Surface casting of the tropical Metaphire posthuma increases soil erosion and nitrate leaching in a laboratory experiment

In tropical regions, earthworm casts can be globular or granular, depending on the earthworm species that produced them. Although the effects of globular casts on soil erosion have been extensively studied, less is known regarding the effects of granular casts. This study aimed to investigate how granular casts produced by Metaphire posthuma affect soil erosion. To recreate the patchy accumulation of aggregates in the field, small piles of earthworm casts (CAST) or non-ingested soil (NIS) were placed on a layer of bare soil in microcosms. Rainfall simulations were then carried out to measure the fragmentation dynamics of the different types of aggregates, soil detachment and water runoff quantity and quality (NH4+ and NO3− content, pH and conductivity). Our results clearly show that CAST aggregates are more prone to water-disintegration than NIS aggregates. As a consequence, M. posthuma activity significantly contributes to a decrease in soil roughness and an increase in soil detachment. The quantity of water runoff collected after the rainfall simulation was similar between both types of aggregates but higher NO3− concentrations were found with CAST than with NIS aggregates. In conclusion, this study suggests that due to production of granular labile casts at the soil surface, M. posthuma has a detrimental effect on soil roughness and detachment, and NO3− leaching. More studies are now required to determine if our results are general attributes of granular casts.