Soil conditions, erosion and nutrient loss on steep slopes under mixed dipterocarp forest in Brunei Darussalam

Abstract

Soil physical and chemical characteristics are reported under virgin, mixed dipterocarp forest growing on steep slopes in the Temburong District of Brunei Darussalam. Hydrological, sedimentological, soil organic matter and nutrient losses from experimental erosion plots are presented for both wet and dry season conditions. Total annual runoff losses from bare soil surfaces under an intact rainforest canopy exceed 107 litres per hectare, while total annual sediment losses exceed 46 tonnes per hectare. These values are approximately 6 and 20 times higher respectively than those measured from the undisturbed forest floor, comprising leaf litter layer and root mat. The combined root mat and litter layers are more important in protecting the soil surface against sediment loss than they are in reducing overland flow. These high values indicate how serious runoff and soil erosion can be on such steep slopes with very low permeability clay textured soils. Losses of soil nitrogen, phosphorus and potassium in eroded sediments are serious, particularly from the bare soil treatment, where 388 kg N ha⁻¹, 13 kg P ha⁻¹ and 196 kg K ha⁻¹ were lost in twelve months.