Characteristics of Iron Oxide in Soils Different in Cultivation Age in the Pearl River Delta Plain
[Objective] The Pearl River Delta Plain,located in South China,has a more-than-one-thousand-year cultivation history,so that the soil therein has been subjected to impacts of intensive anthropogenic activities in its evolution.The knowledge of how the soil evolves under anthropogenic activities is sure helpful to sustainable utilization and management of the soil resources.Being highly active and geochemically sensitive,iron oxides in soil tend to be affected by any slight change in environmental condition in formation and property,so they are often used as an indicator of soil pedogenic process,and considered as a function of soil genesis and development.Therefore,characterization of soil iron oxides has been one of the major fields of the study on soil genesis.In this study an investigation was performed of characteristics of iron oxides in bulk soil and soil clay relative to cultivation age and affecting factors in the Pearl River Delta Plain,in an attempt to reveal how the soil forms and evolves under anthropogenic cultivation in the region.[Method] Samples were collected from genetic horizons of the soils derived from marine deposits with cultivation age being 30 a,100 a,600 a and 1 000 a,separately,of the soils derived from fluvial deposits with cultivation age being 200 a,400 a,900 a and 1 000 a separately and of the soils derived from delta deposits with cultivation age being 50 a,90 a,150 a and 350 a,separately,in the Pearl River Delta Plain for analysis of physicochemical properties,and total iron (Fe,),free iron (DCB-extractable iron,Fed),ratio of Fed to Fet (Fed / Fe,) in bulk soil and clay (Fed (clay)/Fet (clay)),Fed sedimentation coefficient (ratio of Fed in the genetic horizon beneath the surface horizon to Fed in surface horizon) and enrichment rates of Fet and Fed in clay (Fet(clay)/Fet and Fed(clay)/Fed).[Result] Results show that:(1) Among the soils derived from marine deposits,those,30 a in cultivation age,were all ＞ 1 in Fed sedimentation coefficient,regardless of horizon;those,100 a and 600 a in cultivation age,were ＞ 1 only in some genetic horizons,and those,1 000 a in cultivation age,were all ＜ 1.In the soils derived from fluvial deposits,the generic horizon where Fed sedimentation occurred went deeper with the cultivation age.Among the soils derived from delta deposits,those,50 a,90 a,150 a and 350 a in cultivation age,were found to have genetic horizons as deep as 40-cm,45-cm,80-cm and 90-cm,respectively.(2) In the soils derived from marine deposits,the distribution of Fed in clay (Fed (clay)) in soil profile varied irregularly with cultivation age,while in the soils derived from fluvial and delta deposits the content of Fed (clay) in soil layers below 40-cm and 60-cm depth,respectively,declined with cultivation age.(3) The contents of Fe,and Fed in clay were higher than their corresponding ones in bulk soil,Among the soils derived from marine deposits,those,30 a and 100 a in cultivation age were lower than those,600 a and 1000 a in cultivation age,in Fe,and Fed enrichment rate.In the soils derived from fluvial deposits Fet and Fed enrichment rates increased with cultivation age in the soil layers below 40-cm.And among the soils derived from delta deposits,those,350 a in cultivation age,were lower than those younger in cultivation age in Fet and Fed enrichment rate.And (4) Fed and Fet in bulk soil was extra-significantly and positively related to Fed and Fet in clay,and so was Fe,(clay)/Fe,and Fed (clay)/Fed to Fe,(clay) Fed (clay) and Fed (clay)/Fe,(clay),and Fe,(clay)/Fet to to Fed (clay)/Fed,however Fet (clay)/Fe,and Fed(clay)/Fed was extra-significantly and negatively related to Fe,,Fed,Fed/ Fe,and clay content.[Conclusion] With agricultural cultivation going on,Fed in the soils derived from fluvial and delta deposits tends to leach down and deposit in the lower and lower soil layers;Fed content tends to decline in the hydroponic redox layer of the soils derived from marine deposits;iron oxides tend to increase in enrichment rate in clay of the soils derived from marine deposits,but decline in clay of the soils derived from fluvial and delta deposits.However,the iron oxides accumulating in clay of the soils were dominated with Fed,and clay content in the soils is the major factor affecting iron oxide enrichment rate.
Iron oxidesEluviation-illuviationCultivation ageSoil-forming parent materialPearl River Delta
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