Soil microbial biomass (Cmic.) and is relation with soil carbon and nutrient contents in a tropical rain forest (Bawangling Nature Reserve, Hainan Island, P.R. China) were studied. Cmic. was determined by the substrate induced respiration (Sir) method; bacterial and fungal fractions were measured using a selective inhibition technique. For the A horizon, Cmic ranged between 350 and 700μgg-1 soil in April (beginning of the rainy season) and 250–600μg g-1 in October (end of the rainy season). Cmic. contents of the AB and B horizons were lower, ranging from 80 to 350 μg Cmic 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. Cmic was closely correlated with Corg 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.

We have studied the chitin and ergosterol contents of ectomycorrhizal roots in three sets of experiments to evaluate them as indicators of fungal biomass. The first set of experiments showed that ageing had a marked effect on ergosterol concentrations. The ergosterol content of 7-month-old, brown, shrunken Pinus sylvestris L.–Paxillus involutus (Fr.) Fr. mycorrhizas was found to be only 10% of that found in white, turgid, 1- or 4-month-old specimens. This supports the hypothesis that the compound is a good indicator of living fungal biomass. Ageing had a lesser effect on chitin concentrations since the chitin levels found in 7-month-old mycorrhizas were still 60% of the levels found in 1- and 4-month-old specimens.

Consequently, the chitin[ratio ]ergosterol ratio increased from about 14 to 19 in 1- and 4-month-old mycorrhizas respectively to about 110 in 7-month-old mycorrhizas. In the second set of experiments, we found that variation in plant growth had no effect on the chitin[ratio ]ergosterol ratio in whole root systems of either Alnus incana (L.) Moench or Pinus sylvestris mycorrhizal with Paxillus involutus. In the third set of experiments, we found a constant relationship between the two marker concentrations in 10-month-old root systems of Pinus sylvestris, regardless of fungal species involved, using Paxillus involutus, Piloderma croceum Erikss. & Hjorts and Suillus variegatus (Fr.) O. Kuntze as test organisms. Taken together, the results of this study suggest that both chitin and ergosterol give reliable, but different, relative measures of fungal biomass in mycorrhizal roots. Furthermore, we demonstrate that, in combination, the two chemical markers can be used to estimate both total and living fungal biomass (derived from the chitin[ratio ]ergosterol ratio).