SOIL MICROBIAL BIOMASS IN AMAZONIAN SOILS - EVALUATION OF METHODS ANDESTIMATES OF POOL SIZES

The suitability of the fumigation-incubation (FI), fumigation-extracti on (FE) (with specific k-factor determined by C-14-labelling) and subs trate induced respiration (SIR) methods to obtain measures of the micr obial biomass in forest soils from the Amazonian basin was examined. S everal representitive topsoil (0-10 cm) samples were studied. A reason able estimate of microbial biomass C in two acid Oxisols (pH <4.5) wit h a high clay content was obtained by the FI method, provided no corre ction was applied for basal CO2 respiration. However, on two Ultisols and one Alfisol more consistent estimates of microbial C were obtained by the FI method, where correction for basal respiration was obtained using the respiration of non-fumigated (NF) soils between 0-10 d, or fumigated (F) soils between 10-20 d. Estimates of microbial C by the F E method did not differ significantly from those obtained by the FI me thod when calculated using a specifically-determined k-factor of 0.26 to convert the extractable-C flush to microbial biomass C, In the Ulti sols and the Alfisol, the SIR method gave similar estimates of microbi al C to those obtained by the FI and FE methods, but in both Oxisols m icrobial C was underestimated when a standard conversion factor was ap plied. Possibly the SIR assay was adversely affected by a high proport ion of dormant organisms and a slow gaseous release of respired CO2 fr om these heavy clay soils. When estimated using the FE method, microbi al C was in the range 890-1100 mu g C g(-1) soil in the Oxisols, 340-4 70 mu g C g(-1) soil in the Ultisols and around 580 mu g C g(-1) soil in the Alfisol. Microbial C comprised 3-4% of the total soil organic C and microbial N 4-7% of total N. Microbial C-to-N ratios averaged 6.9 +/- 0.8. Although the FI method can be used on these Amazonian soils, the FE method is more convenient and generally appears suitable for e stimating both microbial C and N in the same extract. The SIR method i s not recommened for use with Oxisols.