Nitrogen budget for fertilized carrot cropping systems in a Quebec organic soil

After reclamation, organic soils tend to shift from net N immobilization to net N mineralization. The N amounts mineralized annually can be sufficient to cover the N needs of the crop, reducing the need for supplemental N fertilization. The objective of this study was to assess N budgets and to infer the apparent N mineralization in fertilized and unfertilized carrot (Daucus carota L.) crops in an organic soil in southwestern Quebec. Five pre-plant N fertilization treatments (0, 25, 50, 75, and a split 25 + 25 kg N ha(-1)) were applied over a 3-yr period to non-irrigated carrots. Crop yield and plant and soil N contents were measured. Apparent net N mineralization was an important source of N, averaging 117 kg N ha(-1) yr(-1). Crop yield was not affected by the N fertilization rate, by splitting of the fertilizer application or by the previous year's crop. Increasing the N fertilizer rate increased the soil nitrate content below the root zone (similar to 40 cm below the surface), enhancing the risk of nitrate leaching. Thus, because soil N mineralization was sufficient for the carrots' requirements in this organic soil, supplemental N fertilization was found to be unnecessary and could exacerbate the environmental risk of nitrate leaching.