Accelerated export of nitrogen-containing dissolved organic matter (DOM) or dissolved organic nitrogen (DON) to
streams and rivers from agricultural watersheds has been reported worldwide. However, few studies have examined
the dynamics of DOMmolecular composition with the attention paid to the relative contributions of DON from various
sources altered with flow conditions. In this study, end-member mixing analysis (EMMA) was conducted with the optical
properties of DOMto quantify the relative contributions of several major organic matter sources (litter, reed, field
soil, and manure) in two rivers of a small agricultural watershed. DOC and DON concentration increased during the
storm events with an input of allochthonous DOM as indicated by an increase in specific ultraviolet absorbance at
254 nm (SUVA254) and a decrease in biological index (BIX), fluorescence index (FI), and protein-like component (%
C3) at high discharge. EMMA results based on a Bayesian mixing model using stable isotope analysis in R (SIAR)
were more accurate in source tracking than those using the traditional IsoSource program. Manure (>30%) and
field soil (also termed as “manure-impacted field soil”) (>23%) end-members revealed their predominant contributions
to the riverine DOM in SIAR model, which was enhanced during the storm event (up to 56% and 38%, respectively).
The molecular composition of the riverine DOM exhibited a distinct footprint from the manure and manure-impacted field soil, with a larger number of CHON formulas and abundant polyphenols and condensed aromatics in
peak flow samples in the studied rivers. The riverine DOMduring peak flow containedmany uniquemolecular formulas
in both rivers (4980 and 2082) of which >60%originated frommanure andmanure-impacted field soil. Combining
the EMMAwith DOMmolecular composition clearly demonstrated the effect of manure fertilizer on the riverine DOM
of the watershed with intensive agriculture. This study provides insights into the source tracking and regulation of
DON leaching from anthropogenically altered river systems worldwide.
