Anecic earthworms generate more topsoil than they contribute to erosion – Evidence at catchment scale in northern Vietnam

Soil is considered as a non-renewable resource, which may be lost in sloping land more rapidly than it is formed thus leading to loss of fertility and ecosystem deterioration. We hypothesized that earthworms could counteract this process due to their cast forming activity. To test this hypothesis, we quantified the production of casts in small plots of 0.25 m2 established in three vegetation units (woodland, shrubland and meadow) in a catchment of 46 ha for 2.5 years in relation to their (micro-) pedoclimatic conditions. We also assessed the impact of cast formation on water runoff and soil detachment. Moreover, we quantified the mass of casts deposited in the entire catchment on a regular grid of 50 m and we measured soil erosion at the outlet of the catchment. Our results showed a high and variable production of casts (from 16 to 219 t ha−1 year−1) depending on vegetation, season and year. The mass of casts found in the entire catchment represented on average 7.4 t ha−1 with an annual production rate of 35 t ha−1. Since the annual erosion rate measured for the entire catchment (3 t ha−1 year−1) was much lower than the cast production rate, our results indicated that most of the soil bioturbated by earthworms remained in the catchment. Indeed, water runoff and soil detachment measured in small plots showed that casts were not transported in the water runoff but degraded by raindrop impacts with the material remaining at the place of deposition. This process led to the generating of a new soil horizon at the culmination point of the catchment of up to 6.5 cm after 2.5 years. We conclude that the surface activity of anecic earthworms could influence soil generation at scale and conteract the effects of soil erosion. © 2021 Elsevier B.V.