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

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^-1_soil in the top 10 cm of soil (O and A horizons) and 547 µg P g^-1_soil 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^-1_soil) and decreases six fold over time to its lowest in fall (145 µg P g^-1_soil).  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.