As root depth increases they access mineral potassium from deep in the soil profile. As leaf litter is deposited on the soil, this potassium is thought to be stored in the surface soil. The results of this study suggest that mixed layered clays can serve as a reservoir for this potassium. We found that in the near surface of pine forested plots, which were planted on previously cultivated plots in the 1950s, there is an increase in the abundance of potassium bearing clays as compared to samples from deep in the soil (>5 m) and from plots that have been continuously cultivated. Confidence in modeled abundances of mixed-layer clays was increased by using a multi-specimen modeling approach.
Modeling subtle changes in the low angle region (<14 °2θ) of X-ray diffraction (XRD) patterns of oriented clay (<2 μm) slides from sites with different land use histories suggests that mixed-layer clay minerals in long-term deeply rooted systems play a role in the uptake and storage of potassium in the shallow surface (<1 m) even in highly weathered Ultisols.
The clay fractions of Ultisols from pine forested and cultivated plots in the Calhoun Critical Zone Observatory, located on the Piedmont of South Carolina, United States were examined by XRD. XRD patterns were measured for deep (>5 m) and shallow (<1 m) samples that were treated with Mg2+ and K+ cation saturations (Mg-sat and K-sat) under both ethylene glycol (EG) EG and heated states (110° C). Mixed-layer clay mineral abundances were quantified by calculated XRD patterns forward modeled using NEWMOD2. Changes in the XRD patterns for different specimens of the same sample (i.e. Mg-Sat/EG vs Mg sat/110° vs K sat/EG vs K sat/110°) allowed for the identification of mixed-layer clays. Strict balancing of modeled results to expected changes based on specimen preparation allowed for increased confidence in modeled difference between sample depths and sites.
Mixed-layers of varied proportions were exhibited in all samples. For example, the deep pine site progressively had in the Mg-sat/EG (IS73, IV74, IV64, KI90, KI50); K-sat/EG (IV85, IV72, KI90, KI34), Mg-sat/110° (IS68, IV79, IV64, KI96, KI47); K-sat/110° (IV85, IV72, KI34, KI90). XRD patterns for shallow samples exhibited less variability across all treatments because they contain lower abundances of vermiculite layers (smectite layers are not detected). Subtle changes in the intensity and peak position of the kaolinite (001) peak exhibited in the K-sat/110° pattern suggests that there are multiple combinations of mixed-layered phases, possibly including three component mixed-layer structures but these are not able to be modeled using the two-layer algorithms in NEWMOD2.
Model results indicate a change in K-bearing layer types progressing from cultivated to pine forested locations hints that these mixed-layer phases play an important role in the uplift of K in vegetative ecosystems.
Austin, Jason, Paul A Schroeder, Daniel deB. Richter (2019): Biological Cycling of Potassium by Roots as a Control on Mixed-layer Clays. American Geophysical Union Fall Meeting, San Francisco, CA, December 9-13, 2019.
This Paper/Book acknowledges NSF CZO grant support.