Impacts of Invasive Australian Acacias on Soil Bacterial Community Composition, Microbial Enzymatic Activities, and Nutrient Availability in Fynbos Soils

A nature-positive future with biological invasions: theory, decision support and research needs

In 2050, most areas of biodiversity significance will be heavily influenced by multiple drivers of environmental change. This includes overlap with the introduced ranges of many alien species that negatively impact biodiversity. With the decline in biodiversity and increase in all forms of global change, the need to envision the desired qualities of natural systems in the Anthropocene is growing, as is the need to actively maintain their natural values. Here, we draw on community ecology and invasion biology to (i) better understand trajectories of change in communities with a mix of native and alien populations, and (ii) to frame approaches to the stewardship of these mixed-species communities. We provide a set of premises and actions upon which a nature-positive future with biological invasions (NPF-BI) could be based, and a decision framework for dealing with uncertain species movements under climate change. A series of alternative management approaches become apparent when framed by scale-sensitive, spatially explicit, context relevant and risk-consequence considerations. Evidence of the properties of mixed-species communities together with predictive frameworks for the relative importance of the ecological processes at play provide actionable pathways to a NPF in which the reality of mixed-species communities are accommodated and managed. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.

The invasive legume Lupinus polyphyllus has minor site-specific impacts on the composition of soil bacterial communities

Plant invasions can have major impacts on ecosystems, both above- and belowground. In particular, invasions by legumes, which often host nitrogen-fixing symbionts (rhizobia), are known to modify soil bacterial communities. Here, we examined the effect of the invasive herbaceous legume Lupinus polyphyllus on the alpha diversity and community composition of soil bacteria. We also explored the relationships between these bacterial communities and vegetation cover, the cover of other (non-invasive) legumes, or the number of vascular plants present. For this, we sampled rhizosphere soil and surveyed vegetation from ten paired sites (uninvaded versus invaded more than 10 years ago) in southwestern Finland, and identified bacterial DNA using 16S rRNA gene amplicon sequencing. The presence of the plant invader and the three vegetation variables considered had no effect on the alpha diversity of soil bacteria in terms of bacterial richness or Shannon and Inverse Simpson diversity indices. However, the composition of soil bacterial communities differed between invaded and uninvaded soils at four out of the ten sites. Interestingly, the relative abundances of the top bacterial families in invaded and uninvaded soils were inconsistent across sites, including for legume-associated rhizobia in the family Bradyrhizobiaceae. Other factors-such as vegetation cover, legume cover (excluding L. polyphyllus), number of plant species-also explained a small proportion of the variation in bacterial community composition. Our findings indicate that L. polyphyllus has the potential to modify the composition of local soil bacterial community, at least in sites where it has been present for more than a decade.

Microbial communities in terrestrial surface soils are not widely limited by carbon

Microbial communities in soils are generally considered to be limited by carbon (C), which could be a crucial control for basic soil functions and responses of microbial heterotrophic metabolism to climate change. However, global soil microbial C limitation (MCL) has rarely been estimated and is poorly understood. Here, we predicted MCL, defined as limited availability of substrate C relative to nitrogen and/or phosphorus to meet microbial metabolic requirements, based on the thresholds of extracellular enzyme activity across 847 sites (2476 observations) representing global natural ecosystems. Results showed that only about 22% of global sites in terrestrial surface soils show relative C limitation in microbial community. This finding challenges the conventional hypothesis of ubiquitous C limitation for soil microbial metabolism. The limited geographic extent of C limitation in our study was mainly attributed to plant litter, rather than soil organic matter that has been processed by microbes, serving as the dominant C source for microbial acquisition. We also identified a significant latitudinal pattern of predicted MCL with larger C limitation at mid- to high latitudes, whereas this limitation was generally absent in the tropics. Moreover, MCL significantly constrained the rates of soil heterotrophic respiration, suggesting a potentially larger relative increase in respiration at mid- to high latitudes than low latitudes, if climate change increases primary productivity that alleviates MCL at higher latitudes. Our study provides the first global estimates of MCL, advancing our understanding of terrestrial C cycling and microbial metabolic feedback under global climate change.

Strategies for the Management of Aggressive Invasive Plant Species

Current control methods for invasive alien plants (IAPs) have acceptable short-term outcomes but have proven to be unfeasible or unaffordable in the long-term or for large invaded areas. For these reasons, there is an urgent need to develop sustainable approaches to control or restrict the spread of aggressive IAPs. The use of waste derived from IAP control actions could contribute to motivating the long-term management and preservation of local biodiversity while promoting some economic returns for stakeholders. However, this strategy may raise some concerns that should be carefully addressed before its implementation. In this article, we summarize the most common methods to control IAPs, explaining their viability and limitations. We also compile the potential applications of IAP residues and discuss the risks and opportunities associated with this strategy.

