Phytotoxicity of organic amendments on activities of select soil enzymes

Three organic materials, a municipal solid waste compost (MWC), a paper slu dge (PS) and an agroforest compost (AC), were tested to relate their stabil ity to the corresponding soil enzyme activity resulting from their applicat ion. PS and AC were stable materials, free of phytotoxic substances, althou gh AC presented direct toxicity due to its high salinity and inhibited seed germination. The presence of heavy metal does not seem playing an importan t role in the phytotoxicity of the tested materials since, the highest valu es of germination index were obtained with PS material despite its high hea vy metals concentration. However, MWC presented a low level of maturity aff ecting negatively seed germination, and indicating an inaccurate composting process. The application of the three organic materials to a sandy soil (at a rate e quivalent to about 50,000 kg ha(-1)) increased soil enzyme activities (dehy drogenase, beta -glucosidase, urease and benzoyl-argininamide hydrolyzing ( BAA)) with respect to the control (soil without organic matter application) during one month of incubation. The positive effect of the organic materia ls on enzyme activities was more pronounced in the case of the MWC, a fresh organic residue having low organic matter stability. It suggested that the presence of phytotoxic substances did not affect negatively soil enzyme ac tivities. Probably, molecules or promoters released by decomposing organic compounds which affect negatively seed germination, enhance enzyme activiti es.