Evaluation of three organic wastes for reclaiming burnt soils: improvement in the recovery of vegetation cover and soil fertility in pot experiments

The efficacy of three abundant organic wastes: poultry manure (PM), cattle slurry (CS) and sewage sludge (SS) for the reclamation of burnt soils was evaluated. A forest soil, previously furnace-heated in order to simulate exposure to a high-intensity wildfire, was labelled with nitrogen-15 (15N) to evaluate the contribution of N derived from the organic waste to the burnt soil and vegetation. Four treatments were performed with the heated 15N-labelled soil: an unamended control soil (S) and three waste amended soils (S+PM, S+CS and S+SS) at a dose waste of 167 mg total N kg-1 soil. Lolium perenne was grown in all the pots for 3 months. In each treatment the phytomass produced and its N content decreased significantly in the following order of treatments: S+PM much greater than S+CS > S+SS much greater than S. The percentage of plant N derived from the waste was similar in the S+PM (22.8%) and S+CS (24.0%) treatments, but significantly lower in the S+SS treatment (16.5%). At the end of the 3 month experimental period, the available N reserves (phytomass N + soil inorganic N) in the control soil accounted for 51.5-71.5% of those in the S+PM, S+CS and S+SS treatments, whereas the yield of the plants was only 13.4-29.8% of that in the manured soils. These results demonstrated the importance of the addition of organic wastes, particularly PM, for the recovery of the vegetation cover and for the stabilization of the soil ash layer. They also showed that the level of N was not the main controlling factor of plant growth in the control soil, which, moreover, did not show evidence of a shortage of macronutrients, i.e. phosphorus, potassium, calcium or magnesium. It is hypothesized that, as occurs in heat-sterilized soils, phytomass production in the control-heated soil could have been inhibited by the heat-induced production of phytotoxic compounds, their negative effects being microbially or chemically suppressed by the addition of organic wastes.