2002-07-08_GENERAL DOCUMENTS - M2002004 (3)

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SMITH, REDENTE, and HOOPER ration and equilibrium level of on under the proposed post- .' that they supply nutrients to • site conditions affecting plant ystem. has been reclaimed in recent ✓sing by livestock and /or wild - en returned to forest land or igeland reduces organic matter n in organic matter of up to 25 ig soil is a good target for pro - rangeland areas should be 100 n level of organic matter for a ent undisturbed area. But one rbed systems are in the process ;anic matter may expedite that liar set of soil- forming factors and processes of soil organic undisturbed land, equilibrium nperature, rainfall, vegetation ind land use management (4). iorizons of native soils in min - rom 0.5 to 8 percent (9, 16, 17, is of the West organic matter to 6 percent in southern and r levels in mountainous areas ser textured soils in the south - vel of 1 percent (26). Finer tex- a stable fraction level of 3 to 5 have been done on reclaimed alvaging or replacement tech - 0- year -old mine soil near Col - rofile than in adjacent natural nine soil organic C levels were ils in Saskatchewan, Canada, In a North Dakota mine soil, itural soils (87). In Montana, 4 gm /yr in a 50- year -old mine line soil that was subjected to ;orous introduced perennial SOIL ORGANIC MATTER 199 grasses and alfalfa (68). Accumulation rates of about 28 gm /yr have been calculated in Canada (5). Generally, equilibrium organic matter levels increase with increasing pre- cipitation and decrease with increasing temperature (9). Therefore, the organic matter content of soils in western mining areas should be highest in eastern Montana, followed in order by eastern Wyoming, eastern Colo- rado, and eastern New Mexico. Furthermore, with all other conditions be- ing equal, organic matter levels generally decrease with decreasing clay and /or silt content. That is, organic matter content in clay is greater than that in loam, which is greater than that in sand. Finally, more concave or depositional physiographic positions have higher organic matter levels in the upper horizons because of the additional organic matter - enriched runoff received from upslope surfaces or increased plant production facilitated by additional moisture. Equilibrium levels of organic matter also vary with the type of land man- agement. Organic matter levels are commonly lower in cultivated areas ad- jacent to virgin soils having the same aspect and landscape position than or- ganic matter levels in the virgin soil. In addition, organic matter levels in cultivated soils tend to remain lower because tillage breaks up organic resi- dues and brings them into contact with soil organisms, "thereby increasing the rate of decomposition" (9). In most mining regions of the western United States, topsoil use is becom- ing more common. In these situations low organic matter content will not hinder revegetation efforts as much as other undesirable soil conditions, such as low N or P levels, adverse (high) sodicity or salinity levels, a skeletal particle size class (greater than 35 percent by volume rock fragments), or a sand or clay texture. Even though organic matter levels in surface soil horizons at any given mine location will vary as much as 5 percent, the immediate adverse effect of low organic matter levels on revegetation is less than other undesirable soil conditions, such as excess Na. In the long term, however, organic matter is probably a controlling influence insofar as con- tinued nutrient cycling relates to revegetation success. Mitigating undesirable soil or site conditions with organic matter. Our assumption is that ample topsoil generally is available for use at most min- ing sites in the West. But "ample" topsoil varies from one site to the next. Moreover, some western mining operations are on very shallow, sodic /sa- line, or skeletal soils or on steeply sloping areas providing little or no top- soil. In such areas as the San Juan Basin in New Mexico or sodic /saline locations in southwestern Wyoming revegetation is being attempted on spoil or nontopsoil material. In some of these cases applied organic matter may be beneficial. Organic matter probably would increase the water - holding capacity of skeletal spoil, but no more than loamy soil material. In sodic/ saline subsoil or spoil, which is often medium to fine textured, organic mat-