The relationship between stream flow and stream nitrate concentrations under altered conditions of nitrate availability: Will dilution be the solution in a stormier world?

Background/Question/Methods

Enhanced nitrate loss is a critical event in the process of nitrogen saturation that may lead to significant changes in downstream ecosystems. Nitrate loading into receiving waters is a function of both stream flow and its nitrate concentration.  A relationship between stream flow and stream-water concentrations could diminish or enhance the loss of nitrate from a watershed by either diluting or increasing nitrate concentrations.  Since the frequency of high stream flows may change in the future, I used long-term stream-flow and stream-chemistry data from the Fernow Experimental Forest to determine whether the nature of the relationship between stream flow and nitrate concentrations changes in response to spatial and temporal differences in nitrate availability.  The slopes of the relationship between monthly streamflow and monthly mean stream nitrate concentration were compared for three reference watersheds that differ in stream-water nitrate concentrations.  Similar comparisons were made for a single reference watershed before and after it underwent a significant change in nitrate losses, and for an experimental watershed before and after fertilization with ammonium sulfate.

Results/Conclusions

Small reference watersheds that currently (1989-2013) have low (~15 μM) and intermediate (~40 μM) concentrations of stream water nitrate exhibit a "dilution effect" in which nitrate concentrations decrease as flows increase. Furthermore, the strength of this effect (the slope of the flow vs. concentration relationship) was greater (p < 0.001) in the watershed with the lowest mean concentration of stream-water nitrate. In contrast, the reference watershed with the highest current concentration of stream-water nitrate (~50 μM) exhibits a "concentration effect" in which nitrate concentrations increase with enhanced flow. Within a single reference watershed that has undergone a transition from low to high losses of nitrate, the relationship between flow and concentration shifted from a dilution effect prior to the transition, to a concentration effect after the transition (p < 0.001). A similar temporal shift (p < 0.001) from a dilution to a concentration effect occurred in an experimental watershed following the start of fertilizer additions.  Results of this study suggest that if N-rich systems experience more intense storms in the future, they will discharge waters enriched in nitrate as storm water flushes portions of the watershed (e.g. surface soils) that contain a greater fraction of the available N pool.