The variable effects of soil nitrogen availability and insect herbivory on aboveground and belowground plant biomass in an old-field ecosystem

Nutrient availability and herbivory can regulate primary production in ecosystems, but little is known about how, or whether, they may interact with one another. Here, we investigate how nitrogen availability and insect herbivory interact to alter aboveground and belowground plant community biomass in an old-field ecosystem. In 2004, we established 36 experimental plots in which we manipulated soil nitrogen (N) availability and insect abundance in a completely randomized plot design. In 2009, after 6 years of treatments, we measured aboveground biomass and assessed root production at peak growth. Overall, we found a significant effect of reduced soil N availability on aboveground biomass and belowground plant biomass production. Specifically, responses of aboveground and belowground community biomass to nutrients were driven by reductions in soil N, but not additions, indicating that soil N may not be limiting primary production in this ecosystem. Insects reduced the aboveground biomass of subdominant plant species and decreased coarse root production. We found no statistical interactions between N availability and insect herbivory for any response variable. Overall, the results of 6 years of nutrient manipulations and insect removals suggest strong bottom-up influences on total plant community productivity but more subtle effects of insect herbivores on aspects of aboveground and belowground production.     

Punching above their weight: low-biomass non-native plant species alter soil properties during primary succession

Non‐native invasive plants can greatly alter community and ecosystem properties, but efforts to predict which invasive species have the greatest impacts on these properties have been generally unsuccessful. An hypothesis that has considerable promise for predicting the effects of invasive non‐native plant species is the mass ratio hypothesis (i.e. that dominant species exert the strongest effects). We tested this hypothesis using data from a four year removal experiment in which the presence of two dominant shrub species (one native and the other not), and subordinate plant species, were manipulated in factorial combinations over four years in a primary successional floodplain system. We measured the effects of these manipulations on the plant community, soil nutrient status and soil biota in different trophic levels of the soil food web. Our experiment showed that after four years, low‐biomass non‐native plant species exerted disproportionate belowground effects relative to their contribution to total biomass in the plant community, most notably by increasing soil C, soil microbial biomass, altering soil microbial community structure and increasing the abundance of microbial‐feeding and predatory nematodes. Low‐biomass, non‐native plant species had distinct life history strategies and foliar traits (higher foliar N concentrations and higher leaf area per unit mass) compared with the two dominant shrub species (97% of total plant mass). Our results have several implications for understanding species’ effects in communities and on soil properties. First, high‐biomass species do not necessarily exert the largest impacts on community or soil properties. Second, low‐biomass, inconspicuous non‐native species can influence community composition and have important trophic consequences belowground through effects on soil nutrient status or resource availability to soil biota. Our finding that low‐biomass non‐native species influence belowground community structure and soil properties more profoundly than dominant species demonstrates that the mass ratio hypothesis does not accurately predict the relative effects of different coexisting species on community‐ and ecosystem‐level properties.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

Plant invasion alters nitrogen cycling by modifying the soil nitrifying community

Plant invasions have dramatic aboveground effects on plant community composition, but their belowground effects remain largely uncharacterized. Soil microorganisms directly interact with plants and mediate many nutrient transformations in soil. We hypothesized that belowground changes to the soil microbial community provide a mechanistic link between exotic plant invasion and changes to ecosystem nutrient cycling. To examine this possible link, monocultures and mixtures of exotic and native species were maintained for 4 years in a California grassland. Gross rates of nitrogen (N) mineralization and nitrification were quantified with ¹⁵N pool dilution and soil microbial communities were characterized with DNA‐based methods. Exotic grasses doubled gross nitrification rates, in part by increasing the abundance and changing the composition of ammonia‐oxidizing bacteria in soil. These changes may translate into altered ecosystem N budgets after invasion. Altered soil microbial communities and their resulting effects on ecosystem processes may be an invisible legacy of exotic plant invasions.     

