SOIL ORGANIC-MATTER RECOVERY IN SEMIARID GRASSLANDS - IMPLICATIONS FOR THE CONSERVATION RESERVE PROGRAM

Although the effects of cultivation on soil organic matter and nutrient supply capacity are well understood, relatively little work has been done on the long-term recovery of soils from cultivation. We sampled soils from 12 locations within the Pawnee National Grasslands of northeastern Colorado, each having native fields and fields that were historically cultivated but abandoned 50 yr ago. We also sampled fields that had been cultivated for at least 50 yr at 5 of these locations. Our results demonstrated that soil organic matter, silt content, microbial biomass, potentially mineralizable N, and potentially respirable C were significantly lower on cultivated fields than on native fields. Both cultivated and abandoned fields also had significantly lower soil organic matter and silt contents than native fields. Abandoned fields, however, were not significantly different from native fields with respect to microbial biomass, potentially mineralizable N, or respirable C. In addition, we found that the characteristic small-scale heterogeneity of the shortgrass steppe associated with individuals of the dominant plant, Bouteloua gracilis, had recovered on abandoned fields. Soil beneath plant canopies had an average of 200 g/m(2) more C than between-plant locations. We suggest that 50 yr is an adequate time for recovery of active soil organic matter and nutrient availability, but recovery of total soil organic matter pools is a much slower process. Plant population dynamics may play an important role in the recovery of shortgrass steppe ecosystems from disturbance, such that establishment of perennial grasses determines the rate of organic matter recovery.