Secretions for P. australis were higher in presence of sludge and differences between treatments were significant in spring for POD specific activity (p = 0.03) ( Table 1). It could be thought that these differences result from differences in above-ground biomass, as 5–25% of the photosynthetically fixed carbon can be released in the rhizosphere via a wide range of organic compounds including enzymes, but similar biomass was observed between treatments at this season, in spring. Most probably, POD exudation was induced by sludge exposure, which is in accordance with others studies where plants show higher POD activity in presence of xenobiotics such as azo dye acid orange