Short- and long-term effects of a single application of two organic amendments

A frequent side-effect of soil treatment with organic amendments is the slow release of harmful metals deriving from the initial matrices, mainly municipal waste and manure from intensive farming. Contamination is amplified by repeated treatments, which is a common practice to maintain soil fertility. The aim of the present research was to compare, in a mesocosm trial, short (one year)-and long-term (ten years) effects of a single application of compost or a mixture of compost and poultry manure to limestone waste. Attention was focused on pH, organic matter content, metal availability, and microbial biomass and activity. Amendment ecotoxicity at ten years after application was also evaluated. A single application reduced the metal availability and metabolic quotient (an index of stress condition). In the long term, an overall improvement of the environmental conditions has been observed, as the microbial biomass increased, respiration decreased (suggesting low energy requirement) and mineralization activity decreased (likely due to high recalcitrance of residual organic matter). In the brief term, poultry manure played a significant role in improving the environmental conditions as it contributed to reduce the metal availability and to enhance the microbial biomass and activity. In the long term, the overall conditions of both the organic amendments appeared favorable for organisms as low ecotoxicity occurred.