Long-term effects of prescribed underburning on litter decomposition and nutrient release in ponderosa pine stands in central Oregon

The effects of low-intensity prescribed underburning on the rates of litter decomposition and N and P release in ponderosa pine (Pinus ponderosa Dougl. ex. Laws) stands were studied by a litter-bag technique for 18 months in sites burned 0.3, 5, or 12 years earlier. Litter decomposition rates (k) were low, between 0.15 and 0.28 year(-1), 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 year(-1) on the sites burned 12 years earlier, and from 0.172 to 0.167 year(-1) on the sites burned 0.3 year 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, a pattern that may indicate changes in microbial activity in the forest floor. However, there were no significant differences in nutrient dynamics on the remaining sites.