Responses of soil phosphorus cycling and bioavailability to plant invasion in river-lake ecotones

The invasion of exotic plants in the river-lake ecotone has seriously affected the nutrient cycling processes in wetland soil. The South American species Alternanthera philoxeroides (Mart.) Griseb. is rapidly invading the river-lake ecotone in subtropical China, and has become the dominant species in the river-lake ecotone. However, there have been few studies on the effects of A. philoxeroides invasion on soil phosphorus (P) cycling and bioavailability in this ecotone. Here, we measured the bioavailable P fractions, physicochemical properties and nutrient content in the surface soils of the native plant (Zizania latifolia (Griseb.) Turcz and Nelumbo nucifera Gaertn.) communities and the adjacent invasive A. philoxeroides communities in three river-lake ecotones with different nutrient substrates in the subtropical Dongting Lake basin over a 3-year period to reveal the effects of A. philoxeroides invasion on the morphology and concentrations of soil bioavailable P. The principal coordinate analysis results showed that the A. philoxeroides invasion significantly altered the bioavailable P concentrations in the soil of native plant communities in the different river-lake ecotones, and this effect was not disturbed by the heterogeneity of the soil matrix. However, the effects of invasion into different native plant communities on the fractions of soil bioavailable P were different. Compared with native Z. latifolia and N. nucifera communities, A. philoxeroides invasion increased the concentration of inorganic P by 39.5% and 3.7%, respectively, and the concentration of organic P decreased by 32.7% and 31.9%, respectively. In addition, the invasion promoted P cycling and accumulation in the river-lake ecotone, which resulted in average decreases in the soil N:P and C:P ratios of 7.9% and 12.5%, respectively. These results highlight the impact of exotic plant invasions on nutrient cycling in wetland ecosystems in the river-lake ecotone, and this process may be detrimental to the late recovery of native plants.

Identity and Diversity of Invasive Plant Affecting the Growth of Native Lactuca indica

Despite the significant number of studies that have recently focused on plant invasion and invasive plants' success, many uncertainties still exist on the effects of invasive plant identity and diversity on the native plant response under different levels of diversity. A mixed planting experiment was conducted using the native Lactuca indica (L. indica) and four invasive plants. The treatments consisted of 1, 2, 3, and 4 levels of invasive plants richness in different combinations in competition with the native L. indica. Here, the results showed that native plant response depends on the invasive plant identity and invasive plant diversity, which increases the native plant total biomass under 2-3 levels of invasive plant richness and decreases under high invasive plant density. This plant diversity effect was more significant in the native plant relative interaction index, which shows negative values except under a single invasion with Solidago canadensis and Pilosa bidens. The native plant leaf nitrogen level increased under four levels of invasive plant richness, which means more affected by invasive plant identity than invasive plant diversity. Finally, this study demonstrated that native plant response under invasion depends on the identity and diversity of invasive plants.

Nitrate Nitrogen and pH Correlate with Changes in Rhizosphere Microbial Community Assemblages during Invasion of Ambrosia artemisiifolia and Bidens pilosa

The rhizosphere of invasive plants presumably develops different soil microbial assemblages compared with native plants, which may hinder or promote their invasion. However, to date, no studies have clearly explored rhizosphere microbial community assemblages during invasion. The invasive species Ambrosia artemisiifolia L. and Bidens pilosa L. are widely distributed in China and are known to reduce local biodiversity and cause agricultural losses. Monoculture of A. artemisiifolia or B. pilosa, a mixture of each invasive and native species, and monoculture of native species were established to simulate different degrees of invasion. Metagenomic sequencing techniques were used to test microbial community structure and function. The aim was to explore the drivers of the assembly of peculiar functional microbes in the rhizosphere soil of invasive species during the long-term invasive-native species interaction. Compared with the native species, the relative abundance of 34 microbial genera was higher in the rhizosphere soil of the invasive species. The NO₃-N concentration in the rhizosphere soil from the A. artemisiifolia and B. pilosa monocultures was lower than that from monocultures of the three native plants, whereas pH followed the opposite trend. The NO₃-N concentration was significantly and negatively correlated with Sporichthya, Afipia, Actinokineospora, and Pseudolabrys. pH was positively correlated with Bradyrhizobium, Actinoplanes, Micromonospora, Steroidobacter, Burkholderia, and Labilithrix. The differences in soil microbes, NO₃-N concentrations, and pH between native and invasive species suggest that the rhizosphere soil microbial assemblages may vary. The reduced NO₃-N concentration and increased pH corelated with changes in rhizosphere microbial community during A. artemisiifolia and B. pilosa invasion. IMPORTANCE Soil microbial communities play a vital role in the growth of invasive plants. Invasive species may shape peculiar functional microbes in the rhizosphere soil of an invasive species to benefit its growth. However, the drivers of the assembly of soil microbial communities in the rhizosphere soil of invasive species remain unclear. Our study established the relationship between soil microbial communities and soil chemical properties during invasion by A. artemisiifolia and B. pilosa. Additionally, it showed that the presence of the invasive plants correlated with changes in NO₃-N and pH, as well as in rhizosphere microbial community assemblage. Furthermore, the study provided important insights into the difference in the microbial community assembly between native and invasive plant species.

