Analysing soil and canopy factors affecting optimum nitrogen fertilization rates of oilseed rape (Brassica napus)

Implementation of the EU Nitrate Directive in Germany will result in nitrogen (N) balance surpluses being restricted to 60 kg N/ha averaged over 3 years, starting in 2009. With N surpluses of more than 100 kg N/ha, winter oilseed rape (OSR) is a main contributor to N balance surpluses in OSR-based crop rotations in northern Germany. The exact calculation of N fertilization rates therefore becomes increasingly important in order to meet the target or less than 60 kg N/ha N balance average surplus over 3 years at a farm level. Currently, soil mineral nitrogen (SMN) at the beginning of spring growth is commonly used as an indicator for calculation of N fertilization rates in spring. However, amounts of SMN at the beginning of spring growth under OSR are Usually low and canopy N is only taken into account to a very limited extent. This might lead to N fertilization rates exceeding the optimum N fertilization rate (N-opt). In the present study, the effects of SMN in spring and of canopy N in autumn and spring on N-opt were investigated. Multi-site field trials producing different crop canopies, as a result of two sowing dates and two autumn N fertilization levels, with five spring N fertilization levels (0 280 kg N/ha) were carried out in 2005/06 and 2006/07. N-opt in spring was estimated by quadratic response functions using the combine-harvested seed yield data from the spring N fertilization treatments. Regression analyses revealed no relationship between N-opt and SMN at the beginning of spring growth or canopy N at the beginning of spring growth. In contrast, it significant negative correlation between N-opt, and canopy N at the end of autumn growth was found, Based on the results of the present study, it is sensible to take autumn canopy N into account When Calculating N fertilization rates in spring. If canopy N in autumn is high (>50 kg N/ha), as a consequence, N fertilization rates Should be reduced.