Objectives Castor (Ricinus communis L.) is found to be one of the tolerant plants in mine area, and is good at absorbing and accumulating Cu. Mine soils are infertile and thus result in reduced castor growth. The experiment was tried to improve the growth of castor through nitrogen and phosphorus fertilization, to put good use of castor in Cu uptake and accumulation in mine areas.

Methods Both the castor seeds and soils used for a pot experiment were collected from the Cu mine, Daye City, Hubei Province. Four concentrations of nitrogen (NH₄NO₃) and phosphate (NaH₂PO₄) were set up (N 0, 75, 150 and 300 mg/kg; P 0, 20, 80 and 200 mg/kg). Caster seeds were grown in mica substrates until two real leaves were developed, then transferred into mine soils for treatments. After two months of growth, casters were harvested, and SPAD values, dry weight, Cu content, soil pH and different forms of Cu were analyzed, and Cu accumulation and transfer rates were calculated .

Results Under the single application of nitrogen fertilizer, the SPAD values were increased with the increase of nitrogen application rate, and the dry weight and Cu accumulation of castor reached a maximum at N₇₅P₀, which was significantly different with the other treatments. Under the single application of phosphate fertilizer, the castor had the heaviest dry weight in N₀P₂₀, which was significantly different with the other treatments, and the Cu accumulation of castor was increased first, then decreased and finally increased, reaching the maximum at N₀P₂₀₀. There were significant effects of nitrogen and phosphorus on SPAD values, dry weight, Cu content, Cu accumulation and transfer rate (P < 0.05). Low nitrogen and phosphate fertilizer amounts increased the dry weight of castor and significantly reduced the Cu accumulation of castor. The application of high nitrogen and low phosphate amounts inhibited the growth of castor, and the dry weight of castor was lower than that of the N₀P₀. The low nitrogen and high phosphate treatments had the most significant effect on castor’s biomass, the largest amount of Cu accumulation and the lowest transfer rate. The application of high nitrogen and low phosphate could significantly increase SPAD values. The N₇₅P₂₀₀ (N 75 mg/kg+P 200 mg/kg) had been shown to cause 133% increase in total dry weight, N₃₀₀P₂₀ (N 300 mg/kg+P 20 mg/kg) enhanced root Cu concentration of Cu, and the N₇₅P₂₀₀ had the highest Cu accumulation which was 34.93 μg/plant. Soil pH decreased by less than 0.1 during the fertilization and dropped by 0.2 to 0.4 units after castor was planted. The forms of Cu were not significantly affected by the fertilization. The acid extractable Cu accumulation and reducible Cu were increased after planting of castor.

Conclusions Applying appropriate nitrogen fertilizer increased the castor’s biomass and SPAD values, while applying high nitrogen amounts decreased the biomass and SPAD values. Cu uptake was increased with the increase of phosphate fertilizer, and reached the largest at the highest application of phosphate. Taking into account of each index in this experiment, the optimum combination fertilizers is the N₇₅P₂₀₀, and its Cu accumulation is 2.33 times of that of the N₀P₀.