What modulates the impacts of acid rain on the allelopathy of the two Asteraceae invasives?

Most of the allelopathic studies have focused on the independent allelopathy of one invasive plant, but have ignored the co-allelopathy of the two invasives. The variations in the type of acid rain can modulate the invasiveness of invasives via the changes in the allelopathy. Thus, it is vital to elucidate the allelopathy of invasives, particularly the co-allelopathy of the two invasives, under acid rain with different types, to illuminate the mechanisms driving the co-invasion of two invasives under diversified acid rain. However, little progress has been finished in this aspect presently. This study aimed to evaluate the co-allelopathy of two Asteraceae invasives Solidago canadensis L. and Erigeron annuus L. treated with acid rain with different nitrogen-to-sulfur ratios on seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. via a hydroponic experiment. Aqueous extracts of the two Asteraceae invasives generated obvious allelopathy on L. sativa. S. canadensis aqueous extracts caused stronger allelopathy. There may be an antagonistic effect for the co-allelopathy of the two Asteraceae invasives. Nitric acid at pH 5.6 weakened the allelopathy of the two Asteraceae invasives, but the other types of acid rain strengthened the allelopathy of the two Asteraceae invasives. The allelopathy of the two Asteraceae invasives increases with the increasing acidity of acid rain, but the allelopathy of the two Asteraceae invasives decreases with the increasing nitrogen-to-sulfur ratio of acid rain. Accordingly, the species number of invasives, and the acidity and type of acid rain modulated the impacts of acid rain on the allelopathy of the two Asteraceae invasives.

An exotic plant successfully invaded as a passenger driven by light availability

Invasive exotic plant species (IEPs) are widely distributed across the globe, but whether IEPs are drivers or passengers of habitat change in the invaded spaces remains unclear. Here, we carried out a vegetation and soil survey in 2018 and two independent field experiments (Pedicularis kansuensis removal in 2014 and 2015, and fertilization experiment since 2012) and found that the invasive annual P. kansuensis was at a disadvantage in light competition compared with perennial native grasses, but the successful invasion of P. kansuensis was due to the sufficient light resources provided by the reduced coverage of the native species. Conversely, nitrogen enrichment can effectively inhibit P. kansuensis invasion by increasing the photocompetitive advantage of the native species. sP. kansuensis invasion did not reduce species richness, but did increase plant community coverage, productivity and soil nutrients. Furthermore, the removal of P. kansuensis had little effect on the plant community structure and soil properties. Our results suggest that the passenger model perfectly explains the benign invasive mechanism of P. kansuensis. The invasion "ticket" of P. kansuensis is a spare ecological niche for light resources released by overgrazing.

