尼日利亚热带雨林地区的内陆山谷中土壤发育和肥力特征: 矿物成分和粒度分布

The particle-size distribution and mineralogical composition of the clay (< 2 μm) and fine-sand (0.25--0.10 mm) fractions in soils of two inland valleys in Abakaliki and Bende, Southeast Nigeria, were investigated to provide basic information on soil-forming processes and agricultural potentials. These soils were silty or clayey, deriving from Cretaceous or Tertiary shale materials. The particle-size distribution and its computation on a clay-free basis revealed relatively remarkable lithologic breaks in a couple of pedons. The effect of lithologic discontinuities on soil mineralogical composition was not, however, conspicuous. Petrographic investigation revealed that quartz predominantly comprised the fine-sand fraction in the soils at both study sites. Nevertheless, the clay mineralogical composition of the soils was a mixture of kaolinite, irregularly interstratified smectite-illite intergrades (S/I), hydroxyl-Al interlayered 2:1 type clays (HICs), vermiculite, smectite, halloysite and illite along with fine-sized quartz in Abakaliki. The soils of Bende predominantly contained smectite, which was partially interlayered with hydroxyl-Al and kaolinite. It is suggested that seasonal floodwater has slowed the disintegration of weatherable clay minerals inherited from the shale, while quartz originating from the sandstone is predominant in the fine-sand fraction. Additionally, a possible soil-forming process observed at the both study sites was ferrolysis, which was indicated by a clear decreasing pattern of HICs downward in the soil profiles. The entry of S/I and vertical distribution patterns for a couple of clay minerals in the pedon suggested that the soils in Abakaliki have developed under the significant influence of aeolian dust delivered by the harmattan. The findings might describe a site-specific deposition pattern of harmattan dusts as well as hydromorphic soil-forming processes in the wetlands of the inland valleys.     

水稻土发生分类的研究 Ⅰ、铁的淋溶淀积与水稻土的发育

水稻土离铁作用明显,并以耕层最为强烈.耕层中的晶质铁和非游离铁被活化、淋溶,使耕层的全铁、游离铁、晶质铁及非游离铁量均随发育度的提高而渐减.耕层下淋的活性铁在心土层氧化淀积,导致心土层的全铁.游离铁和晶质铁量随发育度的提高而渐增.氧化铁的晶胶率和晶化度在剖面中有显著的分异,其变化趋势可确切地反映出不同水型下的成土特点。     

The composition of mobile matter in a floodplain topsoil: A comparative study with soil columns and field lysimeters

Floodplain soils are characterized by frequent and extreme redox changes caused by inundation with river water or imbibition of groundwater. Depending on the duration and extent of inundation, biogeochemical processes run at sub‐/anoxic conditions, which may result in the mobilization and relocation of dissolved and particulate matter within the soil. In this study, we investigated the effect of inundation events on the composition of mobilized matter in the topsoil horizon of a floodplain soil. We conducted experiments with soil columns in the laboratory and gravitational lysimeters in the field to identify redox‐mediated (im)mobilization processes and to estimate their relevance under field conditions. The lysimeters were filled with topsoil monoliths and run under in situ conditions during a ≈ 2.5‐y period. The soil columns were run with the same soil material either under strictly anoxic or mixed oxic–anoxic conditions. Effluents from mixed oxic–anoxic soil were composed fundamentally different [comparably high: Mn, Al, nitrate, sulfate; comparably low: pH, organic C (OC); not detected: Fe, As] compared to effluents from strictly anoxic soil (comparably high: pH, Fe, Mn, OC, As; comparably low: Al; not detected: nitrate, sulfate). Matter, which was mobile under anoxic conditions (e.g., Fe, As, OC), was effectively immobilized as soon as the mobile phase passed anoxic–oxic boundaries within soil (exception: Mn). We assume that the solution in the soil monoliths always passed such anoxic–oxic boundaries during downwards migration independent of lysimeter flooding with river water. This is indicated by the similar composition of the lysimeter seepage water and the effluents from mixed oxic–anoxic soil columns. Both solutions contained “fingerprints” from anoxic (Mn) and oxic conditions (nitrate). Inundations with river water and the duration of these floods (1–22 d) did not affect the composition of the lysimeter seepage water. In conclusion, immediate changes in the composition of the solution, which enters either the subsoil or nearby receiving waters, cannot be expected from regular topsoil flooding.