Hydrologic controls on nitrogen availability in a high-latitude, semi-arid floodplain

Abstract

Past research shows a discrepancy between apparent nitrogen supply and the annual growth requirements for early successional plant communities along the Tanana River floodplain in interior Alaska. Because previous measurements of nitrogen fixation, mineralization, and deposition can only account for approximately 26% of these communities' nitrogen requirements, other mechanisms of nitrogen supply should be operating. This study examined the potential for subsurface hydrology to supply nitrogen to the rooting zones of early successional plant communities. Three transects of ground water wells were established along the floodplain, and hydrologic characteristics and soil/water chemistry were measured for 2 y. Hydrologic measurements were then used to model the potential nitrogen contributions from advective flow and capillary rise. Ion exchange membranes and stable isotope tracers were used to identify the use of hyporheic nitrogen by vegetation. Nitrogen accumulation on ion exchange membranes showed a large input of nitrogen during a period of high water in 2003. Plants showed uptake of hyporheic 15N-labelled nitrogen at 2 of 4 locations. Models of capillary rise and advective flow estimated that nitrogen supply from subsurface water equalled or exceeded total nitrogen supply from nitrogen mineralization and nitrogen fixation combined. These findings underscore the importance of hyporheic processes in controlling nutrient supply to early successional vegetation on the Tanana River floodplain.