Seasonal variation in nitrogen retention processes in a coupled terrestrial-aquatic ecosystem: A tracer experiment

Background/Question/Methods A variety of ecosystem processes affect the retention and loss of nitrogen (N) from deciduous temperate forests, and these processes are likely to vary seasonally. We followed the fate of nitrate entering an Upper Susquehanna forest through a whole-ecosystem study of coupled terrestrial and aquatic N retention processes. We expected that losses of N to stream export and denitrification would be greatest during spring, plant uptake would be greatest in spring and summer, and soil- and in-stream heterotrophic uptake would be greatest following leaf-fall. Tracer-level doses of 15N-KNO3 (3 x 70 gN/ha) and equimolar KBr were applied to the terrestrial upland (0.25 ha) surrounding a headwater stream in late April, 2007 (post snowmelt, pre-leafout), late July (mid growing-season), and late October (end of leaf-fall) to allow comparison of N retention processes across seasons. The fate of these additions was followed as losses of N gases; assimilation into litter, soils, and roots; through soil water; and into and down the stream reach. Gases, soil and well water, and soil pools were sampled prior to and 1, 2, 7, 30, and 90 days post-application, and stream water was collected near-continuously with 3 automated samplers deployed along a stream transect.

Results/Conclusions

Stream nitrate loss from this catchment follows unusual seasonal patterns of low concentrations (< 20 ugN/L) in spring, highest values in summer (~80 ugN/L), and abrupt decreases at leaf-fall (to 2-5 ugN/L). Nitrate concentrations change little with distance downstream, except during autumn, when concentrations at the spring (75 ug/L) decreased exponentially to <5 ug/L within 80 m. Dissolved organic nitrogen and carbon concentrations increased as nitrate decreased. Seasonal patterns of stream chemistry more closely followed that of shallow (10 cm depth) lysimeters than deep (50 cm) lysimeters. Following each of the tracer additions, sharp peaks of Br- were observed in shallow soil water and stream water. Following the April addition, a small amount of enriched nitrate (d15N = 1000-2000 per mil) leached through shallow soil profiles and out with streamwater in subsequent days. Approximately half of the 15N was recovered in surface litter material (33 ± 2 g/ha, enriched to ~90 per mil), and half in surface mineral soil (33 ± 2 g/ha, 0-10 cm; enriched to 5 per mil). The July addition had similar soils results, and October results are forthcoming. The experiment integrates terrestrial and aquatic perspectives and assesses the relative contribution of watershed subsystems in N retention.