The Distribution And Ecological Risk Assessment Of Heavy Metals And Rare Earth Elements In Soils From The Earth's Critical Zone Observation

The Earth's Critical Zone Observation?CZO?is the interaction region between rock,soil,organism,water and atmosphere,which is of great significance to the ecological environment survival and sustainable development.Soil is usually studied as the core of CZO,which interacts closely with rock,water,biology and atmosphere.The accumulation of hazardous elemtents in soil has posed a serious potential threat to the ecological environment and human health under the influence of global climate change and human activities.Chenqi catchment in Puding,Guizhou and Jiulongjiang River catchment in Fujian,China were selected as the representative observation to study the effects of natural processes and human activities on element cycle in the CZO.Chenqin catchment in Puding dominated by carbonate rocks is characteristic of highly fragile environments with barren soil layer,and the ecological environment is mainly affected by agricultural activities.However,the ecological environment in Jiulongjiang River catchment,which is dominated by carbonate and silicate rocks,is more extensively influenced by human activities,such as grazing,poultry farming,excessive application of chemical fertilizers,and unreasonable mining.This dissertation demonstrates the distribution,migration,and influencing factors of heavy metals and rare earth elements in soils from these two catchments,as well as assesses the ecological risk.The main results and conclusions are summarized as follows:?1?The results of soil physicochemical properties showed soil organic carbon contents in soils of Chenqin catchment in Puding(4.6–138.8 g kg^-1)were much higher than those in Jiulongjiang River catchment(1.2–14.4 g kg^-1).The abundant organic matter in Puding soils was more conducive to the enrichment of Cd and Pb;however,organic matter increased the mobility of Cr,Fe,Ni and rare earth elements?REEs?,which was not beneficial for the enrichment of these heavy metals.The higher contents of clay particle and iron oxide mineral was conducive to the enrichment of REEs in soils of these two catchments.?2?The heavy metal contents in topsoils under different land uses in Puding decreased as followed an order of cropland>shrub land>abandoned cropland>grassland>secondary forest,and an order of abandoned tea plantation>forest in Jiulongjiang River catchment.The may be related to agricultural inputs.The vertical distribution of heavy metal contents in soil profiles showed that most heavy metals were enriched in topsoils from the two catchments.Their contents decreased with depth at0–30 cm;however,the changes were not significant in soils below 30 cm.It implied that heavy metals accumulation in topsoils was closely related to atmospheric deposition and agricultural activities in the two catchments,while the distributions of heavy metals in the deep soils were mainly associated with the geological background of catchments.?3?The contents and distribution patterns of rare earth elements showed?50?REEs in topsoils under cropland and abandoned cropland(231–248 mg kg^-1)were higher than those in secondary forest,grassland,and shrub land(145–223 mg kg^-1)in Puding.?50?REEs in the topsoils of abandoned tea plantation(92–103 mg kg^-1)were higher than those in forest land(41–67 mg kg^-1)in Jiulongjiang River catchment.It may be related to the inputs of REEs due to agricultural activities.The REEs distribution patterns along soil profile under different land uses in Puding were similar,and it showed middle REEs significant fractionation relative to the light and heavy REEs,moreover,REEs fractionations in cropland and abandoned cropland soils were more significant than those in secondary forest,grassland and shrub land.The fractionations of REEs in different soil profiles of Jiulongjiang River catchment were various,which were mainly due to the different parent rocks.By comparing?50?REEs in vertical soil profiles between these two catchments,it was found that?50?REEs contents generally increased as profile depth from top to bottom,which was mainly associated with the continuous leaching in soil formation process,while?50?REEs decreased firstly and then increased in some soil profiles from top to bottom,which was influenced by REEs inputs and leaching in soils.?4?The results of enrichment factor,geoaccumulation index analyses and potential ecological risk analyses indicated most heavy metals in soils from the two catchments were low ecological risk;however,Cd posed a moderate or strong ecological risk in different land-use soils.Rare earth elements in soils from the two catchments were not polluted by human activities,and they were mainly natural sources.On the whole,it showed low ecological risk in Puding soils and undisturbed soils?forest land?in Jiulongjiang River catchment,while it exhibited moderate ecological risk in abandoned tea plantation in Jiulongjiang River catchment.