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Energy is an integral part of society. The major US energy sources of fossil fuels (coal, oil, natural gas); biofuels (ethanol); and wind are concentrated in grassland ecosystems of the Great Plains. As energy demand continues to increase, mounting pressures will be placed on North American grassland systems. In this review, we present the ecological effects of energy development and production on grassland systems. We then identify opportunities to mitigate these effects during the planning, construction, and production phases by using informed methodology and improved technology. Primary effects during energy development include small- and large-scale soil disturbance and vegetation removal as small patches of grasslands are used to host oil or gas wells, wind turbine pads, associated roadways, and pipelines or through the conversion of large grassland areas to biofuel croplands. Direct habitat loss or habitat fragmentation can affect wildlife directly through increased mortality or indirectly through reduction in habitat quantity and quality. During energy production, air and water quality can be affected through regular emissions or unplanned spills. Energy development can also affect the economy and health of local communities. During planning, energy development and production effects can be reduced by carefully considering effects on grasslands during siting and even by selecting different energy source types. During construction, effects on soil and plant systems can be minimized by eliminating weed populations before disturbance, salvaging and stockpiling topsoil for future revegetation, and harvesting native local seed for postsite restoration. During energy production operations, noise and road traffic reduction plans and atmospheric monitoring will enable more informed mitigation measures. Continued research on energy development effects and mitigation measures is necessary to establish best management practices beneficial to grassland health while providing needed energy for the United States.

20 Pág. Departamento de Medio Ambiente y Agronomía​, Restoration of degraded drylands is urgently needed to mitigate climate change, reverse desertification and secure livelihoods for the two billion people who live in these areas. Bold global targets have been set for dryland restoration to restore millions of hectares of degraded land. These targets have been questioned as overly ambitious, but without a global evaluation of successes and failures it is impossible to gauge feasibility. Here we examine restoration seeding outcomes across 174 sites on six continents, encompassing 594,065 observations of 671 plant species. Our findings suggest reasons for optimism. Seeding had a positive impact on species presence: in almost a third of all treatments, 100% of species seeded were growing at first monitoring. However, dryland restoration is risky: 17% of projects failed, with no establishment of any seeded species, and consistent declines were found in seeded species as projects matured. Across projects, higher seeding rates and larger seed sizes resulted in a greater probability of recruitment, with further influences on species success including site aridity, taxonomic identity and species life form. Our findings suggest that investigations examining these predictive factors will yield more effective and informed restoration decision-making., We would like to thank the supporters of the Global Arid Zone Project. The intellectual and energetic input of the network participants made this work possible. We also acknowledge the many employers and funding agencies that supported projects and the authors’ time in preparing this work and contributing data to the GAZP database. Please note that any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US Government

1 Pág. Correción errata., In the version of this Article originally published, the surname of author Tina Parkhurst was incorrectly written as Schroeder. This has now been corrected.

Vietnam’s forests have undergone major transformations since the 1990s, including a transition from net forest loss to net expansion, which is attributable to plantation forests and rehabilitated forests. Our study aimed to better understand the patterns and the causes of forest cover rehabilitation in Vietnam to expand tropical forests in other regions. We used geographic information system tools, a structural regression model and a random effects model based on official Government of Vietnam forest cover maps, and field surveys to quantify the extent of rehabilitated forests and its drivers at the local, commune, scale, in Dien Bien province, Vietnam. Results showed that around 118,000 hectares of forests were rehabilitated between 1990 and 2010. Rehabilitated forests comprised the largest share (above 84%) of total forest gain and this share increased from 1990-2000 to 2000-2010. Rehabilitated forests were associated with biophysical and accessibility conditions (elevation and road density). Expansion of rehabilitated forests was mainly driven by the presence of migration, lower population density, higher income, and the implementation of forestry policies. The empirical results offer policy implications for forest restoration practices as part of forest-based climate change mitigation programs as well as for environmental management, sustainable mountainous rural livelihood development in Vietnam and beyond.

Climate change is a pressing global issue and it negatively affects many developing countries, including Vietnam. To help Vietnam effectively respond to this pressing challenge, the country has recently introduced a major program for reducing carbon emissions arising from deforestation and forest degradation, fostering conservation, managing forests sustainably, and enhancing forest carbon stocks (REDD+). Current policies in Vietnam provide a sound platform for the development of REDD+, and REDD+ can potentially greatly contribute to the reduction of deforestation and forest degradation. However, these policies and the REDD+ program are hindered by limited understanding of the extent of deforestation and forest degradation and their underlying causes. This study employed geographic information system (GIS) tools, a structural regression model (structural model), and a regression tree method to quantify the extent as well as the approximate causes of deforestation and forest degradation in Vietnam. Results show that around 1.77 and 0.65 million ha of forests were lost and degraded, respectively, between 2000 and 2010. Deforestation and forest degradation were most notable in the north central, northeast, central highland, and northwest areas of the nation. There were several underlying indicators of deforestation and forest degradation including initial forest cover, per capita income, agricultural production, governance, population growth, food, and poverty. Our results illustrate several important policy implications for forest restoration and the REDD+ program in Vietnam: Vietnam should focus most strongly on reducing poverty, preserving existing forests, improving provincial-level governance, and controlling population growth.

Revegetation by seeding is an important tool in restoration. Seeding practices for restoration often rely on standard prescriptions for seed mix diversity and seeding rates. Seed mix diversity and rates are generally low within restoration projects and these practices are typically not informed by research. The objective of this study was to explore a new method for determining an optimal seed mix diversity and seeding rate for restoration of a semiarid grassland. We examined restoration success associated with differing seed mix diversity levels (5–50 species) and seeding rates (400–1,600 pure live seeds [PLS]/m2) using a response surface regression (RSR) experimental design at 12 disturbed sites in northeastern Colorado. Overall restoration success was evaluated based on optimizing desirability across nine individual responses: biomass and diversity of seeded, volunteer native, noxious, non-native species, and the density of seeded species. Greatest restoration success after four growing seasons occurred at a seed mix diversity of 35 species and a seeding rate of 1,366 PLS/m2. RSR experimental design and analysis has seldom been used to answer ecological questions. This novel approach to address a pressing restoration challenge provided unique insight into how seed mix diversity and seeding rate, singly or in combination, influence the first 4 years of plant community development and overall restoration success. These results suggest that including more native species and seeding at higher rates than current practice could lead to greater restoration success in grasslands.

