Impacts by invasive plants are easily observed in the aboveground components of communities they invade, but only recently have ecologists started appreciating and quantifying belowground impacts, including changes in soil chemistry, nutrients, soil microbial communities and function. Soil microbial communities are very dynamic and complex and play important roles in ecosystem functioning and in structuring aboveground plant communities. Such feedbacks may manifest as impacts when invasive plants become dominant community components.
Highly invasive nitrogen-fixing legumes, like members of the genus Acacia, can alter soil nutrients as well as microbial community structure and function, possibly to their own benefit. Following removal of invasive biomass (clearing), native species recovery is usually slow, probably as a consequence of altered abiotic and biotic soil conditions.
Using a unique situation in the highly diverse Core Cape Subregion (CCR) of South Africa, we compared whole soil microbial community diversity, composition, function and nutrient loads between pristine and acacia-invaded habitats. Using a combination of soil chemistry, soil microbiome data, and enzymatic assays we aimed to determine whether various invasive acacia species induce similar changes in 1) soil chemistry, 2) soil microbial diversity and composition and 3) soil microbial function across geographically isolated regions in the CCR.
Results/Conclusions
We show that invasive Australian acacias significantly alter soil nutrient profiles, especially NH4. Despite this, microbial enzymatic functions related to N, P and C cycling remained similar across invaded and uninvaded sites, but showed strong seasonal variation. Whole soil microbial diversity was significantly different at a few sites only and without clear directionality, rather showing strong temporal variation. In contrast, we found the presence of invasive acacias to always significantly change soil microbial community structure and composition, irrespective of geography. That is, invaded sites were always significantly enriched for Gammaproteobacteria while being depauperate in Alphaproteobacteria. Gammaproteobacteria are copiotrophs, i.e. microbes found in environments rich in nutrients, particularly carbon and their enrichment may reflect acacia-induced changes in soil nutrients. Taken together these results demonstrate a clear uncoupling of invasive legume impacts on bacterial community composition and function. Using Structural Equation Modelling we show strong causal pathways that link invasion effects on soil nutrients and bacterial communities, microbial function, and their seasonal dependency. Our results illustrate that belowground impacts by invasive plants are complex and they should not be studied in isolation, i.e. a disconnection between the typical soil impacts by invasive plants as measured by soil microbial community function and structure.