Plant diversity drives soil microbial biomass carbon in grasslands irrespective of global environmental change factors

Soil microbial biomass is a key determinant of carbon dynamics in the soil. Several studies have shown that soil microbial biomass significantly increases with plant species diversity, but it remains unclear whether plant species diversity can also stabilize soil microbial biomass in a changing environment. This question is particularly relevant as many global environmental change (GEC) factors, such as drought and nutrient enrichment, have been shown to reduce soil microbial biomass. Experiments with orthogonal manipulations of plant diversity and GEC factors can provide insights whether plant diversity can attenuate such detrimental effects on soil microbial biomass. Here, we present the analysis of 12 different studies with 14 unique orthogonal plant diversity × GEC manipulations in grasslands, where plant diversity and at least one GEC factor (elevated CO2, nutrient enrichment, drought, earthworm presence, or warming) were manipulated. Our results show that higher plant diversity significantly enhances soil microbial biomass with the strongest effects in long-term field experiments. In contrast, GEC factors had inconsistent effects with only drought having a significant negative effect. Importantly, we report consistent non-significant effects for all 14 interactions between plant diversity and GEC factors, which indicates a limited potential of plant diversity to attenuate the effects of GEC factors on soil microbial biomass. We highlight that plant diversity is a major determinant of soil microbial biomass in experimental grasslands that can influence soil carbon dynamics irrespective of GEC.

A Latex Metabolite Benefits Plant Fitness under Root Herbivore Attack

Plants produce large amounts of secondary metabolites in their shoots and roots and store them in specialized secretory structures. Although secondary metabolites and their secretory structures are commonly assumed to have a defensive function, evidence that they benefit plant fitness under herbivore attack is scarce, especially below ground. Here, we tested whether latex secondary metabolites produced by the common dandelion (Taraxacum officinale agg.) decrease the performance of its major native insect root herbivore, the larvae of the common cockchafer (Melolontha melolontha), and benefit plant vegetative and reproductive fitness under M. melolontha attack. Across 17 T. officinale genotypes screened by gas and liquid chromatography, latex concentrations of the sesquiterpene lactone taraxinic acid β-D-glucopyranosyl ester (TA-G) were negatively associated with M. melolontha larval growth. Adding purified TA-G to artificial diet at ecologically relevant concentrations reduced larval feeding. Silencing the germacrene A synthase ToGAS1, an enzyme that was identified to catalyze the first committed step of TA-G biosynthesis, resulted in a 90% reduction of TA-G levels and a pronounced increase in M. melolontha feeding. Transgenic, TA-G-deficient lines were preferred by M. melolontha and suffered three times more root biomass reduction than control lines. In a common garden experiment involving over 2,000 T. officinale individuals belonging to 17 different genotypes, high TA-G concentrations were associated with the maintenance of high vegetative and reproductive fitness under M. melolontha attack. Taken together, our study demonstrates that a latex secondary metabolite benefits plants under herbivore attack, a result that provides a mechanistic framework for root herbivore driven natural selection and evolution of plant defenses below ground.

Shifts in microbial communities do not explain the response of grassland ecosystem function to plant functional composition and rainfall change

Ecosystem functioning in grasslands is regulated by a range of biotic and abiotic factors, and the role of microbial communities in regulating ecosystem function has been the subject of much recent scrutiny. However, there are still knowledge gaps regarding the impacts of rainfall and vegetation change upon microbial communities and the implications of these changes for ecosystem functioning. We investigated this issue using data from an experimental mesotrophic grassland study in south-east England, which had been subjected to four years of rainfall and plant functional composition manipulations. Soil respiration, nitrogen and phosphorus stocks were measured, and the abundance and community structure of soil microbes were characterised using quantitative PCR and multiplex-TRFLP analysis, respectively. Bacterial community structure was strongly related to the plant functional composition treatments, but not the rainfall treatment. However, there was a strong effect of both rainfall change and plant functional group upon bacterial abundance. There was also a weak interactive effect of the two treatments upon fungal community structure, although fungal abundance was not affected by either treatment. Next, we used a statistical approach to assess whether treatment effects on ecosystem function were regulated by the microbial community. Our results revealed that ecosystem function was influenced by the experimental treatments, but was not related to associated changes to the microbial community. Overall, these results indicate that changes in fungal and bacterial community structure and abundance play a relatively minor role in determining grassland ecosystem function responses to precipitation and plant functional composition change, and that direct effects on soil physical and chemical properties and upon plant and microbial physiology may play a more important role.

Plant Adenosine 5′-phosphosulfate Reductase Is a Novel Iron-Sulfur Protein

Adenosine 5′-phosphosulfate reductase (APR) catalyzes the two-electron reduction of adenosine 5′-phosphosulfate to sulfite and AMP, which represents the key step of sulfate assimilation in higher plants. Recombinant APRs from both Lemna minorand Arabidopsis thaliana were overexpressed inEscherichia coli and isolated as yellow-brown proteins. UV-visible spectra of these recombinant proteins indicated the presence of iron-sulfur centers, whereas flavin was absent. This result was confirmed by quantitative analysis of iron and acid-labile sulfide, suggesting a 4Fe-4S cluster as the cofactor. EPR spectroscopy of freshly purified enzyme showed, however, only a minor signal at g = 2.01. Therefore, Mössbauer spectra of 57Fe-enriched APR were obtained at 4.2 K in magnetic fields of up to 7 tesla, which were assigned to a diamagnetic 4Fe-4S2+ cluster. This cluster was unusual because only three of the iron sites exhibited the same Mössbauer parameters. The fourth iron site gave, because of the bistability of the fit, a significantly smaller isomer shift or larger quadrupole splitting than the other three sites. Thus, plant assimilatory APR represents a novel type of adenosine 5′-phosphosulfate reductase with a 4Fe-4S center as the sole cofactor, which is clearly different from the dissimilatory adenosine 5′-phosphosulfate reductases found in sulfate reducing bacteria.