Exotic plant invasion can have dramatic impacts on native plants making restoration of native vegetation at invaded sites challenging. Though invasives may be superior competitors, it is possible their dominance could be enhanced by insect herbivores if native plants are preferred food sources. Insect herbivory can regulate plant populations, but little is known of its effects in restoration settings. There is a need to better understand relationships between insect herbivores and invasive plants with regard to their combined potential for impacting native plant establishment and restoration success. The objective of this study was to assess impacts of grasshopper herbivory and the invasive grass Bromus tectorum (cheatgrass) on mortality and growth of 17 native plant species used in restoration of critical sagebrush steppe ecosystems. Field and greenhouse experiments were conducted using moderate densities of a common, generalist pest grasshopper (Melanoplus bivittatus). Grasshoppers had stronger and more consistent impacts on native restoration plants in field and greenhouse studies than cheatgrass. After 6 weeks in the greenhouse, grasshoppers were associated with 36% mortality over all native restoration species compared to 2% when grasshoppers were absent. Herbivory was also associated with an approximately 50% decrease in native plant biomass. However, effects varied among species. Artemisia tridentata, Chrysothamnus viscidiflorus, and Coreopsis tinctoria were among the most negatively impacted, while Oenothera pallida, Pascopyrum smithii, and Leymus cinerus were unaffected. These findings suggest restoration species could be selected to more effectively establish and persist within cheatgrass infestations, particularly when grasshopper populations are forecasted to be high.

Vietnam&rsquo, s forests have experienced a notable transformation over the past 20 years from net deforestation to reforestation and expanding forests. Continued reforestation that aims to achieve further economic and environmental benefits remains a national priority and strategy. We explore the current status of plantation forests and highlight possible means to facilitate their expansion in the uplands of Vietnam. We employ mixed method triangulation to empirically explore plantation forests and their economic role in household livelihood, to quantify trade-offs between plantation forests and shifting cultivation, and to assess the constraints on plantation forest expansion in Nghe An province, north-central Vietnam. Results show that forest in the study area expanded by 406,000 ha (71.1%) between 1990 and 2016. Plantation forests increased by nearly 500% (from 32,000 ha to 190,000 ha), while natural forests expanded by 48.1% (from 538,000 ha to 797,000 ha). Plantation forests contributed an average of 35.1 percent of total household income in wealthier households and 27.9 percent of income in poor households. Switching from shifting cultivation to plantation forests would increase total household income and average carbon stock but decrease food provision. Total Economic Value would be higher for plantation forest scenarios if increased carbon stocks in plantations can be monetized. This carbon income might drive conversion of shifting cultivation to plantation forests. Constraints on further expansion of plantation forest are low external cooperation, education, market stability, and agroforestry extension services. Our empirical results inform national plantation forest development, sustainable upland livelihood development, and climate change mitigation programs to ultimately facilitate forest transition and improve the resilience and sustainability of socio-ecological systems.

Increased urbanization coupled with increased reliance of urban communities on rural areas for ecosystem service provision is a challenge faced by many nations. The ability of urban households to directly support restoration efforts in surrounding rural regions represents an underappreciated funding stream for ecological restoration. This study explored the willingness of urban households to support forest restoration in Vietnam. We surveyed 211 households (HHs) in the capital city Hanoi, Vietnam. A Maximum Likelihood Estimator (MLE) model allowed us to obtain the parameters of our model and quantify mean Willingness-to-Pay (WTP) for a program of forest restoration in addition to identifying factors influencing the decision of WTP. Generally, over forty percent of the households surveyed are willing to pay for forest restoration and the mean value of WTP is 37,830 VND ($1.73) per household per month. WTP depends on endogenous and exogenous factors including level of education, income, female-to-male ratio in the household, attitude toward payment for monthly electricity consumption, and awareness of payment for environmental service. Our results suggest that urban household’s demand for forest restoration is real, and represents an untapped source of restoration funding. Policy-makers should take actions to apply charges on water bills to turn this potential into reality for restoration projects in Vietnam if the benefits from restoration outweigh the costs based on our findings.

Ecosystem restoration planning near the beginning of the site assessment and management process ("early integration") involves consideration of restoration goals from the outset in developing solutions for contaminated ecosystems. There are limitations to integration that stem from institutional barriers, few successful precedents, and limited availability of guidance. Challenges occur in integrating expertise from various disciplines and multiple, sometimes divergent interests and goals. The more complex process can result in timing, capacity, communication, and collaboration challenges. On the other hand, integrating the 2 approaches presents new and creative opportunities. For example, integration allows early planning for expanding ecosystem services on or near contaminated lands or waters that might otherwise have been unaddressed by remediation alone. Integrated plans can explicitly pursue ecosystem services that have market value, which can add to funds for long-term monitoring and management. Early integration presents opportunities for improved and productive collaboration and coordination between ecosystem restoration and contaminant assessment and management. Examples exist where early integration facilitates liability resolution and generates positive public relations. Restoration planning and implementation before the completion of the contaminated site assessment, remediation, or management process ("early restoration") can facilitate coordination with offsite restoration options and a regional approach to restoration of contaminated environments. Integration of performance monitoring, for both remedial and restoration actions, can save resources and expand the interpretive power of results. Early integration may aid experimentation, which may be more feasible on contaminated lands than in many other situations. The potential application of concepts and tools from adaptive management is discussed as a way of avoiding pitfalls and achieving benefits in early integration. In any case, there will be challenges with early integration of restoration concepts for contaminated ecosystems, but the benefits are likely to outweigh them.

The ecological consequences of slash pile burning are a concern for land managers charged with maintaining forest soil productivity and native plant diversity. Fuel reduction and forest health management projects have created nearly 150,000 slash piles scheduled for burning on US Forest Service land in northern Colorado. The vast majority of these are small piles (

Summary Symphyotrichum ericoides (Asteraceae) from naturally seleniferous habitat (Pine Ridge) was shown previously to have selenium (Se) hyperaccumulator properties in field and glasshouse studies, and to benefit from Se through protection from herbivory. To investigate whether Se hyperaccumulation is ubiquitous in S. ericoides or restricted to seleniferous soils, the S. ericoides Pine Ridge (PR) population was compared with the nearby Cloudy Pass (CP) population from nonseleniferous soil. The S. ericoidesPR and CP populations were strikingly physiologically different: in a common garden experiment, PR plants accumulated up to 40-fold higher Se concentrations than CP plants and had 10-fold higher Se : sulfur (S) ratios. Moreover, roots of S. ericoidesPR plants showed directional growth toward selenate, while CP roots did not. Growth of both accessions responded positively to Se. Each accession grew best on its own soil. Rhizosphere soil inoculum from the S. ericoidesPR population stimulated plant growth and Se accumulation in both S. ericoidesPR and S. ericoidesCP plants, on both PR and CP soils. While the S. ericoidesPR population hyperaccumulates Se, the nearby CP population does not. The capacity of S. ericoidesPR plants to hyperaccumulate Se appears to be a local phenomenon that is restricted to seleniferous soil. Mutualistic rhizosphere microbes of the S. ericoidesPR population may contribute to the hyperaccumulation phenotype.