Above‐ and belowground insect herbivores mediate the impact of nitrogen eutrophication on the soil food web in a grassland ecosystem

Insect herbivores are important drivers of ecosystem processes in grasslands, and can mediate the grassland's response to environmental change. For example, recent evidence shows that above‐ and belowground herbivory, individually and in combination, can modify how a plant community responds to nitrogen (N) eutrophication, an important driver of global change. However, knowledge about how such effects extend to the associated soil food web is lacking. In a mesocosm experiment, we investigated how communities of soil nematodes – an abundant and functionally important group of soil organisms – responded to above‐ and belowground insect herbivory at contrasting N levels. We found that the strongest influence of above‐ and belowground herbivory on the nematode community appeared at elevated N. The abundance of root‐feeding nematodes increased when either above‐ or belowground insect herbivores were present at elevated N, but when applied together the two herbivore types cancelled out one another's effect. Additionally, at elevated N aboveground herbivory increased the abundance of fungal‐feeders relative to bacterial‐feeders, which indicates changes in decomposition pathways induced by N and herbivory. Belowground herbivory increased the abundance of omnivorous nematodes. The shifts in both the herbivorous and detrital parts of the soil food web demonstrate that above‐ and belowground herbivory does not only mediate the response of the plant community to N eutrophication, but in extension also the soil food web sustained by the plant community. We conclude that feedbacks between effects of above‐ and belowground herbivory mediate the response of the grassland ecosystem to N eutrophication.     

Responses of belowground communities to large aboveground herbivores: Meta‐analysis reveals biome‐dependent patterns and critical research gaps

The importance of herbivore–plant and soil biota–plant interactions in terrestrial ecosystems is amply recognized, but the effects of aboveground herbivores on soil biota remain challenging to predict. To find global patterns in belowground responses to vertebrate herbivores, we performed a meta‐analysis of studies that had measured abundance or activity of soil organisms inside and outside field exclosures (areas that excluded herbivores). Responses were often controlled by climate, ecosystem type, and dominant herbivore identity. Soil microfauna and especially root‐feeding nematodes were negatively affected by herbivores in subarctic sites. In arid ecosystems, herbivore presence tended to reduce microbial biomass and nitrogen mineralization. Herbivores decreased soil respiration in subarctic ecosystems and increased it in temperate ecosystems, but had no net effect on microbial biomass or nitrogen mineralization in those ecosystems. Responses of soil fauna, microbial biomass, and nitrogen mineralization shifted from neutral to negative with increasing herbivore body size. Responses of animal decomposers tended to switch from negative to positive with increasing precipitation, but also differed among taxa, for instance Oribatida responded negatively to herbivores, whereas Collembola did not. Our findings imply that losses and gains of aboveground herbivores will interact with climate and land use changes, inducing functional shifts in soil communities. To conceptualize the mechanisms behind our findings and link them with previous theoretical frameworks, we propose two complementary approaches to predict soil biological responses to vertebrate herbivores, one focused on an herbivore body size gradient, and the other on a climate severity gradient. Major research gaps were revealed, with tropical biomes, protists, and soil macrofauna being especially overlooked.     

Linking Above- and Belowground Responses to 16 Years of Fertilization,Mowing, and Removal of the Dominant Species in a Temperate Grassland

Species-rich oligotrophic meadows are affected by a wide range of management interventions that influence their functioning and capacity to deliver ecosystem services, but long-term studies on the above- and belowground adaptations to different management tools are still scarce. We focused on the interactive effects of NPK fertilization, mowing, and removal of the initially dominant species (Molinia caerulea) on plant, soil, and microbial responses in wet oligotrophic grassland in a 16-year full-factorial manipulative experiment. Changes in vegetation composition, soil pH, and nutrient availability were accompanied by altered microbial phospholipid fatty acid (PLFA) composition, whereas treatment effects on soil microbial biomass and carbon (C) mineralization were mainly related to changes in soil organic matter (SOM) content and nutrient availability. Fertilization decreased plant species richness aboveground and lowered SOM storage and microbial activity belowground. Mowing preserved high plant diversity and led to more efficient recycling of N within the grassland, whereas Molinia removal significantly affected only plant community composition. Mowing combined with fertilization maintained high species richness only in the short term. Belowground, mowing reduced N leaching from the fertilized system but did not prevent SOM depletion, soil acidification, and concomitant adverse effects on soil microbes. We conclude that annual mowing is the appropriate type of extensive management for oligotrophic species-rich meadows, but the concomitant nutrient depletion should not be compensated for by regular NPK fertilization due to its adverse effects on soil quality.     

