Nitrogen mineralization in forest floors of three tree species on the same site : the role of litter quality

Forest floor samples from a 25-year-old plantation of three tree species [Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.), and paper birch {Betula papyrifera Marsh.)] growing on the same site were incubated (aerobically) in the laboratory for 29 d. Net N mineralization per g of dry matter in the forest floors of Douglas-fir (144.1 pg/g) was significantly greater than either birch (64.2 pg/g) or lodgepole pine (49.1 pg/g). Initial NH₄-N was highest in the forest floor of Douglas-fir (26.5 pg/g), followed by paper birch (9.8 pg/g) and lodgepole pine (3.5 pg/g). Initial N0₃-N was low for all three species and increased very slightly during the incubation, suggesting that ammonification was the dominant process. N mineralized per unit mass of N in the forest floors of Douglas-fir (6.6 ug/g N) was significantly greater than either birch (3.3 ug/g N) or lodgepole pine (2.9 ug/g N). There were no significant differences among the three species in rates of CO₂-C mineralization in forest floors during the 29 d aerobic incubation. Nitrogen concentrations in the forest floors of the three species differed significantly: Douglas-fir had the greatest % N (2.09), lodgepole pine the least (1.65), and birch was intermediate (1.85). Percent total N and C/N of the forest floor were strongly correlated (r²= 0.81 and r²= 0.54, respectively) with net N mineralization, showing a clear species separation. The litter of Douglas-fir had the highest lignin/N and C/N ratios, and mineralized the most N during the incubation, whereas the litter of paper birch had the lowest lignin/N and C/N ratios yet mineralized the second lowest amount of N. Net N mineralization in the forest floor was not correlated with either lignin/N or C/N of the foliar litter, suggesting that this index does not have widespread applicability to all litter types.Forestry, Faculty ofGraduat