Premise of the study Are there dimensions of symbiotic root interactions that are overlooked because plant mineral nutrition is the foundation and, perhaps too often, the sole explanation through which we view these relationships? In this paper we investigate how the root nodule symbiosis in selenium (Se) hyperaccumulator and nonaccumulator Astragalus species influences plant selenium (Se) accumulation. Methods In greenhouse studies, Se was added to nodulated and nonnodulated hyperaccumulator and nonaccumulator Astragalus plants, followed by investigation of nitrogen (N)-Se relationships. Selenium speciation was also investigated, using x-ray microprobe analysis and liquid chromatography-mass spectrometry (LC-MS). Key results Nodulation enhanced biomass production and Se to S ratio in both hyperaccumulator and nonaccumulator plants. The hyperaccumulator contained more Se when nodulated, while the nonaccumulator contained less S when nodulated. Shoot [Se] was positively correlated with shoot N in Se-hyperaccumulator species, but not in nonhyperaccumulator species. The x-ray microprobe analysis showed that hyperaccumulators contain significantly higher amounts of organic Se than nonhyperaccumulators. LC-MS of A. bisulcatus leaves revealed that nodulated plants contained more γ-glutamyl-methylselenocysteine (γ-Glu-MeSeCys) than nonnodulated plants, while MeSeCys levels were similar. Conclusions Root nodule mutualism positively affects Se hyperaccumulation in Astragalus. The microbial N supply particularly appears to contribute glutamate for the formation of γ-Glu-MeSeCys. Our results provide insight into the significance of symbiotic interactions in plant adaptation to edaphic conditions. Specifically, our findings illustrate that the importance of these relationships are not limited to alleviating macronutrient deficiencies.

Soil nitrogen immobilization by carbon amendment is a management technique used for conservation purposes to increase the competitive ability of late-seral plant species over early-seral species based on their different tolerance of low soil nitrogen content. We immobilized nitrogen over six growing seasons on three ex-arable fields with poor sandy soils in the Hungarian lowland in order to restore endemic sandy grassland. Sucrose and sawdust were applied at rates based on previous laboratory experiments using local soils. We tested the efficacy of long-term carbon amendment for lowering soil nitrogen availability and favouring late-seral native species over early-seral weed species. Carbon amendments resulted in significant increase in soil microbial biomass C and reduced soil nitrogen availability after 2 years. Total vegetation cover was reduced by reducing soil nitrogen availability, but total species richness was not impacted. Cover of early-seral species decreased, and species richness and cover of late-seral species increased irrespective of nitrogen immobilization. However, after 4-6 years reducing soil nitrogen availability hampered the spread of moss under vascular vegetation. Synthesis and applications. This study supports the efficacy of carbon amendment as a tool to immobilize available soil nitrogen in the upper soil layers. However, the desired impact on vegetation was not fully achieved despite application over several years. Nitrogen immobilization was most relevant to bryophytes, lacking deep root systems, which may explain the responsiveness of this group to N limitation. The different impact of N availability on the complex of early-seral, late-seral vascular species and that of the bryophyte layer provides opportunity for directing state transformations in arid grasslands. Bryophyte cover can be suppressed through carbon amendments in order to enhance the germination and establishment of grassland species. The advantage of the method is that it opens bryophyte cover gradually without disturbing the soil surface, possibly avoiding the establishment of invasive species. However, further studies are required for deeper insight. © 2014 The Authors.

Heap leach processing for extraction of gold began in 1985 at Summitville Mine in southwestern Colorado, and by 1994 the site was declared a Superfund Site by the USEPA. In 1995, we began a science-based approach aimed at restoring 200 ha of highly disturbed land. The short-term goal of the restoration was aimed at stabilizing soils and preventing off-site movement of metal-containing sediments. The longer-term goal was to encourage development of a plant community that resembled a reference condition. Here, we describe our systematic approach for restoration that included identification of constraints to plant establishment and growth, a greenhouse experiment to screen 36 growth medium treatments, and a field experiment to test a subset of best performing treatments. A greenhouse study identified several promising treatments that were implemented in the field experiment, which ultimately identified a single best approach (30 cm of waste rock amended with lime and mushroom compost and covered with 15 cm of limed, fertilized stockpiled topsoil). From 1999 to 2001, the site was re-contoured, amended, and seeded. From 2002 to 2009, we documented an increase in uniformity of vegetation cover on restored areas, an increase in species richness, and a significant shift from a plant community dominated by seeded species, to one more similar to a reference plant community as evidenced by a non-metric multi-dimensional scaling analysis. Overall, these results demonstrate how initial well-planned treatments can encourage a favorable trajectory for restoration of a seeded site, while also demonstrating the utility of this science-based approach for dealing with restoration of a highly disturbed landscape.

A survey of the root-nodule symbiosis in Astragalus and its interaction with selenium (Se) has not been conducted before. Such studies can provide insight into how edaphic conditions modify symbiotic interactions and influence partner coevolution. In this paper plant-organ Se concentration ([Se]) was investigated to assess potential Se exposure to endophytes. •Selenium distribution and molecular speciation of root nodules from Se-hyperaccumulators Astragalus bisulcatus, A. praelongus, and A. racemosus was determined by Se K-edge x-ray absorption spectroscopy. A series of greenhouse experiments were conducted to characterize the response of root-nodule symbiosis in Se-hyperaccumulators and nonhyperaccumulators. •Nodules in three Se-hyperaccumulators (Astragalus crotalariae, A. praelongus, and A. preussii) are reported for the first time. Leaves, flowers, and fruits from Se-hyperaccumulators were routinely above the hyperaccumulator threshold (1,000 µg Se g(-1) DW), but root samples rarely contained that amount, and nodules never exceeded 110 µg Se g(-1) DW. Nodules from A. bisulcatus, A. praelongus, and A. racemosus had Se throughout, with a majority stored in C-Se-C form. Finally, an evaluation of nodulation in Se-hyperaccumulators and nonhyperaccumulators indicated that there was no nodulation inhibition because of plant Se tolerance. Rather, we found that in Se-hyperaccumulators higher levels of Se treatment (up to 100 µM Se) corresponded with higher nodule counts, indicating a potential role for dinitrogen fixation in Se-hyperaccumulation. The effect was not found in nonhyperaccumulators. •As the evolution of Se hyperaccumulation in Astragalus developed, root-nodule symbiosis may have played an integral role.

Arbuscular mycorrhizal fungi (AMF) have been implicated in non-native plant invasion success and persistence. However, few studies have identified the AMF species associating directly with plant invaders, or how these associations differ from those of native plant species. Identifying changes to the AMF community due to plant invasion could yield key plant-AMF interactions necessary for the restoration of native plant communities. This research compared AMF associating with coexisting Bromus tectorum, an invasive annual grass, and Artemisia tridentata, the dominant native shrub in western North America. At three sites, soil and root samples from Bromus and Artemisia were collected. Sporulation was induced using trap cultures, and spores were identified using morphological characteristics. DNA was extracted from root and soil subsamples and amplified. Sequences obtained were aligned and analyzed to compare diversity, composition, and phylogenetic distance between hosts and sites. Richness of AMF species associated with Artemisia in cultures was higher than AMF species associated with Bromus. Gamma diversity was similar and beta diversity was higher in AMF associated with Bromus compared to Artemisia. AMF community composition differed between hosts in both cultures and roots. Two AMF species (Archaeospora trappei and Viscospora viscosum) associated more frequently with Artemisia than Bromus across multiple sites. AMF communities in Bromus roots were more phylogenetically dispersed than in Artemisia roots, indicating a greater competition for resources within the invasive grass. Bromus associated with an AMF community that differed from Artemisia in a number of ways, and these changes could restrict native plant establishment.

