Secondary Enrichment,Ecological Risk,Prevention And Control Of Cd And Other Elements In Karstic Soils In Guangxi

Karst environments are globally significant,and the soils that develop on these landscapes often have unique geochemical features,including elevated–levels of metals.And the soil ecological risk has gradually attracted extensive attention.However,it has not been determined whether the enrichment of metals in soils is mainly controlled by parent rocks or secondary enrichment during weathering.The concentrations of Cd and other heavy metals in karstic soils and crops do not fluctuate synchronously.The concentrations of Cd and other heavy metals in some crops including rice cultivated in strict control areas may not exceed the standard and probably cause a waste of massive land resources if the classification management of land resources in karst areas is carried out according to the Chinese Soil Environmental Quality Standard and Soil Pollution and Control Action Plan.In this study,we studied the control of the accumulation of heavy metals in soils by parent materials and/or the process of soil formation.We examined the un–weathered carbonate and clastic bedrocks,surface soils,deep soils,as well as soil profiles in a typical area,where karst and clastic rocks coexist in Guigang,Guangxi.At the same time,1090 sets of rice grains and paired rhizosphere soils were collected as research objects from the outcropped karst areas of carbonate rocks in Guangxi,and the ecological risk of heavy metals were studied.The spatial distribution of heavy metal（Cd,As,Cr,Cu,Hg,Ni,Pb,and Zn）concentrations in soils was mainly controlled by the geological background.Except for Cd,the concentrations of heavy metals in carbonate rocks were much lower than those in clastic rocks,whereas the soils derived from the carbonate rocks were highly enriched with heavy metals than those in clastic area.The enrichment of Cd in surface soils from the carbonate substrates was the result of the dual effect of secondary enrichment and parent rock inheritance,whereas the enrichment of other metals was mainly the result of secondary enrichment.During the weathering process of carbonate bedrock,great majority of the quantities of inherent heavy metals were leached,with only 2%of Cd being retained in situ.The high concentrations of soil Fe,Al,and Mn oxides,organic carbon and clay content（Al₂O₃/Si O₂）were closely related to the enrichment of heavy metals in studied surface soils.Ferromanganese nodules commonly exist in high quantity in karstic soils and thus play a crucial role in the occurrence and bioavailability of heavy metals.They not only promote the enrichment of heavy metals in soils,but also reduce the bioavailability of heavy metals.The concentrations of Cd and other heavy metal elements in the karstic soils were very high.However,the exceedance rates of heavy metals in rice grains were very low.Pb in rice grains from the studied area did not exceed the standard;the exceedance rates of Cr,Hg,and inorganic As were only about 1%;and the exceedance rate of Cd was 12.5%.Soil Cd concentration increased gradually whereas the concentration and exceedance rate of Cd in rice grains decreased gradually with the increase of soil p H.Specifically,the exceedance rate of Cd in rice grains was only 4%when soil p H>6.5.The results indicated that the risk screening values of Cd element in the Chinese Soil Environmental Quality Standard are not satisfactorily applicable to the ecological risk assessment of Cd in karstic soils.Therefore,the recommended threshold values for the safe utilization and risk control of Cd in karstic soils was proposed so as to provide a basis for soil environmental quality hierarchization,pollution risk control and land safety zoning of agricultural land in karst areas in China.