Nitrogen and sulphur fertilization effect on leaching losses, nutrient balance and plant quality in a wheat-rapeseed rotation under a humid Mediterranean climate

An unbalanced S and/or N fertilization may have low N and S use efficiency together with substantial negative implications for yield, nutrient losses and plant quality parameters. The effect of N and S fertilization and their interactions on N-S balances, on N-S losses and on some plant quality parameters were investigated in a field experiment with a wheat (Triticum aestivum L.)-rapeseed (Brassica napus L.)-wheat rotation (2005-2008). The study was conducted under humid Mediterranean climatic conditions on a potentially S deficient soil. The effects of N (0, 140, 180, 220 kg N ha(-1) in wheat; 0, 100, 140, 180, 220 N ha(-1) in rapeseed) combined with S fertilizer rates (0, 16 and 32 kg S ha(-1) in wheat and 0, 30, 60 kg S ha(-1) in rapeseed) were studied. Nitrogen fertilization increased yield by 55 % in wheat and 60 % in rapeseed, N concentration in grain and straw and S concentration in the grain of wheat. However, it led to a reduction in the S concentration of straw and the oil content of the rapeseed seed. The S application did not increase yield but had a positive effect on S concentration in the wheat straw. Glucosinolate concentration, a potentially toxic secondary metabolite in rapeseed, was not influenced by N or S applications. Nitrate leaching tended to increase with N application while sulphate leaching decreased. A net N and S mineralization was observed in each growing season, except for the first year in which a net S immobilization was observed. To make N fertilizer recommendations, the N mineralization from the previous crop residues should be taken into account. For S fertilizer recommendations, N supply is the most important item both from a qualitative point of view (N/S ratio in wheat grain) and an environmental point of view (S leaching).