Publications in peer reviewed journals
Little effects on soil organic matter chemistry of density fractions after seven years of forest soil warming2016 - Soil Biology and Biochemistry, 103: 300-307
Microbial decomposition of 13C- labeled phytosiderophores in the rhizosphere of wheat: Mineralization dynamics and key microbial groups involved2016 - Soil Biology and Biochemistry, 98: 196-207
Being low molecular weight carbon (LMW-C) compounds, phytosiderophores (PS) released by strategy II plants are highly susceptible to microbial decomposition. However, to date very little is known about the fate of PS in soil. Using in-house synthesized 13C4-2′-deoxymugineic acid (DMA), the main PS released by wheat, we investigated DMA mineralization dynamics, including microbial incorporation into phospholipid fatty acids (PLFA), in the wheat rhizosphere and bulk soil of two alkaline and one acidic soil. Half-lives of the intact DMA molecule (3–8 h) as well as of DMA-derived C-compounds (8–38 days) were in the same order of magnitude as those published for other LMW-C compounds like sugars, amino acids and organic acids. Combining mineralization with PLFA data showed that between 40 and 65% of the added DMA was either respired or incorporated into soil microbial biomass after 24 h, with the largest part of total incorporated DMA-13C being recovered in gram negative bacteria. Considering root growth dynamics and that PS are mainly exuded from root tips, the significantly slower mineralization of DMA in bulk soil is of high ecological importance to enhance the Fe scavenging efficiency of PS released into the soil.
Stable isotope signatures reflect dietary diversity in European forest moths2016 - Frontiers in Zoology, 13: 1-10
Background: Information on larval diet of many holometabolous insects remains incomplete. Carbon (C) and nitrogen (N) stable isotope analysis in adult wing tissue can provide an efficient tool to infer such trophic relationships. The present study examines whether moth feeding guild affiliations taken from literature are reflected in isotopic signatures. Results: Non-metric multidimensional scaling and permutational analysis of variance indicate that centroids of dietary groups differ significantly. In particular, species whose larvae feed on mosses or aquatic plants deviated from those that consumed vascular land plants. Moth δ15N signatures spanned a broader range, and were less dependent on species identity than δ13C values. Comparison between moth samples and ostensible food sources revealed heterogeneity in the lichenivorous guild, indicating only Lithosia quadra as an obligate lichen feeder. Among root-feeding Agrotis segetum, some specimens appear to have developed on crop plants in forest-adjacent farm land. Reed-feeding stem-borers may partially rely on intermediary trophic levels such as fungal or bacterial growth. Conclusion: Diagnostic partitioning of moth dietary guilds based on isotopic signatures alone could not be achieved, but hypotheses on trophic relationships based on often vague literature records could be assessed with high resolution. Hence, the approach is well suited for basic categorization of moths where diet is unknown or notoriously difficult to observe (i.e. Microlepidoptera, lichen-feeders). Keywords: δ13C, δ15N, Larval diet, Trophic position Abbreviations: C, Chemical symbol for carbon; IAEA-CH-6, Reference standard for 13C/12C ratios derived from sucrose and provided by the international atomic energy agency (IAEA); IAEA-CH-7, Reference standard for 13C/12C ratios derived from polyethylene and provided by the international atomic energy agency (IAEA); IAEA-N-1, Reference standard for 15N/ 14N ratios derived from ammonium sulfate and provided by the international atomic energy agency (IAEA); IAEA-N- 2, Reference standard for 15N/14N ratios derived from ammonium sulfate and provided by the international atomic energy agency (IAEA); IAEA-NO-3, Reference standard for 15N/14N ratios derived from potassium nitrate and provided by the international atomic energy agency (IAEA); MMDS, Metric multi-dimensional scaling; N, Chemical symbol for nitrogen; NMDS, Non-metric multi-dimensional scaling; SD, Standard deviation; TLE, Trophic level enrichment; δ 13C, Shift in the 13C/12C ratio of the sample relative to the reference standard (i.e. Pee Dee Belemnite); δ 15N, Shift in the 15N/14N ratio of the sample relative to the reference standard (i.e. atmospheric nitrogen)
Microbial carbon use efficiency and biomass turnover times depending on soil depth - Implications for carbon cycling.2016 - Soil Biology and Biochemistry, 96: 74-81
Drought history affects grassland plant and microbial carbon turnover during and after a subsequent drought event2016 - Journal of Ecology, 104: 1453-1465
Exploring the metabolic potential of microbial communities in ultra-basic, reducing springs at The Cedars, CA, USA: Experimental evidence of microbial methanogenesis and heterotrophic acetogenesis2016 - Journal of Geophysical Research Biogeosciences, 4: 1203-1220
Present-day serpentinization generates groundwaters with conditions (pH > 11, Eh < −550 mV) favorable for the microbial and abiotic production of organic compounds from inorganic precursors. Elevated concentrations of methane, C-C alkanes, acetate, and formate have been detected at these sites, but the microbial or abiotic origin of these compounds remains unclear. While geochemical data indicate that methane at most sites of present-day serpentinization is abiogenic, the stable carbon, hydrogen, and clumped isotope data as well as the hydrocarbon gas composition from The Cedars, CA, USA, are consistent with a microbial origin for methane. However, there is no direct evidence of methanogenesis at this site of serpentinization. We report on laboratory experiments in which the microbial communities in fluids and sediments from The Cedars were incubated with C labeled substrates. Increasing methane concentrations and the incorporation of C into methane in live experiments, but not in killed controls, demonstrated that methanogens converted methanol, formate, acetate (methyl group), and bicarbonate to methane. The apparent fractionation between methane and potential substrates (αC = 1.059 to 1.105, αC = 1.042 to 1.119) indicated that methanogenesis was dominated by the carbonate reduction pathway. Increasing concentrations of volatile organic acid anions indicated microbial acetogenesis. αC values (0.999 to 1.000), however, were inconsistent with autotrophic acetogenesis, thus suggesting that acetate was produced through fermentation. This is the first study to show direct evidence of microbial methanogenesis and acetogenesis by the native microbial community at a site of present-day serpentinization.
