Distribution and activity of earthworms and contribution to nutrient recycling in alley cropping

Earthworm surface casting activity was monitored in Leucaena leucoccephala alley cropping and a no-tree control system. Under L. leucocephala hedgerows, five times more casts (116.8 Mg ha-1 year-1) were deposited than in the inter-row space (24.3 Mg ha-1 year-1) and the no-tree control plots (27.8 Mg ha-1 year-1). Hyperiodrilus africanus and Eudrilus eugeniae were the dominant species in alley and no-tree control plots, respectively. Amounts of nutrients returned to the soil surface by casting were lowest in the no-tree control due to low nutrient concentration in casts of Eudrilus sp. In alley cropping (weighted average of amounts under hedgerow and inter-row space), three times more N, K, Ca, and Mg were recycled to the surface than in the no-tree control plots. The relative contribution of casts to nutrient cycling in alley cropping (nutrients in L. leucocephala prunings plus casts = 100%) was 33% N, 16% P, 6% K, 16% Ca, and 34% Mg. Permanent shading of the vicinity of hedgerows was identified as the most important factor enhancing casting activity.