Summary 1. Plant community succession has been a major area of study over the past century with recent research focusing on the importance of initial colonisers following disturbance. Seed addition can accelerate ecosystem regeneration and is a method commonly used by land managers to restore disturbed lands. However, few studies have examined the effects of seeding treatments on long-term community composition. 2. A study was established in 1984 in the Piceance Basin of northwest Colorado to examine how various revegetation seed mixes affect plant community development following disturbance. The site was surveyed again in 2008 and 2009 to assess long-term community development. 3. Initial seed mix resulted in significant differences in plant community composition after 25 years. Seeding with native and exotic early-seral species resulted in a community with significantly more exotic species and mid-seral shrubs, while seeding with native late-seral species resulted in a community dominated by perennial grasses. Additionally, an unseeded control resulted in a vegetation community dominated by both perennial grasses and mid-seral shrubs, but community composition at the species level was considerably different from that of the seeded treatments. However, the plant community composition of each of the three treatments was significantly different from an adjacent undisturbed reference area, which was dominated by the late-seral shrub, Artemisia tridentata, and perennial grasses. 4. Synthesis and applications. Our results illustrate how initial colonisers (seed mix) can strongly affect subsequent community assemblage after 25 years of development. Restoration ecologists should give considerable thought to the species used in a restoration seed mix to ensure the success of restoration designs and to create the desired community assembly and associated ecosystem services.

Chemical restrictions, ecological concerns, liability issues, and public sentiment present challenges to land managers attempting to control highly invasive plants like Canada thistle (Cirsium arvense [L.] Scop.). Although herbicide application can be an effective control strategy, increasing limitations force managers of sensitive environments (e.g., national parks, wildlife refuges, protected water-bodies or waterways) to search for effective control alternatives. A greenhouse study was conducted to test the effectiveness of clipping (to simulate field mowing) and grass seeding as alternatives for Canada thistle control. Two native North American grasses (western wheatgrass [Pascopyrum smithii {Rydb.} A. L?ve] and streambank wheatgrass [Elymus lanceolatus {Scribn. & J.G. Sm.}Gould ssp. lanceolatus]) and one sterile hybrid cross between common wheat (Triticum aestivum L.) and tall wheatgrass (Thinopyrum ponticum [Podp.] Z.W. Liu & R.C. Wang) called RegreenTM were used. The effects of clipping and grass seeding on Canada thistle growth, and the effect of Canada thistle on grass growth, were evaluated using 14 unique treatments applied to potted Canada thistle and grass plants. Clipping inhibited Canada thistle growth (by 60%), while grass seeding had no effect. Presence of Canada thistle inhibited grass growth for all seeding treatments except when RegreenTM and western wheatgrass were seeded together with Canada thistle. Planting multiple species for restoration of Canada thistle-infested sites may be important (RegreenTM + western wheatgrass treatment), and cutting Canada thistle may be useful for reducing its growth in restored areas.

Arbuscular mycorrhizal fungi (AMF) are an important driver of plant community assembly, and as such may be critical for restoring plant communities. Previous work has shown that as plant communities develop, AMF density is a good predictor of what type of plant community is supported. However, interactions between plants, particularly facultative hosts, and AMF are often assumed from plant growth responses and lack concomitant AMF growth response data. We examined both plant and AMF responses in association using early- and mid-seral plant hosts and a homogenous AMF community. The goal was to determine how variable interactions are between facultative plant hosts and an AMF community. Plant responsiveness was measured using field soil with and without AMF. AMF density was measured by observing root colonization by AMF in a bioassay host plant grown in soils trained by the individual host plant species used in the plant responsiveness study. Plant species studied were highly variable in their interactions with AMF, and mutualisms, parasitisms, amensalisms and commensalisms were all prevalent. The presence of certain AMF facilitators may have a strong founder effect on plant communities and, where such feedbacks exist, identifying and utilizing these key interactions might facilitate the restoration of degraded ecosystems.

Semiarid ecosystems of Western North America are expe-riencing a boom in natural gas development. However,these systems are slow to recover from the disturbancescreated. The purpose of this study was to develop improvedrestoration techniques on natural gas well pads in West-ern Colorado. This study examined effects and interac-tions of seedbed modifications, soil amendments, seed mix-tures, and seeding methods. The experiment was conductedin pinyon-juniper and semidesert shrub plant communi-ties on five natural gas well pads beginning in 2006. Soiland plant cover data were collected to assess the effec-tiveness of 16 different treatment combinations. After twogrowing seasons, we found that patches of soil salinity( > 4 dS/m) reduced plant cover to less than 20% on 55 ofour 240 experimental plots. These patches of salinity, suchas where reserve pits were buried, may need to be treatedto completely restore cover on the total gas pad area,although causes of salinity patches needs further investi-gation. After removing the 55 saline plots from our dataanalyses, we found that wood chips (WC) as a soil amend-ment increased organic matter content and reduced non-native species. Rough seedbed modifications increased theestablishment of native species, especially during years ofbelow average precipitation. Island broadcasting resultedin an increase of noxious plant cover during the secondgrowing season. From these findings we recommend thatdisturbed well pads in a similar environment be restored byseeding native species on sites that are amended with WCand physically modified to create a roughened seedbed.Key words: ecological processes, microcatchments, nativeplants, natural gas, revegetation techniques, soil amend-ments.Introduction

Some plants hyperaccumulate metals or metalloids to levels several orders of magnitude higher than other species. This intriguing phenomenon has received considerable attention in the past decade. While research has mostly focused on the above-ground organs, roots are the sole access point to below-ground trace elements and as such they play a vital role in hyperaccumulation. Here we highlight the role of the root as an effective trace element scavenger through interactions in the rhizosphere. We found that less than 10% of the known hyperaccumulator species have had their rhizospheres examined. When studied, researchers have focused on root physical characteristics, rhizosphere chemistry, and rhizosphere microbiology as central themes to understand plant hyperaccumulation. One physical characteristic often assumed about hyperaccumulators is that their roots are small, but this is not true for all species and many species remain unexamined. Transporters in root membranes provide avenues for root uptake, while root growth and morphology influence plant access to trace elements in the rhizosphere. Some hyperaccumulators exhibit unique root scavenging and direct their growth toward elements in soil. Studies on hyperaccumulator rhizosphere chemistry have examined the role of the root in altering elemental solubility through exudation and pH changes. Different interpretations have been reported for mobilization of non-labile trace element pools by hyperaccumulators. However, there is a lack of evidence for a novel role for rhizosphere acidification in hyperaccumulation. As for microbiological studies, researchers have shown that bacteria and fungi in the hyperaccumulator rhizosphere may exhibit increased metal tolerance, act as plant growth promoting microorganisms, alter elemental solubility, and have significant effects on plant trace element concentrations. New evidence suggests that symbiosis with arbuscular mycorrhizae may not be rare in hyperaccumulator taxa, even in some members of the Brassicaceae. Although there are several reports on the presence of mycorrhizae, a cohesive interpretation of their role in hyperaccumulation remains elusive. In summary, we present the current state of knowledge about how roots hyperaccumulate and we suggest ways in which this knowledge can be applied and improved.

