Plant responses to amendment with compost enriched with exhausted fire-extinguishing powders in a microcosm study

Multi-purpose ABC-E fire-extinguishing powders (FEP) are constituted by a finely divided mixture of mineral salts composed of mono-ammonium phosphate and ammonium sulfate (>95%, by weight). To improve their rheological properties and water-repellence, FEP are externally coated with silicone-based additives which make problematic their disposal after the service life has expired (36 months), with no chance of reuse their inner mineral core. In a previous study we developed an innovative method to biologically lyse the particle external coating and recover the phosphate-containing salts for the production of a nutrient-enriched compost for use in agriculture. In order to assess plant growth and root system responses to soil amendment with a FEP nutrient-enriched compost a plant bioassay was carried out by using lettuce (Lactuca sativa L.) seedlings in a microcosm-scale experiment. One pre-germinated lettuce (Lactuca sativa L.) seed was transplanted into each microcosm at the beginning of the trial and left growing for 28 days in a growth chamber under controlled conditions. Soil moisture content was kept constant at 70% field capacity. Microcosms were randomly set up with 4 compost rates (0, 1, 2 and 3 kg m-2), 2 compost types (enriched and non-enriched with FEP) and 2 plant treatments (with and without lettuce plants). Soluble soil nutrients (phosphate, ammonium, sulfate, chloride, potassium) increased with increasing compost rates in all amended microcosms, especially in FEP-enriched compost treatments. Compost increased plant growth compared to untreated plants and this positive result was further amplified by FEP-enriched compost addition, which provided the highest values of fresh and dry weight of both lettuce roots and leaves. Biomass distribution between the root (RMR) and the leaf apparatus (LMR) significantly varied according to the treatment, with an increased growth of the shoot over the root portion in soils added with FEP-enriched compost. Moreover, root morphological parameters (length, diameter, volume and surface area) were differently influenced, suggesting a different acquisition ability of below-ground resources, depending on the treatment. These results confirm that the addition of FEP-enriched compost increased the availability of soluble plant nutrients, and the morphological changes observed in the root system were aimed at absorbing water and solutes more quickly so as to support a faster growth rate.