Micromorphology and Genesis of Soils from Topolitosequences in the Brazilian Central Plateau

ABSTRACT The micromorphology of deeply weathered soils (Ferralsols/ Latossolos ) from the Central Plateau of Brazil remains little studied, and its affiliation to different parent materials, poorly known. To clarify the processes of soil formation of these acric, gibbsitic, Fe-oxide rich Ferralsols, three lithotoposequences on local ultrabasic to basic intrusive rocks were studied. The influences of mixing and pedobioturbation are evident in all soils, and Ferralsols of the Central Plateau of Brazil are polygenetic, based on the coarse mineral composition, with ultrabasic and metapelitic rock contributions. The typical oxic microstructure with stable microaggregates encompasses a gibbsite/Fe rich micromass with random inclusions of charcoal, allochthonous quartz, Ti-magnetite, and perovskite grains. Shallow Cambisols ( Cambissolos ) on tuffites also display a similar “oxic” microstructure, but a much lower degree of weathering. These Cambisols possesses apatite, and mafic and pelitic minerals as residuals minerals grains, indicating the polygenetic colluvia mixture of different substrates. The common occurrence of perovskite (CaTiO3) as a residual grain in deep-weathered, acric Ferralsols on ultrabasic rocks, not yet reported in the literature, shows an unusual resistant Ca reserve in oxic soils, though nothing is known about its implications to soil fertility.

摘要 巴西中部高原的强风化土壤（铁铝土（Ferralsols）/拉托索土（Latossolos））的微形态学特征鲜有研究，其与不同母质的归属关系也鲜为人知。为阐明这类强酸性、含三水铝石、富铁氧化物的铁铝土的成土过程，本研究选取当地超基性至基性侵入岩上的3个岩性地形序列开展分析。所有供试土壤均表现出混合作用与土壤生物扰动的显著影响；基于粗粒矿物组成可知，巴西中部高原的铁铝土为多成因土壤，其物质来源包含超基性岩与变质泥质岩。这类具有稳定微团聚体的典型氧化型微形态，由富三水铝石/铁的微基质构成，其中随机分布炭屑、外源石英、钛磁铁矿（Ti-magnetite）与钙钛矿（perovskite）颗粒。凝灰岩上发育的浅层雏形土（Cambisols / Cambissolos）同样呈现类似的“氧化型”微形态，但风化程度显著更低。这类雏形土含有磷灰石、镁铁质矿物与泥质矿物作为残余矿物颗粒，表明其为多成因的不同母质崩积物混合体。目前文献尚未报道，在超基性岩上发育的强酸性强风化铁铝土中普遍存在作为残余颗粒的钙钛矿（CaTiO3），这表明氧化型土壤中存在一种罕见的稳定性钙储库，但其对土壤肥力的影响尚不明确。