Retention and removal of nitrogen and phosphorus in saturated soils of arctic hillslopes

Interaction of hydrologic and biogeochemical processes on hillslopes may contribute significantly to export of nutrients from soils to stream networks, yet hillslopes remain poorly understood components of catchments. In the arctic, hillslopes are underlain by permafrost, and drained by zero-order channels called water tracks that contain perennial subsurface flow. We conducted in situ experiments to measure retention of inorganic nitrogen (N) and phosphorus (P) by saturated soils of water tracks and investigated the roles of water residence time, flowpath depth, and substrate availability in determining the balance of reaction and transport of nutrients. Net retention of P was observed in 46 % of experiments, and net retention or removal of nitrate () was observed in 57 % of experiments, whereas net retention of ammonium () occurred in only 16 % of experiments performed. Net production of occurred in 42 % of experiments, was more frequently observed than retention, and was most rapid where water residence time was shortest. P retention was enhanced by P availability, suggesting strong capacity to buffer downslope fluxes of inorganic P in water tracks. Net retention or removal of tended to occur in shallow flowpaths, but was not detected in deeper soils. Strong retention of inorganic P by saturated arctic soils indicates that hillslopes contribute to regulating the flux of P to downstream ecosystems, whereas weaker retention of inorganic N, particularly where flows are deep or rapid, suggests that increased discharge from hillslopes and deeper thaw will contribute to increased export of N.