Complex Interactions Between Fertilizers and Subsoils Triggering Reactive Nitrogen Speciation in Lowlands

The present research was performed in the Ferrara Province (Italy), an intensively cropped low-lying landscape covering 2636 km², situated in the southern portion of the Po River valley. The foremost used fertilizer in this zone is synthetic urea which is known to induce nitrate leaching towards the unconfined aquifer. Approximately 800 soil samples, distributed throughout the most representative soil types of the area, were collected from the ploughed layer (0–50 cm below ground) and subsoils (50–100 cm below ground) in 2010. Soil samples were analyzed for: soil porosity, dry bulk density, soil water content, ammonium, nitrate and nitrite. A subset was also analyzed for: urea, soil pH, total organic carbon, and total and organic nitrogen. Scatter diagrams showed an accumulation of ammonium in peaty subsoils, whereas in drained sandy soils ammonium was, in general, very low. The most acidic peaty subsoils were characterized by high ammonium concentrations (20–70 mM-N/kg of dry soil), while the peaty soils amended with poultry chicken manure showed the lowest ammonium concentrations. Instead, the largest accumulation of NO₃⁻ was observed in sandy and loamy subsoils (10–27 mM-N/kg of dry soil), where the deep water table and neutral-alkaline soils pH delivered the best conditions for nitrification.