Transformations of 2:1 phyllosilicates in 41-year-old soils under oak and pine

Abstract

The large, unconfined lysimeters at the San Dimas Experimental Forest, in southern California, provide a unique setting in which to study decade-scale vegetation effects on mineral weathering. We investigated the 2:1 phyllosilicate mineralogy of lysimeter soils under 41-year-old monocultures of scrub oak (Quercus dumosa Nutt.) and Coulter pine (Pinus coulteri B. Don), and compared the results to archived original fill material. X-ray diffraction showed that mica increased relative to vermiculite in the clay and medium silt fractions of A horizons under both oak and pine, compared to the archived original fill material. The increase, however, was far greater under oak than under pine. No mineralogical differences were observed in the subsurface horizons of either oak or pine, compared to archived material. Nonexchangeable K increased by 23% in the clay fraction of the oak A horizon, and increased by 5% in the clay fraction of the A horizons under pine, relative to archived parent material. Strong evidence supports biocycling as the basis for observed decade-scale mineral transformations. We conclude that the return of K to the soil surface through litter decomposition, and subsequent fixation by vermiculite, resulted in increased mica contents in A horizons. More K may have been fixed by vermiculite in the oak A horizon compared to pine due to greater K concentration in the oak litter pool; earthworm-mediated mineralization of K from organic matter under oak; and presence of fewer roots at the surface under oak, and, consequently, less plant removal of K from the A horizon.