## 4 Discussion
Soil urease is the only enzyme that acts on urea fertilizers and can be used as an indicator of metal pollution by measuring its activity through soil nitrogen transformation [70]. The soil urease activity (Figure 5c) in all treatments was significantly (\(p<0.05\)) increased compared to the control. The addition of waste cement reduced Cd availability, thereby reducing the toxicity of heavy metals to soil microorganisms, promoting the growth of soil microorganisms, and increasing the activity of urease, which is consistent with [PERSON] et al. 's research [71]. Soil pH is an important factor for microbial growth [72], and some studies had shown that highly alkaline environments increase the activity of urease [62]. The addition of waste cement increased the pH and CEC of the soil, thereby improving the growth and metabolism of soil microorganisms, significantly increasing the activity of urease in the soil. It was found that the uptake of cadmium by plant tissues was significantly reduced (Figure 6) after the application of calcium hydroxide (CH) alkaline amendment, which effectively reduced the uptake of Cd by Chinese cabbage [73]. The bioavailability of Cd in soil and lettuce was decreased. The solubility and leachability of cadmium were directly related to the absorption of Cd by plant roots and shoots [74; 75; 76; 30]. Application of Si and Ca\(-\)containing amendments in Cd-contaminated soil increased Cd immobilization and decreased Cd concentration in rice grains, as demonstrated in a study by [PERSON] et al. [77]. The addition of waste cement increased the pH of the soil and introduced carbonate and Si into the soil, which precipitated and adsorbed cadmium in the soil, thereby reducing the migration of cadmium to the above-ground parts of the plants. The application of waste cement had positive effects on plants in cadmium-contaminated soils, with higher or lower additions increasing plant height, root length, and total plant dry biomass (Figure 7). With the increase in the amount of waste cement added, the promotional effect on plants showed a trend of first increasing and then decreasing. In this experiment, the adding of 2.0% waste cement had the best effect. This trend was mainly due to the initial low-dose addition, which reduced the bioavailability of Cd and neutralized the acidity of the soil caused by heavy metal pollution, thereby reducing the toxicity of Cd to plants and improving the soil microenvironment to make it more suitable for plant growth. However, as the amount of waste cement added was increased, the precipitation and adsorption of heavy metals in the soil reached the maximum. At this time, heavy metals were no longer the primary threat to plant growth and the microecology had become the dominant factor. With the increase in waste cement application rate, the alkalinity of the soil increased, exceeding the optimum pH for plant growth; high alkalinity will also inhibit the growth of plants [78], so with the increase in waste cement application rate, the growth of the plants showed a tendency to first increase and then decrease. In general, the addition of waste cement could promote the restoration of soil and the growth of plants. When plants were subjected to biotic and abiotic stress, the body was stimulated to produce excessive reactive oxygen species (ROS: O\({}_{2}\)\({}^{\bullet-}\), H\({}_{2}\)O\({}_{2}\), and OH\({}^{\bullet}\)), which can lead to damage or loss of cell function, or even cell death [79]. POD activity is sensitive to stress, and it is used as a biomarker to assess heavy metal toxicity stress [80]. The CAT enzyme directly decomposes H\({}_{2}\)O\({}_{2}\) into H\({}_{2}\)O and O\({}_{2}\) in plant cells, and CAT activity can indirectly reflect the damage of plant cells under environmental stress [80; 81]. SOD can directly reduce the toxic effect of O\({}_{2}\)\({}^{\bullet-}\) in plants, enhance plant tolerance to biotic and abiotic stress, and is the first line of defense in the antioxidant system [35; 82; 83].