Selenium (Se) hyperaccumulation, when plant species accumulate upwards of 1,000 mg Se kg �1 dry weight (DW), protects plants from a variety of herbivores and pathogens. The objective of this study was to determine the effect of plant Se concentration on the rate of litter decomposition by invertebrates and microbes in a seleniferous habitat. Decomposition, Se loss, the decomposer community and soil Se concentration beneath leaf litter were compared between litter from two populations of the Se hyperaccumulator Astragalus bisulcatus (one population with 350 and the other with 550 mg Se kg �1 DW) and from the related non-accumulator species Astragalus drummondii and Medicago sativa containing 1- 2m g Se kg �1 DW using a litterbag method. High-Se litter decomposed faster than low- Se litter and supported more microbes and arthro- pods than low-Se leaf litter after 8 and 12 months, respectively. Soil collected from under high-Se litter had higher Se concentration than soil from beneath low-Se litter after 8 months. The higher decompo- sition rate and abundance of decomposers in high- Se litter indicates the presence of Se-tolerant decomposers in this seleniferous habitat that may have contributed to increased decomposition rates of high-Se litter.

Increased soil N availability may often facilitate plant invasions. Therefore, lowering N availability might reduce these invasions and favor desired species. Here, we review the potential efficacy of several commonly proposed management approaches for lowering N availability to control invasion, including soil C addition, burning, grazing, topsoil removal, and biomass removal, as well as a less frequently proposed management approach for lowering N availability, establishment of plant species adapted to low N availability. We conclude that many of these approaches may be promising for lowering N availability by stimulating N immobilization, even though most are generally ineffective for removing N from ecosystems (excepting topsoil removal). C addition and topsoil removal are the most reliable approaches for lowering N availability, and often favor desired species over invasive species, but are too expensive or destructive, respectively, for most management applications. Less intensive approaches, such as establishing low-N plant species, burning, grazing and biomass removal, are less expensive than C addition and may lower N availability if they favor plant species that are adapted to low N availability, produce high C:N tissue, and thus stimulate N immobilization. Regardless of the method used, lowering N availability sufficiently to reduce invasion will be difficult, particularly in sites with high atmospheric N deposition or agricultural runoff. Therefore, where feasible, the disturbances that result in high N availability should be limited in order to reduce invasions by nitrophilic weeds.

Clone libraries of nifH gene fragments specific for the nitrogen-fixing actinomycete Frankia were generated from six soils obtained from five continents using a nested PCR. Comparative sequence analyses of all libraries (n=247 clones) using 96 to 97% similarity thresholds revealed the presence of three and four clusters of frankiae representing the Elaeagnus and the Alnus host infection groups, respectively. Diversity of frankiae was represented by fewer clusters (i.e., up to four in total) within individual libraries, with one cluster generally harboring the vast majority of sequences. Meta-analysis including sequences previously published for cultures (n=48) and for uncultured frankiae in root nodules of Morella pensylvanica formed in bioassays with the respective soils (n=121) revealed a higher overall diversity with four and six clusters of frankiae representing the Elaeagnus and the Alnus host infection groups, respectively, and displayed large differences in cluster assignments between sequences retrieved from clone libraries and those obtained from nodules, with assignments to the same cluster only rarely encountered for individual soils. These results demonstrate large differences between detectable Frankia populations in soil and those in root nodules indicating the inadequacy of bioassays for the analysis of frankiae in soil and the role of plants in the selection of frankiae from soil for root nodule formation.

Water treatment residuals (WTRs) and biosolids are byproducts from municipal water treatment processes. Both byproducts have been studied separately for land application benefi ts. Th ere are possible environmental benefi ts of WTRs and biosolids co-application but these studies are limited. Our objectives were to determine relative long-term (13-15 yr) eff ects of a single and short-term (2-4 yr) eff ects of repeated WTR-biosolids co-applications on soil chemistry, microbiology, and plant community structure in a Colorado semiarid grassland. Only relative changes associated between co-applications were studied, as we assumed WTR application would only occur if used as a management practice. Th ree WTR rates (5, 10, and 21 Mg ha -1 ) were surface co-applied (no incorporation) with a single biosolids rate (10 Mg ha -1 ) once in 1991 (long-term plots) and again in 2002 (short-term plots). Soil 0- to 8-, 8- to 15-, and 15- to 30-cm depth pH, electrical conductivity (EC), NO 3 -N, NH 4 -N, total C, and total N were not aff ected by WTR application in 2004, 2005, or 2006. Ammonium-bicarbonate diethylenetriaminepentaacetic acid (AB-DTPA)- extractable soil Al was unaff ected by WTR application, but extractable P and Mo decreased with increasing WTR rate because of WTR adsorption. Plant tissue P and Mo content decreased with specifi c plant species and years due to adsorption to WTR; no defi ciency symptoms were observed. Plant community composition and cover were largely unaff ected by WTR application. Soil microbial community structure was unaff ected by WTR co-application rate (total ester-linked fatty acid methyl ester (EL-FAME) concentrations ranged from 33.4 to 54.8 nmol g -1 soil), although time since biosolids-WTR application aff ected a subset of microbial community fatty acids including markers for Gram-positive and Gram-negative bacteria. Overall, WTR-biosolids co-applications did not adversely aff ect semiarid grassland ecosystem dynamics. Abbreviations: AB-DTPA, ammonium-bicarbonate diethylenetriaminepentaacetic acid; EC, electrical conductivity; EL-FAME, ester-linked fatty acid methyl ester; PCA, principal component analysis; WTR, water treatment residual.

Cheatgrass (Bromus tectorum L.) is an exotic annual grass that has invaded approximately 40,000,000 ha of rangelands in the United States, including montane ecosystems that are important habitats for wildlife and livestock. In addition to well-understood mechanisms by which Cheatgrass gains competitive advantage, recent studies have shown that Cheatgrass may also change the associated soil microbial community to impact native perennial plants and promote the persistence of Cheatgrass. Furthermore, reducing plant-available N represents a tool for initiating conditions that accelerate successional change from annual- to perennial-dominated communities. At a montane, mixed shrub–grassland Cheatgrass-dominated site in Colorado, we applied sucrose to reduce available N, and we added soil from a native plant community in order to reestablish the microbial community. This approach tested the idea that intact native soil microbial communities may enhance the beneficial effect of reducing soil N availability in a restoration setting. By the end of the experiment, reduced N availability decreased Cheatgrass by 9.8%, non-native annual/biennial plant cover by 15.0%, and increased relative perennial plant cover by 13.4%; soil inoculation reduced Cheatgrass by 7.6% and increased perennial abundance by 11.3%. Soil inoculum additions and reduced N availability both contributed toward restoring a perennial-dominated community and demonstrates that addition of native soil inoculum may be a useful tool for restoration efforts.

