Abstract:

To evaluate the reduction effects of combined application of different nitrogen, phosphorus fertilizers and biochar on cadmium (Cd) availability and crop growth in Cd contaminated farmland soils, so as to provide scientific basis for the safe utilization of Cd contaminated farmland and the rational selection of nitrogen and phosphorus fertilizers. A pakchoi (Brassica chinensis L.) pot experiment was conducted to study the effects of combined application of three nitrogen fertilizers such as ammonium sulfate (L), urea (N), and calcium nitrate (X), two phosphorus fertilizers such as superphosphate (S) and calcium magnesium phosphate (C), and biochar (B) on soil Cd availability, pakchoi growth and Cd uptake from a farmland soil contaminated by Cd. The results showed that compared with the control treatment without fertilizer application, combined application of nitrogen and phosphorus fertilizers significantly increased the biomass of pakchoi by 28.6%-65.7%, and the combined application of nitrogen, phosphorus fertilizers and biochar significantly increased pakchoi biomass by 11.9%-40.0% compared to the treatment of combined application of nitrogen and phosphorus fertilizers. The combined application of calcium nitrate and calcium superphosphate significantly increased soil pH by 0.23 units, whereas other nitrogen and phosphorus fertilizer treatments had no significant impact on soil pH. However, content of soil available Cd in the treatments of combined application of all nitrogen and phosphorus fertilizers was decreased by 16.5%-38.8%, content of Cd in pakchoi was reduced by 5.87%-25.0%, compared with the control treatment. Compared with the combined application of nitrogen and phosphorus fertilizers, the combined application of nitrogen, phosphorus fertilizers and biochar increased soil pH in varying degrees with a maximum increase of 0.42 units, decreased the content of soil available Cd by 5.31%-56.3%, and decreased the content of Cd in pakchoi by 25.5%-4.8%. The results confirmed that reasonable selection and application of nitrogen and phosphorus fertilizers are crucial during the process of safe utilization of farmland contaminated by Cd, which can not only achieve the best economic benefits, but also generate positive environmental effects. Meanwhile, the application of biochar further enhances the passivation effect of nitrogen and phosphorus fertilizers on Cd activity in the contaminated soil.

Key words: nitrogen fertilizer, phosphorus fertilizer, biochar, cadmium contamination, bioavailability, soil, soil Cd availability, vegetable growth