NITRATE AND AMMONIUM IN SOIL SOLUTION IN TOBACCO MANAGEMENT SYSTEMS

Tobacco farmers of southern Brazil use high levels of fertilizers, without considering soil and environmental attributes, posing great risk to water resources degradation. The objective of this study was to monitor nitrate and ammonium concentrations in the soil solution of an Entisol in and below the root zone of tobacco under conventional tillage (CT), minimum tillage (MT) and no-tillage (NT). The study was conducted in the small-watershed Arroio Lino, in Agudo, State of Rio Grande do Sul, Brazil. A base fertilization of 850 kg ha(-1) of 10-18-24 and topdressing of 400 kg ha(-1) of 14-0-14 NPK fertilizer were applied. The soil solution was sampled during the crop cycle with a tension lysimeter equipped with a porous ceramic cup. Ammonium and nitrate concentrations were analyzed by the distillation and titration method. Nitrate concentrations, ranging from 8 to 226 mg L(-1), were highest after initial fertilization and decreased during the crop cycle. The average nitrate (N-NO(3)(-)) concentration in the root zone was 75 in NT, 95 in MT, and 49 mg L(-1) in CT. Below the root zone, the average nitrate concentration was 58 under NT, 108 under MT and 36 mg L(-1) under CT. The nitrate and ammonium concentrations did not differ significantly in the management systems. However, the nitrate concentrations measured represent a contamination risk to groundwater of the watershed. The ammonium concentration (N-NH(4)(+).) decreased over time in all management systems, possibly as a result of the nitrification process and root uptake of part of the ammonium by the growing plants.