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：巴西中部高原的重度风化土壤——铁铝土（Ferralsols/Latossolos），其微观形貌研究较为匮乏，且其与不同母质的归属关系亦尚不明确。为阐明这类强酸性、含三水铝石且富氧化铁的铁铝土的成土过程，本研究针对当地超基性至基性侵入岩发育的3个地形母质序列（lithotoposequence）展开系统研究。所有供试土壤均表现出显著的土壤混合与生物扰动（pedobioturbation）特征；结合粗粒矿物组成分析，巴西中部高原的铁铝土属于多成因土壤，其母质来源包含超基性岩与泥质变质岩。该类土壤具有典型的氧化型微结构：稳定的微团聚体由富三水铝石/铁的微基质构成，其中随机分布有木炭颗粒、外源石英、钛磁铁矿及钙钛矿晶粒。凝灰岩上发育的浅层雏形土（Cambisols/Cambissolos）同样呈现类似的“氧化型”微结构，但其风化程度显著更低。该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坡积物混合体。在超基性岩发育的重度风化强酸性铁铝土中，钙钛矿（CaTiO₃）作为残余颗粒普遍存在，这一现象尚未见诸文献报道，揭示出氧化型土壤中存在一种罕见的难溶性钙储库，但目前学界对其在土壤肥力方面的潜在影响仍一无所知。