Epsilon Open Archive

Abstract

Physical degradation of soil, reductions in plant growth, and increases in the frequency of wild fires are common consequences of forest operations in humid tropical forests. However, there is little available information on the interaction of plant growth, soil nutrients and microbial properties, especially in connection with man made rehabilitation of these soils. Therefore, the objectives of the work described in this thesis were as follows. Firstly, to quantify the effect of skid tracks in a forest plantation in Sabah, Malaysia, on the soil´s physical, chemical and biological properties, as well as plant production. Thirdly, to test and develop assays based on measurements of microbial respiration kinetics for assessing soil degradation and rehabilitation. Fourthly, to assess the effects of fire in a humid tropical Dipterocarp forest on the soil´s chemical and microbial properties. In the study area, where rehabilitation were tested, the growth, soil nutrient content, and soil pore distribution were severely affected by crawler tractors. About 10 days with little rain resulted in water potentials close to the permanent wilting point (-1500 kPa) irrespective of the type of amendments applied, while outside skid tracks the soil dried at a slower rate. Fertilization and tilling of the tracks increased basal area growth of Acacia mangium from 1 to 7 m²ha⁻¹yr⁻¹, despite high soil bulk densities. Two years after the treatments many soil microbial properties had improved, although they had not returned to the levels observed outside the tracks. Microbial growth kinetics were shown to be promising tools for assessing the effect of soil disturbance and rehabilitation. The recommended water content for measurements of microbial respiration kinetics in Acrisols was -15 to -50 kPa. The effect of fire was either minor or positive on soil nutrient pools (partly due to the extensive leaf shedding that followed it), as measured by chemical methods. In contrast, microbially available N decreased by about 50%.