Nitrogen in sugarcane derived from fertilizer under Brazilian field conditions

Nitrogen (N) fertilization of sugarcane crops is a common practice used to reach sustainable levels of productivity, both for plant cane and especially for the ratoon. However, when evaluating the amount of N in the plant derived from fertilizer (NDDF) at harvest, this contribution is approximately 20% of total plant biomass N, which raises questions regarding the efficiency of N fertilization. The goal of this study was to evaluate the N derived from fertilizer (NDFF) during the sugarcane crop development, for both plant cane and first ratoon crop cycles. Two field experiments were performed in São Paulo State, Brazil, in Arenic Kandiustults and in Typic Eutrustox. The sugarcane was mechanically harvested without burning. N fertilizer for both the plant cane (doses of 40, 80 and 120 kg ha⁻¹ of N as urea) and the first ratoon (doses of 50 and 100 kg ha⁻¹ of N as ammonium sulfate) was labeled ¹⁵N. The results showed that NDFF contributed up to 40% of the total N in the plant cane at initial stages of development. The magnitude of this contribution decreased during stages of maturity to approximately 10% of total N at harvest. In the first ratoon, application of N fertilizer was more effective for crop nutrition, constituting up to 70% of total N in initial stages of development and decreasing through the cycle, reaching approximately 30% at harvest. Therefore, studies that evaluate NDDF only at harvest can lead to underestimating the role of N fertilizer for sugarcane nutrition. The higher NDFF in ratoon explains why this crop cycle presents a more consistent response to N fertilization than plant cane, as observed in several studies developed under Brazilian conditions in the last decades.