Transport and transformation of nitrogen in permafrost-influenced catchments

Background/Question/Methods

In permafrost-influenced catchments, flow of water and transformations of nutrients occur within a shallow active layer of soils that thaw seasonally. Nitrogen (N) cycling in these catchments is characterized by rapid internal fluxes and recycling relative to small inputs and outputs. However, increasing evidence suggests that rising temperature in high-latitude catchments causes increased inorganic N availability in soils, and export to streams. Several mechanisms may explain unexpected export of N. First, thawing of deeper soils may introduce previously frozen organic N into the actively cycling pool. Second, thawing beneath current rooting depths could constrain plant uptake of newly available N. Finally, lower C:N content of mineral soils compared to surface organic horizons may promote N mineralization and nitrification over assimilation and denitrification by microorganisms. We addressed the potential for increased depth of soil–stream flowpaths to influence export of inorganic N from high-arctic catchments by measuring rates of inorganic N uptake and denitrification in saturated, valley bottom soils of a boreal and an arctic catchment across a seasonal gradient of increasing thaw depth.

Results/Conclusions

Rates of net inorganic NH₄⁺ uptake, NO₃^– uptake, and denitrification ranged 0-1651, 0-4131, and 0-131 mg N m^-2 d^-1, respectively. Net N uptake and denitrification were greatest in snowmelt and summer when thaw depth was less than 20 cm, whereas little uptake or denitrification occurred when thaw depth exceeded 20 cm. Comparison of the time scales of transport and reaction using the Damköhler number indicated that the timescales of uptake and denitrification tended to be more rapid than transport despite large hydrologic fluxes during snowmelt runoff. Transport was more rapid than, or nearly equivalent to the timescale of reaction later in the thaw season, indicating greater potential for export of inorganic N as the soil–stream flowpath deepens. Using seasonal increases in thaw depth as a proxy for long-term changes in active layer depth, these patterns imply that shallow flowpaths contribute to retention and removal of inorganic N, whereas a deepening active layer may result in flowpath bypass of N sinks, and increased export of N from soils to streams.