Effects Of Three Cropping Patterns On Cd Accumulation Of Soil-Crop In Farmland Under Straw Removal/Return Treatment For Three Consecutive Years

As a big agricultural producer,China produces about 700 million tons of straw every year.Crop straw contains a large number of nutrients needed by soil and crop,so straw fertilizer utilization is the most extensive and the main treatment is returning to the field.With the rapid development of industry and the extensive use of pesticides and fertilizers in recent years,the situation of heavy metal pollution in China’s farmland is becoming more and more serious.Among the common heavy metals,Cadmium is harmful to crops and human health due to its high activity.The ground part will absorb and accumulate Cd when crop growth,and the crop straw will accumulate more Cd due to the biomass assembly.The results showed that straw could affect soil physical and chemical properties in the process of decomposition,thus affecting the migration,transformation,absorption and enrichment of Cd in soil-crop system.Therefore,if straw comes from Cd-contaminated farmland,which will increase the risk of Cd pollution in farmland soil with the increase of returning years?On the contrary,if the Cd-contaminated straw is removed from the field,which can reduce the risk of Cd damage to farmland soil and crops with the increase of the times of removal?In this paper,we take two moderately Cd-contaminated farmland of liuyang and Zhuzhou as the research object in Hunan Province,which to explore whether it is good or bad to remove and return Cd-contaminated straw to the field.Rice and rape straw are two of the most important biomass energy in Hunan province.We selected three conventional planting patterns of rice-oilseed rape,double cropping rice and single cropping rice in Hunan province to explore the effects of straw removal and returning treatment on Cd accumulation in soil and crops in Cd contaminated areas for three consecutive years.The main results are as follows:（1）After three consecutive three years of straw removal and return treatment,the biomass of different parts of crops in liuyang and Zhuzhou test sites showed no significant difference in different years under three planting modes;In addition,three consecutive years of straw removal from the field did not significantly reduce soil organic matter.（2）Under the condition of straw removal from the field,the Cd content in all parts of the crops in the three planting modes was lower than that in the returning treatment,especially in the root and straw.Straw returning increased Cd content in various parts of crops.At the same time,the biological concentration factor of Cd in the roots and straws of all crops was significantly higher under straw return than removal.（3）After straw removal from field,soil Eh,DOC and CEC decreased and were significantly positively correlated with soil available Cd.In liuyang and Zhuzhou,the available Cd content decreased by 12.0%、5.6%、5.3%and 6.3%、7.8%、4.9%under rice-oilseed rape planting mode,respectively.Soil available Cd content decreased by8.4%、4.1%、13.3%and 15.6%、16.7%、11.8%after single cropping rice planting,respectively.There was no significant change in soil available Cd under double cropping rice pattern.On the contrary,after three years of straw returning,the soil available Cd increased significantly by 44.3%and 34.6%,respectively.Rice-oilseed rape and single cropping rice modes could significantly reduce soil total Cd after three consecutive years of straw removal.In Liuyang and Zhuzhou,soil total Cd decreased by 28.3%、13.5%and 24.8%、15.9%,respectively.（4）Straw removal is one of the main output ways of Cd.After straw removal for three consecutive years,23.0 g·ha^-1、32.0 g·ha^-1、32.3 g·ha^-1and 29.8 g·ha^-1、34.0 g·ha^-1、42.2 g·ha^-1of Cd could be removed from soil in liuyang and Zhuzhou under three planting modes,respectively.Due to different crop biomass and Cd accumulation in different years,the theoretical phytoremediation efficiency of Cd-contaminated farmland soil was different among the three planting modes.Through three years of research,it was found that it was feasible to use conventional straw removal treatment to restore Cd contaminated farmland,and the rice-oilseed rape and single-cropping rice model was better than the double-cropping rice model.