The Preliminary Study On Existing Forms Of Cadmium In Organic Fertilizers And Their Bioavailability

The cadmium is not the indispensable chemical element for plant growth, it has a strong living toxicity. So, people pay more attention to environment pollution caused by cadmium. Organic fertilizer is an important fertilizer in agricultural production. The use of organic fertilizer plays an important role in the aspect of offering plants nutrient and keeping soil fertilizer efficiency. Organic fertilizer is regarded as a very safe fertilizer for a long time, in the production of safe agricultural products as no environmental pollution food, green food and organic food, the use of organic fertilizer is even emphasized. However, no pollution of organic fertilizer itself is the premise of the great effect of organic fertilizer. With the development of the social economy and the increasing of environment pressure in recent years, the pollution of organic fertilizer itself is becoming outstanding, the appearance of environmental pollution caused by unsuitable use of organic fertilizer is frequently. Lasting use of organic fertilizer with highly-contained heavy metal makes the heavy metal accumulated in the soil, however, the cadmium has a strong bioavailability in the soil, it is easy for plant to absorb by roots and then get into human food chain. If human beings absorb it, it is bad for healthy. Up to now, the effect of organic fertilizer to the forms and content of originally-existed heavy metal in soil, the accumulation of heavy metal which is taken from outside in soil and plant absorbing lacks further study.This article uses the method of classifying forms, chemistry analysis and potted plant experiment to research the existing form of cadmium in organic fertilizer and bioavailability, initially discovers the form changes pattern of input cadmium in organic fertilizer and the bioavailability of different cadmium forms. In theory, it can prompt and rich the basic research of heavy metal in organic fertilizer; in practice, it can provide a reasonable and safe reference for organic fertilizer production.The tested organic fertilizer contains three typical fertilizer—pig excrement, chicken excrement and merchandise organic fertilizer. The tested soil of potted plant experiment is the neutral and purple soil which is widely distributed in Chongqing, and the tested plant is lettuce. According to the principle of comparison about various organic fertilizer, combination effect of putting input cadmium in organic fertilizer and comparison about different dosage of input cadmium, this potted plant has 29 treatments, which is divided into 3 major groups—adding cadmium directly, adding organic fertilizer containing the same cadmium dosage and adding organic fertilizer with no cadmium, and each group contains five levels. Each treatment is repeated 3 times, this research tries to study the different treatment effect on lettuce growth and cadmium absorbing; taking a sample according to treatment after harvest, and classifying the form of cadmium in the soil. The interior experiment contains two parts—adsorption of cadmium to organic fertilizer and classifying the form of cadmium in the organic fertilizer, and try to explore the adsorption ability of cadmium to different organic fertilizer and the form transform of input cadmium in the organic fertilizer. Cadmium forms classified in organic fertilizer, draws lessons from heavy metal classifying method of soil. And initially classifies the form of cadmium in organic fertilizer into four forms—water-solubility form,acidity-solubility form,alkali-solubility form and remainder form.The experiment on adsorption of cadmium to organic fertilizer shows that, when the content of Cd²⁺ is below 100μg, organic fertilizer can absorb Cd²⁺ nearly 100%, while putting Cd²⁺ as 120μg, the absorbing isotherm inclines gradually, the capacity of absorbing nearly reaches saturation point; The largest absorbing content of the tested pig excrement, chicken excrement and merchandise organic fertilizer is 200.00μg/g, 123.46μg/g and 117.65μg/g.When the input cadmium of water-solubility comes into the organic fertilizer, its forms have transformed. Get water-solubility cadmium of 3mg/g into the organic fertilizer, the proportion order of different forms of cadmium in the three kinds of organic fertilizer is: remainder form (54.77%～60.00%)＞acid-solubility form (19.32%～23.24%)＞water-solubility form (11.62%～14.69%)＞alkali-solubility form (2.45%～10.37%); in the three kinds of organic fertilizer, the content of acid-solubility form in pig excrement is the lowest, it's 0.51mg/g, the content of water-solubility form in chicken excrement is the lowest, it's 0.28mg/g, the content of alkali-solubility form in merchandise organic fertilizer is the lowest, it's 0.06mg/g. The correlation analysis indicates that, the acid-solubility cadmium in the organic fertilizer has the closest relationship with the exchange form of cadmium in the soil, the correlation coefficient is marked, r=0.8549; the water-solubility cadmium in organic fertilizer show significant positive correlation with the same in soil, r=0.8496; at the same time, the water-solubility cadmium and the exchangeable cadmium show significant positive correlation with remainder form of cadmium in organic fertilizer, the r of the two is 