中国北方第四纪黄土发育土壤铁锰结核形成环境及空间分布

Distribution and Formation Environment of Fe-Mn Nodules in Soils Derived from Quaternary Loess in North China

土壤铁锰结核是土壤形成过程的产物,记录着土壤发育过程和成土环境变化信息,也是土壤类型划分的重要依据,具有很高的研究价值。通常认为铁锰结核主要分布在地势较低、地下水位较浅的地区,但在我国东北地区位于高平地、地下水位较深,发育于第四纪黄土状物质的土壤中有铁锰结核的分布,对该地区土壤分类造成了很多问题。为了摸清我国北方温带地区第四纪黄土状物质发育土壤中铁锰结核的分布规律及形成原因,通过实地调查,并收集整理第二次全国土壤普查资料,对中国北方10个省份的345个处于高平地(地下水位深)、发育在第四纪黄土状物质的土壤剖面资料进行整理和统计分析。研究表明,有铁锰结核存在的土壤剖面分布具有明显规律,即主要分布在温带湿润地区(41.19°～49.01°N,120.82°～133.37°E),该区域年均降水量在370.6～917.7 mm之间,年均气温在0.9～14.9℃之间,气候的共同特点是降水量大、蒸发量小、冬季土壤结冻持续时间长、春季土壤冻融交替持续时间久,且土壤"返浆现象"明显。研究进一步表明,位于高平地、发育在第四纪黄土状物质土壤中铁锰结核的形成与土壤冻融交替有关。

【Objective】Fe-Mn nodules in soils,pedogenic products of the soils,contain a lot of information related to soil-forming processes and variation of soil-forming environment and can be used as important basis for soil classification.So distribution of Fe-Mn noudles plays an important role in understanding of the mechanism of Fe-Mn noudles formation and soil classification.It is generally believed that Fe-Mn noudles are mainly distributed at lowlands with shallow groundwater table.However,FeMn nodules are found in uplands with deep groundwater table in Northeast of China,which causes a lot of problems in soil classification in this region.The objectives of this study were to explore the spatial distribution of soil Fe-Mn nodule and explore the environment and mechanism of their formation in North China.The principles for selecting soil profiles were:(1)choose soil profiles situated in stable uplands with deep groundwater table,to ensure that the soils are not affected by groundwater,and in uplands free of obvious evidence of erosion and deposition in the processes of soil formation;(2)pick soils developed from the Quaternary loess-like materials,to ensure that soils were derived from the same parent materials as much as possible.【Method】A total of 345 soil profiles,based on the soil series survey in 2010’s and the legacy data of the Second National Soil Survey(SNSS)during 1980’s,were prepared in 10 provinces in the temperate zone of North China.Whether the Fe-Mn nodules were presence or not in each soil horizon was examined.The mean annual precipitation(MAP)and mean annual temperature(MAT)of each soil sample site was retrieved from the China Meteorological Science Data Sharing Service.Relationships between formation of Fe-Mn nodules in soils and climate conditions of soil-forming were analyzed.【Result】Results show that Fe-Mn nodules were found in 74 profiles or 21.5%of 345 soil profiles studied in total.The soil profiles with Fe-Mn nodules are distributed mainly in a region,120.82°~133.37°E and 41.19°~49.01°N,a part of the humid temperate zone,where MAP is 370.6~917.7 mm,and MAT 0.9~14.9℃.In winter the soils there in remain frozen for a long time,and in spring they undergo a long period of freezing-thawing alternation,with the surface soil getting pulpy,because soil water keeps moving up in the topsoil layer when it gets frozen in winter and when the frozen topsoil begins to thaw,soil water remain in the topsoil layer making it pulpy in early spring because the subsoil layer is still not thawed,preventing the water in the topsoil layer from percolating down.This results in a reducing environment close to the freezing layer for a period of time before the frozen soil thaws thoroughly,which lays a solid foundation for the formation of Fe-Mn nodules.【Conclusion】It is,therefore,concluded that the formation of Fe-Mn nodules in stable upland soils is closely related to the alternation of freezing and thawing in spring.