Temperature effects on organic matter and microbial biomass dynamics in temperate and tropical soils

The aim was to investigate whether soils developed under tropical conditions had different organic matter and microbial biomass dynamics than soils developed under temperate ones. Three soils formed under temperate climatic conditions (U.K.) and three under tropical conditions (Brazilian) were selected to be as comparable as possible in terms of organic matter, clay content and pH. They were then incubated moist at 15°C or 35°C for 150 d. Carbon dioxide evolution and microbial biomass were measured at intervals during the incubation. The biomasses in the tropical soils declined more slowly at both temperatures than in the temperate soils, although at 15°C the differences were mainly small. At 35°C the decline was generally much more marked in the temperate soils (60–75% of the initial value) than in the tropical ones (15, 40 and 60%). Soil organic matter was mineralised more rapidly in the temperate than the tropical soils: at 35°C up to 9–10 times more CO2–C was evolved than was contained in the temperate biomasses during the 150 d incubation. The comparable maximum value for the tropical soils was 4.5 times. These results seem to indicate that the organic matter in the tropical soils was more degraded, or humified, than that in the temperate soils. An attempt to quantify the extent of humification was made using differential thermal analysis (DTA) and thermogravimetric analysis (TG). Both methods also indicated that the organic matter was generally more humified in the tropical than temperate soils. It was concluded that DTA and TG may both be useful techniques in studying soil organic matter dynamics, especially when linked to studies of soil microbial dynamics.