1329 4.2.2 Readily available nutrients of different substrates before and after crop planting Figures 4-5 show the available nitrogen and rapidly available phosphorus contents of different substrates before and after pak choi planting. The average available nitrogen content of all substrates before planting is close to that of the CK, while the rapidly available phosphorus content is much lower than that of the CK. Both contents and the ratio of microbial fertilizer added show a step-by-step increasing trend, among which R7-R9 have the highest content and R1–R3 have the lowest content. Figure 4 shows that the available nitrogen content of different substrates after planting follows a dynamic trend of first decreasing and then increasing. The available nitrogen content of R1-R3 decreases by 1.45 mg/kg, 1.01 mg/kg, and 1.42 mg/kg, respectively, while that of R4-R9 and CK increases by 0.95 mg/kg, 1.51 mg/kg, 1.23 mg/kg, 3.56 mg/kg, 5.20 mg/kg, 4.27 mg/kg, and 1.01 mg/kg, respectively. This indicates that the absorption of available nitrogen by plants during planting leads to a decrease in available nitrogen, while the return of pak choi stubbles to the field and the nitrogen fixation and activation of microbial inoculants accelerate the decomposition of organic matter and the release of nitrogen, increase the nitrogen supply level of the substrates, and reflect the internal circulation process of organic nitrogen mineralization and inorganic nitrogen biological fixation during planting. Figure 5 shows that the rapidly available phosphorus content of R1-R9 after planting increases by 1.17 mg/kg, 1.49 mg/kg, 1.78 mg/kg, 2.72 mg/kg, 3.20 mg/kg, 2.85 mg/kg, 4.07 mg/kg, 4.65 mg/kg, and 4.42 mg/kg, respectively, and that of the CK decreases by 1.60 mg/kg. This indicates that the rapidly available phosphorus in the substrates is absorbed by plant growth during the early stage of planting, but the insoluble phosphorus in the substrates is decomposed and activated by phosphate-solubilizing bacteria. The coprocessing of pak choi stubbles and microbial inoculants reduce the fixation of phosphorus and increase the capacity of rapidly available phosphorus. Due to the difference in substrates, microbial fertilizer amounts, and pak choi biomasses, changes in rapidly available nutrients in substrates vary after planting, with the maximum change observed for R8, which has a significant effect compared to soil nutrient. Figure 4 - Available nitrogen contents of different substrates before and after pak choi planting Figure 5 - Rapidly available phosphorus content of different substrates before and after pak choi planting According to a comprehensive analysis of the influence of different substrates on pak choi growth, as well as the changes in nutrient elements after planting, the 1:1 blending ratio of fine-grained gangue and tailing coal slime, combined with a 10% microbial fertilizer ratio, is most conducive to crop growth. Based on the research results, a small-scale field experiment was conducted in CCTEG Tangshan Research Institute. Spinach, rapeseed, and pak choi were sown to assess the growth performance of leaf vegetables under natural field conditions. The leaf vegetable seeds germinated within 3 to 5 days, achieving an emergence rate of over 95%. The plants reached an average height of approximately 7 cm around half a month after sowing and entered maturity by day 30. A notable observation was the change in leaf color from dark green to a shiny, black-green hue. For comparison, spinach was sown concurrently in a nearby plot using the same cultivation practices but in the native soil. While the growth of the soil-grown spinach was comparable to that in the substrate, the final harvest results of spinach grown in the soil–in terms of both plant height and leaf coloration–were superior than those in the prepared substrate. The product has undergone heavy metal testing by Tangshan Academy of Agricultural Sciences, and the test results meet the limit requirements for food contaminants in national standards.
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