Litter Deposition and Nutrient Cycling of Invaded Environments by Cryptostegia madagascariensis at Tropical Cambisols from Northeastern Brazil

Cryptostegia madagascariensis is an invasive plant species that covers 11% of the Brazilian northeastern territory, but its role on the litter trait in tropical ecosystems remains unclear. Here, we analyzed and compared the litter deposition, litter nutrient content, soil organic matter, and the litter decay rate from invaded and non-invaded environments by C. madagascariensis at a tropical Cambisol. The PCA analysis revealed that litter deposition, litter quality, and soil organic matter were correlated with the invaded environment. We grew plant species in greenhouse conditions to obtain a standard litter material to use in our litter bags in field conditions. We found that litter decay rate was higher in the invaded environment than in the non-invaded one. Our results suggest that C. madagascariensis changes litter traits in tropical ecosystems that in turn create negative plant–soil feedback to the native species by creating a physical barrier on soil surface and to promote its own rhizosphere.

Quercus rubra invasion of temperate deciduous forest stands alters the structure and functions of the soil microbiome

Invasive plants can modify the diversity and taxonomical structure of soil microbiomes. However, it is difficult to generalize the underlying factors as their influence often seems to depend on the complex plant-soil-microbial interactions. In this paper, we investigated how Quercus rubra impacts on the soil microbiome across two soil horizons in relation to native woodland. Five paired adjacent invaded vs native vegetation plots in a managed forest in southern Poland were investigated. Soil microbial communities were assessed along with soil enzyme activities and soil physicochemical parameters, separately for both organic and mineral horizons, as well as forest stand characteristics to explore plant-soil-microbe interactions. Although Q. rubra did not significantly affect pH, organic C, total N, available nutrients nor enzymatic activity, differences in soil abiotic properties (except C to N ratio) were primarily driven by soil depth for both vegetation types. Further, we found significant differences in soil microbiome under invasion in relation to native vegetation. Microbial richness and diversity were lower in both horizons of Q. rubra vs control plots. Moreover, Q. rubra increased relative abundance of unique amplicon sequence variants in both horizons and thereby significantly changed the structure of the core soil microbial communities, in comparison to the control plots. In addition, predicted microbial functional groups indicated a predominant soil depth effect in both vegetation plots with higher abundance of aerobic chemoheterotrophic bacteria and endophytic fungi in the organic horizon and greater abundance of methanotrophic and methylotrophic bacteria, and ectomycorrhizal fungi in the mineral horizon. Overall, our results indicate strong associations between Q. rubra invasion and changes in soil microbiome and associated functions, a finding that needs to be further investigated to predict modifications in ecosystem functioning caused by this invasive species.

Effects of Moss-Dominated Biocrusts on Soil Microbial Community Structure in an Ionic Rare Earth Tailings Area of Southern China

Moss-dominated biocrusts are widespread in degraded mining ecosystems and play an important role in soil development and ecosystem primary succession. In this work, the soil microbial community structure under moss-dominated biocrusts in ionic rare earth tailings was investigated to reveal the relationship between different types of moss and taxonomy/function of microbiomes. The results showed that microbial community structure was significantly influenced by four moss species (Claopodium rugulosifolium, Orthotrichum courtoisii, Polytrichum formosum, and Taxiphyllum giraldii). The microbial assembly was more prominent in Claopodium rugulosifolium soil than in the other moss soils, which covers 482 bacterial genera (including 130 specific genera) and 338 fungal genera (including 72 specific genera), and the specific genus is 40% to 1300% higher than that of the other three mosses. Although only 141 and 140 operational taxonomic units (OTUs) rooted in bacterial and fungal clusters, respectively, were shared by all four mosses grown in ionic rare earth tailings, this core microbiome could represent a large fraction (28.2% and 38.7%, respectively) of all sequence reads. The bacterial population and representation are the most abundant, which mainly includes Sphingomonas, Clostridium_sensu_stricto_1, and unclassified filamentous bacteria and chloroplasts, while the fungi population is relatively singular. The results also show that biocrust dominated by moss has a positive effect on soil microbe activity and soil nutrient conditions. Overall, these findings emphasize the importance of developing moss-dominated biocrusts as hotspots of ecosystem functioning and precious microbial genetic resources in degraded rare-earth mining areas and promoting a better understanding of biocrust ecology in humid climates under global change scenarios.

