Effect Of Decay Process Of Wheat Straw On Cd Solubility And Its Relation With DOC In Flooded Soil

With the rapid development of economy and modernization, especially industry and traffic system, heavy metal pollution in agricultural field has become an increasing threat to food safety in China. Of the factors affecting the bioavailability of heavy metals in polluted soil, soil organic matter (SOM) can change the lability of heavy metals by affecting their behaviors including transformation and migration in soil. In China, high yielding crops produce massive straws that may cause environmental problems if not properly treated. Currently, direct incorporation of crop straws into field has been widely adopted as an important measure to elevate soil fertility and mitigate environmental pressure from open-field burning of crop straws. However, massive input of crop straws to heavy metal polluted soil may cause the increase of lability of heavy metals. As an important environmental substance, dissolved organic carbon (DOC) is the most active part of soil organic matter and can affect transformation, migration and bioavailability of heavy metals in polluted soil. The intermediate organic substances produced during the decay process of incorporated straws is the main source of soil DOC. However, information of the effect of straw incorporation on heavy metals lability in polluted soil and its relation with soil DOC is currently in scarce. With wheat straw and simulated Cd-polluted soil as materials, an incubation experiment was conduct in this study. Different rates of wheat straw and N fertilizer were added to Cd-polluted soil, then the soil mixture was incubated at 30℃ under flooded condition. Electric conductivity (EC), pH and concentration of DOC Cd in soil solution, and Cd fractions in soil solid part were measured at different date of incubation to analyze the effect of decay process of wheat straw on Cd solubility and its relation with DOC in flooded soil. Results obtained showed as following:(1) Addition of straw and N significantly elevated EC value in soil solution. With the increase of adding rates of straw and N, EC of soil solution increased, with average increase percentages of 20.3% and 20.9% over CK with no straw or N addition, respectively. This indicated more electrolytes entered soil solution during the decay process of wheat straw.(2) Cd concentration in soil solution was significantly elevated by the addition of wheat straw and N, and this happened at all sampling dates. The average increase rates (over CK) of Cd concentration in soil solution caused by straw and N addition reached 53.7% and 38.9%, respectively. This indicated that straw incorporation enhanced Cd solubility in Cd-polluted soil.(3) During incubation, DOC concentration in soil solution increased in early stage then decreased, and the peak concentration appeared at 40d. Compared with CK, straw and N addition significantly increased DOC level in soil solution, with the average rates of 43.9% and 17.7%, respectively.(4) Under the condition of wheat straw addition, Cd concentration in soil solution was positively correlated with DOC concentration in the solution. The regression equation was: y=0.112x-3.776(r=0.586**, n=90), which indicated that a 100mg/L increase of DOC could cause 11.2μg/L increase of Cd in soil solution. Further analysis showed that the correlation between Cd and DOC concentration in soil solution was closer during late stages of incubation.(5) With the increase of straw addition, soil available Cd decreased. Solution Cd was negatively correlated with available Cd (r=-0.612**, n=90), which indicated that part of soil available Cd released to soil solution under straw addition.(6) Of four fractions of soil Cd, residual Cd tended to decrease and acid-extractable Cd tended to increase with incubation time under straw addition, but no apparent change of reducible Cd and oxidizable Cd was observed. Correlation analysis indicated that Cd concentration in soil solution was significantly negatively correlated with residual Cd and significantly positively correlated with acid-extractable Cd. The correlation between solution Cd and oxidizable Cd was relatively weak, and solution Cd was not apparently related with reducible Cd.