Laboratory investigation of organic matter mineralization and nutrient leaching from earthworm casts produced by Amynthas khami

Earthworms are considered soil engineers due to their effects on soil properties including the creation of cast-aggregates which are usually enriched in organic matter compared to the surrounding soil. How earthworms influence the dynamics of soil organic matter (SOM) and mineral nutrients is still poorly understood, partly because the relationships between cast dynamics and SOM and mineral nutrient cycling are unknown. In this study, two laboratory experiments were carried out on free aggregates (casts or soil clods) sampled from the soil surface in a tropical fallow field before the rainy season (March 2008). In the first experiment, a 12h rainfall simulation (intensity: 44mmh⁻¹) experiment was used to monitor fragmentation and nutrient leaching from Amynthas khami earthworm casts and soil clods. Control soil aggregates>5000μm were rapidly (∼2h) broken into large water-stable aggregates (>500μm) and then into smaller water-stable aggregates (<500μm). Conversely, the fragmentation of earthworm casts was progressive and more than 12h were needed to totally fragment the casts into small size aggregates (<500μm). Soil nutrient loss varied depending on aggregate size but it was always higher from cast than from control soil aggregates. NH₄ ⁺ and NO₃ ⁻ releases showed linear dynamics vs. time, whereas K and P were tightly associated with the soil aggregates and their losses were significant only after 4h of rainfall. The total amount of N-NH₄ ⁺, N-NO₃ ⁻, K and P losses were, respectively, 8-, 7-, 12- and 2-fold greater from casts than from the control soil aggregates. In the second experiment, different aggregate sizes (50–250, 250–500 and 500–2000μm) from casts and control soil aggregates were sampled during the rainfall simulation and incubated at 28°C for 21 days. The total amount of K and P was not affected by soil aggregate size. The aggregate hierarchy concept was not confirmed because more C was observed in the smaller aggregate size fractions. Organic C and N mineralization, however, increased with increasing aggregate size for casts, but the opposite trend was observed for the control soil aggregates. This study therefore highlights the importance of earthworm activity for the transfer of mineral nutrients and the protection of SOM within microaggregates.