| The dissertation is focused on the geochemical properties of the selenium(Se)-enriched soils in Hubei Province,the most famous Se-rich province in China.Based on the large database obtained through a multi-objective geochemical survey by China Geological Survey,and in the light of eco-geochemical theories,the dissertation studied systematically features of geochemical indexes,pedogenesis of different types of Se-rich soils and ways of utilization of the Se-rich soil resources,by means of statistic analysis of the large database,isotope tracing technique,and indoor and field cultivation experiments.The research can hopefully provide a scientific solid background and theoretical guidance to promote a healthy development of the regional Se-rich agriculture.The main understanding and conclusions obtained are summarized as follows:(1)Soils in Hubei Province were classified into four zones and ten divisions by synthesizing their geological and geochemical characteristics.According to the geochemical properties of the soils obtained through a detailed geochemical survey,Hubei Province was initially divided by using SOM neural network,K-means clustering algorithm,and other big data technologies.Then,soils in Hubei Province were furtherclassified into four zones and ten divisions by combining their geological and geochemical features.The four zones and ten divisions include QinlingDabie orogenic belt(zone A),Nanxiang-Jianghan basin Holocene geochemical subzone(zone B),peripheral plain Pleistocene geochemical subzone(zone C),Yangtze cratonic sedimentary geochemical zone(zone D),Yangtze River alluvial zone division(division I),Hanjiang River alluvial zone division(division II),piedmont pluvial-alluvial division(division III),upper Mesozoic-Cenozoic continental sedimentary parent material division(division IV),Jingmen-Jingshan marine sedimentary parent material division(division V),marine sedimentary parent material division in southwest Hubei(division VI),marine sedimentary parent material division in Southeast Hubei(division VII),Neoproterozoic metamorphic rock series parent material division(division VIII),Paleoproterozoic metamorphic rock series division(division IX),and granite series parent material division(division X).(2)Evaluation of relative contributions of materials to the soil of Jianghan Plain by the Yangtze River and the Hanjiang RiverBy using soil lead isotope tracing technique,the contribution rate of sources of the Enshi-Yangtze River and the Shiyan-Hanjiang River to Jianghan Plain were estimated,being about 18.7% and about 81.3%,respectively.It was therefore assumed that the contribution of the Enshi area to the source of the Se-rich soil in Jianghan Plain could not be too high(<18%).In contrast,the alluvial deposits of the Hanjiang river and the distal materials of the upper reaches of the Yangtze River seems to contribute more to the Serich soil of Jianghan Plain.This result was achieved on the premise of ignoring the contribution of the circumferential soil parent materials and hence certain errors should be enevitable,but the relative contribution of the Yangtze River(especially the Enshi area)-Hanjiang material sources to Jianghan Plain was initially confirmed,which imply that the Hanjiang River contributed more to the Se enrichment of Jianghan Plain than the Enshi area.(3)Comprehensive analysis of the Se source in soils in Hubei provinceThe Se source in the investigated area was first analyzed by using factor analysis of the concentrations of 54 geochemical indexes analyzed in each of the surface soil samples from Hubei province.A multi-fractal analysis was then conducted to peer into the Se enrichment conditions in the surface and deep soilof each zone.The results revealed that Se in the soil of divisions I and II in zone C originated from the weathering,transportation,and deposition of the parent material originated from black rock series in the upper reaches of the Yangtze River and the Hanjiang River,characterized by strong surface enrichment and good uniformity.Divisions I and II were favorable areas for the formation of Se-rich soil in Hubei province.Besides,there exist still some differences in the two divisions,i.e.,Se in division II had the characteristics of upstream parent material,and that in division I liked to have underwent a secondary organismal alteration.While Se in the soil of divisions VI and VII in zone D were mainly originated from the weathering of the Permian carbonaceous siliceous rocks.Besides,secondary enrichment due to the physicochemical action was found in some areas.Divisions VI and VII were also favorable areas for the formation of Se-rich soil.