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

Agriculture is widely recognized as one of the human activities that have a major impact on the pollution of water resources. In agricultural fields, the formation of salt crusts during dry periods and their fast dissolution due to irrigation or rainfall events can induce the leaching of water with an elevated content of dissolved species towards surface and ground waters. This process is rather common in arid environments but it also occurs in coastal plains in temperate environments. The formation of salt crusts was studied in a 6.3-ha experimental site located in the Po River Delta (Northern Italy). 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 and for horizontal spatial heterogeneity collecting numerous surface soil samples. Extreme drought conditions were recorded over the monitoring period (summer–autumn 2012), leading to soil fracturing and then to fast water percolation during the first rainfall events in autumn. Major ion concentration, measured in pore-water, showed nitrate peaks of several grams per litre, suggesting the dissolution of nitrate salts. Results from this study highlighted the following: (i) that the fertilizers applied to the filed site were evapoconcentrated in the top soil; (ii) a marked spatial heterogeneity in the salt crust formation, which was unevenly distributed over the field with a preferential appearance in the hollows; and (iii) a rapid mobilization of nitrate towards tile drains after the first rainfall events, due to preferential flow through soil cracks developed during the summer season.