RESEARCH ARTICLE Successional changes in soil and hyporheic nitrogen fertility on an alluvial flood plain: implications for riparian vegetation Michael R. Morris • Brook O. Brouwer • Jeremy K. Caves • Mary J. Harner • Jack A. Stanford Received: 14 January 2010 / Accepted: 7 June 2010 / Published online: 24 June 2010 Ó Springer Basel AG 2010 Abstract In floodplain primary succession, vegetation colonizes nitrogen-poor alluvial deposits and fertility improves as soil nitrogen accumulates over time. It is generally assumed that vegetation assimilates the vast majority of its nitrogen from the soil; however, recent studies have suggested that the hyporheic zone also may be an important nitrogen source. We investigated the potential relative importance of hyporheic nitrogen by comparing fertility indices, specifically total (TN), dissolved inorganic (DIN), potentially mineralizable (PMN) and ion exchange resin nitrogen (IERN) in both soils and the hyporheic zone at early, mid and late succession stands on an expansive river flood plain. We also constructed mesocosms to assess growth of cottonwood cuttings with access to soil and/or hyporheic water. We found TN and PMN increased from early to mid succession in both the soil (to 10 cm) and hyporheic zone (in a 10 cm layer). While TN, DIN and PMN were an order of magnitude higher in the soil than in the hyporheic zone, IERN was higher in the hyporheic zone, indicating that subsurface flow through the flood plain may be important in delivering nitrogen to the root zone. However, even when flux was added to the hyporheic PMN pool, nitrogen availability in the hyporheic zone (in a 10 cm layer) was vastly lower than soil PMN (to 10 cm). Further, the instantaneous standing stock of DIN in the surface soil alone was about equal to the sum of the DIN pool, the mean subsurface flux and the PMN pool in a 10 cm layer of hyporheic zone. In the mesocosm experi- ment, cottonwood cuttings with access to both soil and hyporheic water grew fastest; however, they also had the lowest foliar nitrogen concentrations, indicating that this was not due to greater nitrogen availability. In the field, nitrogen content of cottonwood foliage increased along with soil (but potentially hyporheic as well) nitrogen accumulation during succession, suggesting the vegetation responded to increasing nitrogen fertility. We conclude that at least on a per unit-volume basis, the hyporheic zone probably provides little nitrogen relative to the surface soil, except on new alluvial bars that characteristically are nitrogen poor. Therefore, the hyporheic zone is probably a much smaller nitrogen source for mature forests relative to the surface soil unless the vegetation exploits a much larger volume of the hyporheic zone than surface soil. Keywords Flood plain Á River Á Nitrogen Á Soil Á Hyporheic Á Ground water Á Populus Á Riparian Introduction In gravel bedded flood plains, flooding continually reworks terrestrial surfaces by eroding and redepositing sediments (Stanford et al. 2005; Lorang and Hauer 2006). As a con- sequence, riparian forests exist as mosaics of vegetation patches in various stages of successional development (Latterell et al. 2006; Nakamura et al. 2007; Whited et al. 2007). For example, on the Nyack Flood Plain of the Middle Fork of the Flathead River in northwestern M. R. Morris Á J. K. Caves Á J. A. Stanford (&) Flathead Lake Biological Station, Division of Biological Sciences, The University of Montana, 32125 Bio Station Lane, Polson, MT 59860, USA e-mail: jack.stanford@umontana.edu B. O. Brouwer Department of Biology, The Colorado College, Colorado Springs, CO 80946, USA M. J. Harner Department of Biology, University of Nebraska, Kearney, Kearney, NE 68849, USA Aquat. Sci. (2010) 72:519–532 DOI 10.1007/s00027-010-0153-8 Aquatic Sciences