N. Grimm, J. Schade, D. Lewis (ASU)

River-riparian ecosystems are characterized by dynamic interactions between terrestrial and aquatic subsystems, and between surface and subsurface waters. These dynamic interactions produce whole-ecosystem functions such as nitrogen retention, or net removal of N during transport. The central objective of this project is to understand how hydrology and biogeochemistry interact to influence nitrogen retention in arid-land riparian ecosystems.

The functioning of ecosystems may be contingent on the spatial arrangement and functioning of their constituent subsystems. Fluvial ecosystems can be conceptualized, using a framework of the Telescoping Ecosystem Model, as a series of concentric cylinders (Fisher et al. 1998). These cylinders, or zones, lie along a core-to-distal gradient. The surface channel runs through the core, and saturated hyporheic sediments envelop this core both vertically and laterally. The riparian zone constitutes the most distal element considered here. This zone exhibits vertical structure that includes the rooting zone of riparian vegetation underlain by a region of episodic saturation. We used this framework to understand the spatial and temporal dynamics of nutrient concentrations in the San Pedro River ecosystem.

Science goals

• Characterize the nutrient concentrations and speciation in different stream/riparian corridor zones.
  
  
• Determine how direction and strength of hydrologic linkage between stream and riparian zone influence nutrient concentrations and retention.



• Describe the patterns and uncovering the mechanisms of nutrient retention by the riparian zone, including location of hotspots for plant and microbial activity.

Results

Two sites along the Upper San Pedro River, Gray Hawk Ranch (gaining reach) and Boquillas Ranch (losing/intermittent reach) were instrumented in year 2 with 90 piezometers, installed on cross-river transects located every 50-100 m along the 400-500-m reaches. Subsurface water was sampled approximately bimonthly. Our findings from the year-long subsurface water sampling underscore the importance of hydrology for controlling variation in nutrient concentrations. Along the core-to-distal gradient, hydrologic flux decreases. That is, flow is fast in the surface channel, slower through gravel bar sediments, and slower yet through riparian soils. Along this gradient, consequently, temporal variation in nutrient concentrations declines, but spatial variation in nutrient concentrations increases (Figure 3-5).

We found flowpaths that moved in both directions across the riparian zone-stream interface. The interface was clearly an active site ("hot spot") of nutrient retention. However, the relative importance of these hot spots depends to a great extent of the total flux of water across the interface. At the locations where we have done these detailed studies, discharge is very low, so high rates of retention will not contribute much to overall ecosystem retention. Therefore, we intend to focus future efforts on locations where subsurface discharge is higher, such as the Gray Hawk Ranch toposequence site.

Plans

• Results from the year-long subsurface water sampling campaign are being summarized in a manuscript by Lewis, et al. (in preparation).
• Seasonal changes in nutrients and major ions are being described in a manuscript by Schade, et al (in preparation).

Future Science Questions

• As to plant-soil-water-nutrient interaction along a relict channel toposequence, questions include: What is the hydrologic flux among the individual segments of this toposequence, and between the entirety of the series an its surroundings (i.e., the mesquite terrace uplands and the riparian floodplain). To what extent does the nutrient budget of a toposequence segment (defined here as a distinct ecosystem) derive from retention and internal recycling versus from exchange with other ecosystems? Does any exchange vary seasonally or in response to storms and the flux of water? What are the feedbacks between the processing of organic matter plus nutrients and the structure and diversity of the plant and detritivore communities?



• Solute injection experiments are needed to trace nutrients through hydrologic storage and biotic compartments.



• Beaver, microbes, and mammalian diversity: a project is being developed in collaboration with John Sabo (ASU), to address the following questions: What are the effects of localized beaver activity on forest biomass and species composition? What factors limit the spatial extent of beaver herbivory? How does large tree removal by beaver change resource and predator abundance for omnivorous mammals? Do strong negative feedbacks exist between beaver activities and subsequent reestablishment of preferred tree species?