亚热带花岗岩坡地富铁土铁氧化物相分配对团聚体稳定性的影响

The Effects of Iron Oxide Phases Distribution on Aggregate Stability of Ferrisol Along a Subtropical Slope Derived from Granite

铁氧化物是环境敏感矿物,也是土壤团聚体的重要胶结物。自然铁氧化物相存在多尺度分异现象,可能影响团聚体的形成与稳定。本文以闽中丘陵区花岗岩坡地上发育的富铁土序列为研究对象,通过湿筛法和吸管法分离团聚体;运用土壤化学和漫反射光谱(Diffuse reflectance spectroscopy,DRS)方法测定了全铁(Fe_(t))、游离铁(Fe_(d))、无定形铁(Fe_(o))、赤铁矿(Hm)和针铁矿(Gt)含量,并系统探讨了坡地、剖面和团聚体尺度铁氧化物相分异特征及其对团聚体稳定性的影响。研究表明,坡地尺度上,沿坡面向下,全样Fe_(d)与游离度(Fe_(d)/Fe_(t))减小,Fe_(o)与活化度(Fe_(o)/Fe_(d))增加,Hm与Hm/(Hm+Gt)明显降低,Gt变化不明显;剖面尺度上,上层土壤的Fe_(d)、Gt多高于下部,Fe_(o)和Hm则多低于下部;团聚体尺度上,Fe_(d)、Fe_(o)和Gt趋向微团聚体中富集,Hm在微团聚体和大团聚体中含量相当,Hm/(Hm+Gt)则随粒级增长而增大。坡地土壤团聚体粒级组成以微团聚体为主,但团聚体稳定性由>0.25 mm大团聚体主导。Fe_(o)、Fe_(o)/Fe_(d)和Hm(Hm/(Hm+Gt))分别与0.5~0.25 mm、1~0.5 mm和2~1 mm大团聚体显著正相关。不同尺度上铁氧化物存在明显分异,其中坡地尺度铁氧化物相的强烈分配导致Hm和Fe_(o)分别在坡顶和坡底相对富集,使得坡地两端团聚体稳定性高于坡中过渡区。

【Objective】Iron oxides are sensitive to the pedogenic environment and an important cement of soil aggregates.The differentiation of iron oxide phases occurs at multiple scales in natural systems,which may affect the formation and stability of soil aggregates.This study aimed to explore the differentiation of iron phases,including total iron(Fe_(t)),free Fe-oxide(Fe_(d)),amorphous Fe-oxide(Fe_(o)),hematite(Hm),and goethite(Gt)at the hillslope,profile and aggregate scales and evaluate their effects on the stability of soil aggregates.【Method】A topo-sequence of Ferrisol,derived from granite in the hilly area of central Fujian Province,was fractionated into aggregates using wet-sieving and pipette methods.Fe_(d) and Fe_(o) in bulk soils and different aggregates were extracted with the citrate-bicarbonate-dithionite(CBD)solution and the acid ammonium oxalate(AAO)solution,respectively.Diffuse reflectance spectroscopy(DRS)was applied to determine Hm and Gt content.The percentage of water-stable aggregate(WSA)and the mean weight diameter(MWD)were evaluated.【Result】At the hillslope scale,the Fe_(d) and Fe_(d)/Fe_(t) in bulk soils decreased,while the Fe_(o) and Fe_(o)/Fe_(d) increased downslope.Meanwhile,the Hm and Hm/(Hm+Gt)significantly declined,while the Gt kept a little variation downslope.At the profile scale,the upper layers generally possessed higher contents of Fe_(d) and Gt,as well as lower levels of Fe_(o) and Hm in contrast to the deeper layers.At the aggregate scale,Fe_(d),Fe_(o) and Gt were enriched in micro-aggregates.The contents of Hm were comparable in the micro-and macro-aggregates,while Hm/(Hm+Gt)increased with the aggregate size.Micro-aggregates were the main components,but the>0.25 mm macro-aggregates dominated the aggregate stability along the hillslope.The WSA of 0.5-0.25 mm and 1-0.5 mm aggregates were significantly positively correlated with Fe_(o) and Fe_(o)/Fe_(d) respectively,and the WSA of 2-1 mm macro-aggregate was significantly positively correlated with the Hm and Hm/(Hm+Gt).【Conclusion】The iron oxides demonstrated significant differentiation from hillslope scale to aggregate scale.The soil aggregate stability in the top-slope and bottom-slope profiles were higher than those in the transitional profiles due to the upslope enrichment of Hm and downslope enrichment of Fe_(o) along the hillslope.