Abstract
Abstract. Aggregation affects a wide range of physical and biogeochemical soil properties with positive effects 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 aluminosilicates, aluminum hydroxides, and Fe oxides are part of the clay-sized fraction it is hard to separate how certain mineral phases modulate aggregation. In addition, it is not known what consequences this will have 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 land uses, 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, and changes in aggregation and OC contents under forest and cropland land use. Patterns in soil aggregation were rather similar across the different mineralogical combinations (high level of macroaggregation and high aggregate stability). Nevertheless, we found some statistically significant effects of aluminous clay and pedogenic Fe oxides on aggregation and OC storage. An aluminous clay content > 250 g kg−1 in combination with pedogenic Fe contents < 60 g kg−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 < 250 g kg−1 promoted OC storage and persistence even under agricultural use. The combination with low aluminous clay and high pedogenic Fe contents displayed the highest OC persistence, despite conversion of forest to cropland causing substantial disaggregation. This indicates that aggregation in these tropical soils 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 these highly weathered soils, where OC storage is more regulated by direct mineral–organic interactions.
Highlights
Many functions of soils such as food production, water purification, and climate regulation are tightly linked to soil structure (Bronick and Lal, 2005; FAO, 2015; Six et al, 2004)
We found a positive relationship between the storage of mineral-associated organic carbon (OC) and the ratio of pedogenic Fe to aluminous clay under forest and cropland land use, suggesting that a larger share of Fe oxides is linked to larger OC storage and persistency against land-use change (Kirsten et al, 2021)
Our data suggest that the mean weight diameter (MWD) under forest is significantly positively influenced by the Fed content (e.g., MWDForest 0–5 cm: r2 = 0.40, p < 0.001; Table S1), whereas nearly no effect was observed for aluminous clay
Summary
Many functions of soils such as food production, water purification, and climate regulation are tightly linked to soil structure (Bronick and Lal, 2005; FAO, 2015; Six et al, 2004). Aggregates are the structural backbone of soil, and changes in aggregation impact various processes such as root development, soil erosion, and soil organic carbon (OC) accumulation (Chaplot et al, 2010; Le Bissonnais et al, 2018) Based on their size, soil aggregates are typically classified into small microaggregates (< 20 μm), large microaggregates (20–250 μm), and macroaggregates (> 0.25 mm) (Tisdall and Oades, 1982). Barthès et al (2008) showed that texture had no effect on macroaggregation over a range of tropical soils characterized by lowactivity clay minerals Such kinds of uncertainty may derive from the fact that the clay size particle fraction (< 2 μm) contains OM and different types of clay minerals and variable contents of pedogenic Fe and aluminum (Al) oxides (Barré et al, 2014; Fernández-Ugalde et al, 2013; Wagai and Mayer, 2007). Such mutual interactions between typical aluminous clay-sized minerals (e.g., kaolinite, gibbsite) and pedogenic Fe oxides are possible drivers of aggregation in weathered tropical soils (Durn et al, 2019)
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