Evidence for a Regime Shift in Nitrogen Export from a Forested Watershed

Abstract

In this study, we document a functional regime shift in stream inorganic nitrogen (N) processing indicated by a major change in N export from a forested watershed. Evidence from 36 years of data following experimental clearcut logging at Coweeta Hydrologic Laboratory, NC, suggests that forest disturbance in this area can cause elevation of dissolved inorganic N (DIN) loss lasting decades or perhaps longer. This elevation of N export was apparently caused by an initial pulse of organic matter input, reduced vegetation uptake, increased mineralization of soil organic N, and N fixation by black locust-associated bacteria following clearcut logging. In forested reference watersheds at Coweeta, maximum DIN concentration occurs in summer when base flow is low, but the clearcut watershed shifted to a pattern of maximum winter DIN concentration. The seasonal pattern of DIN concentration and export from reference watersheds can be explained by terrestrial and in-stream processes, but following clearcutting, elevated DIN availability saturated both terrestrial and in-stream uptake, and the N export regime became dominated by hydrologic transport. We suggest that the long-term elevation of stream DIN concentration and export along with the changes in seasonality of DIN export and the relationship between concentration and discharge represent a functional regime shift initiated by forest disturbance.

Effects of instream processes, discharge, and land cover on nitrogen export from southern Appalachian Mountain catchments

“…Our results and previous studies (e.g., Lutz et al, ; Mulholland & Hill, ) suggest that instream processes play a major role in seasonal patterns of baseflow DIN concentrations (Figure ). We did not see the unimodal, strong summer peak pattern that typically occurs in small forested, headwater streams in this area (Webster, Knoepp, et al, ; Webster, Newbold, & Lin, ). Apparently, there is sufficient springtime light in the third‐order, forested streams included in this study (Ball Creek and Ray Branch) to stimulate algal production and minimize DIN concentration in spring.…”

“…The high DIN concentrations of Crawford Branch and perhaps Bates Branch apparently saturated instream uptake, resulting in a single winter peak. This is similar to the results reported by Webster, Knoepp, et al (). Using a 42‐year data set, they showed that a reference stream and prelogging data from another stream both exhibited a strong summer peak in DIN concentration.…”

“…Throughout the southern Appalachian Mountains, black locust ( Robinia pseudoacacia ) is a rapidly growing tree that shows a strong response following clear‐cut logging (Boring & Swank, ; Elliott, Boring, Swank, & Haines, ). Nitrogen fixation by Rhyzobium bacteria associated with the roots of black locust is a major contributor of terrestrial nitrogen and ultimately to stream DIN export from logged catchments (Webster, Knoepp, et al, ). Both Ball Creek and Ray Branch are approximately third‐order catchments and include areas that were clear‐cut within the past 40 years, perhaps explaining why DIN export from these catchments is higher than DIN export from smaller reference catchments that have not been logged for over 90 years (e.g., Webster, Knoepp, et al, ).…”

“…Nitrogen fixation by Rhyzobium bacteria associated with the roots of black locust is a major contributor of terrestrial nitrogen and ultimately to stream DIN export from logged catchments (Webster, Knoepp, et al, ). Both Ball Creek and Ray Branch are approximately third‐order catchments and include areas that were clear‐cut within the past 40 years, perhaps explaining why DIN export from these catchments is higher than DIN export from smaller reference catchments that have not been logged for over 90 years (e.g., Webster, Knoepp, et al, ). Similar clear‐cut areas once dominated by black locust are likely present in all of our study catchments, and nitrogen fixation by other leguminous plants may contribute to DIN export from some agricultural areas.…”

Stream phosphorus dynamics of minimally impacted coastal plain watersheds

“…In the case of non‐self‐averaging time series, higher‐frequency sampling may not confer any advantage in detecting meaningful trends or predicting future trajectories (Kirchner & Neal, ). More nuanced methods need to be developed, potentially looking for temporal shifts in C‐Q relationships (e.g., Webster, Knoepp, Swank, & Miniat, ) though these too may be affected by long travel times.…”