Nitrogen fertilization of sugarcane ratoon: long-term effects of sources and rates of nitrogen fertilizers

Abstract

Mechanized harvesting of sugarcane (Saccharum spp.) without previous burning of straw ("green cane") is a reality in the sugarcane sector. The main concerns with this harvesting system is related to the use of nitrogen fertilizers, especially urea, which is the most common source of nitrogen (N) in Brazil and can suffer high N losses by volatilization when applied over the straw. In order to reduce tillage costs, the current trend in the "green cane" system is to apply nitrogen fertilizers above the crop residues (straw) layer without fertilizer incorporation into the soil. In this context, the objective of this project is to evaluate the agronomic efficiency of conventional inorganic sources of N: ammonium nitrate (AN), urea (U) and ammonium nitrate with calcium and magnesium (Nitromag - NI) for five consecutive growing seasons of sugarcane ratoon (regrowth), in two areas with sandy soils. Two field trials are being carried out in randomized complete blocks, with four replications. The treatments included four sources of N: AN and U in the 100 kg N ha-1 and NI at doses of 50, 100, 150 and 200 kg N ha-1, and a control (without application of inorganic N). In the fourth cropping cycle of one experimental site will be installed microplots with 15N-Urea, Ammonium Nitrate-15N (15NH4NO3 and NH415NO3) applied over the straw in order to evaluate the use efficiency of N- sources by sugarcane. Each of the sources enriched with isotope 15N is provided at a dose of 100 kg N ha-1 . We will assess the response curves of sugarcane to N fertilization (Nitromag fertilizer) as compared to urea and ammonium nitrate at rate of 100 kg ha-1 (using productivity in TCH and TPH per agricultural cycle and accumulated in successive years), and the concentration and accumulation of macronutrients in shoots, rhizomes and residual straw, as well as soil fertility measurements throughout the ratoons. An experiment will also be conducted under greenhouse conditions (using 15N fertilizer) to evaluate the preferential absorption of N sources by sugarcane (amide, nitrate and ammonia). This project is a partnership between University and companies and has the following collaborations: Yara International ASA (Germany) with partial funding for field activities through the Foundation for Agrarian Studies Luiz de Queiroz (FEALQ); Sugarcane Technology Center (CTC), Piracicaba - SP, for logistical support and Usina São José da Estiva, Novo Horizonte - SP, in providing the areas, supplies and field personnel.