Effects of manure acidification on the soil nutrient leaching and accumulation of heavy metals in the continuous administration

This study aims to explore the effect of acid manure water on soil nutrient leaching and heavy metal accumulation in the field. Two types of fresh and acid manure water were also selected with different pH values in the experiment of continuous application. Taking the concentrated sulfuric acid as the acidifying agent, a series of indoor simulation experiments were performed on the soil column leaching. The fresh fecal water and acid septic water with three pH values (6.5, 6.0, and 5.5) were set as the control group. Six times of leaching with fecal water were then used once every 15 days in each treatment. Prior to leaching, the soil samples were taken from the different depths of soil layers (0-5 cm, 15-20 cm, and 35-40 cm) in each soil column using a syringe tube. The multi-point mixing sampling was adopted for each layer of soil. One soil sample was formed to mix evenly after drying. Each soil sample was air-dried and then bagged to store for testing. Three equal soil samples were taken from the different positions and then repeated three times for data analysis. The test results showed that there was an increase in the total soil nutrient content (N, P, and K) after the application of fresh and acidified manure water. Specifically, the fresh manure water with pH6.5, pH6.0, and pH5.5 increased by 1%-40%, 15%-66%, and 5%-21%, respectively. It was found that the total soil nutrient concentration with the application of acid manure water was higher than that of the control group. The acidification degree of the manure water increased with the amount of manure water. Furthermore, the soil heavy metals of Cu, Zn Cd, and Pb increased by 4%-48% and 4%-11%, 2%-14%, and 1%-18%, respectively. It infers that the continuous application of acidification manure water reduced the soil pH, whereas, gradually increased the soil EC value and the accumulated heavy metals in the soil. The exogenous acidulants were also introduced to change the soil sulfur element with the increase of acidification degree. As such, the risk factor was triggered to damage the soil environment during this time. Therefore, special attention should be paid to the actual application process in the field. It is very necessary to monitor the soil pH value, soil EC value, and soil heavy metal concentration every three to five years, in order to ensure that the indicators are below the critical value of risk for the better healthy growth of crops. More importantly, it is recommended to apply the manure water with the pH value of 6.5 once every two crops, or the pH value of 6.0 once every three crops, or the pH value of 5.5 once every four crops. Anyway, the soil nutrient concentration and heavy metal accumulation were compared to clarify the effect of acid manure water returning to the field, particularly after the continuous application of fresh and acid manure water with the pH values of 6.5, 6.0, and 5.5. Since the acid manure water can harm the soil after returning to the field, the finding can provide the scientific basis and technical support to the actual continuous application of acid septic water to the field.