Influence of sulfur on apparent N-use efficiency,yield and quality of oilseed rape (Brassica napus L.) grown on a calcareous soil

In the Lorraine region, major soils used for winter oilseed rape are calcareous. Across two pot and two field experiments, we studied the influence of sulfur applied at different levels on apparent N-use efficiency (ANU), yield, glucosinolate (GLS) and oil content of seeds. The soil received a constant dose of 200 kg N ha^?1 as ammonium nitrate, urea or cow slurry and three levels of S: 0, 30 and 75 kg ha^?1 as ammonium thiosulfate (ATS), MgSO₄ or ATS plus MgSO₄. Apparently, oilseed rape is a N-inefficient crop as revealed by low ANU values which varied within 36 and 53% from field experiment versus 25 and 61% under controlled conditions. In both cases, S additions improved N-use efficiency only at the highest dose of 75 kg S ha^?1, which is not attained by ATS with 35 kg S ha^?1 (10% v/v). S fertilization increased the GLS contents that were found to be negatively correlated with plant N/S uptake ratios observed at maturity. The most important increase in GLS content by 52% was noted with cow slurry in the pot experiment. But, as a whole, the GLS levels remain below the European norm of 18 μmol g seed^?1. Moreover, the oil content (% DM) of seeds decreased (but the total production increased) when the soil was fertilized with N and with or without S. The results showed that N and S nutrition during the growth were tightly linked. Their interactions, as reflected by plant uptake, are synergistic at optimum rates and antagonistic at excessive levels of one of the both. Collectively, the results indicate that S fertilization is required to improve N-use efficiency and thereby maintaining a sufficient oil level and fatty acid quality.