Although it is generally acknowledged that invasions by exotic plant species represent a major threat to biodiversity and ecosystem stability, little attention has been paid to the potential impacts of these invasions on nutrient cycling processes in the soil. The literature on plant–soil interactions strongly suggests that the introduction of a new plant species, such as an invasive exotic, has the potential to change many components of the carbon (C), nitrogen (N), water, and other cycles of an ecosystem. I have reviewed studies that compare pool sizes and flux rates of the major nutrient cycles in invaded and noninvaded systems for invasions of 56 species. The available data suggest that invasive plant species frequently increase biomass and net primary production, increase N availability, alter N fixation rates, and produce litter with higher decomposition rates than co-occurring natives. However, the opposite patterns also occur, and patterns of difference between exotics and native species show no trends in some other components of nutrient cycles (for example, the size of soil pools of C and N). In some cases, a given species has different effects at different sites, suggesting that the composition of the invaded community and/or environmental factors such as soil type may determine the direction and magnitude of ecosystem-level impacts. Exotic plants alter soil nutrient dynamics by differing from native species in biomass and productivity, tissue chemistry, plant morphology, and phenology. Future research is needed to (a) experimentally test the patterns suggested by this data set; (b) examine fluxes and pools for which few data are available, including whole-site budgets; and (c) determine the magnitude of the difference in plant characteristics and in plant dominance within a community that is needed to alter ecosystem processes. Such research should be an integral component of the evaluation of the impacts of invasive species.

Effects of Exotic Plant Invasions on Soil Nutrient Cycling Processes

Although the impacts of exotic plant invasions on community structure and ecosystem processes are well appreciated, the pathways or mechanisms that underlie these impacts are poorly understood. Better exploration of these processes is essential to understanding why exotic plants impact only certain systems, and why only some invaders have large impacts. Here, we review over 150 studies to evaluate the mechanisms underlying the impacts of exotic plant invasions on plant and animal community structure, nutrient cycling, hydrology and fire regimes. We find that, while numerous studies have examined the impacts of invasions on plant diversity and composition, less than 5% test whether these effects arise through competition, allelopathy, alteration of ecosystem variables or other processes. Nonetheless, competition was often hypothesized, and nearly all studies competing native and alien plants against each other found strong competitive effects of exotic species. In contrast to studies of the impacts on plant community structure and higher trophic levels, research examining impacts on nitrogen cycling, hydrology and fire regimes is generally highly mechanistic, often motivated by specific invader traits. We encourage future studies that link impacts on community structure to ecosystem processes, and relate the controls over invasibility to the controls over impact.

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Mechanisms underlying the impacts of exotic plant invasions

▪ Abstract In the search to identify factors that make some plant species troublesome invaders, many studies have compared various measures of native and alien invasive plant performance. These comparative studies provide insights into the more general question “Do alien invasive plants usually outperform co-occurring native species, and to what degree does the answer depend on growing conditions?” Based on 79 independent native-invasive plant comparisons, the alien invaders were not statistically more likely to have higher growth rates, competitive ability, or fecundity. Rather, the relative performance of invaders and co-occurring natives often depended on growing conditions. In 94% of 55 comparisons involving more than one growing condition, the native's performance was equal or superior to that of the invader, at least for some key performance measures in some growing conditions. Most commonly, these conditions involved reduced resources (nutrients, light, water) and/or specific disturbance regimes. I...

https://www.jstor.org/stable/pdfplus/30033774.pdf

PERFORMANCE COMPARISONS OF Co-OCCURRING NATIVE AND ALIEN INVASIVE PLANTS: Implications for Conservation and Restoration

The episodic nature of water availability in arid and semiarid ecosystems has significant consequences on belowground carbon and nutrient cycling. Pulsed water events directly control belowground processes through soil wet-dry cycles. Rapid soil microbial response to incident moisture availability often results in almost instantaneous C and N mineralization, followed by shifts in C/N of microbially available substrate, and an offset in the balance between nutrient immobilization and mineralization. Nitrogen inputs from biological soil crusts are also highly sensitive to pulsed rain events, and nitrogen losses, particularly gaseous losses due to denitrification and nitrate leaching, are tightly linked to pulses of water availability. The magnitude of the effect of water pulses on carbon and nutrient pools, however, depends on the distribution of resource availability and soil organisms, both of which are strongly affected by the spatial and temporal heterogeneity of vegetation cover, topographic position and soil texture. The 'inverse texture hypothesis' for net primary production in water-limited ecosystems suggests that coarse-textured soils have higher NPP than fine-textured soils in very arid zones due to reduced evaporative losses, while NPP is greater in fine-textured soils in higher rainfall ecosystems due to increased water-holding capacity. With respect to belowground processes, fine-textured soils tend to have higher water-holding capacity and labile C and N pools than coarse-textured soils, and often show a much greater flush of N mineralization. The result of the interaction of texture and pulsed rainfall events suggests a corollary hypothesis for nutrient turnover in arid and semiarid ecosystems with a linear increase of N mineralization in coarse-textured soils, but a saturating response for fine-textured soils due to the importance of soil C and N pools. Seasonal distribution of water pulses can lead to the accumulation of mineral N in the dry season, decoupling resource supply and microbial and plant demand, and resulting in increased losses via other pathways and reduction in overall soil nutrient pools. The asynchrony of resource availability, particularly nitrogen versus water due to pulsed water events, may be central to understanding the consequences for ecosystem nutrient retention and long-term effects on carbon and nutrient pools. Finally, global change effects due to changes in the nature and size of pulsed water events and increased asynchrony of water availability and growing season will likely have impacts on biogeochemical cycling in water-limited ecosystems.

