On‐Farm Monitoring of Soil Nitrate‐Nitrogen in Irrigated Cornfields in the Ebro Valley (Northeast Spain)

The irrigated area served by the Canal d'Urgell, a semiarid region in the Ebro Valley (northeast Spain), presents problems of ground water pollution by nitrates. Corn ( Zea mays L.) is widely grown in this area, and N fertilization in corn production is the major source of N input in the area, with application rates in excess of crop requirements. In this study, we monitored several commercial cornfields for soil NO 3 –N levels, crop N uptake, and crop productivity over a 2‐yr period to quantify the relationship among soil NO 3 –N, N fertilizer rates, crop N use, and N loss through leaching. Monitoring soil NO 3 –N profiles showed that in some fields, soil NO 3 –N was transported to deeper layers in the soil during the growing season. In many cases, important accumulation of NO 3 –N was observed at the bottom of the soil at physiological maturity, increasing the risk of winter leaching. Soil N availability, calculated as preplanting soil nitrate test + N fertilizer, was neither related to plant N uptake nor final biomass and grain yield. In some plots, the occurrence of drought during the growing season was more decisive than soil‐available N in explaining crop N uptake and grain yield differences. Available N levels found in the soil were above 370 kg ha −1 in all cases. Apparently, these levels were enough to satisfy the crop N requirements to achieve yields above 11 to 12 Mg ha −1 . Overall data showed that there is an excess of N in the system.