Although atmospheric inputs of nitrogen deposition and streamwater nitrate export from forested catchments in the northeastern U.S. have declined dramatically in recent decades, occasional, brief periods of high streamwater N loss have been observed. At the Hubbard Brook Experimental Forest (HBEF), prior spikes in nitrate export have been attributed to N deposition inputs, drought, pest outbreaks, ice storm disturbance, soil temperature, and soil freezing. The most recent spike in streamwater nitrate flux, which resulted in net catchment N export, occurred in 2013-14. In order to determine whether the cause of the increased nitrate export was (1) increased N inputs, (2) increased production within the catchment, (3) reduced biotic uptake/consumption, (4) enhanced transport, or (5) a multi-faceted scenario (a particular set of conditions), we examined the long-term record and recent high-frequency sensor data using multiple modeling approaches including deterministic (e.g., catchment biogeochemical modeling) and empirical (e.g., machine learning) analyses.
Results/Conclusions
The results of the different modeling approaches suggest that atmospheric inputs of N are not likely to play a significant role in the 2013-14 nitrate spike. However, preliminary results using an algorithm that predicts N mineralization rate as a function of soil temperature and moisture, indicate the N mineralization was elevated during this period which partially accounts for the nitrate export. Preliminary analyses also suggest that wind damage in June 2013 which decreased stem growth of trees in the catchment resulted in reduced N uptake and, hence, higher N export from the catchment. Finally, transport during this period appears to be enhanced via a combination of precipitation timing and an unusually high water table in soils. These conditions mobilized nitrate from hotspots for production in the catchment that are not always connected to the stream. These preliminary results suggest that a combination of factors led to the period of net nitrate export at the HBEF: an increase in nitrate produced, a decrease in N uptake by trees, and an increase in transport of nitrate to the stream outlet.