Magnitude and controls of microbial nitrate production in the streams and till of a glaciated alpine catchment, Canadian Rocky Mountains, Alberta
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In the summer of 2010, fieldwork was conducted in the Robertson Valley, Canadian Rocky Mountains, Alberta to assess the magnitude and controls of microbial nitrification in proglacial till and in supraglacial, subglacial, and proglacial streams. Seasonal precipitation and glacial and proglacial runoff was sampled for hydrochemical and stable isotope analyses (δ18O and δ15N of nitrate [NO3-]). Lower Ca:Mg ratios, higher mean Σmajor ions, and an increased importance of reactions with slower dissolution kinetics in subglacial streams and proglacial seeps indicated waters here experienced longer rock-water contact time than in dilute supraglacial streams. Additionally, waters emanating from longer residence time flowpaths acquired substantial NO3- from nitrification reactions. Using δ18O-NO3- in a simple end-member mixing model, the fraction of NO3- derived from microbial nitrification was estimated to be 44 to 56% in the two subglacial streams, and greater than 80% in proglacial seeps. These results show that atmospherically-derived nitrogen (N) in this glacial valley undergoes substantial biological cycling prior to export in surface runoff. Water flowing from the east subglacial stream (RE) received a larger portion of its melt from a sediment-rich, slow drainage system and had a higher proportion of nitrified NO3- compared to the west subglacial stream (RW), where runoff was similar in composition to supraglacial runoff, indicating that the nature of subglacial flowpaths is an important factor in determining the amount of microbially-cycled nutrients that are exported from a glacier. Sixteen 34-day in situ soil incubations revealed that net mineralization and net nitrification occurred at all four sampling sites in the glacier forefield along a 1.6 km chronosequence; however, there was no significant difference among these rates with time since deglaciation or temperature. Instead, net mineralization and net nitrification rates were significantly correlated (p < 0.05, n = 16) with measured physical and chemical soil variables, including total organic carbon, total N, bulk density, pH, and clay content, suggesting that substrate availability is a larger control on N-cycling processes than time since deglaciation. High variability in inorganic soil N pools and N-cycling rates indicates that there are likely hot spots of biogeochemical activity within glacial till.