Kinetics of microbial phosphorus uptake in cultivated soils

Knowledge about the role of microorganisms in P cycling at conditions of constant soil respiration rates and constant size of microbially bound P is lacking. To study the kinetics of microbial P uptake and cycling under such conditions, soils differing in biological activity were (PO4)-P-33 labelled by introducing a carrier-free tracer solution and incubated for 56 days. The (PO4)-P-33 incorporation into the fraction of microbial P releasable by chloroform treatment (P-chl) was assessed and the isotopic composition [=specific activity (SA); SA=(PO4)-P-33/(PO4)-P-31] of P-chl and soil solution P compared. Soils were taken from a 20-year-old field experiment including a non-fertilised control (NON), a minerally fertilised conventional (MIN) and two organic farming systems [bioorganic (ORG); bio-dynamic (DYN)]. Tracer P incorporation continuously increased during incubation in DYN, ORG and MIN soils. It decreased in the order DYN > ORG > MIN, with differences in (PO4)-P-33, uptake between the farming systems being higher than suggested by the differences in the amount of P-chl. In the P-deficient NON soil, the highest initial incorporation of tracer P was found, but no additional uptake could be detected thereafter. In all soils, the SA of P-chl converged to the SA of the soil solution with increasing time. Since P-chl remained almost constant during the experiment, the findings suggest an intensive uptake of P from the soil solution into P-chl and concomitant release of P back to the soil solution and, thus, a rapid cycling through P-chl. Intensive P cycling between P-chl and the soil solution was confirmed in an additional experiment where microbial activity was stimulated by glucose and N additions.