Effects of nitrogen addition on above-standard Cd-contaminated soils in vegetable fields

The aim of this study was to explore the effects of nitrogen application on Cd uptake by different kinds of leafy vegetables and on soil available cadmium (Cd) content in above-standard Cd-contaminated soils in vegetable fields. The study also comprehensively evaluated the effects of different nitrogen sources application with the aim of developing strategies to reduce Cd concentration in leafy vegetables by managing the application of nitrogen fertilizers. The experiment was carried out in an above-standard Cd-contaminated vegetable soil to investigate the effects of urea CO(NH₂)₂, calcium nitrate Ca(NO₃)₂, ammonium nitrate (NH₄NO₃) and ammonium bicarbonate (NH₄HCO₃) on yield, quality, Cd content, nitrogen content of two kinds of leafy vegetables — Brassica chinensis L. (BC) and Amaranthus mangostanus L. (AM). Soil DTPA-Cd content and pH as well as comprehensive effects of nitrogen addition on two vegetables were also investigated. The results showed that all nitrogen sources increased yields and decreased Cd contents in both shoot and root systems of the two leafy vegetables. Furthermore, the highest increase of BC yield (59.7%) was caused by NH₄HCO₃ treatment, while the largest increase of AM yield (47.5%) was caused by CO(NH₂)₂ treatment, respectively, compared with control. However, the lowest Cd contents in two kinds of leafy vegetables were observed under Ca(NO₃)₂ treatment among all nitrogen treatments. Ca(NO₃)₂ decreased Cd content by 41.6% and 24.2%, respectively, in AM shoot and root, by 32.2% and 25.9%, respectively, in BC shoot and root. Moreover, the total content of Cd, nitrate, nitrite, Vitamin C and soluble sugar in the shoot system of the two kinds of leafy vegetables varied with the application of different nitrogen resources. Also similar variances were noted for the changes in soil pH and DTPA-Cd content. NH4NO3 application decreased soil pH by 0.12 and 0.25 and increased soil DTPA-Cd content by 15.3% and 14.6%, respectively, in AM and BC. However, the reverse trend was noted under NH₄HCO₃ treatment. Comprehensive evaluation results showed that the four kinds of nitrogen resources had much higher synthetic weighted value than that of control. Among all treatments, the highest value was observed in Ca(NO₃)₂ treatment, which suggested that Ca(NO₃)₂ had the best comprehensive application effect in above-standard Cd-contaminated soils. Thus Ca(NO₃)₂ could be used as optimum nitrogen source in above-standard Cd-contaminated soils in vegetable fields.