Mycorrhizal associations of temperate forest seedlings mediate rhizodeposition, but not soil carbon storage, under elevated nitrogen availability.

Tree-mycorrhizal associations are associated with patterns in nitrogen (N) availability and soil organic matter storage; however, we still lack a mechanistic understanding of what tree and fungal traits drive these patterns and how they will respond to global changes in soil N availability. To address this knowledge gap, we investigated how arbuscular mycorrhizal (AM)- and ectomycorrhizal (EcM)-associated seedlings alter rhizodeposition in response to increased seedling inorganic N acquisition. We grew four species each of EcM and AM seedlings from forests of the eastern United States in a continuously C-labeled atmosphere within an environmentally controlled chamber and subjected to three levels of N-labeled fertilizer.

Trees adjust nutrient acquisition strategies across tropical forest secondary succession.

Nutrient limitation may constrain the ability of recovering and mature tropical forests to serve as a carbon sink. However, it is unclear to what extent trees can utilize nutrient acquisition strategies - especially root phosphatase enzymes and mycorrhizal symbioses - to overcome low nutrient availability across secondary succession. Using a large-scale, full factorial nitrogen and phosphorus fertilization experiment of 76 plots along a secondary successional gradient in lowland wet tropical forests of Panama, we tested the extent to which root phosphatase enzyme activity and mycorrhizal colonization are flexible, and if investment shifts over succession, reflective of changing nutrient limitation.

Urbanization reduces soil microbial network complexity and stability in the megacity of Shanghai.

Urbanization is altering the co-occurrence networks of ecological communities that are critical to maintaining ecosystem functions and services. Soil microbial communities play key roles in various ecosystem processes, but how soil microbial co-occurrence networks respond to urbanization is unclear. Here we analyzed co-occurrence networks in soil archaeal, bacterial, and fungal communities from 258 soil sampling sites across the megacity of Shanghai along large urbanization gradients.

Nitrogen and light regulate symbiotic nitrogen fixation by a temperate forest tree.

Symbiotic nitrogen fixation (SNF) is a critical mechanism of ecosystem recovery, and in forests of the eastern United States, the most common tree species that supports SNF is black locust (Robinia pseudoacacia L.). Despite its prevalence, black locust's fixation strategy-whether it maintains fixation at a constant rate (obligate fixation) or reduces its fixation rate (facultative fixation)-is unknown.

Differences in soil organic matter between EcM- and AM-dominated forests depend on tree and fungal identity.

As global change shifts the species composition of forests, we need to understand which species characteristics affect soil organic matter (SOM) cycling to predict future soil carbon (C) storage. Recently, whether a tree species forms a symbiosis with arbuscular (AM) versus ectomycorrhizal (EcM) fungi has been suggested as a strong predictor of soil C storage, but there is wide variability within EcM systems. In this study, we investigated how mycorrhizal associations and the species composition of canopy trees and mycorrhizal fungi related to the proportion of soil C and nitrogen (N) in mineral associations and soil C:N across four sites representing distinct climates and tree communities in the eastern US broadleaf forest biome.

A starting guide to root ecology: strengthening ecological concepts and standardising root classification, sampling, processing and trait measurements.

In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting-edge, meaningful and integrated knowledge. Consideration of the below-ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies.

Drought sensitivity of an N -fixing tree may slow temperate deciduous forest recovery from disturbance.

Increased drought intensity and frequency due to climate change may reduce the abundance and activity of nitrogen (N )-fixing plants, which supply new N to terrestrial ecosystems. As a result, drought may indirectly reduce ecosystem productivity through its effect on the N cycle. Here, we manipulated growing season net rainfall across a series of plots in an early successional mesic deciduous forest to understand how drought affects the aboveground productivity of the N -fixing tree Robinia pseudoacacia and three co-occurring nonfixing tree species.

Symbiotic N fixation is sufficient to support net aboveground biomass accumulation in a humid tropical forest.

Symbiotic nitrogen (N) fixation has been shown to support carbon storage in young regenerating tropical forests, but N-fixing trees can also be strong competitors with non-fixing trees, making it unclear which mechanism drives long term patterns in biomass accretion. Many tropical forests have excess N, but factors such as rising atmospheric CO or selective cutting practices might induce additional N demand. Here we combine decades of stem inventory data, in-situ measures of symbiotic N fixation, and simulations of N demand to evaluate demographic and biogeochemical controls on biomass dynamics in legume-rich lowland forests of Trinidad.

Nitrogen fixation does not balance fire-induced nitrogen losses in longleaf pine savannas.

Fire is a critical force in structuring ecosystems, but it also removes substantial amounts of nitrogen (N), which can limit plant growth. Biological N fixation (BNF) may alleviate fire-induced N deficiencies that inhibit ecosystem recovery, yet if and how BNF achieves this under frequent fire is unclear. This problem is further complicated in the context of modern human influences (such as land-use history and atmospheric N deposition), which may confound the relationship between fire and fixation.

Phosphorus and species regulate N fixation by herbaceous legumes in longleaf pine savannas.

