AN ABS'I'RACT OF THE THESIS OF

The effects of prescribed burning on the rates of recent litter decomposition, nitrogen and phosphorus release from litter, soil total and inorganic nitrogen pools, and net nitrogen mineralization were determined in ponderosa pine sites that had been burned 0.3, 5 or 12 years earlier. Prescribed burning decreased litter decomposition rates significantly (p >0.1), in the sites burned 0.3 and 12 years previously, although the differences in litter decomposition rates between burned and control plots were small. Nitrogen and P release from recent litter was significantly higher in the plots burned 5 years previously, but there were no significant differences in the plots burned 0.3 or 12 years earlier. Soil inorganic N concentration significantly increased shortly after prescribed burning, but declined thereafter to reach the levels of the control plots at the end of the next growing season. Both inorganic and total soil N pools in soil were significantly lower in the plots burned 5 years previously, and there were no differences in any of the N pools measured for the sites burned ABSTRACT The effects of low intensity prescribed underburning on the rates of litter decomposition and N and P release were studied using a litter-bag technique for an 18 month period in sites burned 0.3, 5, or 12 years earlier. Litter decomposition rates were low, between 0.15 and 0.28 year1, and were significantly (p <0.1) reduced by prescribed fire on the sites burned 0.3 and 12 years earlier. However, the reduction in decomposition rates was small, from 0.22 to 0.19 on the sites burned 12 years earlier, and from 0.172 to 0.167 on the sites burned 0.3 years earlier. Nitrogen tended to be immobilized in the decomposing litter, while P was rapidly released, suggesting that these ecosystems are limited by N, but not by P. Nitrogen showed a distinctive seasonal pattern of net immobilization during winter, and a net release during summer. Prescribed burning significantly increased the release of N and P from the litter on the sites burned 5 years earlier, which may indicate changes in microbial activity in the forest floor. However, there were no significant differences in nutrient dynamics on the remaining sites. ABSTRACT The effects of prescribed underburning on soil total C pools, total and inorganic N pools, and in situ net N mineralization were determined during a one-year study on ponderosa pine sites that had been burned 0.3, 5 or 12 years earlier. In the sites burned 0.3 years previously, prescribed burning caused a significant (p <0.1) increase in total C concentration and inorganic N concentration. However, inorganic N concentration declined during the one-year duration of this study to reach the levels of the control plots at the end of the next growing season. In the sites burned 5 years previously, prescribed burning significantly decreased total C and N concentrations, inorganic N concentration, and net N mineralization. At the sites burned 12 years previously, prescribed burning did not affect N and C pools, but it significantly decreased net N mineralization. The decrease in net N mineralization is likely caused by a decrease in substrate quantity 5 years after burning, and by changes in substrate quality 12 years after burning. A long-term decrease in net N mineralization in the N-poor ponderosa pine stands of Central Oregon may result in a decrease in long-term site productivity and may explain the observed pattern of long-term decrease in stand growth following prescribed burning. This study was carried out on five sites located in the Deschutes near Bend (44°N, Oregon. The sites are part of a long-term study on the effects of prescribed burning in ponderosa pine ecosystems established and maintained by personnel of Silviculture in Oregon. One of the sites, Lava Butte, was burned in 1979; another site, Annabelle, in 1986; and the remaining sites, Swede Ridge, Sugar Cast, and Far East, in 1991. The sites burned in 1991 follow a gradient in productivity; site indices are 35, 31, and 25 m at 100 years (Barrett, 1978), respectively. Measurements started in September, 1991 at the sites, 12 years, 5 years, and 4 months after the burning.