Soil chemical and microbial properties after disturbance by crawler tractors in a Malaysian forest plantation

Extracting timber with heavy machinery on humid tropical soils degrades the soil structure and decreases plant growth. We investigated how disturbance by crawler tractors affected soil chemical and biological properties in a second-generation plantation in Sabah, Malaysia. Total N, total P, and anion resin-extractable P contents were analyzed. N- and P-limited microbial respiration kinetics were bioassayed (by measuring them before and after the addition of glucose + P and glucose + N, respectively), to assess changes in the soil's biological properties. Three months after planting, the soil's organic content were 25% lower on tracks compared to control plots that had not been affected by crawler tractors, while microbial indices of N and P availability were about 50% lower. Two years after planting acid-digestible P, N, and anion resin-extractable P contents were still lower along the tracks, as was one of two indices of microbial P and N availability, respectively. Therefore, the results indicate that soil conservation needs to be improved during the establishment phase to maintain the fertility of forest plantations such as the one studied. If crawler tractors are used, skid-tracks should be designated and their coverage minimized.