Effects of Invasive Plant Diversity on Soil Microbial Communities

Native plant communities can be invaded by different numbers of alien plant species or by the same number of alien plant species with different levels of evenness. However, little is known about how alien invasive plant species richness and evenness affect soil microbial communities. We constructed native herbaceous plant communities invaded by exotic plants with different richness (1, 2, 4 and 8 species) and evenness (high and low) and analyzed soil physico-chemical properties and the diversity and composition of soil fungal and bacterial communities by high-throughput Illumina sequencing. Overall, the species richness and evenness of invasive plants had no significant effect on bacterial and fungal alpha diversity (OTUs, Shannon, Simpson, Chao1 and ACE) or the soil physico-chemical properties. However, invasive species richness had a significant impact on the relative abundance of the most dominant fungi, Ascomycota and Bipolaris, and the dominant bacteria, Actinobacteriota, which increased with increasing invasive species richness. The relative abundance of the dominant microbial groups was significantly correlated with the relative abundance of some specific invasive plants in the community. This study sheds new light on the effects of plant co-invasion on soil microbial communities, which may help us understand the underlying mechanisms of multiple alien plant invasion processes from the perspective of soil microorganisms.

Interaction between Nitrogen, Phosphorus, and Invasive Alien Plants

Plant invasion is significantly affected by environmental factors in the recipient habitats and affects the stability and sustainable development of society. The invasiveness of alien plants may be increased by anthropogenic-mediated disturbances, such as fluctuations in nutrients caused by excessive emissions of nitrogen (N) and phosphorus (P). To improve our understanding of the interactions between N and P fluctuations and invasive alien plants, the current report focuses on the biogeochemical behavior of N and P among invasive alien plants, native plants, and the soil within the plant–soil ecosystem. Our research, together with a synthesis of the literature, shows that fluctuations in N and P resources provide more opportunities and competitiveness for plant invasion. At the same time, the biogeochemical cycles of N and P are promoted because of their efficient and increased utilization and rate of release by invasive alien plants. However, there is no consensus on whether the N and P compositions of invasive species are different from those of the natives in their habitat. Quantitative studies that compare N and P contents in plant, litter, and soil between native plant communities and invaded communities on a global scale are an indispensable area of research focus for the future.

More widespread alien tree species do not have larger impacts on regeneration of native tree species in a tropical forest reserve

There is insufficient information regarding the factors affecting the environmental impacts of alien species. In particular, little is known about whether there is any relationship between the invasiveness (establishment and spread) of an introduced species and its per capita impact. We experimentally assessed the relationship between the extent of spread of up to 29 alien plant species and their impact on recruitment of native tree species in Amani Botanical Garden, Tanzania. We also studied the effects of allelochemicals of selected alien on native plant species to assess potential mechanisms of impact. We found no relationship between the extent of spread of an alien tree species and their impact on seed germination, seedling survival, and seedling communities of native trees in their understory, and no indication that allelochemicals consistently explain their effects on recruitment of the studied species. These results suggest that extent of spread cannot be used as a proxy for impact. Hence, managers should continue assessing both the spread and the impact of alien species when prioritizing alien species for management.

Erigeron annuus (L.) Pers. and Solidago canadensis L. antagonistically affect community stability and community invasibility under the co-invasion condition

The successful invasion of one invasive alien plant (IAP) can generate a favorable habitat in the invaded communities that beneficial to the successful invasion of the subsequent IAP. Advanced variations in the species number of IAP have the potential to alter the functional similarity and dissimilarity between IAP and co-existing native plant species (NPS), plant taxonomic diversity, plant functional diversity, community stability, and community invasibility. This study aims to evaluate the effects of the co-invasion of two notorious IAP, Erigeron annuus (L.) Pers. and Solidago canadensis L., on the functional similarity and dissimilarity between IAP and co-existing NPS, plant taxonomic diversity, plant functional diversity, community stability, and community invasibility in East China by using a comparative study. Results presented that: (I) IAP and co-existing NPS tend to converge functionally under E. annuus invasion and the functional similarity between IAP and co-existing NPS under E. annuus invasion supports the habitat filtering; (II) IAP and co-existing NPS tend to diverge functionally under S. canadensis invasion and the co-invasion condition and the functional dissimilarity between IAP and co-existing NPS under S. canadensis invasion and the co-invasion condition supports the niche differentiation; (III) plant taxonomic diversity was dramatically reduced under invasion condition, especially under S. canadensis invasion; (IV) Mason's α functional diversity was remarkably elevated under S. canadensis invasion and the co-invasion condition; (V) E. annuus and S. canadensis antagonistically affect community stability and community invasibility under the co-invasion condition compared with their independent invasion.

