Impact of soil nitrogen availability and pH on tropical heath forest organic matter decomposition and decomposer activity

Decomposition is a key process for rain forest nutrient cycling, and this may be altered by the increasing rate of deposition of reactive nitrogen on rain forests. Tropical heath forests are characterised by slow litter decomposition rates due to low quality litter, along with acidic soil pH and low availability of soil nitrogen (N). To investigate whether soil N or pH is most important in regulating tropical heath forest organic matter decomposition, we used a factorial N and CaCO3 addition experiment, and measured wood and leaf litter decomposition rates and mesofaunal activity (using bait-lamina sticks and numbers of worm casts). Our heath forest site in the Kabili-Sepilok Forest Reserve (Sabah, Malaysia) had a decomposition rate (calculated using standard tea material) comparable to that of temperate forest soils. Overall, the experimental modification of both soil pH and available N had weak effects on decomposition rates, although different materials responded differently to the experimental treatments. There was a clear, but transient, increase in number of worm casts in the CaCO3 and N + CaCO3 treatment. Although we obtained some evidence that low soil acidity reduced decomposer activity, it may take longer than one year for the decomposer community, and associated processes, to be influenced by the experimentally altered edaphic conditions.