David Lewis (ASU - Bio) takes a stroll along the San Pedro River near Boquillas credit: J. Villinski

The goal of this research is to understand the coupling between nutrient cycles (nitrogen and carbon) and the hydrologic cycle in semi-arid riparian systems. Initially, we plan to focus on the nitrogen cycle. The work plan includes the development of innovative methods to characterize the amount of nitrogen (nitrate, ammonia, organic nitrogen) and the rate of nitrogen retention (e.g., plant uptake and denitrification) in semi-arid riparian systems. Specific aims include determining nitrogen retention along different flow paths in the riparian system, developing a nitrate/chloride (in collaboration with others) balance for the watershed, and to develop a model that links stream and hyporheic zone processes for nitrogen cycling.

• Two sites were chosen along the Upper San Pedro River for a comparative analysis of nitrogen and carbon chemistry at each site. Sites chosen were at Grayhawk Ranch (gaining reach) and Boquillas Ranch (losing/intermittent reach).
  
• The two reaches have been instrumented with 90 piezometers, installed in gravel bars and the riparian zone, arranged in transects perpendicular to the flow of the river. Transects are spaced 50-100 apart longitudinally, and wells range from a few centimeters to 100 meters laterally away from the river. All wells and transects have been surveyed.
  
• We have sampled these wells five times since February 2001 for nutrients, anions, and cations.
  
• In early July, we performed a short-term tracer injection at both reaches.
  
• Graduate student Anne Kramer Huth is determining the sources of nitrogen entering the San Pedro.
  • Two ISCO stream autosamplers were installed at the Boquillas Ranch site at the 0 m and 450 m transects. A 300 cfs flood during July 17-18, 2001, was sampled every half hour during the storm event.
    
  • Three nests of soil lysimeters were placed on the left bank of Boquillas with a lysimeter at depths of 1-2 feet, 2-3 feet, and approximately 4 feet. Samples were also collected within a few days after the storm event.
    
  • In June/July 2001, the groundwater, baseflow, and precipitation were sampled.
    
  • The samples were analyzed for chloride, nitrate, and ammonium within 2-3 days of collection. Remaining samples were frozen and will be analyzed for dissolved organic nitrogen, dissolved organic carbon, and 18O when facilities become available.
    
• Graduate student Jennifer Hamblen's project addresses how changes in geomorphology due to flood events affect hyporheic respiration. She will look at both a losing (Boquillas) and gaining (Grayhawk) reach.
  • Prototype scour chains were installed in March 2001 at both sites. Thirty scour chains have been made and will be installed at both sites.
    
  • In June 2001, 48 bank stability pins were installed at the 8 established cross sections in the 500 m reach at Boquillas. These pins will be used to monitor erosion/aggradation within the reach after flood events.
    
  • Respiration chambers have been built from PVC pipe and initial measurements conducted. Respiration will be measured in 15 cm depth increments to depths depending on degree of microbial activity at the location.
    
  • We have built pressure sensors that will have two purposes: stage measurements and scour and fill measurements. These will be installed this fall in conjunction with the scour chains.
    
• An integrated research effort was started with TA2. James Hogan, as part of his bi-annual Rio Grande sampling trip agreed to collect samples for us. These samples will be taken along Rio Grande twice annually, from near the head of flow to south of El Paso, as well as from tributaries and major agricultural drains that flow into the Rio Grande. The samples will be analyzed for various form of nitrogen (nitrate, ammonium, and dissolved organic nitrogen) as well as dissolved organic carbon, and major anions. As one of the goals of this task is to scale up from the San Pedro to the Rio Grande, this collaboration will enable us to begin to develop a baseline understanding of the general nitrogen balance in the Rio Grande.

Our results so far suggest that the form and concentration of nitrogen varies by subsystem, with riparian zones tending to be low in nitrate and relatively high in ammonium, and gravel bars showing the reverse. Surface water tends to show greater variation in time, with concentrations of nitrogen appearing to vary with changes in algal biomass. In general, temporal variation in nitrogen was highest in surface water, lower in gravel bar and lowest in the riparian zone. The predominant form of nitrogen is significantly related to redox potential, with low oxygen wells tending to be dominated by ammonium.

Plans

• For the remainder of the calendar year, we plan to continue monitoring chemistry and water table depth in the existing wells to look for changes in relation to the leaf fall period.
  
• We also plan to install well grids at a finer scale in preparation for a longer-term tracer injection this fall, with the goal of determining flow paths of hydrologic exchange between river and riparian zone.
  
• Next calendar year we will shift our focus to gaining understanding of nitrogen retention along riparian flow paths, using stable isotope tracer experiments to determine retention rates and mechanisms.
  
• Installation of the chains and scour sensors will occur this fall. Data collection for J. Hamblen will be initiated during the winter rains.
  
• Continue our collaboration with TA2 through sampling and analysis of the Rio Grande River.
  
• Expand our coordination with TA2 by sampling along well transects on the middle Rio Grande being established by Professor Robert Bowman at New Mexico Tech.