Impacts of invasive Australian acacias on soil bacterial community composition, microbial enzymatic activities, and nutrient availability in fynbos soils

Invasive nitrogen-fixing plants often impact the soils they invade, notably through changes in soil chemistry and microbial community composition, which in turn could lead to alterations in soil functionality. We aimed to determine the impacts that invasive Australian Acacia trees have on soil chemistry and function (carbon, nitrogen, and phosphorus cycling) in South Africa's Core Cape Subregion, and whether any differences in soil function are linked to differences in soil chemical properties and bacterial community composition between neighbouring acacia-invaded and uninvaded sites. We do so by using Next Generation Sequencing (NGS) methods together with soil chemistry and soil enzyme functioning profiles. Acacias significantly increased levels of soil nitrogenous compounds (NO3-, NH4+, and total N), C, and pH. Acacias also significantly elevated microbial phosphatase activity, but not β-glucosidase, while having contrasting effects on urease. Acacia-induced changes in soil chemical properties were correlated with changes in enzyme activities for both urease and phosphatase. Changes in soil bacterial community composition due to the presence of invasive acacias were also correlated with phosphatase enzymatic activity. While we found evidence for acacias altering soil function by changing soil chemical properties and bacterial community composition, the legume-soil feedbacks underlying these impacts appear to be context specific.