Modifications of 2:1 Clay Minerals in a Kaolinite-Dominated Ultisol under Changing Land-Use Regimes

Abstract

Chemical denudation and chemical weathering rates vary under climatic, bedrock, biotic, and topographic conditions. Constraints for landscape evolution models must consider changes in these factors on human and geologic time scales. Changes in nutrient dynamics, related to the storage and exchange of K+ in clay minerals as a response to land use change, can affect the rates of chemical weathering and denudation. Incorporation of these changes in landscape evolution models can add insight into how land use changes affect soil thickness and erodibility. In order to assess changes in soil clay mineralogy that result from land-use differences, the present study contrasts the clay mineral assemblages in three proximal sites that were managed differently over nearly the past two centuries where contemporary vegetation was dominated by old hardwood forest, old-field pine, and cultivated biomes. X-ray diffraction (XRD) of the oriented clay fraction using K-, Mg-, and Na-saturation treatments for the air-dried, ethylene glycol (Mg-EG and K-EG) solvated, and heated (100, 350, and 550°C) states were used to characterize the clay mineral assemblages. XRD patterns of degraded biotite (oxidized Fe and expelled charge-compensating interlayer K) exhibited coherent scattering characteristics similar to illite. XRD patterns of the Mg-EG samples were, therefore, accurately modeled using NEWMOD2® software by the use of mineral structure files for discrete illite, vermiculite, kaolinite, mixed-layer kaolinite-smectite, illite-vermiculite, kaolinite-illite, and hydroxy-interlayered vermiculite. The soil and upper saprolite profiles that formed on a Neoproterozoic gneiss in the Calhoun Experimental Forest in South Carolina, USA, revealed a depth-dependence for the deeply weathered kaolinitic to the shallowly weathered illitic/vermiculitic mineral assemblages that varied in the cultivated, pine, and hardwood sites, respectively. An analysis of archived samples that were collected over a five-decade growth period from the pine site suggests that the content of illite-like layers increased at the surface within 8 y. Historical management of the sites has resulted in different states of dynamic equilibrium, whereby deep rooting at the hardwood and pine sites promotes nutrient uplift of K from the weathering of orthoclase and micas. Differences in the denudation rates at the cultivated, pine, and hardwood sites through time were reflected by changes in the soil clay mineralogy. Specifically, an increased abundance of illite-like layers in the surface soils can serve as a reservoir of K+.

A coupled microscopy approach to assess the nano-landscape of weathering

“…Fungi promote soil aggregation and structural development by embedding grains into their hyphal network, and by forming secondary minerals that precipitate, in part, from the release of base cations (e.g., K + , Mg 2+ , Ca 2+ ) during mineral weathering 23 . Biominerals formed by free-living and symbiotic fungi (e.g., mycorrhizae) include aggregated iron hydroxides, calcium and magnesium oxalates, among others 53,57 .…”

“…Unresolved questions pertaining to mineral transformation mechanisms involve assessing relative controls of biochemical versus biomechanical weathering, indirect or direct microbe-mineral interactions, and whether microbes enhance or slow mineral dissolution 17,18 . Microscopy studies have examined biomechanical and biochemical weathering mechanisms that: weaken mineral structures through the fungal-driven oxidation of Fe(II) in biotite 19 , induce secondary mineral formation and biomineralization 20 , enhance microbial growth around nutrient-rich zones 21 , incongruently leach major and trace elements 10,22 , and exert control of K nutrient uptake and related clay mineral modifications under changing land-use regimes 23 . Atomic force microscopy has been used to detect the simultaneous occurrence of indirect and direct biochemical weathering, where it was observed that abundant small etching pits formed on mineral surfaces exposed to siderophores (molecular Fe chelators) released to solution by microbes, whereas fewer yet larger “biopits” formed on surfaces colonized directly by bacteria 24 .…”

Iron-mediated organic matter decomposition in humid soils can counteract protection

“…Soil pH (1:2 ratio of soil: water) was 6.2. XRD analysis revealed that the clay mineralogy of this soil is dominated by kaolinite and illite 78 .…”

Exploring the Effect of Aspect to Inform Future Earthcasts of Climate‐Driven Changes in Weathering of Shale

“…In fact, the variation in these processes during ecosystem development can affect mineral weathering rates in shallow soils (e.g., Lawrence, Harden, & Maher, ,Lawrence, Steefel, & Maher, ). Recent work by Austin et al () shows that the translocation of K at depth to the surface by vegetation in a highly weathered, temperate forested ecosystem leads to the formation of 2:1 clays such as illite preferentially over 1:1 clays such as kaolinite, while work in rain forests have shown that cycling of Si by vegetation helps maintain kaolinite in the topsoil (Lucas, ; Lucas et al, ). Another phenomenon that is not included in our modeling is the effect of light molecular weight organic acids in promoting chemical weathering at shallow depths while inhibiting primary mineral weathering and increasing secondary mineral precipitation at greater depths (Lawrence, Harden, & Maher, ; Lawrence et al, ).…”