Reducing nitrate leaching losses from vegetable production in Mediterranean greenhouses

Vegetable production in greenhouses in southeastern (SE) Spain can be considered to be representative of Mediterranean greenhouse vegetable production. In SE Spain, simple plastic greenhouses have contributed substantially to rapid recent economic development. Of the approximately 41,000 ha of greenhouses in SE Spain, 31,600 ha are located in the province of Almeria, the rest in the nearby provinces of Murcia, Granada and Malaga. Approximately 90% of cropping is in soil, the rest in soilless systems. Very few of the soilless-grown crops collect and recycle drainage. Nearly all of the areas where the greenhouses are located have been declared Nitrate Vulnerable Zones in accordance with EU legislation, and are required to adopt management practices that reduce nitrate (NO3-) contamination of water bodies. In the region of Murcia, considerable public concern over eutrophication of the salt water lagoon "Mar Menor" will likely lead to strict controls on nutrient management in greenhouse and open field crops. Nitrate leaching from agriculture is the major cause of NO3- contamination of aquifers and a major contributing factor to eutrophication of surface water. Data of the NO3- leaching losses from soil- and substrate-grown greenhouse crops are presented. Results of surveys of grower practice are presented that indicate the practices and attitudes that contribute to excessive applications of irrigation and nitrogen (N) that cause NO3- leaching. Data from regional studies are presented to demonstrate the cumulative effect of excessive irrigation and N applications. The reduction of NO3- leaching loss requires that excessive application of irrigation and N be appreciably reduced, that is that both water and N be used more efficiently. As N is often applied in all or most irrigations through fertigation, improved management of both irrigation and N has to be integrated. More efficient management of irrigation and N will require the integration of 1) planning approaches whereby crop requirements are estimated e.g., that consider all sources of N; and of 2) monitoring approaches that assess either the immediately available supply of water and N, and/or the crop water and N status. The available and most promising tools and technologies are described, and examples of their combined use are presented.