Nitrogen cycling in leucaena (Leucaena leucocephala) alley cropping in semi-arid tropics: II. Response of maize growth to addition of nitrogen fertilizer and plant residues

In an alley cropping system, prunings from the hedgerow legume are expected to supply nitrogen (N) to the associated cereal. However, this may not be sufficient to achieve maximum crop yield. Three field experiments with alley-cropped maize were conducted in a semi-arid environment in northern Australia to determine: (1) the effect of N fertilizer on maize growth in the presence of fresh leucaena prunings; (2) the effect of incorporation of leucaena and maize residues on maize yield and the fate of plant residue ¹⁵N in the alley cropping system; and (3) the ¹⁵N recovery by maize from ¹⁵N-labelled leucaena, maize residues and ammonium sulphate fertilizer. Leucaena residues increased maize crop yield and N uptake although they did not entirely satisfy the N requirement of the alley crop. Additional N fertilizer further increased the maize yield and N uptake in the presence of leucaena residues. Placement of leucaena residues had little effect on the availability of N to maize plants over a 2 month period. The incorporation of leucaena residues in the soil did not increase the recovery of leucaena ¹⁵N by maize compared with placement of the residues on the soil surface. After 2 months, similar proportions of the residue ¹⁵N were recovered by maize from mulched leucaena (6.3%), incorporated leucaena (6.1%) and incorporated maize (7.6%). By the end of one cropping season (3 months after application) about 9% of the added ¹⁵N was taken up by maize from either N-labelled leucaena as mulch or ¹⁵N-labelled maize residues applied together with unlabelled fresh leucaena prunings as mulch. The recovery of the added ¹⁵N was much higher (42.7%) from the ¹⁵N-labelled ammonium sulphate fertilizer at 40 kg N ha⁻¹ in the presence of unlabelled leucaena prunings. Most of the added ¹⁵N recovered in the 200 cm soil profile was distributed in the top 25 cm soil with little leached below that. About 27-41% of the leucaena ¹⁵N was apparently lost, largely through denitrification from the soil and plant system, in one cropping season. This compared with 35% of the fertilizer ¹⁵N lost when the N fertilizer was applied in the presence of prunings.