Impacts of acacia longifolia invasion on soil nutrient cycles : from invasion to solution

Anthropogenic alterations of nutrient cycles and the global redistribution of plant species have a profound impact on soils and the ecosystems relying on them. Invasive woody legumes can engineer oligotrophic ecosystems towards increased biomass production and nutrient turnover. This increased biomass is detrimental to native ecosystems, but could also be useful as a compost feedstock for agricultural purposes. The aim of this study was to observe soil changes after Acacia longifolia invasion in Portuguese dune systems, using Corema album foliar δ15N as a tracer for invasion impact. It provides novel insight into initial invasion of nursery shrubs and changes in soil fractions. It also relates community-scale aboveground invasion impact with organic matter pools and fluxes underneath A. longifolia canopy, compared with the native legume S. spectabilis. Furthermore, A. longifolia compost was evaluated for the first time as a potential agricultural soil amendment. Degradation of this compost in soil was observed in controlled conditions and its effects on maize growth and kernel quality studied in an urban garden setting. A. longifolia dune invasion increases the siltclay fraction, root and rhizosphere biomass. Increased soil phosphorus cycling and lower tissue phosphorus concentrations create an N/P imbalance in the oligotrophic system. Co-composted A. longifolia litter/biomass and biomass compost alone are increasing soil microbial activity. Biomass compost has beneficial effects on maize growth and provides, mixed with nutrient-rich compost, maize productivity levels comparable to mineral fertilization. Further results show open-pollinated maize varieties exhibit increased kernel micronutrient concentrations under compost fertilization, compared to hybrid maize. Summarized, N/P co-limitation needs to be considered when observing oligotrophic ecosystem invasion. Also, belowground processes precede aboveground effects, making early removal of young plants imperative for ecosystem conservation. Composted biomass can be safely employed as a soil amendment with beneficial effects on various soil and plant parameters, potentially encouraging future eradication