Cadmium influences the litter decomposition of Solidago canadensis L. and soil N-fixing bacterial communities

It is important to illuminate the effects of litter decomposition of invasive alien species on soil N-fixing bacterial communities (SoNiBa), especially under heavy metal pollution to better outline the mechanisms for invasion success of invasive alien species. This study attempts to identify the effects of litter decomposition of Solidago canadensis L. on SoNiBa under cadmium (Cd) pollution with different concentrations (i.e., low concentration, 7.5 mg/kg soil; high concentration, 15 mg/kg soil) via a polyethylene litterbags-experiment. Electrical conductivity and total N of soil were the most important environmental factors for determining the variations of SoNiBa composition. S. canadensis did not significantly affect the alpha diversity of SoNiBa but significantly affect the beta diversity of SoNiBa and SoNiBa composition. Thus, SoNiBa composition, rather than alpha diversity of SoNiBa, was the most important determinant of the invasion success of S. canadensis. Cd with 15 mg/kg soil did not address distinct effects on alpha diversity of SoNiBa, but Cd with 7.5 mg/kg soil noticeably raised the number of species and species richness of SoNiBa mainly due to the hormonal effects. The combined S. canadensis and Cd with 15 mg/kg soil obviously decreased cumulative mass losses and the rate of litter decomposition (k) of S. canadensis, but the combined S. canadensis and Cd with 7.5 mg/kg soil evidently accelerated cumulative mass losses and k of S. canadensis. Thus, Cd with 7.5 mg/kg soil can accelerate litter decomposition of S. canadensis, but Cd with 15 mg/kg soil can decline litter decomposition of S. canadensis.

The invasive tree staghorn sumac affects soil N2 -fixing bacterial communities in north China

Soil N₂ -fixing bacterial communities (SNB) can enhance soil N availability and the invasiveness of invaders. Some invaders display different degrees of invasion across different climate regions. Given that bacterial communities may change with different climate regions, it is important to understand soil micro-ecological mechanisms driving the successful invasion of invaders across different climate regions. This study performed cross-site comparisons to comprehensively analyse effects of the invasive tree staghorn sumac (Rhus typhina L.) on the structure of SNB. In north China, we selected three sites within two sampling regions (a warm temperate region and a cold temperate region). Staghorn sumac invasion did not significantly affect soil physicochemical properties and the diversity and richness of SNB. LEfSe analysis showed that numerous SNB taxa changed significantly during staghorn sumac invasion. This may be attributed in part to the selective effects of allelochemicals released by staghorn sumac via leaf litter and/or root exudates. Consequently, staghorn sumac invasion may alter the structure, rather than the diversity and richness, of SNB to facilitate its invasion process by establishing a favourable soil microenvironment in the invaded habitats. The number of species and richness of SNB under staghorn sumac invasion were significantly lower in the warm temperate region than in the cold temperate region. A possible reason for the increased diversity and richness of SNB under staghorn sumac invasion in the cold temperate region may be because staghorn sumac in the cold temperate region can provide more nutrients into the soil sub-ecosystem, presumably to support a higher diversity and richness of SNB via the nutritional requirements of SNB. The changed structure of SNB under staghorn sumac invasion, especially across different climate regions, may play an important role in its successful invasion across most regions of north China.

Enhanced activity of soil nutrient-releasing enzymes after plant invasion: a meta-analysis

Plant invasion can significantly alter soil nutrient cycling of ecosystems. How these changes are linked to soil enzyme activities is still unknown, however, even though these are proximate agents of organic matter decomposition and nutrient release. We performed a meta-analysis of 60 case studies examining responses of 10 unique soil enzymes to plant invasion, and tested whether invaded soils differed in their enzyme activities from uninvaded soils. We also examined whether increases in soil nutrient-releasing enzyme activity were paralleled by enhanced soil nutrient availability after plant invasion. Overall, we found that plant invasion had significant impacts on the activities of seven types of soil enzymes. Plant invasion had inconsistent impacts on C-decomposing enzymes, but invaded sites had significantly higher activities of soil enzymes related to N- and P-release than noninvaded sites. Increases in nutrient-releasing enzyme activity after plant invasion ranged from +23% to +69%, which potentially results in a linear increase of soil nutrient availability in response to enhanced enzyme activities. Invaded soils also had higher nutrient stocks and soil microbial biomass than uninvaded soils. Our results suggest that enhanced activity of soil nutrient-releasing enzymes after plant invasion may accelerate nutrient cycling, potentially creating a nutrient-rich soil environment that benefits invaders and promotes their persistence, as invasive plants often appear to be more resource-demanding and competitive than native species.

