Effects Of Exogenous Water-soluble Organic Matter On The Occurrence Of Cadmium In Paddy Soils And Accumulation Of Cadmium On Rice

Water-soluble organic matter(WSOM)was one of the most active components in environment,which can influence the forms of heavy metals and vary physical-chemical properties of soil.Rice absorbed and accumulated cadmium easily,which was harmful for the growth of rice and produced rice with cadmium in a degree to threat human health.It is significant to study the effects of WSOM on the occurrence forms of cadmium in soil,which have a importance to control cadmium pollution in fields and ensure rice quality and safety.A indoor pot experiment was conducted in this study.Three water-soluble organic matter of which P-WSOM was extracted from peat,R-WSOM was extracted from rice straw and F-WSOM was extracted from fertilizer were added to the soils with different cadmium concentrations after transplanting rice for 15 days,respectively.Soil,rice roots and rice plants were collected continuously during the whole growth cycle of rice.And husks and grain rice were collected at harvest stage.Available cadmium contents in soil and total cadmium contents of plant biological samples were determined.The effect of WSOM on available cadmium transported in paddy soil,rice growth and development and the differences in uptake and accumulation of cadmium in rice tissues and absorption of cadmium in rice were analyzed as to provide theoretical support for the safe production of rice.The main results are as follows:Under different cadmium stress conditions,the effects of three exogenous WSOM on soil available cadmium were varying.At low,middle and high levels of cadmium stress,the content of available cadmium in soils decreased with time after the addition of P-WSOM and reached the minimum value at the harvest period of rice.Under the R-WSOM and F-WSOM,the available cadmium in soil showed different changing trends with time,especially,the content of available cadmium decreased and then increased with time in blank soils under the R-WSOM,and the content of available cadmium decreased continuously with time under the F-WSOM,and the content of available cadmium decreased and then was increasing under the F-WSOM and R-WSOM when the cadmium stress was 0.5 mg/kg.The available cadmium content of soil remained constant almost with time under the influence of R-WSOM and displayed decreased continuously with time when the cadmium stress level was 5 mg/kg under the action of F-WSOM.When cadmium stress level was 10 mg/kg,the content of available cadmium did not change significantly at R-WSOM and the available cadmium content in rice soil decreased with time under the action of F-WSOM.The content of available cadmium in soil increased with the increase of cadmium stress concentration under the addition of WSOM.The content of available cadmium in soil decreased in varying degrees with the addition of exogenous water-soluble organic matter when the same level of cadmium stress level,specially,the decrease of available cadmium content was more obvious with the addition of P-WSOM.Under different cadmium stress levels,the effects of three exogenous WSOM on the growth and development of rice were different,but with the increase of cadmium stress concentration,the growth and development of rice were inhibited to varying degrees,which showed as plant height,number of tillers and ear length was decreased.the maximum plant height and ear height were obtained under the action of P-WSOM in blank soil,and the number of tillers is the most under the F-WSOM.When the cadmium stress was 0.5 mg/kg,plant height of rice reached the maximum under the F-WSOM,and the maximum number of tillers and the maximum ear length were obtained after P-WSOM treatment.When the the cadmium stress was 5 mg/kg,the plant height,ear height and tiller number of rice were the largest by treating with F-WSOM.the plant length was the most when the amount of soil stress was 10 mg/kg under the P-WSOM,and the maximum number of tillers and ear height of rice reached with F-WSOM treatment.Under low concentration cadmium stress,P-WSOM had the best effect on alleviating cadmium stress according to plant height×tiller number.Under middle and high concentration cadmium stress,the effect of F-WSOM was the best on reduce Cd stress in rice was the best using plant height×tiller number as evaluation index.Rice plant height was the least affected under the action of P-WSOM and under the action of R-WSOM,rice plant height was the most affected.The number of rice tillers was the least affected under the action of F-WSOM and the effect of rice tillers was the greatest under the action of R-WSOM.Under the action of F-WSOM,the panicle length of rice was the least affected,and the effect of panicle length on rice was the biggest under the action of P-WSOM.Under different cadmium stress levels,exogenous WSOM could inhibit the adsorption of cadmium by rice roots,but the effects on cadmium accumulation in plants and rice were different.The total cadmium content of rice roots decreased with the growth of rice on the effects of exogenous WSOM in blank soil and under the cadmium stress were 5 mg/kg and 10 mg/kg.In blank soil and Under low and middle concentration Cd stress,the total Cd content of root increased with rice growth in control treament,which indicated that the three exogenous WSOM can change the accumulation trend of cadmium in rice roots.The total cadmium content in the roots of rice was significantly lower than that of the control treament at harvest stage after adding water-soluble organic matter.P-WSOM and F-WSOM can prevent cadmium accumulation in rice plants,but R-WSOM can promote cadmium accumulation in plants.With the growth of rice,the cadmium content in the plants increased gradually on the effect of exogenous WSOM.Both P-WSOM and F-WSOM can inhibit the accumulation of cadmium in rice under different adding concentrations cadmium stress,obviously,the inhibitory effect of P-WSOM was better.The accumulation of cadmium in rice was changed from promoting to inhibiting under R-WSOM with the increase of exogenous cadmium concentration.Compared to the hulls,rice grain is easier to accumulate cadmium,and the trend is more obvious with the increase of cadmium content in paddy soil.In general,root with the most capable of cadmium enrichment,followed is rice plants,rice grains and rice husks.There was a positive correlation between available cadmium and rice cadmium content.There was also a positive correlation between cadmium content in grain hull and rice cadmium content.