NUTRIENTS IN GRASSLAND SOILS OF THE CENTRAL COLORADO PLATEAU—SPATIAL CONTROLS BY SUBSTRATE COMPOSITION, GEOMORPHIC SETTING, AND LAND-USE EFFECTS
Most of the sediment is derived from local bedrock sources, as indicated by geomorphic patterns (spatial relations between sediments and bedrock), as well as comparable mineralogic and textural properties. Nevertheless, some of the silt in these dominantly sandy deposits is from far-traveled dust, as revealed by mineralogic and chemical properties that cannot be attributed to nearby bedrock. Nutrient additions from dust are relatively more vital in settings underlain by nutrient-poor eolian sandstone (e.g., Navajo and Cedar Mesa sandstones) than by fluvial sandstone (arkose) and lacustrine mudstones (e.g., Cutler Group redbeds and Chinle Formation), which contain higher amounts of potential nutrients. On gentle slopes (even less than 5 degrees), important controls on the abundance of nutrient-rich silt include depletion by eolian winnowing especially in upslope areas and down-slope accumulation by slope-wash processes.
Grazing has disturbed nutrient status as revealed by comparison among never-grazed, previously grazed, and currently grazed soils. Grassland areas grazed until about 35 years ago exhibit lower nutrient and dust contents than never-grazed areas on similar nutrient-poor sandstone. These differences reflect past disturbance of soils and consequent soil erosion. In addition, there may have been insufficient time or incapacity of these lands to re-stabilize and thereby regain nutrients through dust inputs. Observations since 1998 document continued wind erosion at a previously grazed site especially during drought, whereas a nearby never-grazed site remained stable. Our studies show that recovery of the previously grazed area has been retarded by failure to regenerate adequate abundance and type of vegetation and biologic soil crust.