• SanClements, Michael D.
  National Ecological Observatory Network
  
• Ivan J. Fernandez
  University of Maine, School of Forest Resources, Climate Change Institute, School of Food and Agriculture
  ivanjf@maine.edu
  2075812932
  
• Stephen A. Norton
  University of Maine, Earth and Climate Sciences
  

We used P fractionation techniques to study the accumulation, mobilization, and availability of soil P in six watersheds of the eastern United States and Europe, two of which included paired long-term acidifi cation experiments. Although total soil P concentrations varied widely among these watersheds, the proportions of P fractions were relatively uniform. Th e mean for the P fraction operationally defi ned as being associated with Al in the reference watersheds was 71% of total extractable P (SE ≤ 1%). Experimental whole-watershed acidifi cation resulted in signifi cant depletion of Al-P concentrations from the upper mineral soil in treated watersheds due to the dissolution of Al hydroxide by acidic solutions traveling along shallow fl ow paths. Acidic soil solutions mobilize both Al and P, leading to P depletion from the Al-P fraction in the mineral soils. Across this suite of watersheds, lower pH appears to decrease Al/P ratios in the Al-P fraction of these mineral soils through changes in Al solubility. Biocycling in these forests can play a critical role in linking subsurface mineral soil P to surface O horizon available P. In this study, sites with the lowest mineral subsoil Al/P ratios generally had the lowest mineral soil pH values and the highest O horizon available P concentrations. Th e net eff ect was to leave subsoil P more bioavailable at the lower pH values because of a lower Al/P ratio in the mineral subsoil at the pH range of 3.1 to 4.6 found in this study. Th ese results suggest that changes in soil acidity due to management, air pollutants, or pedogenesis could shift P availability by altering acidity and the Al/P balance.