Plant genotype affects the quality of oilseed rape (Brassica napus) for adults and larvae of the pollen beetle (Meligethes aeneus)

Plant quality is one of the main factors influencing the fitness of phytophagous insects. Plant quality can vary not only among genotypes of the same host plant species, but also relative to the insect sex or its life stage. In the present study, the performance of larvae and adults of the pollen beetle (Meligethes aeneus F., Coleoptera: Nitidulidae), a major insect pest of oilseed rape crops, is compared on six genotypes of oilseed rape (Brassica napus). All of the traits that are measured vary among genotypes, and comprise larval developmental duration, life span of unfed emerging adults and survival time of field-sampled adults fed with pollen from the different genotypes. No correlation is found between insect performance and quantity of food available, showing that the quality of the food (i.e. pollen) is the fitness determinant for this insect species. Additionally, the performance of larvae and adults is also not correlated despite use of the same plant genotypes, suggesting that the determinants of pollen quality differ at least partially between both life stages. It is hypothesized that this may be a result of extensive differences in diet breadth between the life stages: larvae are specialists of brassicaceous plants, whereas adults are generalists. Finally, it is suggested that the manipulation of plant quality to increase pollen beetle development time may comprise a valuable strategy for favouring biological control by natural enemies of this pest; for example, as a result of extending the vulnerability window of larvae to attack by parasitoids.

Species Diversity is a Good Predictor of Prairie Plant Persistence in Restorations

Prairie restoration projects sometimes fail because of heavy invasion by invasive weeds, especially if they are not intensively managed. Few restoration projects are sampled after the first few years post-establishment, and little is known about what predictors are significant in maintaining restored communities over the very long term. Here, we stopped weeding experimental restoration plots to determine if persistence (that is, remaining unchanged after weeds are allowed to invade) of native prairie in western Iowa was related to planted species diversity

Carbon/Nitrogen ratio of plant tissues and biomass of belowground consumers after 11 years of experimental treatment, Toolik lake

Theory and observation indicate that changes in the rate of primary production can alter the balance between the bottom-up influences of plants and resources and the top-down regulation of herbivores and predators on ecosystem structure and function. The Exploitation Ecosystem Hypothesis (EEH) posited that as aboveground net primary productivity (ANPP) increases, the additional biomass should support higher trophic levels. We developed an extension of EEH to include the impacts of increases in ANPP on belowground consumers in a similar manner as aboveground, but indirectly through changes in the allocation of photosynthate to roots. We tested our predictions for plants aboveground and for phytophagous nematodes and their predators belowground in two common arctic tundra plant communities subjected to 11 years of increased soil nutrient availability and/or exclusion of mammalian herbivores. The less productive dry heath (DH) community met the predictions of EEH aboveground, with the greatest ANPP and plant biomass in the fertilized plots protected from herbivory. A palatable grass increased in fertilized plots while dwarf evergreen shrubs and lichens declined. Belowground, phytophagous nematodes also responded as predicted, achieving greater biomass in the higher ANPP plots, whereas predator biomass tended to be lower in those same plots (although not significantly). In the higher productivity moist acidic tussock (MAT) community, aboveground responses were quite different. Herbivores stimulated ANPP and biomass in both ambient and enriched soil nutrient plots; maximum ANPP occurred in fertilized plots exposed to herbivory. Fertilized plots became dominated by dwarf birch (a deciduous shrub) and cloudberry (a perennial forb); under ambient conditions these two species coexist with sedges, evergreen dwarf shrubs, and Sphagnum mosses. Phytophagous nematodes did not respond significantly to changes in ANPP, although predator biomass was greatest in control plots. The contrasting results of these two arctic tundra plant communities suggest that the predictions of EEH may hold for very low ANPP communities, but that other factors, including competition and shifts in vegetation composition toward less palatable species, may confound predicted responses to changes in productivity in higher ANPP communities such as the MAT studied here.

(Supplemental Table 2) Plant characteristics, GHG fluxes, and soil chemical and microbial properties in experimental treatments in Alexandra Fiord

We evaluated above- and belowground ecosystem changes in a 16 year, combined fertilization and warming experiment in a High Arctic tundra deciduous shrub heath (Alexandra Fiord, Ellesmere Island, NU, Canada). Soil emissions of the three key greenhouse gases (GHGs) (carbon dioxide, methane, and nitrous oxide) were measured in mid-July 2009 using soil respiration chambers attached to a FTIR system. Soil chemical and biochemical properties including Q10 values for CO2, CH4, and N2O, Bacteria and Archaea assemblage composition, and the diversity and prevalence of key nitrogen cycling genes including bacterial amoA, crenarchaeal amoA, and nosZ were measured. Warming and fertilization caused strong increases in plant community cover and height but had limited effects on GHG fluxes and no substantial effect on soil chemistry or biochemistry. Similarly, there was a surprising lack of directional shifts in the soil microbial community as a whole or any change at all in microbial functional groups associated with CH4 consumption or N2O cycling in any treatment. Thus, it appears that while warming and increased nutrient availability have strongly affected the plant community over the last 16 years, the belowground ecosystem has not yet responded. This resistance of the soil ecosystem has resulted in limited changes in GHG fluxes in response to the experimental treatments.