Nitrate leaching through the unsaturated zone following pig slurry applications

As the increase of nitrate concentration in groundwater has often been ascribed to an inappropriate use of liquid manure, the main purpose of this study was to better understand the factors controlling nitrate dynamics in the unsaturated zone of soils subjected to characteristic agronomic practices, and to contribute to improving Action Programmes, with reference to EU Directive 91/676, for nitrate vulnerable zones (NVZ). Water infiltration and nitrate leaching have been studied in experimental fields located inside nitrate vulnerable zones of the Emilia-Romagna region (Northern Italy), characterized by different pedological and hydrogeological properties and equipped with meteorological station, tensiometers, ceramic-cup samplers and piezometers. This article describes the results obtained from one of these sites, monitored over a 6-year period, which was cereal cropped and treated with pig slurry. MACRO and SOILN field-scale models have been used in order to verify the reliability of simulated water flow and nitrogen transport. The results demonstrate how nitrogen inputs from slurry, substantially higher than crop uptake, cause nitrate accumulation in the surface layer of the soil especially in warm periods (concentrations of up to 300 mg NO3–N l K1 were found in soil water). Even if the soil texture was fine, the shrinking–swelling properties of clay minerals determined fast drainage conditions (related to macroporosity), so that during the early rainy periods nitrates leached through the first meters of the unsaturated zone, at least down to 4 m. This shows that nitrate accumulation should be limited before these periods, i.e. by reducing manure application rates, especially if the soil is to be left uncultivated. Journal of Hydrology 316 (2006) 195–212 www.elsevier.com/locate/jhydrol 0022-1694/$ see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.04.026 * Corresponding author. Tel.: C39 0522 436999; fax: C39 0522 435142. E-mail addresses: [email protected] (P. Mantovi), [email protected] (L. Fumagalli), [email protected] (G.P. Beretta), [email protected] (M. Guermandi). 1 Fax: C39 0264 482895. 2 Fax: C39 0250 315494. 3 Fax: C39 0512 84208. P. Mantovi et al. / Journal of Hydrology 316 (2006) 195–212 196 The model results confirm the observed role of macroporosity in accelerating the breakthrough of surface applied soluble compounds and provide evidence that MACRO and SOILN may be suitable tools for predicting such phenomena, even though their calibration requires some further refinements. q 2005 Elsevier B.V. All rights reserved.