A step-wise approach to assess the fate of nitrogen species in agricultural lowlands

Nitrate leaching from agricultural lands is a worldwide concern and in Europe a large part of farmed areas is affected by nitrate pollution since decades. A comprehensive case study is given by the Pianura Padana lowland (Northern Italy), an intensively exploited area, which partly has been already declared vulnerable to nitrate from agricultural sources (WFD; 2000/60 CE). Although groundwater nitrate contamination is a well-known phenomenon, the key factors governing N transport and transformations, through the vadose zone to the water table, are far from being fully clarified. In order to untangle the complexity of the processes affecting nitrate leaching and define good fertilization practices, dedicated also to groundwater protection, it is crucial to pick a representative case study and apply a clear rationale and a step-wise approach on it. That is, first to perform a well-designed and continuous monitoring of physical and chemical parameters dedicated to properly estimate cumulative infiltration, which is an important parameter, especially in lowlands were the runoff is minimal. Beside, tracer tests have to be used to quantify the field velocity and the dispersion coefficient to determine the conservative mass transfer. As second step, parallel tests on other relevant soil parameters have to be conducted at different scales in a laboratory controlled environment (for example, batch test leaking, 1D columns elution and 3D tank experiments) in order to gain the necessary flow and transport parameters for the quantification of the reactive mass transfer. Finally, a numerical assessment via forward and inverse modelling, based on the achieved parameters, can be performed to quantify indirectly other biological functions that are involved in the complex fate of nitrogen species and nitrate. The presented results follow the proposed stepwise approach and end up with some practical operative guidelines transferred into agricultural practice for the reduction of nitrate pollution.