| China is the world’s largest rice producer with 27%of cropland area being used to cultivate rice.However,the contamination of Cd,Pb and As has become a serious problem in Chinese rice paddy soils.The objective of this experiment was to study the effects of biochar amendment on soil physicochemical properties,heavy metal concentration and microbial biomass in the red paddy soil from Changsha county,Hunan province.Biochar was applied once at rates of 0,10,20,30,and 40 t/hm2(marked as CK,A10,A20,A30 and A40,respectively)and the topsoil samples(0-17 cm)were collected in September 2017 and October 2018.Path analysis was used to study the direct and indirect effect between soil factors and soil DTPA extraction Cd content,and the soil integrated fertility index(IFI)was used to assess the impact of biochar application on soil fertility in heavy metals polluted red paddy soil so as to clarify the feasible application rate of biochar,providing a scientific basis and support for heavy metal remediation.The main results were shown as follows:(1)Biochar amendment could improve the physical properties of red paddy soil.Application of biochar could not only improve the acidity of the red soil of paddy soil,reduce soil bulk density,increase soil porosity,but also improve the soil water-stable aggregates distribution,large aperture water-stable aggregates content,whereas reduce the<0.25 mm micro water-stable aggregates,the soil aggregates distribution,especially in 40 t/hm2 treatments.(2)Biochar amendment could improve the nutrient status in red paddy soil.The application of biochar can improve the content of soil organic matter and cation exchange capacity(CEC),nitrogen content and phosphorus level of red paddy soil.The concentration of DTPA extractable Cd and Pb were significantly decreased with the increasing amount of biochar added,whereas no significant effect in DTPA extractable As was observed among treatments.In addition,biochar amendment could also increase the yield by 35.54%in 2017 and 23.99%in 2018 and 2018,respectively.(3)Biochar amendment could significantly improve the soil microbial biomass carbon(MBC),soil microbial biomass nitrogen(MBN)and soil breathe intensity,but there is no significant difference between biochar application rate and soil breathe intensity.In addition,biochar amendment could improve soil enzyme activity,such as soil fluorescein diacetate hydrolase(soil FDA hydrolase),urease,sucrase,acid phosphatase,catalase and polyphenol oxidase,and also improved the synthase activity in red paddy soil.(4)An indoor simulation experiment showed that microbial biomass and sucrase activity increased at first,and then decreased with the addition of exogenous Cd;and soil catalase,dehydrogenase and urease activities significantly decreased with the increasing of exogenous Cd concentration.Path analysis showed that soil microbial biomass carbon,nitrogen,catalase,urease and dehydrogenase had a highly significant negative correlation with the addition of exogenous Cd(p<0.01),but sucrase had a highly significant positive correlation with the addition of exogenous Cd(p<0.01).Among them,Cd stress concentration had the highest correlation with dehydrogenase,which could be used to predict the degree of Cd pollution in red loamy paddy soil in the study area,and the direct path coefficient of microbial biomass carbon was higher than its indirect path coefficient.(5)Based on correlation and path analysis,the DTPA extractable Cd had the highest correlation with soil available phosphorus.Soil porosity,organic matter,available phosphorus and urease activity had the high direct impact on the DTPA extractable Cd,the soil bulk density,pH,porosity and CEC had the high indirect impact on the DTPA extractable Cd.The IFI showed that the soil fertility composite quality and the soil fertility composite quality among treatments decreased in the order:A30>A40>A20>A10>CK. |
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