Invasive Goldenrod (Solidago gigantea) Influences Soil Microbial Activities in Forest and Grassland Ecosystems in Central Europe

A giant goldenrod plant, Solidago gigantea, native to North America is rapidly spreading in Europe and may have serious impact on ecosystems that inhabit. There is a lack of information about the effects of this species on soil biochemical properties and distribution and activity of microbial community. We analyzed soil physicochemical properties (soil reaction, soil moisture content, organic carbon and total nitrogen content) associated with activity of microbial population (activity of fluorescein diacetate (FDA), beta-glucosidase, urease and phosphatases enzymes) between invaded and adjacent uninvaded control sites in two habitats, forest and grassland, in the lowland of southeast Slovakia during years 2016 and 2017. The results revealed that invasion of S. gigantea significantly altered several soil properties and is associated with different soil properties. Soil acidity increased, organic carbon and moisture content decreased, while total nitrogen content was not significantly affected by invasion. FDA and urease activity were significantly higher in uninvaded sites. In contrast, beta-glucosidase and alkaline phosphatase activity were enhanced by S. gigantea invasion in both ecosystems studied. Acid phosphatase was not affected by the invasion. Our study proved that S. gigantea can influence several soil microbial properties while others remained unaffected, despite its significant impact on basal soil physicochemical properties.

Phenotypic variations alter the ecological impact of invasive alien species: Lessons from Parthenium hysterophorus

Invasive plant species constantly adjust their behavior with ecological shifts by virtue of phenotypic plasticity and/or local adaptations. Changes in the phenotype of an invasive species may also trigger variations in its community level impacts, which is an acceptable, yet unexplored aspect of invasion biology. Our study attempts to fill important knowledge gaps on the basic behavior and ecological interactions of invasive species. Parthenium hysterophorus, a widely distributed invasive alien species of tropical and sub-tropical regions, was evaluated for variations in its morpho-functional traits and ecological performance at a common spatial and temporal scale. Field surveys were conducted in Chandigarh, India, in five sites identified as invaded with P. hysterophorus. Individuals of P. hysterophorus randomly sampled from these sites, showed from trait analyses that the population is differentiated into two morphotypes, P_A and P_B. Morphotype P_B exhibits traits comparable to the shrub life-form in terms of woody stem (with higher stem circumference [+32.26%], stem specific density [+128.57%], twig dry matter content [+25.15%]); profuse branching (+46.38%); larger canopy (+91.16%); and better reproductive output (+190.29%) compared to P_A. P_A, on the other hand, reflected herbaceous characteristics with greater leaf area (+67.58%) and higher content of chlorophyll (+21.92%) compared to P_B. Based on these morphotypes, the plots were divided into three invasion categories: areas invaded by P_A [IP_A], areas invaded by P_B [IP_B] and uninvaded areas [UI]. Ecological indices and soil chemical properties were compared across IP_A, IP_B and UI. Shannon's index (p < 0.001), evenness index (p = 0.008), and richness index (p < 0.001) were significantly lower in IP_B compared to IP_A. UI areas were found to have higher soil pH, phenolics, organic matter, and concentrations of N, P and K, compared to IP_A and IP_B, but lower Ca and Mg. Results suggest that phenotypic variations within population of P. hysterophorus regulate its ecological impact on associated vegetation. Conservation managers would benefit from studying its invasion patterns and identifying the morphotype with higher ecological impact to prioritize management efforts. Monitoring these behavioral and ecological patterns in P. hysterophorus over the long-term may also help in anticipating challenges to preventive measures.

