Changing Soil Properties – Relevance to State and Transition Models

Arlene J. Tugel, USDA-Natural Resources Conservation Service
Soil Quality Institute, Las Cruces, New Mexico

• A summary of the presentation made at the workshop is below.  Topics included an example of a modified soil; existing soil survey methods for identifying modified soils; new soil classification and database activities that address changing soil properties; and suggestions for including soil information in state and transition models.
• The pertinent question is “what kinds of soil changes cause an ecological site to become a new site?”  Soil properties change both in response to management and natural events or cycles. Those changes that effect the ability of the soil (site) to support the original suite of states (plant communities) should be considered when assigning an ecological site. In the example of the black grama grassland shift to mesquite duneland, the soil modification occurs as both erosion and deposition.  In some landscapes, the soil in the shrub inter-space is eroded and the dune soil is wind deposited soil material.  In other landscapes, the soil in the inter-space has an intact soil profile with an A horizon that extends under the dune soil as a buried soil. In both cases, the soils are no longer the same as the original soil under the black grama grassland and can no longer support the original plant community. Examples of soil changes also occur in plant communities other than black grama-mesquite.
• Current methods of identifying modified soils in soil survey include phases of soil series, unnamed inclusions, new soil series and new soil classification.  The genetic link between the original soil and the modified soil is only maintained through the phase naming method. (eg. Alpha soil and Alpha soil, eroded).
• The International Committee for Anthropogenic Soils has been charged with developing a provisional classification scheme for human modified soils. When proposed, criteria for classifying anthropogenic soils should be evaluated for their impact on rangeland soils and procedures for assigning ecological sites.
• The capacity of soil to function in rangeland systems depends on both 1) inherent (static) soil features and 2) dynamic soil properties that are susceptible to change in response to management. Some “essentially” static properties include texture, mineralogy, horizon sequence, soil depth, slope, and aspect. Examples of dynamic properties include soil moisture and temperature, organic matter, nutrients, topsoil depth, aggregation, mineral crusts, salinity, soil microbes and soil fauna.
• The NRCS Soil Survey Center and Soil Quality Institute are currently doing initial evaluation work for the development of a “Use-dependent Soil Property Database.”  The database will include near-surface dynamic soil properties and their measured or estimated values under various land uses and management systems. The various states in a site could be included.
• Some suggestions for including information within state and transition models to reflect changes in soil properties are:  1) a representative soil series or phase of soil series for each state, 2) information that describes the genetic linkage between the original soil and the modified soil of a new site, and 3) dynamic soil properties that define thresholds or early warning indicators of vegetation changes. The challenges will be to determine the dynamic soil information needed to predict vegetation changes and the functioning level of rangelands, and then to gather the soil property data.  Possible benefits of including dynamic soil information in STM’s include: 1) documentation of soil properties for various states, 2) reference “values” for rangeland inventories and planning, and 3) aides for the prediction of vegetation and soil stability changes.

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Changes in Soil Properties: Relevance to State and Transition Models

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