Photochemical and microbial degradation of external dissolved organic matter inputs to rivers

Photochemical and microbial degradation of external inputs of dissolved organic matter (DOM) from both natural (forests) and anthropogenic (swine and equine pastures) non-point sources to rivers were examined through field and laboratory microcosm experiments. Little or no photochemical degradation of DOM to inorganic N or P occurred in either the agricultural or forest runoff. The dissolved organic carbon (DOC), dissolved organic nitrogen (DON), NH4, and PO4 concentrations did not change during the photochemical experiments for any of the sources examined. A small, but significant increase in the NO3/NO2 (+0.7 μM; p = 0.004) concentration was detected in the forest runoff, suggesting that a small fraction (4 to 9%) of the DON may be photochemically degraded to dissolved inorganic nitrogen (DIN). Bacteria readily utilized DOC and DON in the agriculture and forest runoff. The percent of DOC and DON consumed by the bacteria ranged from 6 to 14% and 21 to 25%, respectively. Light exposure did not alter the biological availability of the DOC and DON in either the equine pasture or forest runoff. Our results emphasize the importance of microbial processes in degrading DOM in riverine environments; they appear to degrade DOM more rapidly than photochemical processes and may be more important in affecting the quantity and quality of the DOM exported from rivers to estuaries.