Available Organic Soil Phosphorus Has an Important Influence on Microbial Community Composition

Soil acidity can have a potent influence on soil microbial composition and function, but it remains unclear if this influence is a direct response to the concentration of H+ or an indirect response caused by the influence of pH on other soil chemical properties. We hypothesized that the composition and function of soil microorganisms would shift in response to decreases in the availability of inorganic P caused by mobilized Al. We tested this hypothesis at Dysart Woods, a mosaic of old growth and second growth mixed, mesophytic forest in eastern Ohio. We evaluated microbial community composition (phospholipid fatty acid [PLFA] analysis) and function (extracellular enzyme activity) in the context of P availability and common metrics of soil fertility. We observed a threshold response where available organic P (P-o, i.e., bicarbonate-extracted, molybdate-unreactive P) dominated (similar to 80%) the availability of total available P when mobilized Al was present (>0.1 cmol(c) kg(-1)), but when Al was immobilized, available inorganic P (P-i, i.e., bicarbonate-extracted, molybdate-reactive P) content was significantly greater (similar to 50%). Available P-o significantly correlated (r = 0.54) with acid phosphatase activity. Using nonmetric multidimensional scaling, we observed that PLFA composition ordination was significant along a gradient of soil pH and available P-o. Because the composition and function of the microbial community changes due to available P-o, results suggest that microbial communities in acidic forest soils are functionally P limited. The availability of P-o did not influence the microbial biomass, however, indicating that the microbial community compensates for the apparent P limitation by altering its composition.