Amendment significantly improves soil structure and promotes crop growth. To combat soil degradation and low crop yields in facility agriculture, it is crucial to study the optimal application rate of amendments. This study analyzed the effects of biochar, vermicompost, and mineral-source potassium fulvic acid on the stability of aggregate structure, soil nutrient content, and tomato yield in cambisols, providing a theoretical basis for improving the soil quality of plastic greenhouses in Southern China. A pot experiment on tomato cultivation was carried out in yellow-brown soil in plastic greenhouses. The experiment included eight treatments: 1% biochar (B1); 3% biochar (B3); 5% biochar (B5); 3% vermicompost (V3); 5% vermicompost (V5); 0.1% mineral-source potassium fulvic acid (F1); 0.2% mineral-source potassium fulvic acid (F2); and the control condition without adding soil amendments (CK). The results showed that the biochar and vermicompost treatments effectively reduced soil bulk density and increased total soil porosity. Compared to the control, treatments with soil amendments significantly increased soil pH and had different effects on soil nutrients: F2 showed the most significant improvement in the content of available nitrogen, available phosphorus, and available potassium, with an increase of 133.33%, 834.59%, and 74.34%, respectively; B3 treatment had the highest increase in dissolved organic carbon (DOC), while B5 treatment had the highest organic matter content. Compared to the CK, the particle size of the biochar treatment was mainly 0.053~0.25 mm, while the V3, F1, and F2 mainly occurred with a particle size > 0.25 mm; and V3 has the best aggregate stability. Biochar, vermicompost, and mineral potassium fulvic acid can all promote tomato yield, with the F2 and V3 treatments having a yield increase effect of over 30%. Furthermore, Pearson’s correlation analysis showed a highly significant positive correlation between geometric mean diameter (GMD) and mean weight diameter (MWD), water-stable macroaggregate content (R_0.25), and a positive correlation between alkaline-dissolved nitrogen, available phosphorus, dissolved organic carbon content, and aggregate stability indicators. Adding 0.2% mineral-source potassium fulvic acid optimizes cambisols’ properties, enhances aggregate formation and stability, boosts tomato yield, and shows great application potential.

Corn stover biochar amendment significantly influences nitrogen (N) and phosphorus (P) transformations, microbial community composition, and enzyme activities in continuous cropping soils. This study aimed to identify the optimal biochar application rate for enhancing N and P nutrient availability in Solanum lycopersicum L. continuous cropping systems, providing theoretical and technical foundations for mitigating continuous cropping obstacles. A soil experiment under rain-out shelters employed four treatments: 1% biochar (BA1), 3% biochar (BA3), 5% biochar (BA5), and a non-amended control (BA0). The results indicated that biochar amendment significantly elevated available phosphorus content in the soil while effectively suppressing its vertical migration; nitrate N content increased under BA1 treatment but decreased in the BA3 and BA5 groups; and the strength of the inhibition effect of biochar treatment on the vertical migration of nitrate N was BA1 > BA5 > BA0 > BA3. The addition of biochar treatment had no significant effect on the content of ammonium N but could inhibit the vertical migration of ammonium N. The addition of biochar treatment could increase the soil’s ammonium N content. The addition of biochar treatment increased soil catalase and urease and sucrase activities, decreased alkaline phosphatase activity, led to the promotion of nitrate reductase activity at low doses and its inhibition at high doses, and resulted in BA1 treatment having the largest soil enzyme index (SEI), which was the most favorable to increase the overall level of soil enzyme activities. Biochar significantly increased the relative abundance of Patescibacteria and Ciliophora while reducing Gemmatimonadota, Acidobacteriota, Nitrospirota, Ascomycota, and Chlorophyta. Comprehensive evaluation using gray relational analysis (GRA) demonstrated that the addition of 5% biochar resulted in the optimal overall performance, enhancing nitrogen and phosphorus transformation, improving microbial community structure, and harmonizing enzyme activities, thereby exhibiting considerable potential for alleviating the nutrient limitations of nitrogen and phosphorus in continuous cropping soils.