Degradation of soil nutrients and slow recovery of biomass following shifting cultivation in the heath forests of Sarawak, Malaysia

Heath forests which are tropical forests on sandy soils are widely distributed in Borneo and provide many ecosystem services, such as carbon storage and non-timber forest products. Over the past several decades, such forests have been disturbed by non-traditional shifting cultivation, which is usually conducted in unsuitable land with short fallow duration. Anthropogenic disturbances in forests growing on sandy soils that have low nutrient retention capacities may promote further reductions in soil fertility. Long-term monitoring of soil nutrients and biomass accumulation is rare in degraded tropical rainforests growing on sandy soils despite the known negative effects of non-traditional shifting cultivation practices on soil fertility, vegetation and biomass recovery, and the growth of trees planted to rehabilitate forests. The objective of the current study was therefore to elucidate changes in soil fertility and biomass recovery potential in degraded forests on sandy soils in Borneo. We monitored soil nutrients and chemical properties in fifty-four 12 × 12 m plots located on different topographical positions along upper and lower slopes. Samples were collected in plots aged 7 and 14 years after abandonment of shifting cultivation. The biomass of secondary forest trees in all plots 12 years after the abandonment was also measured to determine the relationship between biomass accumulation and changes in soil nutrients. We found that soil pH and cation exchange capacity, and the concentrations of most nutrients, including nitrogen, magnesium, and phosphorus, had decreased significantly 7-14 years after abandonment; biomass accumulation in the plots was also limited 12 years after abandonment (<0.8 Mg ha−1 year−1). These changes in soil traits within the study plots were similar regardless of topographical position, and the changes were not related to the amount of biomass. Soil nutrients in the plots may have leached out as a result of removal of thick root mats in the surface soil, high sand content, and large amounts of rainfall in the area rather than uptake by recovering trees. These results indicate that it may be difficult to recover soil fertility on steep slopes in sandy soil conditions because of high leaching rates and erosion when the soil surface layer is destroyed by shifting cultivation. It is therefore important to carefully manage shifting cultivation practices in tropical forests on sandy soils in Borneo.