PARENT SESSION
Oral Session #35: Carbon and Nitrogen Cycling.
Presiding: D. Rothstein
Tuesday, August 6. 8:00 AM to 11:30 AM. Palo Verde Room, Radisson.
Ecological significance of dissolved organic nitrogen exported from minimally disturbed watersheds.
Kaushal, Sujay*,1, Lewis, William, 1 Center for Limnology, Boulder, CO
ABSTRACT- Dissolved organic nitrogen (DON) comprises a substantial proportion of the soluble nitrogen exported from undisturbed and disturbed watersheds. Relatively little is known about the ecological significance of DON in ecosystems and how it may be altered in response to enrichment with inorganic nutrients. We investigated seasonal changes in the metabolism of DON in two streams draining watersheds that receive low rates of N deposition (ca. 3 kg/ha/yr). Concentrations of DON peaked during early spring runoff as concentrations of nitrate and soluble reactive phosphorous began to decline to very low levels (<5 micrograms/L) throughout the growing season. In these nutrient deficient watersheds, the composition and seasonal patterns of DON in stream water were weakly related to dissolved organic carbon (DOC). Instead, concentrations of non-humic and humic DON showed a stronger inverse relationship to concentrations of nitrate suggesting that the dynamics of organic and inorganic N may be linked in undisturbed watersheds. Over an annual period, the bioavailability of DON and DOC was positively related to the C:N of non-humic substances. Consumption of DON and DOC in incubations increased substantially during snow melt. At this time, approximately 50% of DON and 38% of DOC in stream water was bioavailable. Consumption of DON and DOC in incubations declined substantially during base flow in response to decreases in substrate quality and limitation by inorganic N and P. The metabolism of dissolved organic nitrogen in streams draining minimally disturbed watersheds is primarily influenced by seasonal changes in substrate quality, but can also be affected by the availability of inorganic nutrients.
KEY WORDS: dissolved organic nitrogen, nitrate, decomposition, nitrogen deposition