(4)Systematic classification and optimization of typical Se-rich areas in Hubei provinceBased on the Se source in the soil,conditions for the Se enrichment,and the Se enrichment status in the soils,two major genetic types of the Se enrichment and four typical Se-rich divisions were delineated in Hubei province.The twogenetic types are:secondary enrichment after protolith weathering,which was mainly found in the area of Jianghan Plain and corresponded to the geochemical division I-II,and weathering of the sedimentary parent material,which can be mainly found in the Enshi area and the Huangshi area,representative,andcorresponded to thegeochemical division VI-VII.(5)Exploring of new assessment methods of Se-rich soilsThe probabilities of Se enrichment and cadmium(Cd)exceedance in rice in typical Se-rich areas were predicted by neural networks,using concentrations of Se and heavy metals in rice and its root soil as seeds of training to setup a model which was later applied to regional soil analysis data as an input layer.This method could supplement the existing evaluation of Se-rich soil.The results showed that the probability of Se enrichment in rice was significantly higher in the Enshi area(division VI)than in Jianghan Plain(divisions I+II).It suggested a higher potential of the Enshi area for producing Se-rich rice in Serich soil.The probability of Se enrichment in rice could be used as a supplementary index for the evaluation of Se-rich soil,while the probability of Cd exceedance in rice represents the safety of probable rice Cd content growing in the soil environment in Jianghan Plain(divisions I + II)as a standard,the soil environmental quality in the Enshi area(division VI)was calibrated and converted.It was found that the clean soil in the Enshi area accounted for 88.09% in terms of soil environmental quality.(6)Measures of growing crops of Se enrichment and simultaneous Cd reduction were proposed for safe utilization of Se-rich soil in the Enshi areaAfter the addition of three ameliorants(lime,biomass charcoal,and hydroxyapatite),the bioavailable Se content in the soil was obviously increased,while the bioavailable Cd content was reduced.The orthogonal test results revealed that the three amendmentsadded at ratios of 1:1:2,1:2:1,and 1:1:1 improved the soil quality.The bioavailable Se content rose by 121.0-133.1%,whereas the Cd content declined furthest(by more than 90%)when the three amendments wereadded at a ratio of 2:1:1.Besides,itwas discovered through cultivation experiments that the addition of the three amendments significantly increased the fresh weight of pakchoi and improved the pakchoi growth in the Se-and Cd-rich soil.Firstly,the bioavailable Cd content in the soil decreased after adding amendments,reducing the Cd absorption by pakchoi.Secondly,the bioavailable Se content in the soil increased by 28.3-39.4% after adding amendments,promoting the Se absorption by pakchoi leaves.Thirdly,the addition of amendments also enhanced the ability of pakchoi to resist oxidative stress.Specifically,after the addition of amendments,the content of GSH and As A in pak choi leavesincreased to a certain extent.The rise in the content of chlorophyll a,chlorophyll b,and carotenoid in pakchoi leaves was also observed.(7)Ways of development and utilization of Se-rich cold waterlogged paddy soils in Jianghan Plain was discussedThe bioavailable Se content in the cold waterlogged paddy soils could be increased by three ways of soil utilization(drainage,supplementation of a Se fertilizer,and supplementation of a Se fertilizer after drainage).This was beneficial for the Se uptake of Se in the soil by rice,significantly increasing the Se content in rice seeds.The supplementation of a Se fertilizer after drainage of cold waterlogged paddy soils exhibited the best effects.The Se content in rice roots and seeds increasedby the drainage mode.The supplementation of a Se fertilizer significantly increased the total Se content,the available Se content,the potentially available Se content in the soil,and the Se content in rice.The bioavailable Se content in the soil and the Se content in rice roots,stems,and seeds exhibited the largest increases by the supplementation of a Se fertilizer after drainage of the cold waterlogged paddy soils,increasing by 50.0,41.8,46.1,and 47.9%,respectively,compared with the control group. |
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