Interactions between plant growth and soil nutrient cycling under elevated CO2: a meta-analysis

free air carbon dioxide enrichment (FACE) and open top chamber (OTC) studies are valuable tools for evaluating the impact of elevated atmospheric CO₂ on nutrient cycling in terrestrial ecosystems. Using meta‐analytic techniques, we summarized the results of 117 studies on plant biomass production, soil organic matter dynamics and biological N₂ fixation in FACE and OTC experiments. The objective of the analysis was to determine whether elevated CO₂ alters nutrient cycling between plants and soil and if so, what the implications are for soil carbon (C) sequestration. Elevated CO₂ stimulated gross N immobilization by 22%, whereas gross and net N mineralization rates remained unaffected. In addition, the soil C : N ratio and microbial N contents increased under elevated CO₂ by 3.8% and 5.8%, respectively. Microbial C contents and soil respiration increased by 7.1% and 17.7%, respectively. Despite the stimulation of microbial activity, soil C input still caused soil C contents to increase by 1.2% yr^?1. Namely, elevated CO₂ stimulated overall above‐ and belowground plant biomass by 21.5% and 28.3%, respectively, thereby outweighing the increase in CO₂ respiration. In addition, when comparing experiments under both low and high N availability, soil C contents (+2.2% yr^?1) and above‐ and belowground plant growth (+20.1% and+33.7%) only increased under elevated CO₂ in experiments receiving the high N treatments. Under low N availability, above‐ and belowground plant growth increased by only 8.8% and 14.6%, and soil C contents did not increase. Nitrogen fixation was stimulated by elevated CO₂ only when additional nutrients were supplied. These results suggest that the main driver of soil C sequestration is soil C input through plant growth, which is strongly controlled by nutrient availability. In unfertilized ecosystems, microbial N immobilization enhances acclimation of plant growth to elevated CO₂ in the long‐term. Therefore, increased soil C input and soil C sequestration under elevated CO₂ can only be sustained in the long‐term when additional nutrients are supplied.     

Effects of increased N and P availability on biomass allocation and root carbohydrate reserves differ between N‐fixing and non‐N‐fixing savanna tree seedlings

In mixed tree‐grass ecosystems, tree recruitment is limited by demographic bottlenecks to seedling establishment arising from inter‐ and intra‐life‐form competition, and disturbances such as fire. Enhanced nutrient availability resulting from anthropogenic nitrogen (N) and phosphorus (P) deposition can alter the nature of these bottlenecks by changing seedling growth and biomass allocation patterns, and lead to longer‐term shifts in tree community composition if different plant functional groups respond differently to increased nutrient availability. However, the extent to which tree functional types characteristic of savannas differ in their responses to increased N and P availability remains unclear. We quantified differences in above‐ and belowground biomass, and root carbohydrate contents in seedlings of multiple N‐fixing and non‐N‐fixing tree species characteristic of Indian savanna and dry forest ecosystems in response to experimental N and P additions. These parameters are known to influence the ability of plants to compete, as well as survive and recover from fires. N‐fixers in our study were co‐limited by N and P availability, while non‐N‐fixers were N limited. Although both functional groups increased biomass production following fertilization, non‐N‐fixers were more responsive and showed greater relative increases in biomass with fertilization than N‐fixers. N‐fixers had greater baseline investment in belowground resources and root carbohydrate stocks, and while fertilization reduced root:shoot ratios in both functional groups, root carbohydrate content only reduced with fertilization in non‐N‐fixers. Our results indicate that, even within a given system, plants belonging to different functional groups can be limited by, and respond differentially to, different nutrients, suggesting that long‐term consequences of nutrient deposition are likely to vary across savannas contingent on the relative amounts of N and P being deposited in sites.     

Linking soil food web structure to above‐ and belowground ecosystem processes: a meta‐analysis

Soil fauna can be an important regulator of community parameters and ecosystem processes, but there have been few quantitative syntheses of the role of soil fauna in terrestrial soil communities and ecosystems. Here, we conducted a meta‐analysis to investigate the impacts of invertebrate soil micro‐ and mesofauna (grazers and predators) on plant productivity and microbial biomass. Overall our results indicate that an increase in the biomass of soil fauna increased aboveground plant productivity across ecosystems by 35% and decreased microbial biomass by 8%. In addition, we found no evidence for trophic cascades in terrestrial soil food webs, but the bacterivorous component of soil fauna influenced plant productivity and microbial biomass more than did the fungivorous component. Furthermore, changes in the biomass of soil fauna differentially affected plant productivity among plant functional groups: a higher biomass of soil fauna increased aboveground productivity by 70% in coniferous systems. However, in ecosystems dominated by legumes, a functional group with lower inorganic nitrogen requirements, there was no response of aboveground productivity to increases in the biomass of soil fauna. In sum, the results of this meta‐analysis indicate that soil fauna help to regulate ecosystem production, especially in nutrient‐limited ecosystems.     

