Occurrence form of cadmium in soil aggregates in cadmium-contaminated brown calcareous soil parent material paddy field in karst area
[Objective]The present paper aimed to study the occurrence form of cadmium(Cd)accumulation in soil aggregates of Cd-contami-nated brown calcareous soil matrices of paddy fields in the Karst area of northwestern Guangxi,and to provide data reference for safe rice pro-duction in this type of Cd-contaminated paddy fields in the region.[Method]The sampled soil aggregates were separated into five aggregates as＞2.000 mm,1.000-2.000 mm,0.050-1.000 mm,0.074-0.250 mm and＜0.074 mm,the composition of soil aggregates parti-cle size,Cd content of different particle size aggregates and their main occurrence patterns from 0-20,20-30,30-50 and 50-80 cm depth profiles in the Karst area of northwest Guangxi were studied.[Result]The proportion of 0.250-1.000 mm size aggregates in each pro-file was stable,above 30.00％,with no significant difference between profiles(P＞0.05),the proportion of＞2.000 mm size aggregates in the 0-20 cm soil profile(tillage layer)was the largest(40.20％),and as the depth of the profile increased,the proportion of＞2.000 mm size aggregates decreased rapidly,and the proportion of＜0.250 mm size aggregates showed a gradual increasing trend.Analysis of the Cd content of agglomerates of different particle sizes showed that,in 0-20,20-30 and 30-50 cm soil profiles,Cd was mainly enriched in＞2.000 mm particle size aggregates,with contents as high as 2.22,1.66 and 0.95 mg/kg,respectively,and in the 50-80 cm soil profile,Cd was mainly enriched in 1.000-2.000 mm particle size aggregates.＞2.000 mm particle size aggregates contributed most to the whole soil Cd content in the 0-30 cm soil layer.The occurrence form of Cd in the aggregates at different profile depths showed obvious differ-ences,the proportion of Cd in aggregates of all particle sizes in the 0-20 cm profile was mainly in the organic matter binding state,with the smallest proportion of Cd in the exchangeable state(except for＜0.074 mm particle size aggregates),and the proportion of Cd in the ex-changeable state in the aggregates increased greatly with the increase of the depth of the profile,when the profile depth was increased to un-der 30 cm,the Cd in the aggregates mainly existed in carbonate bound state,and with minimal content in the residue lattice state.[Conclu-sion]In the typical Cd-contaminated brown calcareous soil matrices of paddy fields in the Karst area of northwestern Guangxi,the Cd is main-ly enriched in aggregates of＞2.000 mm and 1.000-2.000 mm particle size,the Cd in the aggregates of each particle size in the 0-20 cm tillage soil is mainly in the form of organic matter bound state,and with the smallest proportion in the form of exchangeable state.With the increase of the profile depth,the proportion of exchangeable Cd increases greatly,while the proportion of Cd in the stable form decreases sig-nificantly.Therefore,in the actual production process,it should focus on improving the content of large particle size soil aggregates and avoi-ding excessive intensive ploughing and deep tilling,reducing the exchangeable state Cd content in the ploughed soil,and ultimately reducing the risk of excessive Cd in rice during rice production.
Cadmium contaminationBrown calcareous soilPaddy fieldSoil aggregatesCadmium occurrence form
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