0.8299 and 0.8259, it shows that, any form of cadmium in organic fertilizer can transform to the effective form to plant, when it comes into the soil.Different treatment has significant diverse effect on cadmium forms and availability in soil. In the soil of selected treatment, Exch-Cd and Res-Cd have got great proportion, the percent of the two forms is above 50%, the other forms have got smaller proportion compare to them. In the level 3 treatment of organic fertilizer with cadmium (0.3mg input cadmium in 100mg organic fertilizer), the proportion of cadmium forms in soil is: Res-Cd of pig excrement with cadmium 43.78%＞Res-Cd of merchandise organic fertilizer with cadmium 35.19%＞Res-Cd of chicken excrement with cadmium 35.03%; after transforming to balance in different organic fertilizer, the existing forms of the input cadmium with the same total content have changed greatly. Take WS-Cd and Exch-Cd that have great bioavailability for example, the proportion of distribution in different organic fertilizer is: merchandise organic fertilizer 6.67%＞pig excrement 4.42%＞chicken excrement 2.04% and merchandise organic fertilizer 22.96%＞chicken excrement 15.65%＞pig excrement 13.25%, It shows that, the bioavailability of cadmium in different organic fertilizer have significant diversity. In the treatment of pig excrement with input cadmium of different level, as the level of input cadmium increases, the proportion of Exch-Cd in soil increases significantly, from 10.69% to 22.84%, the proportion of WS-Cd increases from 3.44% to 5.77%, but the proportion of Res-Cd shows the trend of decline, reduces from 47.33% to 37.5%. It shows that, as the level of input cadmium increases, much more cadmium has the existing forms of WS-Cd and Exch-Cd that have great availability. The cadmium of adding directly and adding through the organic fertilizer in soil (x,mg/kg) both make available cadmium in soil (y, mg/kg)had linear increase, the relation of them can be described by this equation: y=bx+a, but the available cadmium in soil (b) increased by unit content of adding cadmium differs greatly. Adding cadmium through pig excrement, chicken excrement, the tested organic fertilizer and adding cadmium directly in soil, the content of available cadmium in soil increased by unit content of adding cadmium is: 0.1478, 0.175, 0.301 and 0.3217. It is obvious that, the use of organic fertilizer can restrain the availability of cadmium in soil, and make the content of availability cadmium reduced, the restraint effect of pig excrement is the most significant, the reduced extent of availability cadmium of their own level is 3.76%～28.10%; compare to the blank contrast, the content of availability cadmium of the organic fertilizer with input cadmium has increased, however it is still lower than the content of availability cadmium of adding cadmium directly with the same cadmium level.Different form of cadmium in soil has different effect on cadmium accumulation in plant. Exchangeable cadmium has the greatest effect on plant's accumulation of cadmium.The correlation coefficient of cadmium concentration of the part on the ground, the part under the ground and Exch-Cd is 0.698**～0.713** (n=29), which shows very significant positive correlation. The water-solubility cadmium and the cadmium bond to carbonate show significant positive correlation with cadmium concentration, of plant, and correlation coefficient is 0.391*～0.405* and 0.372*～0.389* (n=29). The cadmium strictly bond to organic matter and residual cadmium have negative correlation with plant's uptake and the correlation coefficient is -0.412*～-0.448* (n=29), which reaches to the significant level. And the cadmium weakly bond to organic matter and the cadmium bond to Mn oxides did not. The available cadmium extracted by the lmol/L ammonium acetate solutions is consistent with plant's uptake, and can reflect the bioavailability of cadmium. The correlation coefficient of available cadmium and cadmium content in two parts of the plant is 0.6641** and 0.6819** (r_0.01=0.4785 n=28), which show significant correlation.Different treatment has different effect on plants production, when the cadmium level of different treatment is 0.3mg: adding input cadmium in the soil directly leads to the decline of lettuce biomass, each plant reduces 2.16g compare to the contrast, reduces 45.38%; adding 3 kinds of organic fertilizer sepemtely can increase lettuce biomass, increased by 7.56% with pig excrement, increased by 4.41% with chicken excrement and increased by 1.47ï¼…with merchandise organic fertilizer, but the different organic fertilizer can cause different increase extent of pig excrement＞chicken excrement＞merchandise organic fertilizer; the treatment of organic fertilizer with cadmium all can reduce the lettuce biomass compare to the treatment of organic fertilizer with no cadmium, reduced by 21.09% with pig excrement, reduced by 36.82% with chicken excrement and reduced by 43.06% with merchandise organic fertilizer, however it is still higher than the way of adding input cadmium with the same level directly, at the same time, input cadmium begins to restrain lettuce growth strongly, it covers the adjusting and controlling effect of organic fertilizer to cadmium toxicity. The result has consistency with the effect of organic fertilizer on cadmium forms and availability in soil.