Spatial heterogeneity in soil nutrient supply modulates nutrient and biomass responses to multiple global change drivers in model grassland communities

Changes in the atmospheric concentration of carbon dioxide ([CO₂]), nutrient availability and biotic diversity are three major drivers of the ongoing global change impacting terrestrial ecosystems worldwide. While it is well established that soil nutrient heterogeneity exerts a strong influence on the development of plant individuals and communities, it is virtually unknown how nutrient heterogeneity and global change drivers interact to affect plant performance and ecosystem functioning. We conducted a microcosm experiment to evaluate the effect of simultaneous changes in [CO₂], nutrient heterogeneity (NH), nutrient availability (NA) and species evenness on the biomass and nutrient uptake patterns of assemblages formed by Lolium perenne, Plantago lanceolata and Holcus lanatus. When the nutrients were heterogeneously supplied, assemblages exhibited precise root foraging patterns, and had higher above‐ and belowground biomass (average increases of 32% and 29% for above‐ and belowground biomass, respectively). Nutrient heterogeneity also modulated the effects of NA on biomass production, complementarity in nitrogen uptake and below: aboveground ratio, as well as those of [CO₂] on the nutrient use efficiency at the assemblage level. Our results show that nutrient heterogeneity has the potential to influence the response of plant assemblages to simultaneous changes in [CO₂], nutrient availability and biotic diversity, and suggest that it is an important environmental factor to interpret and assess plant assemblage responses to global change.     

Invasive plants differentially affect soil biota through litter and rhizosphere pathways: a meta‐analysis

Invasive plants affect soil biota through litter and rhizosphere inputs, but the direction and magnitude of these effects are variable. We conducted a meta‐analysis to examine the different effects of litter and rhizosphere of invasive plants on soil communities and nutrient cycling. Our results showed that invasive plants increased bacterial biomass by 16%, detritivore abundance by 119% and microbivore abundance by 89% through litter pathway. In the rhizosphere, invasive plants reduced bacterial biomass by 12%, herbivore abundance by 55% and predator abundance by 52%, but increased AM fungal biomass by 36%. Moreover, CO₂ efflux, N mineralisation rate and enzyme activities were all higher in invasive than native rhizosphere soils. These findings indicate that invasive plants may support more decomposers that in turn stimulate nutrient release via litter effect, and enhance nutrient uptake by reducing root grazing but forming more symbioses in the rhizosphere. Thus, we hypothesise that litter‐ and root‐based loops are probably linked to generate positive feedback of invaders on soil systems through stimulating nutrient cycling, consequently facilitating plant invasion. Our findings from limited cases with diverse contexts suggest that more studies are needed to differentiate litter and rhizosphere effects within single systems to better understand invasive plant‐soil interactions.     

Cascading effects of belowground predators on plant communities are density‐dependent

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density‐mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density‐dependent population change in predators, that is, an increase in low‐density treatments, but a decrease in high‐density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density‐dependent cascading effects on detritivore and soil microbial communities.     

Microbial responses to inorganic nutrient amendment overridden by warming: Consequences on soil carbon stability

Eutrophication and climate warming, induced by anthropogenic activities, are simultaneously occurring worldwide and jointly affecting soil carbon stability. Therefore, it is of great interest to examine whether and how they interactively affect soil microbial community, a major soil carbon driver. Here, we showed that climate warming, simulated by southward transferring Mollisol soil in agricultural ecosystems from the cold temperate climate zone (N) to warm temperate climate (C) and subtropical climate zone (S), decreased soil organic matter (SOM) by 6%–12%. In contrast, amendment with nitrogen, phosphorus and potassium enhanced plant biomass by 97% and SOM by 6% at the N site, thus stimulating copiotrophic taxa but reducing oligotrophic taxa in relative abundance. However, microbial responses to nutrient amendment were overridden by soil transfer in that nutrient amendment had little effect at the C site but increased recalcitrant carbon‐degrading fungal Agaricomycetes and Microbotryomycetes taxa derived from Basidiomycota by 4‐17 folds and recalcitrant carbon‐degrading genes by 23%–40% at the S site, implying a possible priming effect. Consequently, SOM at the S site was not increased by nutrient amendment despite increased plant biomass by 108%. Collectively, we demonstrate that soil transfer to warmer regions overrides microbial responses to nutrient amendment and weakens soil carbon sequestration.     

