Earthworm dynamics in relation to alterations of vegetation: effects of oil palm in comparison to rubber and natural forest in tropical Ultisols

Earthworms are natural soil quality indicators that are highly responsive to their living environment. Due to the recent rapid expansion of oil palm in tropical countries, there are raised concerns on possible negative impacts on the soil environment. This study aims to examine the earthworm distribution through the shallow soil profile and its relation to soil characteristics using oil palm and rubber plantations, compared with a natural forest in Galle District, Sri Lanka, to understand impacts of vegetation alterations. Soil samples were collected from 0 to 10, 10–20, and 20–30 cm depths (9 points/layer/site). The earthworm count was taken manually to determine the earthworm density (EWD). The C stock of forest soil in three soil layers was significantly higher than that of rubber and oil palm. In all sites, the topsoil showed the highest C stock, however, the differences between layers were not significant. Forest soils showed the highest EWD, although there was no significant difference between the three sites. The topsoil layer of all sites showed significantly higher EWD. In forest soil, EWD showed a strong negative correlation (R² = 0.81) with bulk density, a strong positive correlation with porosity (R² = 0.78), and a moderate positive correlation (R² = 0.57) with organic matter content. The EWD in rubber and oil palm soils showed positive correlations with organic matter content, which were not statistically significant. Literature shows that the earthworm abundance has strong positive correlations with N fertilizer. The forest has intact soil environments while the tested oil palm and rubber plantations annually receive high amounts of N. Accordingly, the creation of a favorable soil environment for the growth and development of earthworms with the addition of N fertilizer might be the reason for these results.