MICROBIAL BIOMASS AND RELATIVE CONTRIBUTIONS OF BACTERIA AND FUNGI IN SOIL BENEATH TROPICAL RAIN-FOREST, HAINAN ISLAND, CHINA

Soil microbial biomass (C(mic)) and its relation with soil carbon and nutrient contents in a tropical rain forest (Bawangling Nature Reserve, Hainan Island, P.R. China) were studied. C(mic) was determined by the substrate induced respiration (SIR) method; bacterial and fungal fractions were measured using a selective inhibition technique. For the A horizon, C(mic) ranged between 350 and 700-mu-g g-1 soil in April (beginning of the rainy season) and 250-600-mu-g g-1 in October (end of the rainy season). C(mic) contents of the AB and B horizons were lower, ranging from 80 to 350-mu-g C(mic) g-1 soil. On average, 75% of the biomass was bacterial, in the AB and B horizon this was slightly higher than in the A horizon. C(mic) was closely correlated with C(org) and N content. N and P contents as well as the respiratory response upon glucose addition indicated that towards the end of the rainy season compared with the start the soils were depleted in nutrients. These observations stress the importance of microbial biomass as a factor for nutrient retention in tropical forest ecosystems.