Successional and seasonal variations in soil and litter microbial community structure and function during tropical postagricultural forest regeneration: a multiyear study

Soil microorganisms regulate fundamental biochemical processes in plant litter decomposition and soil organic matter (SOM) transformations. Understanding how microbial communities respond to changes in vegetation is critical for improving predictions of how land‐cover change affects belowground carbon storage and nutrient availability. We measured intra‐ and interannual variability in soil and forest litter microbial community composition and activity via phospholipid fatty acid analysis (PLFA) and extracellular enzyme activity across a well‐replicated, long‐term chronosequence of secondary forests growing on abandoned pastures in the wet subtropical forest life zone of Puerto Rico. Microbial community PLFA structure differed between young secondary forests and older secondary and primary forests, following successional shifts in tree species composition. These successional patterns held across seasons, but the microbial groups driving these patterns differed over time. Microbial community composition from the forest litter differed greatly from those in the soil, but did not show the same successional trends. Extracellular enzyme activity did not differ with forest succession, but varied by season with greater rates of potential activity in the dry seasons. We found few robust significant relationships among microbial community parameters and soil pH, moisture, carbon, and nitrogen concentrations. Observed inter‐ and intrannual variability in microbial community structure and activity reveal the importance of a multiple, temporal sampling strategy when investigating microbial community dynamics with land‐use change. Successional control over microbial composition with forest recovery suggests strong links between above and belowground communities.     

Root exudate cocktails: the link between plant diversity and soil microorganisms?

Higher plant diversity is often associated with higher soil microbial biomass and diversity, which is assumed to be partly due to elevated root exudate diversity. However, there is little experimental evidence that diversity of root exudates shapes soil microbial communities. We tested whether higher root exudate diversity enhances soil microbial biomass and diversity in a plant diversity gradient, thereby negating significant plant diversity effects on soil microbial properties. We set up plant monocultures and two‐ and three‐species mixtures in microcosms using functionally dissimilar plants and soil of a grassland biodiversity experiment in Germany. Artificial exudate cocktails were added by combining the most common sugars, organic acids, and amino acids found in root exudates. We applied four different exudate cocktails: two exudate diversity levels (low‐ and high‐diversity) and two nutrient‐enriched levels (carbon‐ and nitrogen‐enriched), and a control with water only. Soil microorganisms were more carbon‐ than nitrogen‐limited. Cultivation‐independent fingerprinting analysis revealed significantly different soil microbial communities among exudate diversity treatments. Most notably and according to our hypothesis, adding diverse exudate cocktails negated the significant plant diversity effect on soil microbial properties. Our findings provide the first experimental evidence that root exudate diversity is a crucial link between plant diversity and soil microorganisms.     

15N enrichment as an integrator of the effects of C and N on microbial metabolism and ecosystem function

Organic carbon (C) and nitrogen (N) are essential for heterotrophic soil microorganisms, and their bioavailability strongly influences ecosystem C and N cycling. We show here that the natural ¹⁵N abundance of the soil microbial biomass is affected by both the availability of C and N and ecosystem N processing. Microbial ¹⁵N enrichment correlated negatively with the C : N ratio of the soil soluble fraction and positively with net N mineralization for ecosystems spanning semiarid, temperate and tropical climates, grassland and forests, and over four million years of ecosystem development. In addition, during soil incubation, large increases in microbial ¹⁵N enrichment corresponded to high net N mineralization rates. These results support the idea that the N isotope composition of an organism is determined by the balance between N assimilation and dissimilation. Thus, ¹⁵N enrichment of the soil microbial biomass integrates the effects of C and N availability on microbial metabolism and ecosystem processes.     

