Limitations of stream restoration for nitrogen retention in agricultural headwater streams

•Stream restoration effects on nitrogen retention are limited in eutrophic streams.•Channel reconfiguration may enhance in-stream ammonium uptake.•Soil-polluted, nutrient-rich sediments release ammonium to the stream water.•Denitrification in anoxic sediments of nitrate-rich streams is nitrate-limited.

High nutrient loading and channelization reduce the nutrient retention capacity of agricultural streams and lead to increases in nutrient downstream transport. The aim of the current study was to study the effects of channel reconfiguration and riparian reforestation on the nitrogen retention capacity of eutrophic agricultural headwater streams. In addition, we investigated the role of stream sediments as a nitrogen sink or source for the stream ecosystem.We compared two restored reaches with two morphologically pristine and four channelized reaches in an agricultural catchment in the north-east of Austria regarding in-stream ammonium uptake, whole-reach retention of dissolved inorganic nitrogen, potential denitrification enzyme activity, and sedimentary ammonium release.Restored and pristine reaches exhibited significantly shorter ammonium uptake lengths (330 m) and larger mass transfer coefficients (2.7 × 10−5 m s−1) than channelized reaches (2500 m and 1.1 × 10−5 m s−1, respectively). Increased ammonium uptake was positively correlated with increased transient storage in restored and pristine reaches. Total DIN retention was slightly, though not significantly higher in restored sections (average rates 0.06 g DIN m−2 h−1) and showed signs of temporal nitrogen saturation in all reaches. In general, sediments were characterized by small grain sizes (0.04-0.31 mm), high ammonium (60-215 μg g−1 DW), and low nitrate concentrations (0.4-5.7 μg g−1 DW). Ammonium was released from sediments of all reaches below concentrations of 100 μg NH4+-N L−1 in the overlying water column which shows the high potential of nutrient-rich sediments to act as an internal ammonium source for the stream ecosystem. Potential denitrification was lowest in sediments of restored reaches and significantly increased after nitrate amendment to 3-26 mg N m−2 h−1.The study reveals that stream sediments, which are loaded with nutrient-rich soil from the agricultural catchment, may limit the effects of stream restoration in agricultural streams. In order to improve the nutrient retention capacity of agricultural streams, reach-scale restoration measures have to be combined with measures in the catchment which reduce nutrient and soil inputs to streams.