Soybean is the most important grain crop in Brazil. Despite its high N accumulation, mostly from biological nitrogen fixation, crop N balance studies suggest that harvest residues do not contribute significantly to improve soil N. Traditional manual recovery of soybean roots reveal they represent no more than 5 to 10% of total N accumulated by the plant. However, for other plant species like lupin and chickpea, root exudates and dead root turnover measured with a ¹⁵N leaf-labelling technique seem to represent a large proportion of the total amount of below-ground plant N. In the present investigation the leaf-labelling ¹⁵N technique was employed to quantify, under field conditions, the below-ground N of soybean, and also the effect of this N source on the yield and N accumulation of a subsequent sorghum crop. Soybean (cv. Celeste) was seeded at 0.5 m row space, with a density of 14 seeds per linear metre. Twelve PVC soil cores of 25 cm diameter and 60 cm length were inserted into the soil in the rows of a soybean crop at random points. In eight soil cores, two soybean plants were planted per core and the other four remained unplanted. When the plants reached the V3 (mid-vegetative) stage, selected leaves of each plant were dosed with 2 ml of a 0.5% solution of urea labelled at 71 atom % ¹⁵N. At grain maturity the whole plant shoot was harvested from the eight soil cores planted to soybean, and from four of them all roots and soil were removed for the quantification of recoverable roots and non-recoverable root N, the latter based on the ¹⁵N technique. The remaining four cores containing the soybean below-ground material were planted to sorghum (Sorghum bicolor cv. BR 304) along with the four unplanted cores. Non-recovered root N was estimated to be equivalent to 81% of the total amount of N derived from the root system and 16% of the total N accumulated by the plant. Most of the below-ground N (83%) was concentrated in the 0 – 20 cm soil layer. The presence of the soybean root system in the cores did not increase the yield of sorghum, but approximately 10% of the N in sorghum plants was derived from the root system of soybean.

Keywords: below-ground N, ¹⁵N technique, N nutrition, root N.