Long-term data reveals a regime shift in watershed nitrogen export
Reference forest watersheds at Coweeta Hydrologic Laboratory are highly nitrogen retentive, and export is biologically controlled by processes in both the terrestrial and stream systems. However, accumulation of available nitrogen beyond that which is biologically immobilized results in nitrogen saturation and can switch regulation of watershed nitrogen export to hydrologic control. This change in nitrogen export represents a significant regime shift.
Based on 42-yr of weekly data, WS 7 shifted from biologically controlled export to hydrologic controlled export several years after logging. This regime shift was apparently caused by reduced tree uptake, increased mineralization of organic nitrogen, and nitrogen fixation by black locust associated microbes. The data show a long-term increase in the magnitude of nitrogen export and a shift in the seasonal timing. In forested watersheds at Coweeta, maximum dissolved inorganic nitrogen concentration occurs in summer when base flow is low, but WS 7 has now shifted to a pattern of maximum winter concentration. The seasonal pattern of nitrogen concentration and export from undisturbed watersheds can be explained by in-stream processes, but following any disturbance such as logging, elevated nitrogen export saturates these in-stream processes, and the nitrogen export regime becomes dominated by hydrologic transport from the terrestrial system. This shift from biological to hydrological control of nitrogen dynamics appears to be permanent or, at least, will require a very long time for recovery.