Effects of nitrogen and phosphorus fertilization on plant growth and nitrate accumulation in vegetables

Experiments were carried out on a vegetable field with Peking cabbage (Brassica pekinensis lour. rupr.), cabbage (Brassica chinensis var. oleifera makino and nemoto), green cabbage (Brassica chinensis L.), spinach (Spinacia oleracea L.), and rape (Brassica campestris L.) to study the effects of nitrogen (N) forms and rates, and phosphorus (P) fertilization on their growth and nitrate accumulation. The results indicated that application of ammonium chloride, ammonium nitrate, sodium nitrate, and urea significantly increased the yields and nitrate concentrations of Peking cabbage and spinach. Although N forms had no significantly different effect on yields, input of nitrate N fertilizer increased nitrate accumulation in vegetables much more than did ammonium N. Vegetable yields were not increased continuously with N rate, and an excess input of N fertilizer more or less reduced plant growth, leading to yield decline for the earlier harvests. This trend was also true for nitrate concentrations in some vegetables and at some sampling times. However, as a whole, nitrate concentrations in vegetables were positively correlated with N rates. As a result, addition of N fertilizer to soil was the major cause for vegetables increasing their nitrate contents. Effects of P fertilization on vegetable growth and nitrate accumulation were species and sampling-time dependant. By addition of P fertilizer, yields of green cabbage and rape were increased, while those of spinach and cabbage had no significant changes. The nitrate concentration was significantly decreased in green cabbage, but significantly increased in cabbage and rape by P fertilization at the second harvest. Different organs had different amount in nitrate accumulation, and it was found that nitrate concentrations were much higher in roots, stems, and petioles than in blades at any N rate.