Search

Bromine and iodine are halogens with similar chemical properties. Distributions of Br and I in the soil profile provide insights for understanding of their biogeochemical cycles and fate of these elements derived from artificial origin. Although generally not abundant in parent rock materials, they are often found in higher concentrations in soils, particularly volcanic ash soils in Japan. However, the process of Br and I accumulation in soils during soil formation remains unclear. We investigated the vertical distribution of Br and I to characterize the processes of Br and I accumulation in volcanic ash soils. Soils were collected from three pedons in the southern Shimokita Peninsula, Japan. Bromine (1.4 to 253 mg kg-1) and iodine concentrations (1.2 to 90 mg kg-1) in the soils were significantly correlated. Nondestructive speciation analysis by X-ray absorption near edge structure spectroscopy showed that both Br and I mainly exist in their organic forms in soils enriched in those elements. Indices of the degree of Andosol formation (oxalate-extractable Al and Fe, pyrophosphate-extractable Al and Fe, phosphate absorption coefficient, and soil pH [NaF]) were significantly correlated with both Br and I concentrations in the soils. These results suggest that Br and I, derived from atmospheric deposition, were retained in surface soils after transformation to organic form and accumulated during Andosol formation from their parent tephra deposits. [ABSTRACT FROM AUTHOR]

It is important to clarify which factors control the distribution of secondary minerals because secondary minerals affect soil physicochemical properties. This study investigated the effects of parent materials and climate at different elevations on secondary mineral distributions in soils under a humid tropical climate with a hilly landscape and without recent volcanic activity. Soil material from B horizons was sampled at 60 sites on gentle slopes at different elevations (20-1700 m altitude). Each major parent material (sedimentary, felsic and intermediate to mafic) was represented at different elevations. The soil samples were classified based on their total elemental compositions using cluster analysis. Secondary minerals were measured by X-ray diffraction and selective extractions. The samples were divided into ferric (high Fe contents), K&Mg (high K, Mg and Si) and silicic (high Si) groups. The ferric soils were derived from mafic parent materials and characterized by high Fe oxides and gibbsite contents. The K&Mg and silicic soils were derived from felsic or sedimentary parent materials and had similar secondary mineral (kaolinite and vermiculite) contents, although more mica was found in the former group. Climate affected secondary mineral distributions only in the silicic group soils; higher temperatures at lower elevations may enhance weathering and increase kaolinite and crystalline Al and Fe contents. The hilly landscape of this study is different from those of shields in tropical humid regions in Africa or South America. Here, secondary mineral distributions were primarily determined by parent material (mafic vs. felsic or sedimentary) and secondarily by climate, which varied with elevation. [ABSTRACT FROM AUTHOR]

Factors controlling the abilities of Allophanic Andosols (representative of Japanese volcanic ash soils) to retain radiocesium (RCs) were assessed. The hypothesis was that retention is largely controlled by traces of micaceous minerals deposited as components of aeolian dust. The radiocesium interception potentials (RIPs), mica K contents were determined in the 2- to 20- and <2.0-μm particle fractions of 23 soil samples from agricultural fields in areas representative of Japanese Allophanic Andosols. Quartz contents were determined in the 2- to 20-mm particle fractions of the same soil samples. The oxygen isotope ratio (δ18O) in the quartz isolated from each 2- to 20-mm particle fraction was determined to identify the origin of the quartz in each soil sample. The mean RIP for the 2- to 20- and <2-mm particles was 1.7 ± 0.8 and 2.6 ± 1.3 mol kg-1, respectively, and the mica K contents were 3.2 ± 1.3 and 3.4 ± 1.7 g kg-1, respectively, corresponding to micaceous mineral contents of <5% by mass in each size fraction. In spite of their rather low values, the RIPs and mica K contents in each fraction positively correlated (p < 0.01), indicating that the abilities of the soils to retain RCs were mainly controlled by the micaceous mineral contents. The mica K content was proportional to the quartz content in the <20-mm particles, and the mean δ18O in the quartz was +14.8‰, similar to those in fine quartz in Chinese loess. These results strongly indicate that the abilities of Allophanic Andosols to retain RCs are largely controlled by micaceous minerals probably originating in aeolian dust. [ABSTRACT FROM AUTHOR]