Early warning model of heavy metal accumulation and AN in the continuous cropping soils of cauliflower heart planting area in Ningxia - the changes in biomass of cauliflower heart

Abstract In order to better evaluate the environmental safety of continuous cropping soil for Chinese lettuce heart in Ningxia. Five central lettuce heart planting areas in Ningxia were chosen to explore the relationship between soil heavy metals and alkali dissolved nitrogen (AN) and their effects on the growth of Chinese lettuce heart, with the accumulation of soil heavy metals as the research object. The results showed that no soil pollution was caused by the heavy metal accumulation in soil during a five-year continuous cultivation of vegetable hearts. Since the 6th year, the soil suffered mild heavy metal contamination, showing a decline in the biomass and quality of vegetable hearts. AN was significantly correlated with heavy metals and vegetable hearts (p ≤ 0.05). A model was established for AN and heavy metals to estimate that the maximum permissible cropping period is less than 6 years in Helan, Wuzhong and Guyuan as the vegetable heart planting areas, and is 9 years in Pingluo and Yongning, with the maximum threshold of 30 mg kg-1 at the time of severe AN deficiency as the criterion. In planting areas across Ningxia, the AN, TN, TC and TOC contents in soil showed a decreasing trend. Besides, when the content of heavy metals Mn and Zn reaches the alarming level, it signifies an extremely low level of soil fertility. In this circumstance, it is necessary to stop continuous cropping and apply C and N-containing fertilizers to maintain the AN above the minimum production capacity.