Solidago canadensis invasion affects soil N-fixing bacterial communities in heterogeneous landscapes in urban ecosystems in East China

Soil nitrogen-fixing bacterial communities (SNB) can increase the level of available soil N via biological N-fixation to facilitate successful invasion of several invasive plant species (IPS). Meanwhile, landscape heterogeneity can greatly enhance regional invasibility and increase the chances of successful invasion of IPS. Thus, it is important to understand the soil micro-ecological mechanisms driving the successful invasion of IPS in heterogeneous landscapes. This study performed cross-site comparisons, via metagenomics, to comprehensively analyze the effects of Solidago canadensis invasion on SNB in heterogeneous landscapes in urban ecosystems. Rhizospheric soil samples of S. canadensis were obtained from nine urban ecosystems [Three replicate quadrats (including uninvaded sites and invaded sites) for each type of urban ecosystem]. S. canadensis invasion did not significantly affect soil physicochemical properties, the taxonomic diversity of plant communities, or the diversity and richness of SNB. However, some SNB taxa (i.e., f_Micromonosporaceae, f_Oscillatoriaceae, and f_Bacillaceae) changed significantly with S. canadensis invasion. Thus, S. canadensis invasion may alter the community structure, rather than the diversity and richness of SNB, to facilitate its invasion process. Of the nine urban ecosystems, the diversity and richness of SNB was highest in farmland wasteland. Accordingly, the community invasibility of farmland wasteland may be higher than that of the other types of urban ecosystem. In brief, landscape heterogeneity, rather than S. canadensis invasion, was the strongest controlling factor for the diversity and richness of SNB. One possible reason may be the differences in soil electrical conductivity and the taxonomic diversity of plant communities in the nine urban ecosystems, which can cause notable shifts in the diversity and richness of SNB.

Density-dependency and plant-soil feedback: former plant abundance influences competitive interactions between two grassland plant species through plant-soil feedbacks

BACKGROUNDS AND AIMS

Negative plant-soil feedbacks (PSFs) are thought to promote species coexistence, but most evidence is derived from theoretical models and data from plant monoculture experiments.

METHODS

We grew Anthoxanthum odoratum and Centaurea jacea in field plots in monocultures and in mixtures with three ratios (3:1, 2:2 and 1:3) for three years. We then tested in a greenhouse experiment the performance of A. odoratum and C. jacea in pots planted with monocultures and 1:1 mixtures and filled with live and sterile soils collected from the field plots.

RESULTS

In the greenhouse experiment, C. jacea produced less aboveground biomass in soil conditioned by C. jacea monocultures than in soil conditioned by A. odoratum monocultures, while the aboveground biomass of A. odoratum in general did not differ between the two monospecific soils. The negative PSF effect was greater in the 1:1 plant mixture than in plant monocultures for A. odoratum but did not differ for C. jacea. In the greenhouse experiment, the performance of C. jacea relative to A. odoratum in the 1:1 plant mixture was negatively correlated to the abundance of C. jacea in the field plot where the soil was collected from. This relationship was significant both in live and sterile soils. However, there was no relationship between the performance of A. odoratum relative to C. jacea in the 1:1 plant mixture in the greenhouse experiment and the abundance of A. odoratum in the field plots.

CONCLUSIONS

The response of a plant to PSF depends on whether the focal species grows in monocultures or in mixtures and on the identity of the species. Interspecific competition can exacerbate the negative plant-soil feedbacks compared to intraspecific competition when a plant competes with a stronger interspecific competitor. Moreover, the abundance of a species in mixed plant communities, via plant-soil feedback, negatively influences the relative competitiveness of that species when it grows later in interspecific competition, but this effect varies between species. This phenomenon may contribute to the coexistence of competing plants under natural conditions through preventing the dominance of a particular plant species.

Responses of the soil fungal communities to the co-invasion of two invasive species with different cover classes