Russian knapweed (Acroptilon repens) and Spotted knapweed (Centaurea maculosa) are allelopathic weeds invasive in North American grasslands. Both species contain at least one phytotoxic flavonoid root exudate with demonstrated negative influences on other plants. Previous findings indicated that Silky lupine (Lupinus sericeus), among other legumes, was relatively resistant to Spotted knapweed invasion and allelochemistry. We hypothesized that legume species may exhibit resistance to flavonoids in knapweed root exudates and may serve as candidate species for management efforts. Because legumes form symbiotic relationships with rhizobia, these bacteria must also be evaluated for allelochemical resistance before legumes can be recommended for restoration. In this study, we examined four legume species for effects of 7,8-benzoflavone (from Russian knapweed) and (±)-catechin (from Spotted knapweed) on rhizosphere interactions involving legume roots and associated rhizobia. Pure cultures of four rhizobia strains exhibited varied responses when grown with 7,8-benzoflavone or (±)-catechin. Alfalfa (Medicago sativa) and its bacterial symbiont, Sinorhizobium meliloti, exhibited allelochemical resistance that varied with (±)-catechin concentration when grown in vitro. Four legume species were grown under greenhouse conditions. Plants that were inoculated and nodulated generally exhibited no response to 7,8-benzoflavone or (±)-catechin treatments. Plants that were not inoculated exhibited stronger responses. Therefore, inoculation and nodulation may confer resistance to allelochemicals. These results, when coupled with previous research and field observations, suggest that legumes may not be susceptible to knapweed allelopathy and may be good choices in restoration of knapweed infestations when inoculated, particularly on sites with low soil nitrogen.

Bioassays with Morella pensylvanica as capture plant and comparative sequence analyses of nifH gene fragments of Frankia populations in nodules formed were used to investigate the diversity of Frankia in soils over a broad geographic range, i.e., from sites in five continents (Africa, Europe, Asia, North America, and South America). Phylogenetic analyses of 522-bp nifH gene fragments of 100 uncultured frankiae from root nodules of M. pensylvanica and of 58 Frankia strains resulted in a clear differentiation between frankiae of the Elaeagnus and the Alnus host infection groups, with sequences from each group found in all soils and the assignment of all sequences to four and five clusters within these groups, respectively. All clusters were formed or dominated by frankiae obtained from one or two soils with single sequences occasionally present from frankiae of other soils. Variation within a cluster was generally low for sequences representing frankiae in nodules induced by the same soil, but large between sequences of frankiae originating from different soils. Three clusters, one within the Elaeagnus and two within the Alnus host infection groups, were represented entirely by uncultured frankiae with no sequences from cultured relatives available. These results demonstrate large differences in nodule-forming frankiae in five soils from a broad geographic range, but low diversity of nodule-forming Frankia populations within any of these soils.

The potential role of host plant species in the selection of symbiotic, nitrogen-fixing Frankia strains belonging to the Elaeagnus host infection group was assessed in bioassays with two Morella, three Elaeagnus, and one Shepherdia species as capture plants, inoculated with soil slurries made with soil collected from a mixed pine/grassland area in central Wisconsin, USA. Comparative sequence analysis of nifH gene fragments amplified from homogenates of at least 20 individual lobes of root nodules harvested from capture plants of each species confirmed the more promiscuous character of Morella cerifera and Morella pensylvanica that formed nodules with frankiae of the Alnus and the Elaeagnus host infection groups, while frankiae in nodules formed on Elaeagnus umbellata, Elaeagnus angustifolia, Elaeagnus commutata, and Shepherdia argentea generally belonged to the Elaeagnus host infection group. Diversity of frankiae of the Elaeagnus host infection groups was larger in nodules on both Morella species than in nodules formed on the other plant species. None of the plants, however, captured the entire diversity of nodule-forming frankiae. The distribution of clusters of Frankia populations and their abundance in nodules was unique for each of the plant species, with only one cluster being ubiquitous and most abundant while the remaining clusters were only present in nodules of one (six clusters) or two (two clusters) host plant species. These results demonstrate large effects of the host plant species in the selection of Frankia strains from soil for potential nodule formation and thus the significant effect of the choice of capture plant species in bioassays on diversity estimates in soil.

In a greenhouse experiment, we examined the effectiveness of four native cover crops for controlling four exotic, invasive species and increasing success of four western North American grassland species. Planting the annual cover crops, annual ragweed (Ambrosia artemisiifolia) and common sunflower (Helianthus annuus), reduced the biomass of the exotic species cheatgrass (Bromus tectorum), Japanese brome (Bromus japonicus), Canada thistle (Cirsium arvense), and whitetop (Cardaria draba). The annual cover crops also reduced the desired species biomass in competition with the perennial exotics, but either increased or did not affect the desired species biomass in competition with the annual exotics. Planting the perennial cover crops, Canada goldenrod (Solidago canadensis) and littleleaf pussytoes (Antennaria microphylla), rarely inhibited exotic species, but did increase the desired species biomass. Field experiments are needed to test the cover crops under more ecologically relevant conditions, but our results suggested that the annual cover crops may be effective for controlling invasive annuals and for facilitating native perennials.

Knapweeds (Centaurea spp.) are damaging invaders of grasslands and other North American rangelands. A field study was conducted to determine conditions that promote diffuse knapweed (C. diffusa) emergence and establishment in a native Colorado grassland (North America). Knapweed was planted in native grassland under treatments with different opening sizes, levels of competition, knapweed seed burial and season of seeding. There was no effect of opening size where competing natives were alive, but knapweed emergence in 5- and 15-cm openings was higher than 0-cm openings where natives were killed. Reducing competition reduced fall diffuse knapweed emergence, but did not affect spring emergence. Seed burial increased knapweed emergence, but the effect varied by season. Although diffuse knapweed emergence reached 35%, only four plants survived from 3,600 seeds. This native grassland did not prevent knapweed emergence or establishment, but both were so low that rapid knapweed invasion is unlikely.

Co‐application of biosolids and water treatment residuals (WTR) land has not been extensively studied but may be beneficial by sorbing excess biosolid‐borne or soil phosphorus (P) onto WTR, reducing the likelihood of off‐site movement. Reduction of excess soil P may affect the role of specific P‐cleaving enzymes. The research objective was to understand the long‐term effects of single co‐applications and the short‐term impacts of repeated co‐applications on soil acid phosphomonoesterase, phosphodiesterase, pyrophosphatase, and phytase enzyme activities. Test plots were 7.5 × 15 m with treatments consisting of three different WTR rates with a single biosolids rate (5, 10, and 21 Mg WTR ha−1; 10 Mg biosolids ha−1) surface co‐applied once in 1991 or reapplied in 2002. Control plots consisted of those that received no WTR–biosolids co‐applications and plots that received only 10 Mg biosolids ha−1. Plots were sampled to a 5‐cm depth in 2003 and 2004, and soil phosphatases and phytase enzyme activities were mea...

