SOIL MICROBIAL BIOMASS AND MICROBIAL AND MINERALIZABLE N IN A CLEAR-CUT CHRONOSEQUENCE ON NORTHERN VANCOUVER-ISLAND, BRITISH-COLUMBIA

We studied the dynamics of microbial biomass and nitrogen in old-growth forests and in 3- and 10-year-old plantations established after clear-cutting and slash burning of old-growth western red cedar (Thuja plicata Donn ex D. Don) - western hemlock (Tsuga heterophylla (Raf.) Sarg.) stands on northern Vancouver Island. Ten-year-old plantations, after initially growing well, were experiencing declining growth rates. Three forest floor layers: F (fermentation), woody F (Fw), and H (humus) were sampled four times in May, July, August, and October of 1992. Moisture content was significantly greater in the old-growth forests than in the plantations for F on July 16 (p < 0.05) and Fw (p < 0.10), but was not significantly different for H. Microbial biomass C and N were relatively constant throughout the sampling period, resulting in nonsignificant date effects. Microbial C content was in the order: old-growth forests > 10-year-old plantations > 3-year-old plantations. Microbial N content was significantly greater in the old-growth forest than in the young plantations for both F (p < 0.001) and H (p < 0.05) but was not different between the plantations. Therefore, the hypothesis that the microbial biomass acted as a net sink in the 10-year-old plantations by immobilizing N into the microbial N pool is rejected. Microbial C/N ratios were greater (p < 0.05) in the 10-year-old plantations than in the old-growth forests and in the 3-year-old plantations in H and on July 16 in F, indicating that microbial competition for N was probably a factor in the growth declining in the 10-year-old plantations. Extractable C and N and mineralizable N were generally higher in the old-growth forests than in the 3-year-old plantations and higher in the 3-year-old than in the 10-year-old plantations. As a result of better nutritional conditions, tree and understory foliage in the 3-year-old plantations had higher N concentrations and lower C/N ratios than in the 10-year-old plantations. Trees in the 10-year-old plantations displayed chlorotic symptoms and slow growth which were not observed in the 3-year-old plantations.