The role of the unsaturated zone in determining nitrate leaching to groundwater

In order to identify the dominant processes affecting nitrate leaching in the Po River Delta area, field tests were performed to determine the fate and transport of nitrogen species. Nitrogen (urea) was applied at a rate of 300 kg-N/ha/y, in both a sandy and a silty loamy sites cultivated with maize; the sandy soil was amended with chicken manure (700 kg/ha), while the silty loamy soil never received chicken manure amendment. Each field site was equipped with soil moisture probes, suction cups and piezometers to quantify the presence of nitrogen and carbon dissolved species in the subsurface. Nitrate leaching was observed in the silty loamy soil, while in the sandy soil the elevated dissolved organic matter, resulting from chicken manure decomposition, prevented the nitrate migration towards the aquifer. Results highlight the reliability of increasing the labile organic matter in the more permeable and intrinsically vulnerable sandy soil to prevent nitrate leaking.