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

To investigate the choy sum (Brassica chinensis var. parachinensis) yield and soil condition changes under the open-field continuous cultivation, a three-factor three-level orthogonal experiment was conducted. The factors examined were soil heavy metal accumulation, soil fertility, and soil structure. The results indicated that the order of factors influencing choy sum yield was soil structure, heavy metal accumulation, and fertility. The interaction between soil heavy metal accumulation and soil fertility and the interaction between soil structure and soil fertility both had significant effects (P<0.05) on choy sum production; however, the interactions among other factors did not have significant effects on choy sum production. Soil heavy metal accumulation, soil structure, and soil fertility were the first, second and third factors affecting soil nitrate-nitrogen content, repectively. Notably, the interaction effects between soil heavy metal accumulation and soil structure on soil nitrate-nitrogen content were significant (P<0.05). Response surface method analysis revealed that 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 were required to maintain a high level of choy sum yield and soil nitrate-nitrogen content. Logistic regression analysis was conducted over three consecutive cropping years using various indicators, such as SHI, SFI, SSI, yield, and soil nitrate-nitrogen content, to forecast these indicators for continous cultivation of choy sum 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 the 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 five, a significant decrease in choy sum yield, quality, and soil nitrate-nitrogen content was observed, and the values fell below the optimized values of the Box-Behnken model. Therefore, it is recommended that the consecutive cropping period of choy sum be limited to not more than four years to maintain soil productivity at a level that ensures stable yield and good quality of choy sum.