Base cation and silicon biogeochemistry under pine and scrub oak monocultures: implications for weathering rates

Vegetation type has a profound influence over biogeochemical processes, including canopy leaching, stemflow, litterfall, and nutrient uptake. Understanding the relationship between plants and soil processes, such as mineral weathering, is a difficult task considering the interactions among biotic and abiotic soil-forming factors. In this study, soils planted with pure stands of scrub oak (Quercus dumosa Nutt.) and Coulter pine (Pinus coulteri B. Don) were used to identify key biogeochemical processes in each soil-plant system. The experimental design, a true biosequence, allowed an opportunity to link biogeochemical processes to mineral weathering. Precipitation, throughfall, litter leachate, and soil solutions were collected following rain events from November of 1996 to April of 1998. Relatively high Na and Ca concentrations in pine throughfall suggest that atmospheric deposition is a more important process within the pine canopy than is canopy leaching. In contrast to pine throughfall, mean K concentrations were higher than Ca in oak throughfall, reflecting the importance of canopy leaching. Nutrient cycling was more rapid within the scrub oak soil-plant system as evidenced by significantly reduced levels of cations in soil solution at the 7.5 cm depth compared to litter leachate. Mean Si concentrations at the 65 cm depth were 334 μmol L−1 under oak and 431 μmol L−1 under pine. Silicon and base cation fluxes indicate higher weathering rates under pine. Solution chemistry shows that canopy and nutrient cycling processes are different between the two species and are impacting the cation loss rates.