Ecological Engineering 60 (2013) 224–234 Contents lists available at ScienceDirect Ecological Engineering j ourna l ho me pa g e: www.elsevier.com/locate/ecoleng Limitations of stream restoration for nitrogen retention in agricultural headwater streams Weigelhofer Gabriele a,b,∗ , Nina Welti a,1 , Thomas Hein b,a a WasserCluster Lunz, Dr Carl Kupelwieser Promenade 5, A-3293 Lunz/See, Austria b University of Natural Resources and Life Sciences, Institute of Hydrobiology and Aquatic Ecosystem Management, Max Emanuelstr. 17, A-1180 Vienna, Austria a r t i c l e i n f o Article history: Received 21 March 2013 Received in revised form 2 July 2013 Accepted 6 July 2013 Keywords: Agricultural streams Restoration Ammonium uptake Potential denitrification Ammonium release a b s t r a c t 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 ammo- nium (60–215 g g -1 DW), and low nitrate concentrations (0.4–5.7 g g -1 DW). Ammonium was released from sediments of all reaches below concentrations of 100 g NH 4 + -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. © 2013 Elsevier B.V. All rights reserved. Abbreviations: ˛, transient storage exchange coefficient; A, cross-sectional area; As, transient storage zone area; D, dispersion coefficient; DIN, dissolved inorganic nitrogen; DEA, denitrification enzyme activity; DO, dissolved oxygen; GPP, gross primary production; ER, ecosystem respiration. ∗ Corresponding author at: WasserCluster Lunz, Dr Carl Kupelwieser Promenade 5, A-3293 Lunz/See, Austria. Tel.: +43 7486 200 60 43; fax: +43 7486 200 60 20. E-mail addresses: gabriele.weigelhofer@wcl.ac.at, gabriele.weigelhofer@aon.at (W. Gabriele), n.welti@uq.edu.au (N. Welti), thomas.hein@boku.ac.at (T. Hein). 1 Present address: University of Queensland, School of Civil Engineering, Brisbane St. Lucia 4072, Australia. 1. Introduction The intensification of agriculture during the last century has impacted the integrity and function of streams considerably (Bernhardt and Palmer, 2011; Dodds and Oakes, 2008; Hancock, 2002; Riseng et al., 2011). An increasing proportion of the landscape has been turned into arable land, thereby altering the vegetation, soil properties, and the hydrologic regime of the catchment and depriving streams of their natural riparian buffer zones (Gordon et al., 2008; Verhoeven et al., 2006). Elevated nutrient concentra- tions in soils and groundwater, resulting from excessive fertilizer application, decrease the water quality and lead to the eutrophi- cation of agricultural streams (Dodds and Oakes, 2008; Kronvang et al., 2008; Oenema et al., 2005). 0925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2013.07.057