Water Pulses and Biogeochemical Cycles in Arid and Semiarid Ecosystems

Exotic species affect the biogeochemical pools and fluxes of materials and energy, thereby altering the fundamental structure and function of their ecosystems. Rapidly accumulating evidence from many species of both animal and plant invaders suggests that invasive species often increase pool sizes, particularly of biomass, and promote accelerated flux rates, but many exceptions can be found. Ecosystem dynamics are altered through a variety of interacting, mutually reinforcing mechanistic pathways, including species’ resource acquisition traits; population densities; ability to engineer changes to physical environmental conditions; effects on disturbance, especially fire; regimes; the ability to structure habitat for other species; and their impact on food webs. Local factors of landscape setting, history, and other sources of disturbance constrain ecosystem responses to invasions. New research directions are suggested, including the need for whole-system budgets, the quantification of abundance-impact relationships for particular ecosystem processes, and a better exploration of food web impacts on ecosystem processes.

Ecosystem Consequences of Biological Invasions

The introduction of nonnative plant species may decrease ecosystem stability by altering the availability of nitrogen (N) for plant growth. Invasive species can impact N availability by changing litter quantity and quality, rates of N 2-fixation, or rates of N loss. We quantified the effects of invasion by the annual grass Bromus tectorum on N cycling in an arid grassland on the Colorado Plateau (USA). The invasion occurred in 1994 in two community types in an undisturbed grassland. This natural experiment allowed us to measure the immediate responses following invasion without the confounding effects of previous disturbance. Litter biomass and the C:N and lignin:N ratios were measured to determine the effects on litter dynamics. Long-term soil incubations (415 d) were used to measure potential microbial respiration and net N mineralization. Plant-available N was quantified for two years in situ with ion-exchange resin bags, and potential changes in rates of gaseous N loss were estimated by measuring denitrification enzyme activity. Bromus invasion significantly increased litter biomass, and Bromus litter had significantly greater C:N and lignin:N ratios than did native species. The change in litter quantity and chemistry decreased potential rates of net N mineralization in sites with Bromusby decreasing nitrogen available for microbial activity. Inorganic N was 50% lower on Hilaria sites with Bromus during the spring of 1997, but no differences were observed during 1998. The contrasting differences between years are likely due to moisture availability; spring precipitation was 15% greater than average during 1997, but 52% below average during spring of 1998. Bromus may cause a short-term decrease in N loss by decreasing substrate availability and denitrification enzyme activity, but N loss is likely to be greater in invaded sites in the long term because of increased fire frequency and greater N volatilization during fire. We hypothesize that the introduction of Bromus in conjunction with land-use change has es- tablished a series of positive feedbacks that will decrease N availability and alter species composition.

Exotic plant invasion alters nitrogen dynamics in an arid grassland

Invasion of exotic species threatens the stability of ecosystems and is regarded as a significant component of global ecological change. One of the most successful invasive species of the intermountain west of the United States is Bromus tectorum L. (downy brome)—an annual Eurasian grass that was first observed in North America in the late 1800s in areas of Washington, Oregon and Utah and now covers ca. 40,000,000 ha. The prevalence of B. tectorum raises concerns regarding its potential effects on ecosystem structure and function. In this study we investigated the effects of invasion of B. tectorum on an ungrazed grassland in Canyonlands National Park, Utah. Because the invasion occurred on previously established study transects, we were able to assess the short-term effects of an invasion with a known entry date. We conducted long-term soil incubations in the laboratory to obtain data on cumulative nitrogen mineralization which was used to calculate the size of the labile pool of soil nitrogen. ...

R. L. Rimer

Papers

Exotic plant invasion alters nitrogen dynamics in an arid grassland

Invasion of Downy Brome (Bromus tectorum L.) Causes Rapid Changes in the Nitrogen Cycle