Soil pH and organic C dynamics in tropical forest soils: Evidence from Laboratory and simulation studies

Acidic soil pH may affect decomposition of added organic materials in humid tropical forest soils. Our objective was to determine the effects of soil pH on decomposition of added organic materials to tropical forest soils of different soil texture and clay mineralogy. Release of (CO2)-C-14 and microbial biomass C-14 were measured during a 270-dincubation at 25 degrees C after either [C-14]glucose or C-14-labeled blue grama grass (Bouteloua gracilis) material had been added to 13 tropical forest soils with smectitic, kaolinitic, oxidic or allophanic mineralogies. Initial soil pH ranged from 3.9 to 6.7. An additional investigation examined (CO2)-C-14 release from kaolinitic or oxidic forest soils to which either Ca(OH)(2) or CaSO4 had been previously applied to obtain 5 soil pH values. Initial soil pH and cumulative (CO2)-C-14 release in glucose-amended soils were positively related only after 1 and 4 d. In contrast, plant-residue-amended soils had positive relationships between initial soil pH and cumulative (CO2)-C-14 release after 7 d and continued with that relationship up to 270 d. Microbial biomass C-14 was reduced at lower pH values in both glucose-and plant-residue-amended soils after 270 d. Water-extractable C-14 was also higher at pH > 5.5 in plant-residue-amended soils after 58 d. Differences in soil texture and clay mineralogy had no apparent effect on the relationship between soil pH and decomposition. Simulated results of the experiment using the CENTURY Soil Organic Matter Model diverged from observed results for soils with pH < 6.5. Further research is required to determine the effects of acidic soil pH on decomposition rates of stable C pools and to develop functions for simulation models to account for the short- and long-term effects of soil acidity on decomposition.