Iron oxides and aluminous clays selectively control soil carbon storage and stability in the humid tropics

Abstract

Clay minerals and pedogenic metal (oxyhydr)oxides are the most reactive soil mineral constituents controlling the long-term persistence of organic carbon (OC) in terrestrial ecosystems. However, their co-occurrence in most soils complicates direct assessment of their individual contribution to OC persistence. Making use of unique mineralogical combinations in soils located in the East Usambara Mountains of Tanzania, we disentangled the contribution of clay-sized aluminous minerals (kaolinite, gibbsite) and pedogenic Fe (oxyhydr)oxides (predominant goethite and hematite) on OC storage and stabilization under natural forests and croplands. Topsoil samples, varying in contents but not types of aluminous clays and pedogenic Fe (oxyhydr)oxides, were identified by selective extractions, X-ray diffraction, and Mössbauer spectroscopy. Associated abundance of particulate and mineral-associated organic matter (OM) was quantified by density fractionation and their changes during land-use conversion were determined as a measure of OC persistence. Additionally, we assessed the resistance of OC to chemical oxidation as well as microbial decomposition in a 50-day laboratory incubation. We found that the ratio of pedogenic Fe to aluminous clay is more consequential for OC storage and stabilization than their individual contents, despite the fact that Fe (oxyhydr)oxides generally exert a stronger impact on OC than aluminous clays. Conjunction of large amounts of Fe (oxyhydr)oxides with low aluminous clay contents caused the strongest accumulation of mineral-associated OC, a low soil respiration, high OC stability against chemical oxidation, and high OC persistence during land-use change. Our study suggests that certain mineralogical combinations in the humid tropics alleviate OM losses during land conversion because of the strong and selective mineral control on OC stabilization, particular if the weight ratio of pedogenic Fe to aluminous clay exceeds the threshold range of 0.44‒0.56.

Aluminous clay and pedogenic Fe oxides modulate aggregation and related carbon contents in soils of the humid tropics

“…Clay minerals and Fe oxides are considered as important mineral constituents fostering aggregation and subsequent OC storage via physical protection (Denef et al, 2004). The overwhelming portion of OC in the studied topsoils resided in mineral-organic associations (35−81%), whereas OC occluded in aggregates amounted to 7−24%, with a lower share under cropland than forest as determined by density fractionation (Kirsten et al, 2021). he lo clay-high Fe cropland had an OC content more than t ice https://doi.org/10.5194/soil-2020-98 Preprint.…”

“…The mineralogical composition of the study soils was very homogeneous with kaolinite and gibbsite as the 174 main aluminous minerals of the clay fraction, and well-crystalline goethite and hematite as dominant (Table 1). The advanced weathering state of study soils was also reflected in low pH 180 and CEC eff values (Table 1; Kirsten et al, 2021). 181 https://doi.org/10.5194/soil-2020-98 Preprint.…”

“…Mountains of Tanzania. The soils vary in the amount of aluminous clay (kaolinite, gibbsite) and pedogenic Fe oxides (goethite, hematite) but without variation in their mineralogical composition (Kirsten et al, 2021). The small-scale conversion of natural forest to cropland that took place in that region enables us to evaluate the effect of land-use change under each mineralogical combination on soil physical properties and related OC persistence.…”

“…The dominating Acrisols and Alisols, developed from Precambrian crystalline bedrock, are deeply weathered and highly leached, with visible clay illuviation in the subsoil (Kirsten et al, 2019). Criteria for site selection and soil sampling has been described in detail by Kirsten et al (2021).…”

“…Variation in mineral constituents caused different soil OC contents, ranging between 19 to 95 g OC kg −1 across all sites including both land use and depth ( Kirsten et al, 2021).…”