Radiation regime and nitrogen supply in modelled alley cropping systems ofErythrina poeppigiana with sequential maize-bean cultivation

A simulation study relating the effect of the spatial arrangement of trees on the photon exposure above an associated crop (PE_i) was carried out for an alley cropping system of the leguminous treeErythrina poeppigiana (Walpers) O. F. Cook with sequential maize-bean cultivation. These results were combined with estimates of the nitrogen supplied by prunings from different tree densities to rank the arrangements. A significant difference was found in the PE_i between the east-west and north-south-oriented alleys. The latter exhibited more areas of moderate shade, while the former has unshaded centres and heavily shaded parts next to the tree row. When the simulated PE_i data was combined with the N supply from tree residues, the wider between-row tree spacings (6 or 8 m, with 1 or 2 m within-row spacing) appeared to be best. In general, to minimize shading of the crop, yet maintain a sufficient supply of organic matter and nitrogen in prunings, alley cropping tree arrangements of high between-row and low within-row spacings seem to be most promising. The east-west orientation seems to be more promising than north-south.