Nitrato na solução do solo e na água de fontes para consumo humano numa microbacia hidrográfica produtora de fumo

Abstract

The tobacco production system in Rio Grande do Sul state is mainly developed in small farms, using shallow soils in steep slope. In these areas rivers are born forming small watersheds. The areas are fragilized by use of inappropriate tillage systems causing soil degradation. To compensate this, new areas are included in the production system, after deforestation and not following environmental legislations rules. In order to maintain yield of the degraded areas is needed high fertilization rates and agrochemical application, following a technology package recommended by tobacco industry. Adding to this the fact, that tobacco has a low efficiency of nutrient use. The objective of this study, was monitored the nitrate concentration of soil solution in several soil management and use and in two domestic well used for drinking water. The study was carried out at Arroio Lino watershed, located in Nova Boemia district of Agudo county, state of Rio Grande do Sul. The area of the watershed presents high steepness and it prevails Entisols and mollisols.The soil solution was extracted with tension lysimeters equipped with ceramic porous cup installed at rooting depth (0,15 or 0,2 m) and below this depth (0,3 or 0,5 m), after the main rains happened during the tobacco cycle.The precipitation data were obtained from an automated meteorological station installed at central part of watershed. In 2004/2005 agricultural year, the nitrate concentration was measured in lysimeters installed in a soil management experiment, in an Entisol, including tobacco no-tillage (NT), minimum tillage (MT) and conventional tillage (CT). In 2005/2006 agricultural year, the nitrate concentration was measured in five transects distributed in several places of the watershed, including tobacco plantation, forest and grass field. Two domestic wells located at lower position of landscape were monitored to nitrate concentration. One of those was located at a position which had above it 15 m stripe of forest and after had tobacco plantation, and the other had no tobacco plantation at position above it. At the points of each transect was evaluated bulk density, soil porosity, saturated hydraulic conductivity and soil water retention curve. The soil tillage systems experiment showed higher value of nitrate concentration after starter fertilization application and decreased along of plant cycle, with content varying from 8 to 226 mg L-1. The highest concentration was observed at MT and lowest at NT. The average nitrate concentration in the rooting zone (0,2 m) was 75 for PD, 95 for MT and 49 mg L-1 for CT. Below the rooting zone, the average was 58 for NT, 108 for MT and 36 mg L-1 for CT. The soil in the transects presented low bulk density, low water storage, high porosity and high saturated hydraulic conductivity. The nitrate reached depths bellow rooting zone and its concentration was higher at tobacco plantation than at forest and grass field, reaching values as high as 80 mg L-1 in tobacco plantation. The well located below tobacco plantation had higher nitrate concentration, overcoming the critical limit of 10 mg L-1 in some periods.