274-5 Soil P Dynamics In the Sierra Nevada: Exploring the Connection Between Soils and Eutrophication of High-Elevation Lakes.



Tuesday, October 18, 2011: 2:00 PM
Henry Gonzalez Convention Center, Ballroom C-2, Ballroom Level

Peter M. Homyak1, James O. Sickman1 and John M. Melack2, (1)Environmental Sciences, University of California, Riverside, Riverside, CA
(2)Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, Santa Barbara, CA
High-elevation lakes in the Sierra Nevada are experiencing increased nutrient loading with concomitant shifts from P to N limitation of phytoplankton growth during summer.  Because N inputs have remained relatively constant (20+ years), increases in P supply are presumably driving changes in lake trophic dynamics.  However, the source of the excess P is poorly understood.  Since temperature, runoff patterns, and the timing of snowmelt influence N biogeochemistry in high-elevation ecosystems, we hypothesize that climate change may influence P cycling in soils and result in increased P-loading to lakes.  We analyzed entisols and inceptisols derived from granitic and granodioritic parent material in the Emerald Lake watershed (Sequoia National Park) for seasonal changes in soil P pools.  Total P averages 692 µg P g-1soil in the top 10 cm of soil (O and A horizons) and 547 µg P g-1soil in the 10-50 cm depth (B and C horizons).  In the upper 10 cm, 70% of the P is freely exchangeable or associated with Fe and Al, while in soils from 10-50 cm, 60% is exchangeable or bound to Fe and Al.  Inorganic P in labile pools is highest during winter in which a deep snowpack provides moisture and insulates soils from freezing; labile inorganic P decreases overtime and is lowest in fall.  In contrast, soil organic P in NaHCO3- and NaOH-extractable pools is lowest in winter, highest in summer, and decreases during fall.  Microbial biomass P is highest during winter (923 µg P g-1soil) and decreases six fold over time to its lowest in fall (145 µg P g-1soil).  Microbial processes dominate P dynamics in Sierran soils.  Because climate change is predicted to diminish snow cover in the Sierra Nevada, soil freeze-thaw events may lessen winter microbial P immobilization, disrupt soil aggregates, and increase P concentrations in runoff to high-elevation lakes.
See more from this Division: S07 Forest, Range & Wildland Soils
See more from this Session: Carbon and Nutrient Cycling