Longleaf pine savannas house a diverse community of herbaceous N-fixing legume species that have the potential to replenish nitrogen (N) losses from fire. Whether legumes fill this role depends on the factors that regulate symbiotic fixation, including soil nutrients such as phosphorus (P) and molybdenum (Mo) and the growth and fixation strategies of different species. In greenhouse experiments, we determined how these factors influence fixation for seven species of legumes grown in pure field soil from two different regions of the southeastern US longleaf pine ecosystem.

Tolerance or avoidance: drought frequency determines the response of an N -fixing tree.

Climate change is increasing drought frequency, which may affect symbiotic N fixation (SNF), a process that facilitates ecosystem recovery from disturbance. Here, we assessed the effect of drought frequency on the ecophysiology and SNF rate of a common N -fixing tree in eastern US forests. We grew Robinia pseudoacacia seedlings under the same mean soil moisture, but with different drought frequency caused by wet-dry cycles of varying periodicity.

Experimental evidence that mycorrhizal nitrogen strategies affect soil carbon.

Most land plants acquire nitrogen (N) through associations with arbuscular (AM) and ectomycorrhizal (ECM) fungi, but these symbionts employ contrasting strategies for N acquisition, which may lead to different stocks of soil carbon (C). We experimentally test this hypothesis with a mesocosm system where AM and ECM tree seedling roots, or their hyphae only, could access mineral soils with C- and N-enriched organic matter. We quantified loss of soil C and N, plant uptake of N and new inputs of plant C to soil.

Taxonomic identity determines N2 fixation by canopy trees across lowland tropical forests.

Legumes capable of fixing atmospheric N2 are abundant and diverse in many tropical forests, but the factors determining ecological patterns in fixation are unresolved. A long-standing idea is that fixation depends on soil nutrients (N, P or Mo), but recent evidence shows that fixation may also differ among N2-fixing species. We sampled canopy-height trees across five species and one species group of N2-fixers along a landscape P gradient, and manipulated P and Mo to seedlings in a shadehouse.

Fine-root responses to fertilization reveal multiple nutrient limitation in a lowland tropical forest.

Questions remain as to which soil nutrients limit primary production in tropical forests. Phosphorus (P) has long been considered the primary limiting element in lowland forests, but recent evidence demonstrates substantial heterogeneity in response to nutrient addition, highlighting a need to understand and diagnose nutrient limitation across diverse forests. Fine-root characteristics including their abundance, functional traits, and mycorrhizal symbionts can be highly responsive to changes in soil nutrients and may help to diagnose nutrient limitation.

Root and leaf traits reflect distinct resource acquisition strategies in tropical lianas and trees.

In Neotropical forests, lianas are increasing in abundance relative to trees. This increased species richness may reflect a positive response to global change factors including increased temperature, atmospheric CO2, habitat fragmentation, and drought severity; however, questions remain as to the specific mechanisms facilitating the response. Previous work suggests that lianas may gain an ecological advantage over trees through leaf functional traits that offer a quick return on investment of resources, although it is unknown whether this pattern extends to root traits and relationships with fungal or bacterial symbionts belowground.

Drought enhances symbiotic dinitrogen fixation and competitive ability of a temperate forest tree.

General circulation models project more intense and frequent droughts over the next century, but many questions remain about how terrestrial ecosystems will respond. Of particular importance, is to understand how drought will alter the species composition of regenerating temperate forests wherein symbiotic dinitrogen (N2)-fixing plants play a critical role. In experimental mesocosms we manipulated soil moisture to study the effect of drought on the physiology, growth and competitive interactions of four co-occurring North American tree species, one of which (Robinia pseudoacacia) is a symbiotic N2-fixer.

Molybdenum and phosphorus interact to constrain asymbiotic nitrogen fixation in tropical forests.

Biological di-nitrogen fixation (N(2)) is the dominant natural source of new nitrogen to land ecosystems. Phosphorus (P) is thought to limit N(2) fixation in many tropical soils, yet both molybdenum (Mo) and P are crucial for the nitrogenase reaction (which catalyzes N(2) conversion to ammonia) and cell growth. We have limited understanding of how and when fixation is constrained by these nutrients in nature.

Ericoid mycorrhizal root fungi and their multicopper oxidases from a temperate forest shrub.

Ericoid mycorrhizal fungi (ERM) may specialize in capturing nutrients from their host's litter as a strategy for regulating nutrient cycles in terrestrial ecosystems. In spite of their potential significance, we know little about the structure of ERM fungal communities and the genetic basis of their saprotrophic traits (e.g.

Potassium, phosphorus, or nitrogen limit root allocation, tree growth, or litter production in a lowland tropical forest.

We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate potential nutrient limitation of tree growth rates, fine-litter production, and fine-root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 x 40 m each. The addition of K was associated with significant decreases in stand-level fine-root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates.

Soil DIC uptake and fixation in Pinus taeda seedlings and its C contribution to plant tissues and ectomycorrhizal fungi.

Plants can acquire carbon from sources other than atmospheric carbon dioxide (CO(2)), including soil-dissolved inorganic carbon (DIC). Although the net flux of CO(2) is out of the root, soil DIC can be taken up by the root, transported within the plant, and fixed either photosynthetically or anaplerotically by plant tissues. We tested the ability of Pinus taeda L.