Land coapplication of water treatment residuals (WTR) with biosolids has not been extensively researched, but the limited studies performed suggest that WTR sorb excess biosolids-borne P. To understand the long-term effects of a single coapplication and the short-term impacts of a repeated coapplication on soil P inorganic and organic transformations, 7.5- by 15-m plots with treatments of three different WTR rates with a single biosolids rate (5, 10, and 21 Mg WTR ha-1 and 10 Mg biosolids ha-1) surface coapplied once in 1991 or surface reapplied in 2002 were utilized. Soils from the 0- to 5-cm depth were collected in 2003 and 2004 and were sequentially fractionated for inorganic and organic P (Po). Inorganic P fractionation determined (i) soluble and loosely bound, (ii) Al-bound, (iii) Fe-bound, (iv) occluded, and (v) Ca-bound P, while organic P fractionation determined (i) labile, (ii) biomass, (iii) moderately labile, (iv) fulvic acid, (v) humic acid, and (vi) nonlabile associated Po. Pathway analysis showed that humic, fulvic, and nonlabile Po did not play a role in P transformations. Biomass Po and moderately labile Po contributed to the transitory labile Po pool. Labile Po was a P source for Fe-bound and WTR-bound inorganic phases, with the Fe-bound phase transitory to the occluded P sink. The Al-bound phase additionally contributed to the occluded P sink. The Ca-bound phase weathered and released P to both the Fe-bound and WTR-bound P phases. Overall, the WTR fraction, even 13 yr after the initial application, acted as the major stable P sink.

The EDYS (ecological dynamics simulation) model was used to simulate vegetation growth resulting from different experimental treatments for Bromus tectorum control at Yakima Training Center, Washington. The treatments tested in the field for 4 years were seeding, sucrose application, and a combination of seeding and sucrose application. These treatments included burning to favor their implementation. A control plant community with no manipulations was also monitored in the study. The simulations of plant production were not significantly different from the observed field results in 90% of the comparisons, supporting the validity of the model. In long-term simulations, the population of B. tectorum ceased to dominate the plant community in about 12 years regardless of the treatments. Subsequently, the successional patterns were affected by the treatments. The control and sucrose treatments produced similar successional trends dominated mainly by shrubs (Artemisia tridentata and Chrysothamnus nauseosus). In contrast, both seeding treatments, including the one with sucrose, produced successional trends dominated by grasses, which reflected the seed mix composition. Seeding had more lasting effects than sucrose application. However, the seeded species only started to dominate the seeded areas when the dominance of B. tectorum was reduced. The long-term simulations provided projections that were difficult to envision solely based on the results of the 4-year field experiment.

Allelopathy as a mechanism of invasion in plant communities remains a debated topic, partly due to lack of techniques to measure allelochemical output and stability in the field. Evidence has arisen to support allelopathic mechanisms in several invasive knapweeds, including Russian knapweed. Previous studies have shown that a phytotoxin, 7,8-benzoflavone, is produced by Russian knapweed roots under soil-free conditions. Here we describe a high performance liquid chromatography (HPLC) method to detect this compound in soil and we present the first observations of this compound from Russian knapweed infested soils.

The EDYS model was used to simulate plant production in burned and unburned communities dominated by Bromus tectorum at the US Army Yakima Training Centre, Washington. Model results were validated with 4 years of field data obtained in experiments designed specifically for this study. Subsequently, 50-year simulations of plant production were conducted with normal precipitation for the area and with no disturbances such as grazing or military training. Our simulations of plant production were not significantly different (p>0.05) from observed results of burned and unburned communities in 90% of the comparisons, indicating that EDYS adequately simulated the dynamics of this system. Long-term simulations indicated that annual species, primarily B. tectorum, dominated burned and unburned plant communities for 13–15 years, after which, annuals were replaced by perennials. B. tectorum, however, remained five years longer in the burned than in the unburned community, suggesting that fire favors the persistence of this species in invaded areas. Although B. tectorum dominated both plant communities during the initial simulation period, its production fluctuated greatly. The main perennials that replaced B. tectorum were Chrysothamnus nauseosus, Artemisia tridentata, and Agropyron cristatum. Our long-term modelling results are supported by previous field observations in which annual species are replaced by perennials in the absence of disturbances.

Phytotoxicity bioassays and pot experiments using activated carbon both suggest that Centaurea solstitialis (yellow star-thistle) does not rely on phytotoxic root exudates for invasion of California grasslands. Pot experiments in which five native species were grown in the presence/absence of C. solstitialis and in the presence/absence of activated carbon (fully crossed design) showed that C. solstitialis competitively suppressed native species, but did not inhibit them through allelochemicals. In separate experiments examining the role of root exudates in invasion success, treatment with crude root exudates and chloroform-extracted root exudates from C. solstitialis reduced growth of the model plant Arabidopsis thaliana. However, high concentrations of the exudates (50%, v/v or 500 μg mL-1) were required to inhibit A. thaliana growth and did not result in A. thaliana mortality, suggesting the presence of only a weak growth inhibitor. Moreover, high concentrations of C. solstitialis crude root exudates did not affect the growth of five native grass species often displaced by C. solstitialis invasions in California grasslands. Finally, root exudates collected from C. solstitialis had weaker effects on a native California root parasite, Triphysaria versicolor, than root exudates collected from Zea mays, a species not renowned for its competitive or invasive capabilities. Our results suggest that, while C. solstitialis might possibly "be persuaded to yield a product that is toxic to one species or another" (Population biology of plants, Academic, 1977), we find no evidence that allelopathic root exudates play a role in the competitive success of this invasive. Fil: Qin, Bo. Colorado State University; Estados Unidos Fil: Lau, Jennifer A.. University of California; Estados Unidos Fil: Kopshever, Joseph. University of California; Estados Unidos Fil: Callaway, Ragan M.. University of Montana; Estados Unidos Fil: McGray, Heather. University of California; Estados Unidos Fil: Perry, Laura G.. Colorado State University; Estados Unidos Fil: Weir, Tiffany L.. Colorado State University; Estados Unidos Fil: Paschke, Mark W.. Colorado State University; Estados Unidos Fil: Hierro, Jose Luis. University of Montana; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; Argentina Fil: Yoder, John. University of California; Estados Unidos Fil: Vivanco, Jorge M.. Colorado State University; Estados Unidos Fil: Strauss, Sharon. University of California; Estados Unidos

Invasive plants are a widespread problem but the mechanisms used by these plants to become invasive are often unknown. The production of phytotoxic natural products by invasive weeds is one mechanism by which these species may become successful competitors. Here we conducted a bioactivity-driven fractionation of root extracts and exudates from the invasive plant leafy spurge (Euphorbia esula L.), and structurally characterized jatrophane diterpenes and ellagic acid derivatives. Ellagic acid derivatives and one of the jatrophane diterpenes, esulone A, have been previously reported from leafy spurge, but another of the jatrophane diterpenes, kasuinine B, has not. We show that these compounds are phytotoxic but affect plants in different ways, either inducing overall plant necrosis or reducing root branching and elongation.

