Effect and simulation of continuous application of organic fertilizer on heavy metal behavior in vegetable field soil based on geochemical model

Since 2017, the Ministry of Agriculture and Rural Affairs of China has introduced the action plan of chemical fertilizer reduction and efficiency improvement, which required an appropriate increase in organic fertilizer input and the development of circular agriculture. However, the continuous application of organic fertilizers into the soil will affect the soil pH, organic matter and heavy metal content, and change the soil heavy metal behavior. It is essential for scientific evaluation of the impact of organic fertilizer application. It is considered to be unilateral to evaluate the pollution risk only by the total content. The heavy metal content extracted by different chemical extractants cannot completely represent the actual pollution situation. The geochemical model has good applicability and can explain the behavior of heavy metals more comprehensively than the traditional extraction.In the Huang-Huai-Hai Plain, an intensive agricultural planting area, different proportions of manure-source organic fertilizer were continuously applied to dryland vegetable fields. A geochemical model was introduced, combined with a pH-dependent leaching test, to determine the main influence mechanism of continuous fertilization on soil heavy metal behavior. It found that the content of Cu and Zn in organic fertilizer was much higher than that in soil. After application, their leaching concentration in the soil increased significantly as the proportion of application increased, up to more than ten times, and the activity greatly increased, which was positively correlated with the increase in dissolved organic matter（DOM） content after fertilization. The soil under different treatments showed similar leaching trends. The leaching concentration of heavy metals appeared at a minimum at neutral pH, and gradually increased toward the strong acid and alkali（V-shaped）. The geochemical model（LeachXS） showed good simulation results, and its estimated value had a good correlation（71.02%） with the measured concentration. The simulation results showed that the application of organic fertilizer would not significantly change the main adsorption process of heavy metals on the solid surface of the soil, but it could increase the leaching of heavy metals in the soil liquid phase by increasing the DOM and heavy metal content, thereby increasing the activity of heavy metals. Soil organic matter（SOM）, Fe-Al（hdyr）oxides, clay, etc. determined almost all the leaching of heavy metals. Amo