## 4 Discussion  
Effect of Organic Fertilizer Combined with Soil Organic Matter, Total Soil Nitrogen, and Organic-Fertilizer Type on Yield  
The tomato yield increased with an increase in soil organic matter. The nutrient and water retention capacity increased with organic matter accumulation, thus increasing tomato yield [28]. The figure demonstrates that the tomato yield rose more slowly as the total nitrogen concentration of the soil increased compared to when no fertilizer was applied. It is possible that the nitrogen content exceeded the optimal value due to the high application rate of organic fertilizers, causing tomato yields to increase slowly and sometimes decrease [29]. Animal-derived organic fertilizers were better than other organic fertilizers at increasing the tomato yield, possibly because their nitrogen release rate was  
Figure 10: Organic fertilizers combined with soil organic matter (**a**), total soil nitrogen (**b**), type of organic fertilizers (**c**), soil pH (**d**), and cultivation type (**e**) on tomato nitrate percentage changes. Points represent the mean effect, while bars represent 95% confidence intervals. The numbers on the right side of the confidence intervals represent the sample sizes. AOF, animal organic fertilizer; MOF, mixed animal-and-plant organic fertilizer; FOF, plant organic fertilizer; OOF, other organic fertilizer.  
higher than that of other organic fertilizers [30; 31; 32]. They also improved soil physical properties and soil nutrient utilization efficiency [33; 34]. Soil pH affects the water and mineral nutrient uptake by tomato plants [35]. The figure depicts that the yield percentage change of soil pH \(<\) 7 was 1.82 times that of soil pH \(\geq\) 7. Adding organic fertilizers resulted in a higher tomato yield under acidic soil conditions than in alkaline soil. Furthermore, fertilizers application changes the soil microbial community structure, especially AOA and AOB involved in the nitrification process. Applying organic fertilizers is conducive to the digestion process, thereby improving Nr and NO3-N, stabilizing NH4-N, reducing NH\({}_{3}\) volatilization [36], and further improving tomato yield. All cultivation types had a positive effect on tomato yield effect values, with open-field cultivation producing significant differences in tomato yield compared to facility cultivation, with reduced yield variability in greenhouse production compared to field production systems [37]. This could be due to differences in tomato varieties, growth habits, disease resistance, and other characteristics.  
Although organic fertilizers improve tomato yields and quality, they also have some disadvantages in atmospheric and soil pollution compared to conventional cultivation. Studies have shown that methane and carbon dioxide emissions from rice paddies can be increased by the excess application of organic fertilizers. Moreover, improperly disposed food waste can produce bad odors and harbor pathogenic microorganisms that cause environmental pollution [38]. Similarly, soil applied with animal manure may significantly aggravate the heavy metal accumulation in Chinese farm soils coming from Cu and Zn, for example [39]. In recent years, it has been found that animal organic fertilizers could produce large amounts of ammonia, which indirectly or directly has some impact on human health [40; 41]. However, in South, Southwest, and Northwest China, pyrolysis gasification technology has been used to degrade domestic waste, which may provide the possibility to ameliorate the problem of environment pollution and boost the sustainable development of agriculture. Therefore, there are still difficulties and challenges in optimizing the application technique of organic fertilizers in order to avoid environmental pollution as much as possible.