COS 126-9 - A revised perspective of soil surface stability: Dynamic aeolian equilibrium of dryland ecosystem surfaces

Friday, August 7, 2009: 10:50 AM
Grand Pavillion II, Hyatt
Jeffrey J. Whicker, Environmental Programs, Los Alamos National Laboratory, Los Alamos, NM, David D. Breshears, The University of Arizona, Tucson, AZ and Craig D. Allen, U.S. Geological Survey, Fort Collins Science Center, Jemez Mountains Field Station, Los Alamos, NM
Background/Question/Methods

Soil surfaces in drylands can be erodible, with aeolian processes and associated wind erosion increasingly being recognized as important because wind erosion redistributes soil and associated nutrients within and across ecosystems.  The stability of the soil surface is a key characteristic considered in assessing the condition of dryland ecosystems.  While limited wind transport is assumed to occur in drylands, if a net loss of soil is not readily observed or measured, the soil surface is usually assumed to be relatively stable.  However, measurements of total horizontal dust flux have not been compared to net changes at the same location.  Wind-driven processes are omni-directional in that wind can blow sediment back and forth across a given location.  The omni-directional nature of these measurements precludes determination of net horizontal sediment flux into or out of a field.  We measured net soil gain and loss in semiarid grassland and woodland plots, including one highly-degraded woodland site, in New Mexico USA by measuring fluxes in and out of a circular plot at each site using BSNE sediment samplers.  We measured directional fluxes by restricting sampler movement to a sampling angle of 45o and arranging them in a 40-m diameter circle.  

Results/Conclusions

We measured substantial sediment fluxes at each plot.  Horizontal fluxes were greatest in the disturbed site and grassland compared to the undisturbed woodland, consistent with recent research.  Most importantly, omni-directional fluxes into each circular plot did not differ significantly from omni-direction fluxes leaving the plot, even though both were substantial.  Therefore, our results reveal a dynamic aeolian equilibrium in that soil surfaces in these dryland ecosystems are much more dynamic than previously recognized, although there is no net aeolian soil loss occurring.  However, within the plots, aeolian processes are likely redistributing soil and nutrients among vegetation-scale patches similar to water-driven ecohydrological dynamics.  This revised perspective of soil surface stability could have important implications for biogeochemical transport of nutrients and contaminants in drylands.

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