Soil nitrogen dynamics during an oilseed rape (Brassica napus L.) growing cycle in a humid Mediterranean climate.

Nitrogen budgets help explain the supply pattern of N from the soil to the crop. Through budgeting, an improvement of the N fertilization strategy can be achieved. The objective of the present study, which was carried out under humid Mediterranean climate conditions, was to assess the influence of N fertilization, temperature and soil humidity on soil N dynamics during a whole oilseed rape growing cycle. A field experiment was conducted with two treatments: without N (0 N) and with application of 180 kg N ha(180 N). Mineralization was calculated from N balances made throughout the growing cycle, all while taking into account measured N uptake by oilseed rape and N losses by leaching and NO emissions. Nitrogen net mineralization was negative after fertilization, reaching -6.73 kg N ha day but total net mineralization over the year was similar for the 0 N and 180 N treatments (21 and 8 kg N ha, respectively). Temperatures over 5 °C were sufficient for initiating the mineralization processes. In the summer, when the soil water content was below the wilting point, immobilization took place; however, there is a risk of N leaching if rainfall occurs thereafter, mainly in the 180 N treatment.