Soil fungal communities play an important role in the successful invasion of non-native species. It is common for two or more invasive plant species to co-occur in invaded ecosystems. This study aimed to determine the effects of co-invasion of two invasive species (Erigeron annuus and Solidago canadensis) with different cover classes on soil fungal communities using high-throughput sequencing. Invasion of E. annuus and/or S. canadensis had positive effects on the sequence number, operational taxonomic unit (OTU) richness, Shannon diversity, abundance-based cover estimator (ACE index) and Chao1 index of soil fungal communities, but negative effects on the Simpson index. Thus, invasion of E. annuus and/or S. canadensis could increase diversity and richness of soil fungal communities but decrease dominance of some members of these communities, in part to facilitate plant further invasion, because high soil microbial diversity could increase soil functions and plant nutrient acquisition. Some soil fungal species grow well, whereas others tend to extinction after non-native plant invasion with increasing invasion degree and presumably time. The sequence number, OTU richness, Shannon diversity, ACE index and Chao1 index of soil fungal communities were higher under co-invasion of E. annuus and S. canadensis than under independent invasion of either individual species. The co-invasion of the two invasive species had a positive synergistic effect on diversity and abundance of soil fungal communities, partly to build a soil microenvironment to enhance competitiveness of the invaders. The changed diversity and community under co-invasion could modify resource availability and niche differentiation within the soil fungal communities, mediated by differences in leaf litter quality and quantity, which can support different fungal/microbial species in the soil.

Plant invasion is associated with higher plant-soil nutrient concentrations in nutrient-poor environments

Plant invasion is an emerging driver of global change worldwide. We aimed to disentangle its impacts on plant-soil nutrient concentrations. We conducted a meta-analysis of 215 peer-reviewed articles and 1233 observations. Invasive plant species had globally higher N and P concentrations in photosynthetic tissues but not in foliar litter, in comparison with their native competitors. Invasive plants were also associated with higher soil C and N stocks and N, P, and K availabilities. The differences in N and P concentrations in photosynthetic tissues and in soil total C and N, soil N, P, and K availabilities between invasive and native species decreased when the environment was richer in nutrient resources. The results thus suggested higher nutrient resorption efficiencies in invasive than in native species in nutrient-poor environments. There were differences in soil total N concentrations but not in total P concentrations, indicating that the differences associated to invasive plants were related with biological processes, not with geochemical processes. The results suggest that invasiveness is not only a driver of changes in ecosystem species composition but that it is also associated with significant changes in plant-soil elemental composition and stoichiometry.

Effects of Praxelis clematidea invasion on soil nitrogen fractions and transformation rates in a tropical savanna

Plant invasion has been reported to affect a mass of soil ecological processes and functions, although invasion effects are often context-, species- and ecosystem- specific. This study was conducted to explore potential impacts of Praxelis clematidea invasion on contents of total and available soil nitrogen (N) and microbial N transformations in a tropical savanna. Soil samples were collected from the surface and sub-surface layers in plots with non-, slight, or severe P. clematidea invasion in Hainan Province of southern China, which remains less studied, and analyzed for contents of the total and available N fractions and microbial N transformations. Results showed that total N content significantly increased in the surface soil but trended to decrease in the sub-surface soil in the invaded plots relative to the non-invaded control. Slight invasion significantly increased soil alkali-hydrolysable N content in the two soil layers. Soil net N mineralization rate was not significantly changed in both the soil layers, although soil microbial biomass N was significantly higher in plots with severe invasion than the control. There was no significant difference in content of soil N fractions between plots with slight and severe invasion. Our results suggest that invasion of P. clematidea promotes soil N accumulation in the surface soil layer, which is associated with increased microbial biomass N. However, the invasion-induced ecological impacts did not increase with further invasion. Significantly higher microbial biomass N was maintained in plots with severe invasion, implying that severe P. clematidea invasion may accelerate nutrient cycling in invaded ecosystems.

Invasive Shrub Mapping in an Urban Environment from Hyperspectral and LiDAR-Derived Attributes

Proactive management of invasive species in urban areas is critical to restricting their overall distribution. The objective of this work is to determine whether advanced remote sensing technologies can help to detect invasions effectively and efficiently in complex urban ecosystems such as parks. In Surrey, BC, Canada, Himalayan blackberry (Rubus armeniacus) and English ivy (Hedera helix) are two invasive shrub species that can negatively affect native ecosystems in cities and managed urban parks. Random forest (RF) models were created to detect these two species using a combination of hyperspectral imagery, and light detection and ranging (LiDAR) data. LiDAR-derived predictor variables included irradiance models, canopy structural characteristics, and orographic variables. RF detection accuracy ranged from 77.8 to 87.8% for Himalayan blackberry and 81.9 to 82.1% for English ivy, with open areas classified more accurately than areas under canopy cover. English ivy was predicted to occur across a greater area than Himalayan blackberry both within parks and across the entire city. Both Himalayan blackberry and English ivy were mostly located in clusters according to a Local Moran's I analysis. The occurrence of both species decreased as the distance from roads increased. This study shows the feasibility of producing highly accurate detection maps of plant invasions in urban environments using a fusion of remotely sensed data, as well as the ability to use these products to guide management decisions.

