoff mass exports of particulate organic matter (POM) from watersheds, but can also alter the sources, size distribution, and composition of POM. We investigated the quantity, particle size distribution, carbon (C) and nitrogen (N) content, and sources of POM for five locations longitudinally along a forested Piedmont stream. POM was sampled for multiple storm events of varying magnitude and intensity over a two-year period. POM was separated into coarse (CPOM), medium (MPOM), and fine (FPOM) size classes, and sources were estimated using stable iso- topes of 13 C and 15 N with a Bayesian mixing model. CPOM largely resembled less-degraded vascular plant material characteristic of forest floor litter, which was estimated to contribute to *40% of CPOM in upstream locations. FPOM was derived from a more variable mixture of sources with stream beds and stream banks playing a greater role at larger drainage locations (up to *50 and *30%, respectively). Contributions from both forest floor litter and humus to CPOM increased with increasing event runoff, and litter contributions increased during events with higher rainfall intensities. Higher C and N content was noted in coarse sediments and finer POM fractions appeared to be more degraded based on C:N and isotope ratios. Climate-change projections predict intensification of large storm events in the Northeast- ern US. Results of this study suggest that large storms will increase the fluvial exports of coarse, labile, C- and N-rich POM with subsequent impacts on receiving aquatic ecosystems. Keywords Organic carbon Á Nitrogen Á Watersheds Á Aquatic ecosystems Á Storm events Á Runoff Introduction Organic matter (OM) provides a critical energy source that fuels aquatic ecosystem function and serves as a basal component of aquatic food webs (Cole et al. 2007; Tank et al. 2010). Enriched concentrations of organic carbon (OC) and stimulation of algal growth from nutrients such as nitrogen (N) contained in organic matter can lead to eutrophication and thus are concerns for water quality (Stanley et al. 2012; Seitzinger et al. 2002). Lateral fluxes of OC from a watershed with streamflow constitute an important Responsible Editor: James Sickman. R. Rowland Á S. Inamdar (&) Water Science & Policy Graduate Program, University of Delaware, 152 Townsend Hall, 531 S College Avenue, Newark, DE 19716, USA e-mail: Inamdar@udel.edu T. Parr Plant and Soil Science Department, University of Delaware, Newark, DE, USA 123 Biogeochemistry (2017) 133:181–200 DOI 10.1007/s10533-017-0325-x