Diffuse (Centaurea diffusa Lam.) and spotted knapweed (Centaurea maculosa auct. non Lam.) are damaging exotic plant invaders of North American rangelands. Although existing weed management techniques can successfully reduce negative impacts of knapweeds, these plants continue to spread in an unpredictable manner. The successful spread of knapweeds into native plant communities may be driven, in part, by interactions between these plants and native soil communities. This study was conducted to evaluate the relative benefit of native soil communities to two native plants and two knapweeds and to investigate the growth of these plants in soil from knapweed infestations and from adjacent native rangelands. Individual plants of bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) A. Love), yarrow (Achillea millefolium L.), spotted and diffuse knapweed were grown in a greenhouse in field collected soil. The relative benefit of the native soil community was determined by comparing emergence and production of native plants and exotic knapweeds in autoclaved versus unautoclaved native rangeland soil. Emergence and production were also determined in the greenhouse for native plants and exotic knapweeds grown in soil collected from the core and perimeter of knapweed infestations and in soil from the adjacent, uninvaded native rangeland. The native soil community had a negative effect on the growth of bluebunch wheatgrass (P. spicata (Pursh) A. Love) and diffuse knapweed, but a positive effect on spotted knapweed emergence. The interactions between yarrow (A. millefolium L.) and the native soil community were variable. The native soil community appears to be more beneficial to spotted knapweed than to the other plants studied, including diffuse knapweed. Therefore, it appears that two closely related knapweeds have very different interactions with soil biota and perhaps different strategies for invasion. Soil from diffuse and spotted knapweed infestations did not prevent growth of two native plants. Diffuse knapweed growth was not promoted by soil from within a diffuse knapweed infestation, but soil from the core of a spotted knapweed infestation did increase emergence of spotted knapweed.

Spotted knapweed (Centaurea maculosa Lam.) and diffuse knapweed (Centaurea diffusa Lam.) infested plant communities that are being studied for improved management practices to enhance native perennial grass species were evaluated for selected fungal responses during 2000–2003. Replicated plots were treated using combinations of carbohydrate addition to reduce soil N availability, reseeding with desirable mid- and late-seral plant species, and reintroducing a native late-seral soil microbial community using whole soil inoculum. The responses of extraradical filamentous fungi, mycorrhizae (infection intensity, arbuscules and vesicles) and systemic endophytic fungal (SEF) of native perennial grasses were evaluated in relation to plant community relative biomass, litter mass and soil C:N ratio in treated versus replicated untreated control plots. Soil filamentous fungal and plant community changes occurred with combined treatments (sucrose, seeding, soil inoculum) and were most evident in the diffuse knapweed-invaded sites during 2003. Based on overall correlations for the 2000–2003 experimental period, total and active filamentous fungi were inversely related to the relative biomass of invasive plants, particularly of spotted knapweed. The native perennial grasses had lower mycorrhizal infection intensity at the spotted knapweed site, in comparison with the diffuse knapweed infested site, where soil inoculation also appeared to be related to higher SEF and mycorrhizal infection occurrence. These results suggest that a mycocentric approach, involving soil management to maintain filamentous fungal hyphal lengths, while decreasing active fungal development, may create a soil environment more conducive to maintenance of desired indigenous plant species.

Approximately 70,150 dry Mg of biosolids from over 450 wastewater treatment facilities are applied to the semi-arid rangelands of Colorado every year. Research on semi-arid grassland responses to biosolids has become vital to better understand ecosystem dynamics and develop effective biosolids management strategies. The objectives of this study were to determine the long-term (w12 years) effects of a single biosolids application, and the short-term (w2 years) effects of a repeated application, on plant and microbial community structure in a semi-arid grassland soil. Specific attention was paid to arbuscular mycorrhizal fungi (AMF) and linkages between shifts in plant and soil microbial community structures. Biosolids were surface applied to experimental plots once in 1991 (long-term plots) and again to short-term plots in 2002 at rates of 0, 2.5, 5, 10, 21, or 30 Mg ha K1 . Vegetation (species richness and above-ground biomass), soil chemistry (pH, EC, total C, total N, and extractable P, NO3‐N, and NH4‐N), and soil microbial community structure [ester-linked fatty acid methyl esters (EL-FAMEs)], were characterized to assess impacts of biosolids on the ecosystem. Soil chemistry was significantly affected and shifts in both soil microbial and plant community structure were observed with treatment. In both years, the EL-FAME biomarker for AMF decreased with increasing application rate of biosolids; principal components analysis of EL-FAME data yielded shifts in the structure of the microbial communities with treatment primarily related to the relative abundance of the AMF specific biomarker. Significant (p%0.05) correlations existed among biomarkers for Gram-negative and Gram-positive bacteria, AMF and specific soil chemical parameters and individual plant species’ biomass. The AMF biomarker was positively correlated with biomass of the dominant native grass species blue grama (Bouteloua gracilis [Willd. ex Kunth] Lagasca ex Griffiths) and was negatively correlated with western wheatgrass (Agropyron smithii Rydb.) biomass. This study demonstrated that applications of biosolids at relatively low rates can have significant long-term effects on soil chemistry, soil microbial community structure, and plant community species richness and structure in the semi-arid grasslands of northern Colorado. Reduced AMF and parallel shifts in the soil microbial community structure and the plant community structure require further investigation to determine precisely the sequence of influence and resulting ecosystem dynamics. q 2005 Elsevier Ltd. All rights reserved.

Zinc toxicity thresholds for reclamation plants are largely unknown. As a result, ecological risk assessments often rely on toxicity thresholds for agronomic species, which may differ from those of restoration species. Our objective was to provide Zn toxicity thresholds for forb species that are commonly used in reclamation activities. We used a greenhouse screening study where seedlings of yarrow (Achillea millefolium L.), Bigelow's tansyaster (Machaeranthera bigelovii (Gray) Greene var. bigelovii), blue flax (Linum perenne L. var. Appar), alfalfa (Medicago sativa L. var. Ladak), Palmer's penstemon (Penstemon palmeri Gray), and Rocky Mountain penstemon (Penstemon strictus Benth. var. Bandera) were grown in sand culture and exposed to increasing concentrations of Zn. Lethal concentrations (LC50 – substrate Zn concentration resulting in 50% mortality), effective concentrations (EC50 – substrate Zn concentration resulting in 50% biomass reduction), and phytotoxicity thresholds (PT50 – tissue Zn concentration resulting in 50% biomass reduction) were then determined. Phytotoxicity thresholds and effective concentrations for these reclamation species were relatively consistent between species. Our estimates of PT50-shoot for these species range from 1258 to 3214 mg Zn kg−1 . Measures of EC50-plant for these restoration forbs ranged from 82 to 214 mg Zn L−1 . These thresholds might be more useful for risk assessors working on reclamation sites than those based on non-reclamation species that are widely used.