Drainage and nitrate leaching under processing tomato growth with drip irrigation and plastic mulching

Irrigated agricultural areas are particularly susceptible to groundwater pollution because irrigated crops are abundantly fertilized and occasionally overwatered. Our goal was to evaluate the close link between crop production and groundwater protection. The specific objective of our research was to quantify the effect of three irrigation strategies on yield, drainage and nitrate leaching in processing tomato grown using drip irrigation and plastic mulch. We also assess the effect of high soil mineral N content on nitrate leaching. The study was conducted in a field cropped with tomato (Lycopersicum esculentum Mill), cultivated under three different irrigation strategies during 2001 and 2002 in the Ebro Valley. The strategies were combinations of two rates and two frequencies of water application. During the establishment period, water was either applied continuously for 8 h, or as 8 applications of 15 min. During the growth period, water was applied either as a one single daily irrigation of 100% ETc, or as 4–8 daily irrigations to 80% ETc. Monitoring transects, composed of 6 TDR probe access tubes inserted into the soil, provided measurements of soil water content in each treatment. Drainage was calculated weekly by applying the water balance to all tubes located in each transect. Nitrate leaching was evaluated as the drainage volume multiplied by the nitrate concentration of the soil solution extracted in 6 porous ceramic cups located in each treatment. At harvest, total and marketable production were recorded for each plot, and the dry weight of the component aboveground plant parts (stems, leaves and fruits) were determined. Crop N uptake for the plant parts was calculated as the product of total biomass and N concentration. Our results showed that excessive irrigation during the crop establishment period may cause large nitrogen losses. High irrigation frequency ensured appropriate soil water content at planting, and reduced both the amount of water applied and lost by drainage. Treatments receiving water at 80% of ETc, recorded almost no drainage during the growth period and no reduction in crop yield. This suggests that further studies are required to adjust crop coefficients to the actual needs of drip-irrigated tomato under plastic mulching conditions. When soil mineral N content before planting is high, nitrate leaching during the growth period can be relevant due to occasional rain events, even if water application is adjusted to crop requirements.