Drainage and nitrate leaching in a crop rotation under different N-fertilizer strategies: application of capacitance probes

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Quantification of drainage and nitrate leaching from cropping systems is necessary to optimize N-fertilizer application and determine the impact on groundwater quality. The objectives of this work were to (i) assess the use of capacitance probes for the continuous determination of the volume of drainage water and the amount of nitrate leached in a crop production system, and (ii) compare the effect of different N-fertilizer strategies to control nitrate leaching in a crop rotation in humid Mediterranean climate. A factorial (control and three fertilizer strategies) experiment was conducted during three cropping seasons in Navarra (Spain). Wheat (Triticum aestivum L.) was planted in 2002, barley (Hordeum vulgare L.) in 2003, and rapeseed (Brassica napus L.) in 2004. Daily soil water content measurements based on capacitance probes were used to calculate drainage at 1 m depth, by applying the water balance equation. Nitrate leaching was calculated as the drainage volume multiplied by the nitrate concentration of the soil solution extracted in ceramic cups. The results revealed distinct behaviour in three crop phases, viz.: (i) from planting to GS-25, with high risk of drainage and nitrate leaching, (ii) from GS-25 to the end of the drainage period, with little drainage and leaching, and (iii) from then to harvest, when no drainage or nitrate leaching took place. Drainage and soil mineral N content before planting were the main factors determining the amount of N leached. Splitting N-fertilizer application and the use of nitrification inhibitors are not likely to have a significant impact on subsequent N-leaching losses, provided that the N-fertilizer application is adjusted to crop N needs corrected by soil N supply.