KINETIC-PARAMETERS OF PHOSPHATASE-ACTIVITY ALONG A XERIC DOLOMITIC SOIL CHRONOSEQUENCE RELATED TO WILDFIRES

The kinetic parameters of acid and alkaline phosphatase activities were studied across a fire-caused soil and vegetation chronosequence of semi-arid Mediterranean shrublands. All soils skewed two peaks of optimal phosphatase activity, one in the acid range (pH 4) and another in the alkaline range (pH 11), but values of activity were considerably higher at the alkaline pH (about twice those corresponding to the acid range). For low substrate (p-nitrophenyl phosphate, PNPp) concentrations, mid-successional sites showed the highest phosphatase activity values within the sere. However, for substrate concentrations near saturation, a pattern of increasing phosphatase activity with increasing succesional age was apparent. In most cases, phosphatase activity, both in the acid and alkaline range, followed Michaelis-Menten kinetics. However, more complex kinetics, for instance competitive inhibition kinetic, were also observed in samples collected at the most eroded and recently burned sires. Substrate (PNPp) affinity of acid phosphatases significantly increased along the sere (K-m values decreased). Values of V-max did not show any consistent trend. The K-m values of acid phosphatase were more highly correlated with soil phosphorus fractions than K-m values of alkaline phosphatase. Conversely, correlations between K-m values and soil properties such as the ratio of oxidizable organic carbon to total nitrogen (Co/Nt) and organic carbon concentrations were strongest for alkaline phosphatases.