COMPOSTING OF ACACIA LONGIFOLIA AND ACACIA MELANOXYLON INVASIVE SPECIES

Acacias are invasive Fabaceae species that are highly competitive and a serious threat to local biodiversity and natural habitats. Taking into account their high availability and low cost, a valorization approach for acacia shrubs may be composting to produce horticultural organic amendments and substrate components. With this aim, two big piles (100 m³) were set up with grinded and screened Acacia longifolia and Acacia melanoxylon shrubs, and managed with different turning frequency, to analyze the physicochemical characteristics during composting and to model the breakdown of acacia organic matter (OM). Time-temperature conditions in both piles exceeded the more stringent criteria for complete pathogen inactivation as temperatures were between 65°C and 75°C for several months, indicating high amount of biodegradable OM in this material, as found here by the high amount of mineralizable OM (640-690 g kg^-1 of initial OM). High temperature and high pH conditions promoted significant N losses (455-465 g kg^-1 of initial N). Nevertheless, these were smaller compared to C losses and so the C/N ratio decreased from 50 to 29-32 after 231 days of composting. This study indicates that composting acacia can produce organic amendments with high OM content, and low electrical conductivity (< 1.3 dS m^-1). However, a long period of composting (>231 days) is required to achieve full compost maturation.