Phosphorus (P) is an essential element for all living organisms and is necessary for plant growth. The biogeochemistry of P during winter months, when plants are dormant and soil microorganisms are potentially active, is poorly understood. Organic matter constitutes the largest source of available P in the soil. This pool is mineralized via extracellular enzyme acid phosphates produced by both plants and microbes. During the growing season, acid phosphatase activity is mediated by the availability of inorganic P (P_i), but this relationship is unclear during plant dormancy. Our objectives were to quantify microbial function in relation to environmental factors controlling P availability (i.e., topography, pH, Al, Ca, etc) during vegetation dormancy. To test the objective we collected soil from 36 plots located within six mixed-oak forests, covering 40 km², in southeast Ohio during winter. Samples were measured for soil fertility by measuring pH, bicarbonate-extractable P (bicarb P), and base cations. Microbial activity was assessed by measuring the activity acid phosphatase (PHOS).

Results/Conclusions

Soil pH ranged from 3.8 to 6.6. The mean percentage of bicarb organic P (P_o) that constituted the bicarb P fraction was 87% whereas bicarb P_i made up the remaining percentage of the P fraction. Soil pH was negatively correlated with bicarb P_o (p<0.01, r = -0.64), while bicarb P_i was positively correlated with pH (p<0.01, r = 0.58). Activity of PHOS ranged from 102 nmol h^-1g^-1 to 2062 nmol h^-1g^-1. Acid phosphatase activity was found to be positively correlated with concentrations of bicarb P_o (p<0.01, r = 0.65), while a negative correlation was found with bicarb P_i (p<0.01, r = -0.66). PHOS activity during plant dormancy seems to be mediated by the availability of P_i. Similar observations have been made  during the growing season. At this time it is unclear if the observed increase in availability of bicarb P_o was caused by increased soil acidity or increases in PHOS activity. Correlations in the data were not able to be explained by site or topographic position. Results from this study are an important starting point in determining the processes that drive winter P biogeochemistry in acidic mixed-oak forests.