Dissolved inorganic nitrogen dynamics in the hyporheic zone of reference and human-altered southwestern U. S. streams

Canalization and incision are common morphological alterations associated with human land use that reduce hydrological/hydrodynamic linkages between surface and ground waters in stream ecosystems. To explore the impacts of these anthropogenic changes on nutrient spiraling in streams, we measured the linkage between hyporheic and surface zones of reference and human-altered streams using ¹⁵N-nitrate (¹⁵NO₃^-) and bromide (Br^-) tracer injection experiments. Experiments were conducted in 7 streams (3 reference, 3 agricultural and 1 urban) in Arizona and New Mexico, USA, over a 3 yr period during the Lotic Intersite Nitrogen experiment (LINX II). Groundwater wells (6-9 in each stream) were inserted to 30 cm depth and multilevel samplers (MLS) were installed downstream from injection sites to measure inorganic nitrogen (N) and Br^- concentrations in hyporheic water. There was measurable surface water-ground water (SW-GW) connectivity, as indicated by Br^- concentrations in shallow alluvial ground water (0-30 cm depth) at all sites. SW-GW connectivity was higher in reference than in human-altered streams. Concentrations of No₃^-and NH ₄⁺ in MLS cells (0-10, 10-20, and 20-30cm below streambed) were inversely correlated, and ¹⁵N-enrichment of both N species was measurable in groundwater wells. Hyporheic zones with lower surface-water infiltration had higher quantities of ¹⁵NH₄ ⁺than of ¹⁵NO₃^-. Whole stream uptake (k_tot; calculated during the LINX II experiments) was correlated with δ¹⁵NH₄⁺in wells; i.e., high 815NH4+in wells was associated with high stream uptake, and both whole stream NO₃^- uptake and δ¹⁵NH₄ ⁺were highest in the human-altered streams. The presence of enriched ¹⁵NH₄⁺ in many of the wells within 24 h of injection suggested that dissimilatory nitrate reduction to ammonium (DNRA) was occurring in the hyporheic zone.