Contribution of fertilizer nitrogen to the total nitrogen extracted by sugarcane under Brazilian field conditions

With the ordinance that prohibits sugarcane harvesting with prior burning of plantations in Brazil, sugarcane crops absorbs barely 20 % of the N-fertilizer applied. This low recovery is attributed, in part, to nitrogen (N) mineralization in plant straw, high microbial immobilization, and N losses in the soil-plant system, mainly, when sources containing urea are applied to the soil surface. To evaluate N contribution from fertilizer in the sugarcane compartments and total N extracted by plants during the first ratoon cycle, microplots were installed inside plots of sugarcane ratoons. We applied isotope ¹⁵N-labelled urea, ammonium sulfate and ammonium chloride (3 % ¹⁵N atoms) at the dose of 100 kg N ha⁻¹. The soil in the study was classified as a Typic-Hapludox and the experimental design consisted of randomized complete block with four replicates. In addition nitrogen immobilization and mineralization rates was evaluated estimated by aerobic incubation during 20 weeks. Sugarcane residue was incorporated to soil and nitrogenous fertilizers applied to the surface as ammoniacal or amidic-N at 100 mg N kg⁻¹. N sources showed the same efficiency as the results showed no difference for N contents in plant shoots obtained from treatment with N fertilization. The use of ammonium chloride resulted in greater accumulation of N fertilizer in stalks at 218 days after fertilization. The percentage of N fertilizer was approximately 60 % in the early developmental stages, decreasing to 20 % near the harvesting stage. The addition of N-fertilizer in soils covered by sugarcane residue leads to immobilization of available-N in the soil.