Co-occurring nonnative woody shrubs have additive and non-additive soil legacies

To maximize limited conservation funds and prioritize management projects that are likely to succeed, accurate assessment of invasive nonnative species impacts is essential. A common challenge to prioritization is a limited knowledge of the difference between the impacts of a single nonnative species compared to the impacts of nonnative species when they co-occur, and in particular predicting when impacts of co-occurring nonnative species will be non-additive. Understanding non-additivity is important for management decisions because the management of only one co-occurring invader will not necessarily lead to a predictable reduction in the impact or growth of the other nonnative plant. Nonnative plants are frequently associated with changes in soil biotic and abiotic characteristics, which lead to plant-soil interactions that influence the performance of other species grown in those soils. Whether co-occurring nonnative plants alter soil properties additively or non-additively relative to their effects on soils when they grow in monoculture is rarely addressed. We use a greenhouse plant-soil feedback experiment to test for non-additive soil impacts of two common invasive nonnative woody shrubs, Lonicera maackii and Ligustrum sinense, in deciduous forests of the southeastern United States. We measured the performance of each nonnative shrub, a native herbaceous community, and a nonnative woody vine in soils conditioned by each shrub singly or together in polyculture. Soils conditioned by both nonnative shrubs had non-additive impacts on native and nonnative performance. Root mass of the native herbaceous community was 1.5 times lower and the root mass of the nonnative L. sinense was 1.8 times higher in soils conditioned by both L. maackii and L. sinense than expected based upon growth in soils conditioned by either shrub singly. This result indicates that when these two nonnative shrubs co-occur, their influence on soils disproportionally favors persistence of the nonnative L. sinense relative to this native herbaceous community, and could provide an explanation of why native species abundance is frequently depressed in these communities. Additionally, the difference between native and nonnative performance demonstrates that invasive impact studies focusing on the impact only of single species can be insufficient for determining the impact of co-occurring invasive plant species.

Mutualism-disrupting allelopathic invader drives carbon stress and vital rate decline in a forest perennial herb

Invasive plants can negatively affect belowground processes and alter soil microbial communities. For native plants that depend on soil resources from root fungal symbionts (RFS), invasion could compromise their resource status and subsequent ability to manufacture and store carbohydrates. Herbaceous perennials that depend on RFS-derived resources dominate eastern North American forest understories. Therefore, we predict that forest invasion by Alliaria petiolata, an allelopathic species that produces chemicals that are toxic to RFS, will diminish plant carbon storage and fitness. Over a single growing season, the loss of RFS could reduce a plant's photosynthetic physiology and carbon storage. If maintained over multiple growing seasons, this could create a condition of carbon stress and declines in plant vital rates. Here we characterize the signals of carbon stress over a short timeframe and explore the long-term consequence of Alliaria invasion using Maianthemum racemosum, an RFS-dependent forest understory perennial. First, in a greenhouse experiment, we treated the soil of potted Maianthemum with fresh leaf tissue from either Alliaria or Hesperis matronalis (control) for a single growing season. Alliaria-treated plants exhibit significant overall reductions in total non-structural carbohydrates and have 17 % less storage carbohydrates relative to controls. Second, we monitored Maianthemum vital rates in paired experimental plots where we either removed emerging Alliaria seedlings each spring or left Alliaria at ambient levels for 7 years. Where Alliaria is removed, Maianthemum size and vital rates improve significantly: flowering probability increases, while the probability of plants regressing to non-flowering stages or entering prolonged dormancy are reduced. Together, our results are consistent with the hypothesis that disruption of a ubiquitous mutualism following species invasion creates symptoms of carbon stress for species dependent on RFS. Disruption of plant-fungal mutualisms may generally contribute to the common, large-scale declines in forest biodiversity observed in the wake of allelopathic invaders.