Nutrient harvesting - the tree-root safety net

The hypothesis that deep rooting trees intercept nutrients which have leached below the crop rooting zone and compete for nutrients less strongly than trees rooting mainly within the crop root zone was tested in a mixed alley cropping system of Gliricidia sepium and Peltophorum dasyrachis on an Ultisol in North Lampung, Sumatra, by measuring uptake of 15N placed at varying soil depths. The system was established in 1991 and had been intercropped with various crops for 3 yr. In this experiment, the crop was maize. Gliricidia, with its predominantly shallow root system, competed strongly for N with the crop and took up little 15N from lower soil depths. In contrast, Peltophorum roots exhibited a higher nutrient uptake activity at lower soil depth thus providing an active 'safety-net'. It is noted that root activity as well as root length density has to be taken into account when assessing the efficiency of the safety-net. Preliminary modelling results using WaNuLCAS (Water, Nutrient and Light Capture in Agroforestry Systems) suggested that Peltophorum roots in the 40-60 cm soil layer could reduce leaching by 5-10% over the course of a maize crop cycle in the rainy season