Abstract: Downed logs are important carbon and nutrient pools in forest ecosystems. Monitoring of dynamics of microbial biomass and dissolved organic carbon/nitrogen are of great significance to in-depth understanding of the mechanism of downed log decomposition. Microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and ratio of carbon to nitrogen in bark, sapwood and heartwood of downed T. longibracteata logs various in decaying degree from Grade I to Grade V in the Tianbaoyan National Nature Reserve, Fujian Province of China was analyzed. Results show:(1) The downed logs varied from 7.41% to 63.27% in water content, and were 311.66-564.87, 2.34-5.82 and 0.09-0.35 g·kg^-1, respectively, in organic carbon, TN and TP; (2) In terms of MBC and MBN, the three components of the highly decayed downed logs (Grades Ⅳ and Ⅴ in decaying degree) displayed an decreasing order of heartwood > sapwood > bark; (3) Sapwood and heartwood was higher in DOC than bark of the logs the same in decaying degree, while sapwood was higher in DON than bark and heartwood, except for logs of Grade Ⅱ in decaying degree; (4) Microbial biomass C/N ratio in heartwood varied sharply between logs different in decaying degree, whereas DOC/DON ratio in sapwood did; (5) Position in the log, decaying degree and their interactions all had remarkable impacts on MBC, MBN, DOC, DON, MBC/MBN and DOC/DON (P<0.01); and (6) MBC, DOC and DON were extremely and positively related to water content in the log (P<0.01), whereas NBC/MBN and DOC/DON negatively to TP in the log (P<0.05). To sum up, during the downed log decaying or decomposing process, water content is the major factor affecting MBC, MBN, DOC and DON in decaying downed logs.