Effects of resource availability and propagule supply on native species recruitment in sagebrush ecosystems invaded by Bromus tectorum

Resource availability and propagule supply are major factors influencing establishment and persistence of both native and invasive species. Increased soil nitrogen (N) availability and high propagule inputs contribute to the ability of annual invasive grasses to dominate disturbed ecosystems. Nitrogen reduction through carbon (C) additions can potentially immobilize soil N and reduce the competitiveness of annual invasive grasses. Native perennial species are more tolerant of resource limiting conditions and may benefit if N reduction decreases the competitive advantage of annual invaders and if sufficient propagules are available for their establishment. Bromus tectorum, an exotic annual grass in the sagebrush steppe of western North America, is rapidly displacing native plant species and causing widespread changes in ecosystem processes. We tested whether nitrogen reduction would negatively affect B. tectorum while creating an opportunity for establishment of native perennial species. A C source, sucrose, was added to the soil, and then plots were seeded with different densities of both B. tectorum (0, 150, 300, 600, and 1,200 viable seeds m⁻²) and native species (0, 150, 300, and 600 viable seeds m⁻²). Adding sucrose had short-term (1 year) negative effects on available nitrogen and B. tectorum density, biomass and seed numbers, but did not increase establishment of native species. Increasing propagule availability increased both B. tectorum and native species establishment. Effects of B. tectorum on native species were density dependent and native establishment increased as B. tectorum propagule availability decreased. Survival of native seedlings was low indicating that recruitment is governed by the seedling stage.     

Putting the ‘upstairs–downstairs’ into ecosystem service: What can aboveground–belowground ecology tell us?

Interactions between spatially-separated aboveground and belowground biota exert important influences on the functioning of terrestrial ecosystems. Plant root exudates and litter inputs affect root-associated and decomposer sub-communities, which, in turn, regulate nutrient availability and plant growth. Ecosystem services theoretically attributed to specific functional components of aboveground or belowground biota are, therefore, influenced by indirect (plant-mediated) interactions with the wider community. Some recent studies have considered aboveground–belowground interactions in a climate change context, with implications for altered ecosystem service provision. This review is a conceptual discussion of the mechanisms by which aboveground–belowground interactions affect specific ecosystem services: control of herbivores by natural enemies, insect pollination and nutrient mineralization by soil decomposers. While some mechanisms are well-characterized, others are poorly understood. Reducing root and shoot herbivory, in addition to the direct plant benefit, indirectly promotes antagonism of the spatially-separate herbivore by its natural enemies. Soil decomposers and mycorrhizal fungi can increase shoot herbivore performance such that control by natural enemies is weakened, or initiate bottom-up trophic cascades which strengthen antagonism of shoot herbivores. Aboveground herbivory generally stimulates nutrient cycling by decomposers. Root herbivory and mycorrhizal association both appear to increase floral attractiveness to insect pollinators. Mechanisms reflect alterations to plant growth, nutritional quality and chemical defenses. Climate change has considerable potential to alter aboveground–belowground interactions, with largely unexplored implications for biological control, pollination and soil nutrient cycling.     

Grass invasion of a hardwood forest is associated with declines in belowground carbon pools

Invasive plant species affect a range of ecosystem processes but their impact on belowground carbon (C) pools is relatively unexplored. This is particularly true for grass invasions of forested ecosystems. Such invasions may alter both the quantity and quality of forest floor inputs. Dependent on both, two theories, ‘priming’ and ‘preferential substrate utilization’, suggest these changes may decrease, increase, or leave unchanged native plant‐derived soil C. Decreases are expected under ‘priming’ theory due to increased soil microbial activity. Under ‘preferential substrate utilization’, either an increase or no change is expected because the invasive plant's inputs are used by the microbial community instead of soil C. Here, we examine how Microstegium vimineum affects belowground C‐cycling in a southeastern US forest. Following predictions of priming theory, M. vimineum's presence is associated with decreases in native‐derived, C pools. For example, in September 2006 M. vimineum is associated with 24%, 34%, 36%, and 72% declines in total organic, particulate organic matter, mineralizable (a measure of microbially‐available C), and microbial biomass C, respectively. Soil C derived from M. vimineum does not compensate for these decreases, meaning that the sum of native‐ plus invasive‐derived C pools is smaller than native‐derived pools in uninvaded plots. Supporting our inferences that C‐cycling accelerates under invasion, the microbial community is more active per unit biomass: added ¹³C‐glucose is respired more rapidly in invaded plots. Our work suggests that this invader may accelerate C‐cycling in forest soils and deplete C stocks. The paucity of studies investigating impacts of grass invasion on C‐cycling in forests highlights the need to study further M. vimineum and other invasive grasses to assess their impacts on C sink strength and forest fertility.