Contribution of the subsurface drainage system in changing the nitrogen speciation of an agricultural soil located in a complex marsh environment (Ferrara, Italy)

This study focuses on the distribution of major anions and inorganic N species after the installation of a subsurface drainage system in loamy clay soils overlaying paleo-marsh peats and paleo-channel sediments. An agricultural field of 6 ha was chosen as the experimental site. Its stratigraphy was reconstructed thanks to a high resolution core sampling in 13 boreholes. In the lower undisturbed portion of the soil profile a large vertical and horizontal heterogeneity for nitrate and ammonium content was noticed, mainly due to the type of sediment underling the cultivated layer. On the contrary, the upper portion of the soil profile (0.5-0.6 m) displayed an homogeneous texture, organic matter content and permeability, most likely induced by tillage. The presence of the subsurface drainage system, reduces soil salinity and avoids soil waterlogged conditions guaranteeing an oxic/suboxic environment, where nitrate is the dominant N species. Underneath the subsurface drainage system, the elevated ammonium, chloride and bromide concentrations are directly related to the presence of non-drained peaty sediments, suggesting a natural origin of these species. Shallow groundwater samples displayed very similar content of anions with respect to the soils extractions, proving chemical equilibrium conditions between matrix/solution and confirming that advection is not dominant and that, the chemical fluxes are driven by diffusion. This paper shows that subsurface drainage systems are effective in ameliorating soil properties even in agricultural field consisting of low permeability materials with naturally high content of organic matter and salinity and that they can induce changes in redox conditions responsable for the large differences in N speciation found in soil profiles.