SPATIAL AND TEMPORAL GRADIENTS OF EARTHWORM CASTING ACTIVITY IN ALLEYCROPPING SYSTEMS

The amounts of earthworm surface casts were monitored for 200 days aft er commencement of casting in three alley cropping experiments of diff erent ages and hedgerow species. Casts were collected twice per week i n transects from under the hedgerow to the middle of the interrow spac e. Average annual cumulative amounts of casts were higher in alley cro pping systems with one to five years of cropping than in the no-tree c ontrol. After five years of cropping, amounts of casts were similar in all treatments. Within the alley cropping systems, casting activity w as highest immediately under the hedgerows and decreased towards the m iddle of the interrow space. In systems using Leucaena leucocephala as hedgerow species, the hedgerow to interrow space gradient of casting activity became more pronounced with increasing length of cropping. Ca sting activity in the interrow space was reduced by 12%, 55%, 80% and 86% in the first, fourth, sixth and seventh year of cropping, respecti vely, compared to the casting activity under the hedgerows. Senna siam ea, which produced a more recalcitrant mulch, did not show such a stro ng decline in casting in the interrow space. In a Dactyladenia barteri system, the difference in casting between interrow space and hedgerow s was insignificant. With perpendicular distance from the hedge, large st gradients in casting activity occurred close to the hedgerows with up to -4.00 Mg ha(-1) cm(-1) in L. leucocephala but only -1.23 and -0. 76 Mg ha(-1) cm(-1) in S. siamea and D. barteri, respectively. The sha ding effect of trees and a relatively low level of soil disturbance is apparently more beneficial for earthworms in a cropped system than a high supply of readily available food from fast decomposing L. leucoce phala prunings.