Prescribed Fire Impacts on Soil Carbon and Nitrogen

Prior to European settlement of the Mogollon Rim, ponderosa pine forests consisted of open stands of uneven -aged trees with a significant grassy under story (Sackett 1979). Light surface fires occurred on an average interval of 2 -12 years in Arizona and New Mexico (Weaver 1951; Cooper 1960; Dieterich 1980). These fires consumed part of the forest floor litter, burned most of the young trees, and promoted growth of a dense, grassy understory. Crown fires were rare due to lack of ladder fuels and the widely spaced ponderosa pine canopy (Dieterich 1980:44 -48; Sackett 1980). Grass fuels reduction from heavy sheep and cattle grazing in the late nineteenth century, and then forest fire suppression during much of the twentieth century, have resulted in the development of dense, overstocked stands. Forest floor fuels most likely were less than 4 Mg /ha prior to 1870, but have since increased 10 to 100 fold (Sackett 1979; Sackett et al. 1996). Annual accumulations now are in the range of 1.3 to 7.8 Mg /ha /yr. Tree densities that were once < 130 stems /ha have increased to more than 2750 stems/ ha in the densest stands (Sackett 1980; Covington and Sackett 1986). Stand basal areas have also experienced a 3or 4 -fold increase (Marlin Johnson, personal communication). Ponderosa pine stands reached a critical ecological point in the 1990s; fuel loads had built up so much that by the end of the twentieth century, wildfires burned four times the area that they did in the period from 1910 to 1990 (Neary et al. 1999). Fires can greatly alter nutrient cycles of forest ecosystems depending on fire severity, fire frequency, vegetation, and climate (Neary et al. 1996: 107 -117). Responses of total carbon (C) and nitrogen (N) are variable and depend on the site conditions and fire characteristics (DeBano et al. 1998). In most soils, the majority of the N pool is contained in the soil organic matter (OM). As would