Influence of nitrogen forms, pH, and water levels on cadmium speciation and characteristics of cadmium uptake by rapeseed.

Rapeseed (Brassica napus L.) is an ideal crop for remediation in cadmium (Cd)-contaminated soil in farmland. The main objective of this study was focused on the combined effects of four nitrogen forms (urea, ammonium nitrogen, nitrate nitrogen, ammonium nitrate fertilizer), four pH levels (5, 6, 7, 8), and three water levels (low water, middle water, high water) on Cd speciation and characteristics of Cd uptake by rapeseed. A pot experiment was conducted at the Xindu Experimental Park in Sichuan Province, China. Experimental results indicated that the interaction effects of pH and nitrogen forms, three factors on Cd speciation (except organic-bound Cd and exchangeable Cd), were significant and the interaction effects of pH and nitrogen forms on Cd uptake by rapeseed also was significant only under the condition of planting rapeseed. The higher the water level was or the lower the pH value was, the better the repair effect rapeseed to Cd was. High water significantly increased the stem Cd content by 11.89% and 29.55% through significantly increasing the content of exchangeable Cd by 23.40% and 52.63%, respectively, compared with middle water and low water as planting rapeseed, and pH 5 significantly increased the stem Cd content, total Cd enrichment coefficients, bio-availability coefficient, and Cd removal rate by 24.45~40.33%, 49.45~76.62%, 60.00~166.67%, and 16.67~26.00%, respectively, through significantly increasing the content of exchangeable Cd by 37.78~113.79%, compared with pH 7 and pH 8 as the significant decreasing of carbonate-bound Cd, bound to Fe-Mn oxide, and content of organic-bound Cd and residual Cd. Ammonium nitrate also significantly increased total Cd enrichment coefficients, bio-availability coefficient, and Cd removal rate by 45.63~138.10%, 21.05~109.09%, and 40.00~77.50%, respectively, compared with other three nitrogen forms as good growth and Cd resistance of rapeseed. Path analysis structural equation modeling revealed that content of exchangeable Cd and residual Cd had significant and direct path coefficients with variances in stem Cd content of rapeseed. Combined with the safety of edible oil, the best management practices for optimal remediation efficiency of rapeseed to Cd-contaminated soil were ammonium nitrate fertilizer, pH = 5, and high water.