The role of clay minerals and metal oxides for organic matter stabilization in highly weathered tropical soils
Final Report Abstract
The conversion of tropical forests to agricultural land use is considered as a major cause for a decline in soil organic carbon (SOC) stocks. However, the extent and impact of different land uses on SOC stock development is highly uncertain, especially for tropical Africa due to a lack of reliable data. Mechanisms frequently described in the literature, such as mineral interaction with OC and aggregation, can contribute to OC persistence in soils. Clay minerals (e.g. kaolinite and gibbsite; termed ‘aluminous clay’) and pedogenic metal (oxyhydr)oxides (e.g. goethite and hematite; termed ‘Fe oxides’) are commonly found in weathered tropical soils and are reactive under the acid soil conditions. Unfortunately, their co-occurrence in most soils complicates direct assessment of their individual contribution on OC persistence. Aluminous clays and pedogenic Fe oxides have been further suggested as important building units for aggregates in these soils. Like for MAOC storage and stabilization, it is also hard to separate, how certain mineral phases modulate aggregation. Making use of unique mineralogical combinations in soils located in the East Usambara Mountains, NE Tanzania, we disentangled the contribution of commonly found minerals to both mechanisms in weathered soils of the humid tropics. The mean OC stocks of the investigated land uses, down to 100 cm depth were large compared to published data and ranged between 16.8 kg C m^–2 to 20 kg C m^–2. A significant decrease of 1.3 kg C m^-2 was detected, if cropland soils were compared to forest soils in the 0–10 cm depth increment only. Obviously, efficient SOC stabilization mechanisms are counteracting the higher disturbance regime under agricultural land use in these highly weathered tropical soils. A higher OM persistency emerges in soils comprising a high ratio of pedogenic Fe to aluminous clay, rendering them more resilient against disturbances caused by land-use change. We found that stability indicators determined in the laboratory are in good agreement with the persistence of bulk OC and MAOC during land-use change under field conditions. While previous studies often showed land-use changes from forests to cropland drive substantial OC losses, we provide the first evidence that this can be alleviated under certain mineralogical conditions, particularly under conditions with high pedogenic Fe to aluminous clay ratios (0.44‒0.56). Despite the high physical stability, OC contents of macroaggregates declined substantially in most mineralogical combinations if forest was compared with cropland land use. The formation of macroaggregates cannot be considered as a main stabilization process for OC in strongly weathered soils of the humid tropics. In turn, the formation of mineral-organic associations is the most important process that preserves OC during land-use change in these soils. Understanding the formation of reactive mineral surfaces e.g. by interactions between clay minerals and Fe oxides in the complex systems of soils will be key to predict potential stabilization of OC by the formation of MAOC.
Publications
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(2019): Impact of land use on soil organic carbon stocks in the humid tropics of NE Tanzania. In: J. Plant Nutr. Soil Sci. 182 (4), 625–636
Kirsten, Maximilian; Kimaro, Didas N.; Feger, Karl-Heinz; Kalbitz, Karsten
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(2021): Aluminous clay and pedogenic Fe oxides modulate aggregation and related carbon contents in soils of the humid tropics. In: SOIL 7 (2), 363–375
Kirsten, Maximilian; Mikutta, Robert; Kimaro, Didas N.; Feger, Karl-Heinz; Kalbitz, Karsten
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(2021): Iron oxides and aluminous clays selectively control soil carbon storage and stability in the humid tropics. In: Scientific Reports 11 (1)
Kirsten, Maximilian; Mikutta, Robert; Vogel, Cordula; Thompson, Aaron; Mueller, Carsten W.; Kimaro, Didas N. et al.
