Weathering releases lithogenic elements to soil and stream waters, and regulates life in catchment ecosystems. Seasonal and inter-annual variations in hydrologic conditions may change subsurface flowpaths, modifying the influence of weathering on stream water composition. This two-year study aims to determine the seasonal and inter-annual changes in solute sources to stream and soil waters in a seasonally snow-covered headwater catchment in the Jemez Mountains in northern New Mexico using a multi-tracer approach including major cations, strontium isotopes, germanium (Ge)/silica (Si) ratios and trace metals. Climatic forcing was different for the two years studied; 2010 had ample snow accumulation resulting in an increase in stream flow during melt, whereas little snowfall in 2011 caused minimal snowmelt response in the hydrograph. Stream water base cations display relatively constant concentrations with variations in the hydrograph, suggesting that stream water concentrations are not dependent on dilution from snowmelt and storm events, but instead on fluxes from subsurface water supplies. Strontium isotope ratios of stream waters during 2010 and 2011 snowmelt periods (0.707497 - 0.707548) were similar to the La Jara catchment spring water Sr-isotope ratio (0.707510) indicating spring water as a single dominant source of major cations to stream waters. However, concentrations of Ge, Fe, Al, and dissolved organic carbon (DOC), which are generally thought to originate from shallow soil waters, increase with the rise in the 2010 snowmelt hydrograph. This suggests that stream waters contain components of both shallow soil water and spring water during snowmelt. Following snowmelt, Ge, Al, Fe, and DOC concentrations decrease and remain low for the remainder of the study period. This is reflective of a reoccurrence of a single source of spring water to streams even during the monsoon season. This change in solute source was not observed in 2011 due to little snow fall, resulting in freezing soils which prevented the infiltration of melt waters that flush out the soil solutes. The lack of an increase in Ge/Si ratios and Al, Fe, and DOC concentrations during the summer rains for both 2010 and 2011 shows that groundwater dominates stream water composition after the snowmelt period (June-March).

Soil carbon is the largest terrestrial carbon pool. While inputs to the soil carbon system are well constrained, the diverse factors that contribute to soil C efflux remain poorly understood. The O/A horizon, typically rich in organic matter, experiences extremes in wetting and drying, and previous studies have shown that soil carbon dynamics are critically linked to moisture availability. Carbon in surface soils is mobilized via two distinct pathways: CO2 efflux and leaching of dissolved organic carbon (DOC). The goal of this study is to quantify the role of hydrologic variability in mobilizing carbon as gaseous and dissolved fluxes from near-surface soils, and to determine the relative magnitudes of dissolved and gaseous fluxes. Samples were collected through 2010 and 2011 from subalpine sites in Southern Arizona and Northern New Mexico. Both of these sites are characterized by bimodal precipitation and soil moisture seasons- spring snowmelt and summer monsoon. However, sites in NM experience a snow-covered season of variable duration, while sites in AZ experience ephemeral snow cover. We observed no significant variability in DOC leachate, and these values were low at all sites; however in contrast, CO2 fluxes were large (from 0.22gCm-2d-1 to 5.27gCm-2d-1) and varied between sites and between years. Soil moisture is the primary control on CO2 efflux at these sites, both during the winter and during the growing season. Our results suggest that in arid montane forests both soil carbon dynamics and ecosystem carbon balance are critically linked to water availability.

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PDF of abstract here. Colin's post-capstone research project work will be with Amigos de Americas in Nicaragua.

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PDF of abstract here

PDF of abstract here