Effect of nitrogen and sulphur fertilisation on yields, nitrates and cysteine and methionine content in onion (Allium cepa L.)

Abstract

This study explored the interactive effects of two doses of nitrogen and two levels of soil sulphur on onion yields, nitrate content and concentration of the amino acids cysteine and methionine under accurately controlled conditions. Onions were cultivated in Mitscherlich pots with 6 kg of soil with a low content of S-SO42- (5 ppm, S0) or a high content (30 ppm, S1) with added doses of 0.6 (N1) and 1.2 (N2) g N pot-1 as (NH4)2SO 4 and NH4NO3. There were no significant differences (P<0.05) in onion yields between the two N doses. The higher sulphur content (S1) increased yields by 7.0-8.4% compared with S0 for both doses of nitrogen. The nitrate content in fresh onion increased with nitrogen dose (225 and 566 mg NO3- kg -1 for N1 and N2 respectively). High sulphur reduced the nitrate content of onions by 19.3-20.5% depending on nitrogen dose. The contents of cysteine and methionine increased by on average 39.1% and 25%, respectively, with increasing nitrogen dose. Soil sulphur content did not change the cysteine content but higher sulphur content in combination with higher nitrogen dose (N2) significantly increased the methionine content, by 18.7-40.7% compared with the other treatments. Joint fertilisation with nitrogen and sulphur is thus a suitable measure to increase onion yield and quality since it stabilises or increases the content of sulphur amino acids and reduces the level of undesirable nitrates.