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NITROGEN MANAGEMENT

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

^a Dep. of Environ. and Soil Sci., Univ. of Lleida, Rovira Roure 177, 25198 Lleida, Spain
^b Dep. of Biol. Syst. Eng., Washington State Univ., Pullman, WA 99164-6420
^c Soil Fertil. and Analysis Lab., Sidamon, Lleida, Spain

* Corresponding author (villar{at}rectorat.udl.es)

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 NO₃–N levels, crop N uptake, and crop productivity over a 2-yr period to quantify the relationship among soil NO₃–N, N fertilizer rates, crop N use, and N loss through leaching. Monitoring soil NO₃–N profiles showed that in some fields, soil NO₃–N was transported to deeper layers in the soil during the growing season. In many cases, important accumulation of NO₃–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.

Abbreviations: EU, European Union • PMNT, physiological maturity soil nitrate test • PPNT, preplanting soil nitrate test • PSNT, presidedressing soil nitrate test

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