Monitoring and mapping of residual nitrogen in lowlands. The case study of Ferrara province

Nitrogen that remains in soils as inorganic phases (Nitrate and Ammonium), after the growing seasons, represent one of the most serious threat for cultivated land, determining eutrophication and groundwater nitrate contamination. This issue has recently become a priority within the European Community Framework Directive for water protection (WFD) which has established a list of measures and limitations to be applied in areas declared “vulnerable to nitrate from agricultural sources. The Ferrara Province (Northern Italy), an intensively cultivated area affected by serious nitrate pollution since decades. The land is fertile and allows the cultivation of some different crops especially winter cereals, maize and energy plants which cover more than 65% of the land. Ferrara province is a flat area located on Po lowlands covering an area of 2.623 km². It is characterized by a flat topography and is fertilized by more than 91% with synthetic fertilizers, mainly urea. The minimal run-off and an homogeneous inorganic nitrogen load allow to simplify the hydrological and nitrogen balance: excess nitrogen tend to move vertically trough the unsaturated zone, becoming one of the most important causes of groundwater contamination. A monitoring program was started in 2010, to estimate readily available nitrogen before and after maize and wheat cropping cycles. 742 core samples (1 m long and 2,5 cm int. diam.) were manually taken in 67 different agricultural plots, fertilized in 2009 with synthetic Urea, respectively with 240 kg/ha for plots cultivated with wheat and 160 kg/ha for plots cultivated with maize and with 700 - 900 kg/ha of chicken manure (only sandy soils along the coastal zone). Cores were splitted and analyzed into two halves, in order to investigate separately the upper one stressed by tillage, roots growth and weathering (0 to 50 cm of depth) and the lower undisturbed one (50 to 100 cm of depth). Samples were colorimetrically analyzed for urea (CO(NH2)2), NH4+, NO2- and NO3- using a double bean Jasco 550C spectrophotometer and a Tecnicon Autoanalyser II. C and N were obtained using an Elemental Analyzer Fisions Instruments. Data were processed using Geographical Information System applications to obtain maps of the residual mineral nitrogen availability and C/N ratio, for each of considered soil. Results highlights the presence ,at different concentration, of residual Nitrogen as NH4+ and NO3- mainly, in the whole territory. Distributions of NH4+ and NO3- are correlated with C/N ratio values, which are prevalently dependent on the textural characteristics of soils and influenced by the type of used fertilizer. This information can be transferred to agrotechnicians and farmers to calculate accurately the amount of fertilizer to be added in the next cropping season. The proposed procedure highlight the possibility to estimate accurately nitrogen needs in specific vulnerable zones and proceed with fertilization by avoiding to generate excess and risk of nitrate leaching to groundwater.