Effect of a compost mulch on seed germination and plant growth in a burnt forest soil from NW Spain

In this work, a revegetation experiment was performed on a forest soil artificially burnt in the laboratory, and the results of the soil without treatment were compared to those of the soil capped with an organic mulch. Plant biomass, as well as the variation of pH, electric conductivity (EC), and phytotoxicity with depth, were recorded. Only a very low plant growth was observed in the uncapped soil. The substrate-amended soil had a higher production than the uncapped soil, despite showing an increased EC due to the mulch. The increase of pH or salinity during burning can be discarded as the cause of the different plant growth, because the values reached were not high enough to justify the negative effect observed. The determination of phytotoxicity after burning the soil at temperatures between 100°C and 500°C suggested that the problems observed for seed germination in the burnt soil might be linked to the formation of undetermined phytotoxic substances after soil heating at a temperature near 200°C. Although polycyclic aromatic hydrocarbons followed the same trend as phytotoxicity with temperature, their concentrations in soil were too low to be the cause of the effects observed, so they can be discarded as playing an important role in phytotoxicity.