VEGETATION REMOVAL IN 2 SOILS OF THE HUMID TROPICS - EFFECT ON MICROBIAL BIOMASS

Conversion of forests of the humid tropics into pastures and agricultu ral fields is expected to produce, in the long-term, a decline in soil organic matter content and soil fertility. Changes in microbial bioma ss (Biomass C, Bc) following vegetation removal can provide an early i ndication for slower, less easily detectable SOM changes. Microbial bi omass can also provide an index of soil fertility because it represent s an important labile pool of soil nutrients and plays an active role in preventing nutrient loss. There are few published measurements of B c in the humid tropics and fewer of Bc changes due to shifts in vegeta tion cover. We measured Bc in two humid tropical soils (an oxic Humitr opept at 4.5% C and pH of 5.0 and a fluventic Dystrandept at 2.6% C an d pH of 6.4) subjected, for 3 yr, to extreme treatments: soil maintain ed bare, annual harvest of re-growth, and native 20 yr old secondary v egetation. Both soils showed a similar pattern in total SOM and Bc dec line following vegetation removal: after 3 yr, total C and N were redu ced by 20%. Response of Bc was more pronounced. In the bare soil, most of the decline in Bc occurred within the first 6 months (to 50% of in itial values) and after 15 months, Bc appeared to have stabilized at c n 35% of its initial value. Response of Bc to the annual harvest treat ment was more moderate than to the bare soil treatment. Determining th e precise size of microbial biomass is difficult because of variabilit y in time and differences between techniques, but Bc values determined by two techniques and on numerous dates were high: in control plots a round 2000 (oxic Humitropept) and 1300 (fluventic Dystrandept) mu g C g(-1) soil (or 250 and 187 g m(-2) after correction for differences in bulk density), which suggests that Bc in humid tropical soils can be high. Microbial C represented ca 4% of the total C in the control and declined to ca 1.5% of the total C in the bare soil. Eucaryote: procar yote ratios were close to 1 for all treatments except in the Fluventic Dystrandept control, which supported a woody vegetation and had a rat io of 3.3. This study demonstrates the dynamic nature of microbial bio mass following tropical forest clearing and its potential importance f or affecting nutrient loss.