Acid deposition can have a potent influence on soil function because it accelerates soil acidification and alters nutrient availability.  The availability of inorganic phosphorous (P_i) can decrease with pH due to increase in geochemical binding with mobilized aluminum (Al).  We reason that the composition and function of soil microorganism will shift in response to decreases in the availability of P_i cause by mobilized Al.  Our primary objectives were to (1) determine the amount of available P_i and organic phosphorous (P_o) in relation to metrics of soil acidity and (2) evaluate the relationship between P availability and other metrics of soil quality with microbial community composition and function.  The objectives were tested at Dysart Woods, an uneven aged mixed-mesophytic forest in the unglaciated Allegheny Plateau.  Thirty-six plots were arranged throughout a 78 ha area.  A natural pH gradient (4.2 to 6.1) was established using 12 transects parallel with slope.  Bicarbonate extracted P_i and P_o were used as metrics of available P.  Indicators of soil quality were pH, mobilized Al, base cations, NH₄⁺, total C, N, and P.  We evaluated microbial community composition using phospholipid fatty acid analysis (PLFA) and community function with extracellular enzyme acid phosphatase. 

Results/Conclusions

The availability of P_o had a significant correlation with soil pH (r = -0.65) and Al (r = 0.59).  We observed a threshold response where P_o was the dominant (~ 80%) form of available P when mobilized Al was present (> 0.1 cmol_c kg^-1), but was similar or lower than P_i when Al was immobilized.  Acid phosphatase activity significantly correlated with P_o (r = 0.54) and soil pH (r = -0.41).  Using non metric multidimensional scaling (NMS), we observed PLFA composition separated significantly out along a gradient of soil pH (r = 0.37) and available P_o (r = -0.35), but not with other measured metrics of soil quality.  Results cannot be reasonable explained by variations in microclimate or vegetation due to slope position.  It appears available P_o is an important indicator of general microbial community composition, especially fungal PLFAs.  Microbial biomass did not change suggesting soil microbial communities were not suppressed by P availability.  Nevertheless, results indicate microbial communities in acidic soils are functionally P-limited because the composition and function of the microbial community changed due to P availability.