Acacia invasion differentially impacts soil properties of two contrasting tropical lowland forests in Brunei Darussalam

Invasive Acacia species are known to modify soil properties, although effects are often site-specific. We examined the impact of Acacia species on the soils of intact and invaded habitats of two contrasting tropical lowland rain forest types in Brunei Darussalam: heath forest (HF) and mixed dipterocarp forest (MDF). Impacts on soil properties differed between the two forest types. Overall, Acacia-invaded HF soil recorded significantly higher gravimetric water content, pH and total P, K and Ca compared to the intact HF soil. In contrast, invaded MDF soil exhibited significantly higher organic matter content and total soil N, P, K and Mg compared to its intact habitat. Acacia-invaded MDF soils were more nutrient-enriched than Acacia-invaded HF soils by the addition of threefold, threefold and fourfold total soil P, K and Mg, respectively. The positive effect of addition of total soil Ca was, however, fourfold greater in HF soil than MDF soil, indicating that the magnitude of impact on soil properties was strongly site-specific. Overall, Acacia invasion significantly impacted soil properties in nutrient-rich MDF more than those of nutrient-poor HFs, indicating a potential vulnerability of MDFs to invasion.