Root distributions partially explain N-15 uptake patterns in Gliricidia and Peltophorum hedgerow intercropping systems

The relative distributions of tree and crop roots in agroforestry associati ons may affect the degree of complementarity which can be achieved in their capture of below ground resources. Trees which root more deeply than crops may intercept leaching nitrogen and thus improve nitrogen use efficiency. This hypothesis was tested by injection of small doses of ((1)5NH(4))(2)SO4 at 21.8 atom% N-15 at different soil depths within established hedgerow in tercropping systems on an Ultisol in Lampung, Indonesia. In the top 10 cm o f soil in intercrops of maize and trees, root length density (L-rv) of maiz e was greater than that of Gliricidia sepium trees, which had greater L-rv in this topsoil layer than Peltophorum dasyrrachis trees. Peltophorum trees had a greater proportion of their roots in deeper soil layers than Glirici dia or maize. These vertical root distributions were related to the pattern of recovery of N-15 placed at different soil depths; more N-15 was recover ed by maize and Gliricidia from placements at 5 cm depth than from placemen ts at 45 or 65 cm depth. Peltophorum recovered similar amounts of N-15 from placements at each of these depths, and hence had a deeper N uptake distri bution than Gliricidia or maize. Differences in tree L-rv across the croppi ng alley were comparatively small, and there was no significant difference (P <0.05) in the uptake of N-15 placed in topsoil at different distances fr om hedgerows. A greater proportion of the N-15 recovered by maize was found in grain following N-15 placement at 45 cm or 65 cm depth than following p lacement at 5 cm depth, reflecting the later arrival of maize roots in thes e deeper soil layers. Thus trees have an important role in preventing N lea ching from subsoil during early crop establishment, although they themselve s showed a lag phase in N-15 uptake after pruning. Residual N-15 enrichment in soil was strongly related to application depth even 406 days after N-15 placement, demonstrating the validity of this approach to mapping root act ivity distributions.