Utilization by late harvest sugar cane ratoon of nitrogen from 15N-aqua ammonia and 15N-urea applied to the soil as vinasse N-complement

Abstract

The utilization of nitrogen from aqua ammonia and urea applied to the soil as vinasse N-complement by late harvest sugar cane ratoon was evaluated using ¹⁵N tracer technique. The experiment was conducted in São Paulo State on a commercial sugar cane field planted with the variety SP 70-1143, first ratoon crop. Two treatments of nitrogen fertilizer (urea and aqua ammonia) were used. Each treatment: consisted of 19 neighbouring rows of sugar cane, 10 m long and 1.4 m apart. After vinasse application to the soil surface at a rate of 100 m³/ha, the N-fertilizers (90 kg/ha of N) were manually applied and buried to 15 cm deep in furrows located 25 cm from both sides of all cane rows. The ¹⁵N-fertilizers were applied to single microplots of 2 linear meter row segments (4 replicates). The sugar cane yield and N derived from the fertilizer (40% of N-fertilizer recovery) were similar for both treatments of N-fertilizer (aqua animonia and urea), 12 months after fertilization (final harvest). The results of accumulated nitrogen in the shoot, at different growing stages and up to the harvest time, strongly indicated the occurrence of biological nitrogen fixation (BNF). The BNF together with the nitrogen taken up from the soil resulted in a total of 260 kg/ha of N accumulated in the shoot of ratoon crop after 12 months growing period.