Effects Of Exogenous Heavy Metals On Soil Nutrients And Biological Activity In Different Types Of Soil

In recent years, because of human activity, the pollution issue of heavy metal contamination in agricultural soils has become more and more serious which causes serious pollution to soil environment and the safety of agricultural product. This study collected two typical types of soil in the north and south of China--loess soil in Yangling(calcareous soil) and red soil in Jiangxi(acidic soil), which represent the different climatic and zonal characteristics in China. Five kinds of heavy metals, copper(Cu), lead( Pb), cadmium(Cd), mercury(Hg) and chrome(Cr), were took as the objects of this study. Field experiment and chemical analysis were used to analyze the interannual variation situation of the heavy metals in soils, and the effect of heavy metals on soil available nutrients, organic carbon mineralization and microbial activity. This study aims to further understand the environmental chemical behavior of heavy metals in different types of soil, and provide theoretical basis for establishing an effective early warming and assessment system of soil heavy metal pollution. Consequently, these researches have important theoretical and practical significance.The main conclusions are as follows:1) The fixation reaction of exogenous heavy metals occurs mainly in the early stage of the experiment. During this period, heavy metals move and transform quickly, and has obvious dynamic change in different types of soil. The proportions of available fractions gradually decreased as the time growing. The content of available Pb, Cd was slightly low, and easy to be fixed in loess soil, which has the high SOC, CaCO3 and pH, while the time of being stable is longer in red soil with the high concentration of available Pb and Cd.2) The pollution of heavy metal has influence on the nutrient cycling of soil. However, the magnitude of this effect varied among the concentration and variety of heavy metals. In the condition of low concentration of Pb, the available nitrogen in soil has obviously declined; with phosphate radical in soil solution, soluble Pb and Cd can produce slightly soluble salt reducing the content of the rapid available phosphorus, which is against the utilization of the plant; lead-ion has replaced the soil colloid or the K+ in the soil mineral lattice, which increases the content of the rapid available potassium and strengthens its tendentiousness of erosion; the pollution of Cd can reduce the the content of the rapid available potassium.3) The pollution of exogenous lead and cadmium has led to the significant reduction of the content of organic carbon and dissolved organic carbon in the soil, which indicates that the heavy mental pollution promotes the decomposition of soil organic carbon, in this way it influences the long-term carbon sequestration potential in soil, leads to the incapacity of nutrient providing to the plants and threaten the productivity of agroecological system.4) The pollution of low concentration Pb and Cd stimulates the growth of microorganism, increase the content of microbial biomass carbon in soil. However, as the increase of the high concentration heavy metal, the stimulation effect disappears gradually and begins to show its repressive effect. It indicates that the participation of exogenous low concentration heavy metal promotes the mineralization of soil organic carbon and reduces the stability of soil organic carbon, which is against the immobilization of soil organic carbon. It seems that the effect on the stability of soil organic carbon is complex, and it contains the dual effect of heavy metals on the effectiveness of carbon and microbial activity.5) The amount of microorganism in loess soil is bigger than that in red soil while the quality of microorganism in red soil is better. The size of population quantity in soil: bacterium>actinomycetes>fungus. The quantity of bacterium is the main control factor and it also occupies the largest proportion. After adding the exogenous heavy metals in soil, the quantities of the three sorts of microorganism all show the downtrend, the level of their sensibilities: actinomycetes>bacterium>fungus. It indicates that heavy metals has the inhibiting effect on the soil microorganism.6) In the condition of heavy metals pollution, the quantity of fungus changes the least, which indicates that the fungus can endure the high concentration heavy metals to grow. Meanwhile, the ratio of the fungus in soil has increased indicating that because of the heavy metal pollution the quality of soil has degenerated.7) The response of soil urease and catalase activity to heavy metals had a big difference in different season. Pb pollution promoted the activity of urease and catalase activity to a certian extent in summer; while there was an inhibition in winter. Catalase was more sensitive to the pollution of lead in summer, and Urease was more sensitive than catalase in winter.8) But the change trends of the soil urease and catalase activity in Pb pollution of different soils were not completely similar, and the urease activity in alkaline soil was significantly higher than that of acid red soil. Soil urease and catalase activity can be used to token the heavy pollution of soil environmental quality.