Formation and dissolution of salt crusts as a rapid way of nitrate mobilization in a tile drained agricultural field

Agriculture is widely recognized as one of the human activities that have a major impact on pollution of water resources. Agriculture impact on the surrounding environment may consists of the deterioration of surface water and groundwater quality via export of nutrients and pesticides. The formation of salt crusts during dry periods and their fast dissolution and leaching via irrigation or rainfall events can produce extremely elevated concentrations of nitrate in both surface waters and groundwater. This process is rather common in arid environments but due to climate change it will have to be taken into account even in temperate environments. The formation of salt crusts was studied in a 6.3 ha experimental site located in the Po Plain, Northern Italy (45°50’33’’ N and 12°05’40’’ E). The soil, consisting of interfluvial silty-clay deposits recently reclaimed and equipped with tile-drains to avoid water logging conditions, was investigated for vertical spatial heterogeneity via depth profiles (down to -4 m below ground level) and for horizontal spatial heterogeneity collecting numerous surface soil samples. Major ions concentrations were monitored in pore-water using both ceramic suction cups and core samples. Extreme drought conditions were recorded over the monitoring period (summer-autumn 2012), leading to soil fracturing and then to fast water movement during the first rainfall events in autumn. The field monitoring highlighted a marked spatial heterogeneity in the nitrate salt crusts formation, both in the horizontal and vertical directions, showing that the synthetic urea applied was evapoconcetrated in the first 5-10 cm of soil and unevenly distributed over the field with a preferential appearance in the hollows. Monitoring results showed an elevated electrical conductivity of pore-water (>10 dS/m) and nitrate peaks of several grams per liter, suggesting the dissolution of nitrate salts. The field monitoring suggested rapid mobilization of nitrate towards tile-drains after the first rain events due to preferential flow paths into soil cracks developed during the summer season. Results of this study put on evidence the need of sound agricultural practices, accounting for climate change context, to limit fertilizers evapoconcentration and export to surface waters.