Competition in tree row agroforestry systems. 2. Distribution, dynamics and uptake of soil inorganic N.

The effect of tree row species on the distribution of soil inorganic N and the biomass growth and N uptake of trees and crops was investigated beneath a Grevillea robusta A. Cunn. ex R. Br. (grevillea) tree row and Senna spectabilis DC. (senna) hedgerow grown with Zea mays L. (maize) and a sole maize crop, during one cropping season. The hypothesis was that a tree with a large nutrient uptake would have a greater competitive effect upon coexisting plants than a tree that takes up less and internally cycles nutrients. The field study was conducted on a kaolinitic Oxisol in the sub-humid highlands of western Kenya. Soil nitrate and ammonium were measured to 300 cm depth and 525 cm distance from the tree rows, before and after maize cropping. Ammonium concentrations were small and did not change significantly during the cropping season. There was > 8 mg nitrate kg^-1 in the upper 60 cm and at 90–180 cm depth at the start of the season, except within 300 cm of the senna hedgerow where concentrations were smaller. During the season, nitrate in the grevillea-maize system only decreased in the upper 60 cm, whereas nitrate decreased at almost every depth and distance from the senna hedgerow. Inorganic N (nitrate plus ammonium) decreased by 94 kg ha^-1 in the senna-maize system and 33 kg ha^-1 in the grevillea-maize system. The aboveground N content of the trees increased by 23 kg ha^-1 for grevillea and 39 kg ha^-1 for senna. Nitrogen uptake by maize was 85 kg ha^-1 when grown with grevillea and 65 kg ha^-1 with senna. Assuming a mineralisation input of 50 kg N ha^-1 season^-1, the decrease in inorganic soil N approximately equalled plant N uptake in the grevillea-maize system, but exceeded that in the senna-maize system. Pruning and litter fall removed about 14 kg N ha^-1 a^-1 from grevillea, and > 75 kg N ha^-1 a^-1 from senna. The removal of pruned material from an agroforestry system may lead to nutrient mining and a decline in productivity. [ABSTRACT FROM AUTHOR]