Floodplains represent successional chronosequences where vegetation colonizes initially nitrogen (N)-poor alluvial deposits; however, as soils develop, fertility improves.  Floodplains are underlain by shallow alluvial groundwater (hyporheic zone), and some studies have indicated that this may be an important source of N for riparian vegetation.  We investigated the relative importance of the soil and hyporheic N pools for riparian vegetation and how these pools change along a successional chronosequence (open gravel, cottonwood forest, conifer forest).  We directly compared pools and fluxes of N in the hyporheic zone and soil by measuring ion-exchangeable N (IEN), mineralization and nitrification rates, potentially mineralizable N (PMN), total N, sediment and soil C:N, as well as measured foliar C:N of cottonwood trees, a dominant species in this system.  Further, we constructed a hydroponics bioassay to assess the growth of cottonwood cuttings with access to only soil, hyporheic water, soil plus hyporheic water, or river water. 

Results/Conclusions

Soil total N and PMN increased significantly from gravel to mature cottonwood stages but did not change significantly in the conifer stage.  Net mineralization rates and IEN were indistinguishable among successional stages.  Hyporheic sediment total N and PMN followed the same trend as in soil, but average values for hyporheic N (120 ± 38 kg N ha^-1) were significantly lower than for average soil N (1033 ± 151 kg N ha^-1).  Hyporheic IEN did not vary with succession (average of 11.8 ± 1.7 µg N bag^-1 day^-1) but was significantly higher than in soils (0.4 ± 1.7 µg N bag^-1 day^-1).  This flux of N through the hyporheic zone may contribute a substantial portion of N to riparian vegetation despite higher standing stocks of N in soils.  Cuttings grown in soil plus hyporheic water grew significantly more than in soil (followed by hyporheic and river water treatments); yet, foliar C:N of cuttings did not vary significantly among treatments.  The hydroponics bioassay indicated that the hyporheic zone may supply N, but possibly not other essential nutrients, such as insoluble phosphorus.  Thus, though the hyporheic zone appears to be an important source of N, soils remain an important source of other nutrients to riparian vegetation.