Urea and sugarcane straw nitrogen balance in a soil-sugarcane crop system

The objectives of this study were to evaluate nitrogen utilization by sugarcane ratoon from two sources, applied urea and sugarcane straw covering soil surface (trash blanket), besides the recovery of N from both sources in the soil-plant system. The following treatments were established in a randomized block design with four replicates: T1, vinasse-urea (100 kg ha(-1) of urea-N) mixture applied on the total area of the soil covered with cane trash labeled with N-15; T2, vinasse-urea mixture (urea labeled with N-15; 100 kg ha(-1) of urea-N) applied on the total area of the soil covered with non-labeled sugarcane trash; and T3, urea-N-15 (100 kg ha(-1) of urea-N) applied in furrows at both sides of cane rows, with previous surface application of vinasse, onto soil without trash covering. The vinasse was applied at a rate of 100 m(3) ha(-1) in all treatments. The experiment was carried out on a Yellow Red Podzolic soil (Paleudalf), from October 1997 to August 1998, in Piracicaba, SP, Brazil. The nitrogen use efficiency of urea by the sugarcane ratoon was 21%, while that of the sugarcane straw was 9%. The main contributions of N from sugarcane trash, during one cycle, are the preservation and increase of the organic N in soil. The tendency for a lower accumulation of urea-N in the sugarcane plant, in the soil surface covered with sugarcane residue, was compensated by the assimilation of N from trash mineralization. Nitrogen derived from cane trash was more available to plants in the second half of the ratoon cycle.