PATTERNS IN THE AGGREGATE STABILITY OF MANCOS SHALE SOILS

Land management decisions in the Colorado River Basin could have a substantial impact on future salinity and sediment contributions to the Colorado River and its tributaries. Promoting the formation and conservation of stable soil aggregates is one way to reduce future contributions of salts and sediments to water resources. This study was conducted to determine what site characteristics give the best indication of soil aggregate stability on the Mancos Shale soils found in the Gunnison Gorge National Conservation Area in Southwestern Colorado. Ninety-six one meter square plots were intensively sampled to determine plant community composition, biotic and abiotic cover values, physical site characteristics, soil chemistry, and aggregate stability. Surface and sub-surface soil aggregate stability were determined using a simple field test that determines a sample's resistance to slaking when immersed in water. Analysis of variance was used to determine differences in mean aggregate stability values between classes under 20 different parameters. While 13 surface and 3 subsurface parameters showed significant differences in aggregate stability between classes, the differences could always be attributed to the total cover of vegetation and biological soil crust within the class. Plots with no visible cover of biological soil crust showed significantly reduced aggregate stability when compared to plots having a trace to 1 percent cover (p = 0.0046). Similar results were observed for total vegetation cover. Plots with total vegetation cover of less than 2 percent showed significantly reduced aggregate stability when compared to plots with 2 to 10 percent cover (p = 0.0015). Results show that small increases in biotic cover types provide significant increases in aggregate stability.