Soil vs. groundwater: The quality dilemma. Managing nitrogen leaching and salinity control under irrigated agriculture in Mediterranean conditions

A 3-year field trial was carried out in southern Italy on an agricultural farm close to the seacoast of Manfredonia Gulf (Apulia Region) where crop irrigation with saline water is standard practice. Seawater intrusion into the groundwater, and the consequent soil salinization represent a serious environmental threat. Each year, two crop cycles were applied, in spring-summer and autumn-winter seasons, respectively. The crop pairing over the three years was tomato and spinach; zucchini and broccoli; pepper and wheat. Cultivation was performed in a field-unit characterised by three adjacent plots. At the centre of each plot, a hydraulically insulated drainage basin was dug (0.70 m depth) to collect the draining water. The crops were irrigated with saline water and leaching treatments were applied with saline or fresh water whenever soil salinity reached a predetermined electrical conductivity threshold. Since soil salinity control might increase nitrate leaching, operational criteria should optimize the trade-off between the application of higher water volumes to reduce soil salinity and lower water volumes to protect groundwater quality from nitrate leaching. The amount of nitrogen leached from the soil root-zone was considerable (on average, 156 kg N ha−1 year−1) and higher in autumn-winter than spring-summer (72 vs. 28% of the average annual value). In autumn-winter season, nitrogen losses were mainly due to plentiful nitrogen fertilisation and high rainfall. In spring-summer, extra irrigations promoted salt leaching together with nitrogen losses. To manage both irrigation and nitrogen fertilisation a “decoupling” strategy is recommended. This strategy suggests applying leaching preferably at the end of the spring-summer growing season, soon after crop harvesting or at the beginning of the autumn-winter season, before second crop cycle starting. In autumn-winter season, proper nitrogen supplies and timely top-dressing applications, still allow salts to be discharged by rainfalls but prevent nitrogen losses, thus preserving groundwater quality.