Effects Of Organic Materials On The Particulate Organic Matter And Pytoavailability Of Cadmium Within Purple Paddy Soil

Organic matter is the core of soil quality and function, and it is also one of the most important soil properties which affect the transformation and bioavailability of heavy metals in soil. The effect of organic matter on the transformation behavior of heavy metals in soils is both related to its content and composition, especially with particulate organic matter(POM) in soil. POM is soil organic matter which is more than 0.053 mm in soil, it is the active intermediate product of the transformation from the residue of plants and animals to the soil humus, and a kind of organic carbon pool with low degree humification and high activity. Recent studies show that POM is significant in the enrichment of heavy metals, while it is not fully clarified for the relationship between heavy metals in POM and the absorption of heavy metals by plants.Cadmium is the most common heavy metal element in farmland pollution and rice is a kind of crops with strong ability of cadmium accumulation. Organic material is one of the important factors that affect the soil POM and it is also a commonly used repairing agent for heavy metal contaminated soil. In this study, a pot experiment was utilized to study the effects of chemical fertilizer(CF), pig manure biogas residue(PMBR), cattle manure biogas residue(CMBR), compost of straw and cattle manure(CSCM), compost of sludge CS and farmyard manure(FYM) on the change of p H, organic matter content and composition of acidic purple paddy soils(APPS) and calcareous purplish paddy soil(CPPS) in order to investigate the effect of POM on Cd absorbtion in rice under the application of organic matter. The bioavailability of Cd in soil-rice system and the distribution characteristics of organic matter and Cd content in different particle size(including > 0.05 mm POM, > 0.05 mm sand, < 0.05 mm fine soil) were analized. In addition, the effects of POM and its Cd content on the absorption of Cd in rice were discussed,the conclusions obtained are as follows:(1)Compared with CF, except for CMBR treatment, the p H of APPS were significantly increased, while there were no significant change in CPPS. p H were the highest under CSCM and CS treatments in two soils.(2)Effects of different organic materials on soil organic matter contents and compositions were also different. Compared with CF, organic materials significantly increased the organic matter contents of APPS and CPPS, the lifting range were 11.72-27.15% and 8.49-19.38%, respectively. CMBR and PMBR treatments had the most obvious effect in APPS, while CS and CMBR treatments had the most obvious effect in CPPS. During the whole incubation period of rice, content of DOC in each treatment fluctuated and reached the highest at heading stage, the average content of DOC in soil solution was also in an upward trend. The particle size composition of two soils were mainly composed of fine soil, POM component was the lowest, with a proportion of only 2.03-3.58%. Organic matter contents were( > 0.05 mm POM) >( < 0.05 mm fine soil) >( > 0.05 mm sand),the proportion of POM to total organic matter were 31.76-44.70% and 24.86-33.55% in APPS and CPPS, respectively. Compared with CF, organic materials could increase the content of POM in APPS and CPPS with the lifting range of 52.50% and 46.53%, respectively. Organic materials also increased the contribution rates of POM component to organic matter, which increased by 40.74% and 34.96%, respectively.(3)Effects of organic materials on soil Cd content was different according to different soil types. Compared with CF, organic materials application had no significant effect on total Cd in APPS. In CPPS, most organic material treatments decreased the total Cd significantly by 12.26-17.42% except for FYM treatment had no significant change. Compared with CF, CMBR, CSCM, CS and FYM treatments reduced the available Cd significantly in APPS, while no significant change under PMBR treatment; there was also no significant change in CPPS. Correlation analysis showed that POM was significantly negative correlated with available Cd content in CPPS, while a not significant negative correlation in APPS. Indicating that POM was effective to Cd fixation and the effect was stronger in CPPS than APPS, which should be related to the interaction of various factors in soils.(4)The Cd contents and enrichment ability of different size compositions in two soils were both( > 0.05 mm POM) >( < 0.05 mm fine soil) >( > 0.05 mm sand), mainly distributed with fine soil. The POM component had a strong enrichment on Cd, which was up to 5.40 and was significantly higher in APPS than that of CPPS. Effects of organic materials on Cd enrichment coefficients and distribution proportions in POM were relate to organic materials and soil types. Compared with APPS, organic materials improved the enrichment coefficients and distribution proportions in POM of CPPs in a larger degree. In addition, we could found that the changes Cd content in respond to soil organic matter content in APPS(k = 0.017) were more sensitive than that of CPPS(k = 0.009).(5)Organic materials inhibited the absorption of Cd in rice organs in APPS and reduced the Cd content in rice root in CPPS, while PMBR and CSCM treatment promoted the Cd absorption in stems and leaves, CMBR and FYM treatments increased the content of Cd in grain in CPPS, the results showed that the effect of organic materials on Cd uptake of rice varied with the soil. Cd in root and straw were negatively correlated with POM and Cd content in POM component, while Cd in grain was positively correlated with them in this two soils. The correlation coefficients of CPPS were higher than that of APPS. It showed that POM and its Cd content could inhibit the absorption of Cd in root and stem, while promote the absorption of Cd in grain, these 2 kinds of effects were more obvious in CPPS, it might be related to the interaction of other factors in soils.