Retention and release of phosphorus in Forest-Meadow Systems of Lake Tahoe, California

Forests are important non-point sources for phosphorus (P) release to surface waters. Almost a century of fire suppression in lake Tahoe basin has resulted in excessive accumulation of forest litter layer containing large pool of phosphorus. Leachate from such forest floor can potentially act as a long-term source of biologically available phosphorus which can be transported via either leaching, litter flow or by surface runoff to the nearby surface waters. This premise is supported by recent studies that have reported high inorganic phosphorus concentration (25 mg/L P) in overland/litter interflow (Miller et al., 2005) derived from the forest floor O horizons (Loupe et al., 2007). Due to the downstream risk of excessive nutrient load on water clarity in Lake Tahoe the objective of this study was to characterize the fate and transport of P in the upland volcanic and granitic forest-meadow systems. Nine intact soil cores (7.6 cm diameter) representing the top 15 cm of mineral soil were collected from both a volcanic and granitic forest-meadow system. Duplicate air-dried soil samples were used to understand the P adsorption potential using the batch isotherm method. Intact cores were subjected to four flow experiments to characterize the effect of P type (organic vs inorganic) and direction of flow (infiltration and exfiltration) on transport of P. In the experiment 6.6 cm of DI water/spiking solutions (at pH ~ 6.5) were applied to the cores mimicking typical spring precipitation totals. The experiments were completed in the following specific order - (i) rainwater infiltration, (ii) Infiltration with inorganic P, (iii) Infiltration with an organic P solution, (iv) and rainwater exfiltration. The study provides important insights on P fate and transport from forest-meadow systems and have important implications of management on downstream water quality.