Increasing the Contents of Paddy Soil Available Nutrients and Crop Yield via Optimization of Nitrogen Management in a Wheat–Rice Rotation System

To explore the impact of nitrogen (N) rate during the wheat season and N fertilizer management during the rice season on carbon and soil nutrient pools in paddy soil, a wheat–rice rotation system for 2 successive years was implemented. In the rotation system, a conventional N rate (Nc; 150 kg/ha) and a reduced N rate (Nr; 120 kg/ha) were applied in the wheat season. Based on an application rate of 150 kg/hm² N in the rice season, three N management models were applied, in which the application ratio of base:tiller:panicle fertilizer was 20%:20%:60% in treatment M1, 30%:30%:40% in treatment M2, and 40%:40%:20% in treatment M3. Zero N was used as the control (M0). Experimental results indicate that, under Nc, the M2 management model during the rice season, improvements were seen in paddy soil urease, organic carbon, and annual yield relative to other conditions. The average organic matter and total N associated with the M2 rice management model and conventional N application during the wheat season were 5.13% and 4.95% higher, respectively, relative to the use of a reduced N application rate during the wheat season. Similarly, the average total carbon content and annual yields were 6.61% and 5.56% higher under the model M2 with conventional N application during the wheat season, respectively, relative to reduced N application after the two-year study period. These findings indicate that production and carbon fixation in paddy fields can be optimized through a conventional N application rate during the wheat season, and an M2 N management model during the rice season in southern China under a wheat–rice rotation system.

Acacia invasion differentially impacts soil properties of two contrasting tropical lowland forests in Brunei Darussalam

Invasive Acacia species are known to modify soil properties, although effects are often site-specific. We examined the impact of Acacia species on the soils of intact and invaded habitats of two contrasting tropical lowland rain forest types in Brunei Darussalam: heath forest (HF) and mixed dipterocarp forest (MDF). Impacts on soil properties differed between the two forest types. Overall, Acacia-invaded HF soil recorded significantly higher gravimetric water content, pH and total P, K and Ca compared to the intact HF soil. In contrast, invaded MDF soil exhibited significantly higher organic matter content and total soil N, P, K and Mg compared to its intact habitat. Acacia-invaded MDF soils were more nutrient-enriched than Acacia-invaded HF soils by the addition of threefold, threefold and fourfold total soil P, K and Mg, respectively. The positive effect of addition of total soil Ca was, however, fourfold greater in HF soil than MDF soil, indicating that the magnitude of impact on soil properties was strongly site-specific. Overall, Acacia invasion significantly impacted soil properties in nutrient-rich MDF more than those of nutrient-poor HFs, indicating a potential vulnerability of MDFs to invasion.

How does familiarity in rhizobial interactions impact the performance of invasive and native legumes?

Mutualisms can be disrupted when non-native plants are introduced into novel environments, potentially impacting their establishment success. Introduced species can reassemble mutualisms by forming novel associations with resident biota or by maintaining familiar associations when they are co-introduced with their mutualists. Invasive Australian Acacia species in South Africa have formed nitrogen-fixing rhizobium mutualisms using both pathways.

Here we examined the contributions of novel vs familiar rhizobial associations to the performance of Acacia saligna across different soils within South Africa’s Core Cape Subregion (CCR), and the concomitant impacts of exotic rhizobia on the endemic legume, Psoralea pinnata. We grew each legume with and without Australian Bradyrhizobium strains across various CCR soil types in a glasshouse. We identified root nodule rhizobium communities associating with seedlings grown in each treatment combination using next-generation sequencing (NGS) techniques.

Our results show that different CCR soils affected growth performances of seedlings for both species while the addition of Australian bradyrhizobia affected growth performances of A. saligna, but not P. pinnata. NGS data revealed that each legume associated mostly with their familiar rhizobial partners, regardless of soil conditions or inoculum treatment. Acacia saligna predominantly associated with Australian bradyrhizobia, even when grown in soils without inoculum, while P. pinnata largely associated with native South African Mesorhizobium strains.

Our study suggests that exotic Australian bradyrhizobia are already present and widespread in pristine CCR soils, and that mutualist limitation is not an impediment to further acacia invasion in the region. The ability of P. pinnata to sanction Australian Bradyrhizobium strains suggests that this species may be a good candidate for restoration efforts following the removal of acacias in CCR habitats.