Cascade effects of continuous cropping on soil quality, nitrate nitrogen content, yield and quality of choy sum

To investigate choy sum yield and soil condition changes in open field continuous cultivation of choy sum (Brassica chinensis var. parachinensis), a three-factor, three-level orthogonal experiment was conducted. The factors examined were soil heavy metal accumulation, soil fertility, and soil structure. Results indicated that the primary and secondary factors influencing choy sum yield were soil structure, soil heavy metal accumulation, and soil fertility, in that order. Significant effects on choy sum yield were observed for the interaction of soil heavy metal accumulation and soil fertility, as well as soil structure and soil fertility (P<0.05), while interactions of other factors were not significant. The primary and secondary factors affecting soil nitrate nitrogen content were soil heavy metal accumulation, soil structure, and soil fertility. Notably, the interaction effects of soil heavy metal accumulation and soil structure on nitrate nitrogen were significant (P<0.05). Through response surface method analysis, it was determined that maintaining a high level of choy sum yield and nitrate nitrogen content required a soil heavy metal accumulation composite index (SHI)<1.0, soil fertility composite index (SFI)≥6.5, and soil structure composite index (SSI)≥1.95. A logistic regression analysis was conducted over three consecutive cropping years using various indicators like SHI, SFI, SSI, yield, and nitrate nitrogen content to forecast these indicators for 5, 9, and 13 years. Upon replicating the study in the following year and incorporating quality metrics, it was observed that the Root Mean Square Error (RMSE) values between predicted and actual values of these indicators for 5, 9, and 13 years of continuous cropping were consistently below 0.83, indicating a high level of accuracy. Notably, when the continuous cropping years exceeded 5, there was a significant decrease in choy sum yield, quality, and soil nitrate nitrogen content of cauliflower, falling below the optimized values of the Box-Behnken model. Hence, it is recommended to limit the consecutive cropping period of choy sum to no more than 4 years to maintain soil productivity at a level that ensures stable yield and good quality of choy sum.