Influences Of Application Of Sludge On Yield, Quality Of Brassica Chinensis And Formations Of Cadmium, Chromium In Soil

This paper investigated the influences of application of sludge on yield, quality of brassica chinensis and the plant availability of cadmium, chromium in sludge-applied soil, food security of brassica chinensis and risk evaluation of soil environment, from brassica chinensis and soil environment two aspects.Brassica chinensis was planted with the application of dried sludge from four typical sewage works of Xinxiang, Kaifeng, Xuchang, Zhengzhou in Henan Province in pot experiment. The changes of yield and the accumulation of cadmium, chromium in brassica chinensis were studied with application of sludge of different origin and quantity for 4 continuous seasons, and the security of brassica chinensis was evaluated. Meanwhile, the field experiment was carried out to plant brassica chinensis in the Institute of Plant Nutrient, Agricultural Resources and Environmental Science, Henan Academy of Agricultural Science, with the application of dried sludge from Zhengzhou Wulongkou sewage works, in which the changes of yield, quality of brassica chinensis, and the accumulation of cadmium, chromium in soil were studied due to application of sludge in different quantity for 4 continuous seasons, and the risk of total amounts of heavy metal was evaluated, the secure years of sludge used on the soil environment was forecasted; Soil samples were collected at the 10th, 20th, 30th, 60th, 90th days respectively, in the growth period of brassica chinensis, to analyze the changes of formations of cadmium, chromium in soil and the relations between different formations and their availability after planting brassica chinensis. The main results were as follows:(1) The application of sludge increased the yields of brassica chinensis. The range of increased yield of brassica chinensis was closely related to the nutrient contents of sludge. For example, the yields of brassica chinensis were in the order: Xinxiang>Zhengzhou>Kaifeng>Xuchang due to application of sludge with different nutrient contents. The hight, fresh weight, dry material of brassica chinensis increased with increasing sludge dosage, the reverse was observed due to excessive sludge through sludge further used on the field. When sunshine, water and fertilizer were enough supplied, the yield of the second season were obviously higher than other 3 seasons. The excessive use of sludge caused the excessive growth of roots of brassica chinensis by the stress of heavy metals, which produced disadvantageous influences on the dry material accumulation of edible parts in brassica chinensis.(2) The application of sludge influenced the quality of brassica chinensis. The suitable addition of sludge usually increased the chlorophyll, reducing VC, reducing sugar contents and reduced the crude fiber contents of brassica chinensis in some degrees, thus improving the quality of brassica chinensis. But application of sludge also remarkably increased the accumulation of nitrate in brassica chinensis. In this research, although the nitrate contents of brassica chinensis in all treatments did not exceed the standard of national maximum limit of nitrate, the potential risk to human health was observed from the whole food chain.(3) The accumulation of cadmium, chromium in brassica chinensis was closly related to cadmium, chromium contents in sludge. The order of cadmium contents in brassica chinensis was Kaifeng >Xinxiang >Zhengzhou >Xuchang; And Xinxiang >Zhengzhou >Kaifeng >Xuchang, for chromium contents. The cadmium, chromium contents in brassica chinensis increased with increasing dosage and times of sluge applied. The cadmium, chromium contents in brassica chinensis had already exceeded the national food hygiene standard in parts of treatments.(4) The food security issue and the risk evaluation of human health received more and more attention with improvement in the levels of life. It was found that some treatments with application of sludge from Xinxiang, Zhengzhou caused brassica chinensis to exceed the standard when the cadmium, chromium contents in brassica chinensis were converted into the possible cadmium, chromium ingestion of human body per day.(5) According to the pollution indexes of cadmium, chromium in soil< 0.7, soil was not polluted before sludge was added to the soil. But continuous application of sludge caused rapid accumulation of cadmium, chromium. Total amounts of cadmium, chromium in soil would exceed the second standard of national farmland soil when sludge was continuously used for 1year and 10years, respectively, according to the forecast model of soil security years. It was found, application of sludge resulted in increases in total amounts of cadmium, chromium in different layer of soil in some degrees. Cadmium mainly existed in surface soil and was very difficult to move, however, chromium was easier to move into deeper soil. The cadmium toxicity to soil environment was worse than chromium.(6) Cadmium in soil mainly existed in the effective formations, which was very easy to enter food chain through soil; Generally, chromium in soil existed in the stable formations, which was difficult to decompose, so its activity was weak. The contents of formations of cadmium, chromium showed the trend of escalation with increasing sludge continuously, but there were marked differences in the rising speed. The rising speed of cadmium contents of effective formations was obviously faster than chromium as the time of application passing. The plant availability of cadmium was stronger than chromium.(7) The amounts of four effective formations of cadmium, chromium increased in some degrees with increasing time of application of sludge. Increased dosage of sludge was advantageous to the accumulation of cadmium in its exchangeable ion formation (EXCH) and the iron-manganese oxide compound union formation (FeMnOX), but did not favor the carbonate union formation (CARB) and the organic synthesis formation (OM); In contrast, it was advantageous to the accumulation of chromium in its exchangeable ion formation (EXCH), the carbonate union formation (CARB) and the Iron-manganese oxide compound union formation (FeMnOX), but did not favor the organic synthesis formation (OM). On the whole, application of sludge was helpful to accumulate the exchangeable cadmium, chromium, then further to improve the plant availability of cadmium, chromium in soil.In conclusion, it was essential to regulate the scientific maximum limit of application amounts of sludge, and strictly control the years of sludge application to soil. The detriments to soil environment and food security from heavy metal cadmium were much more than chromium because cadmium mainly existed in plant available formations, therefore cadmium must be considered as the critical limited element in application of sludge to farmland; However, the detriment of chromium was also not neglected for its high amount in sludge.