Phosphorus in Soils of Temperate Forests: Linkages to Acidity and Aluminum

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 acidification experiments. Although total soil P concentrations varied widely among these watersheds, the proportions of P fractions were relatively uniform. The mean for the P fraction operationally defined as being associated with Al in the reference watersheds was 71% of total extractable P (SE <= 1%). Experimental whole-watershed acidification resulted in significant 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 flow 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 0 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 0 horizon available P concentrations. The net effect 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. These 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.