Abstract. Aggregation affects a wide range of physical and biogeochemical soil properties with positive feedbacks on soil carbon storage. For weathered tropical soils, aluminous clays (kaolinite and gibbsite) and pedogenic Fe (oxyhydr)oxides (goethite and hematite; termed Fe oxides) have been suggested as important building units for aggregates. However, as both secondary aluminosilicates and Fe oxides are part of the clay-sized fraction it is hard to separate, how certain mineral phases modulate aggregation, and what consequences this has for organic carbon (OC) persistence after land-use change. We selected topsoils with unique mineralogical compositions in the East Usambara Mountains of Tanzania under forest and cropland. Soils are varying in contents of aluminous clay and Fe oxides. Across the mineralogical combinations, we determined the aggregate size distribution, aggregate stability, OC contents of aggregate size fractions as well as changes in aggregation and OC contents under forest and cropland land use. We found the soil aggregation patterns (high level of macroaggregation and aggregate stability) more similar than different among mineralogical combinations. Yet, an aluminous clay content > 250 g kg⁻¹ in combination with pedogenic Fe contents −1 significantly promoted the formation of large macroaggregates > 4 mm. In contrast, a pedogenic Fe content > 60 g kg⁻¹ in combination with aluminous clay content of −1 promoted OC storage and persistence after the change in land use. The low clay-high Fe combination displayed the highest OC persistence, despite conversion of forest to cropland caused substantial disaggregation. Our data indicate that aggregation in this typical soil of the humid tropics is modulated by the mineralogical regime, causing moderate but significant differences in aggregate size distribution. Nevertheless, aggregation was little decisive for overall OC persistence in the highly weathered soils, where OC storage is more regulated by direct mineral-organic interactions.

中文翻译：

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铝土和成土的铁氧化物调节湿热带地区土壤的团聚和相关的碳含量

摘要。聚集作用对土壤的物理和生物地球化学性质具有广泛的影响，对土壤碳储量有积极的反馈作用。对于风化的热带土壤，铝粘土（高岭石和菱镁矿）和成岩的铁（羟基氧化物）（针铁矿和赤铁矿；称为铁氧化物））已被建议用作骨料的重要建筑单元。但是，由于次生铝硅酸盐和Fe氧化物都是黏土级分的一部分，因此难以分离，某些矿物相如何调节聚集以及这对土地用途变化后有机碳（OC）持久性的影响如何。我们在森林和农田下的坦桑尼亚东乌桑巴拉山中选择了具有独特矿物学组成的表土。土壤中铝粘土和氧化铁的含量各不相同。在整个矿物学组合中，我们确定了总粒径分布，总稳定性，总粒径分数的OC含量以及林地和农田土地利用下的聚集体和OC含量的变化。我们发现，在矿物学组合中，土壤聚集模式（高聚集度和高聚集体稳定性）比相似之处更为相似。然而，铝粘土含量> 250 g公斤^-1结合成岩的铁含量-1显着促进了大于4 mm的大型大骨料的形成。相反，成土的铁含量> 60 g kg ^-1与铝粘土含量-1相结合，可促进土地利用变化后的OC储存和持久性。低粘土-高铁结合物表现出最高的OC持久性，尽管森林由耕地转变为农田造成了严重的分解。我们的数据表明，这种湿润热带地区典型土壤中的聚集受到矿物学机制的调节，导致聚集体大小分布出现中等但显着的差异。然而，聚集对于高风化土壤中总的OC持久性几乎没有决定性作用，因为这些土壤中OC的储存更多受直接的矿物-有机相互作用的调节。