Moss δ13C: Implications for subantarctic palaeohydrological reconstructions2016 - Palaeogeography, 453: 20-29
Southern Ocean Islands, despite their equitable oceanic climates, have recently experienced a number of pronounced climate variations. Shifts in water availability in this region are of concern; however, methods of measuring water availability are currently inadequate. Recent advances using stable carbon isotopes (δ13C) in Antarctic mosses to record long-term variations in water availability suggest that this technique might be applicable in other locations where conditions are cold enough to produce meaningful moss growth for reconstructions. Verification of this technique at each new location is essential, however, due to disparity between species and climates. Here, variations in δ13CBULK with growth water availability were measured in three moss species on subantarctic Macquarie Island. We found these subantarctic mosses showed no difference in δ13CBULK signatures between growth water environments and displayed more negative δ13CBULK ranges than those from East Antarctica, suggesting that climatic differences override the microclimate signal. Despite significant differences in leaf cell morphology there was no variation in δ13CBULK between these subantarctic species. It may be that these species are unsuitable as biological proxies due to their growth form being less dense than the turf forming Antarctic species. This underlines the need to carryout preliminary research into moss carbon isotope fractionation for each new region, and for each species, where palaeohydrological reconstructions are planned – a step that is often not given appropriate consideration in palaeo-research.
Metabolism of mineral-sorbed organic matter and microbial lifestyles in fluvial ecosystems2016 - Geophysical Research Letters, 43: 1582-1588
In fluvial ecosystems mineral erosion, carbon (C), and nitrogen (N) fluxes are linked via organomineral complexation, where dissolved organic molecules bind to mineral surfaces. Biofilms and suspended aggregates represent major aquatic microbial lifestyles whose relative importance changes predictably through fluvial networks. We tested how organomineral sorption affects aquatic microbial metabolism, using organomineral particles containing a mix of C, N-labeled amino acids. We traced C and N retention within biofilm and suspended aggregate biomass and its mineralization. Organomineral complexation restricted C and N retention within biofilms and aggregates and also their mineralization. This reduced the efficiency with which biofilms mineralize C and N by 30% and 6%. By contrast, organominerals reduced the C and N mineralization efficiency of suspended aggregates by 41% and 93%. Our findings show how organomineral complexation affects microbial C:N stoichiometry, potentially altering the biogeochemical fate of C and N within fluvial ecosystems.
Synergistic effects of diffusion and microbial physiology reproduce the Birch effect in a micro-scale model2016 - Soil Biology and Biochemistry, 93: 28-37
Stress-induced changes in carbon allocation among metabolite pools influence isotope-based predictions of water use efficiency in Phaseolus vulgaris2016 - Functional Plant Biology, 1149-1158
Understanding how major food crops respond to environmental stress will expand our capacity to improve food production with growing populations and a changing climate. This study uses chemical and physiological adaptations to heat, water deficit and elevated light stresses in Phaseolus vulgaris L. to identify changes in carbon (C) allocation that, combined with post-photosynthetic fractionation of C isotopes, influences water use efficiency (WUE) predictions. The chemical stress response was explored through changes in C allocation to the carbohydrate and cyclitol pools using GC–triple quadrupole MS. Carbon allocation to the sucrose pool fluctuated significantly among treatments, and the putative osmolytes and osmoprotectants (myo-inositol and D-ononitol) accumulated under stress. Significant osmotic adjustment (P < 0.05), quantified via pressure–volume curve analysis, was detected between control and stress treatments, although this was not attributable to active accumulation of the metabolites. Compound-specific 13C isotope abundance was measured using liquid chromatography isotope ratio MS to predict intrinsic WUE. In contrast to other metabolites measured, the δ13C of the sucrose pool fluctuated according to treatment and was proportional to predicted values based upon modelled Δ13C from gas exchange data. The results suggest that the accuracy and precision of predicting WUE may be enhanced by compound-specific analysis of Δ13C and that changes in the allocation of C among metabolite pools may influence WUE predictions based upon analysis of total soluble C. Overall, the plants appeared to use a range of mechanisms to cope with adverse conditions that could be utilised to improve plant breeding and management strategies.
Environmental and landscape controls of soil organic carbon storage in continuous permafrost terrain of the Taymyr Peninsula (N Siberia, Russia)2016 - European Journal of Soil Science, 67: 478-491
Soil microbial carbon use efficiency and biomass turnover in a long-term fertilization experiment in a temperate grassland2016 - Soil Biology and Biochemistry, 97: 168-175
Soil microbial carbon use efficiency (CUE), defined as the ratio of organic C allocated to growth over organic C taken up, strongly affects soil carbon (C) cycling. Despite the importance of the microbial CUE for the terrestrial C cycle, very little is known about how it is affected by nutrient availability. Therefore, we studied microbial CUE and microbial biomass turnover time in soils of a long-term fertilization experiment in a temperate grassland comprising five treatments (control, PK, NK, NP, NPK). Microbial CUE and the turnover of microbial biomass were determined using a novel substrate-independent method based on incorporation of 18O from labeled water into microbial DNA. Microbial respiration was 28–37% smaller in all three N treatments (NK, NP, and NPK) compared to the control, whereas the PK treatment did not affect microbial respiration. N-fertilization decreased microbial C uptake, while the microbial growth rate was not affected. Microbial CUE ranged between 0.31 and 0.45, and was 1.3- to 1.4-fold higher in the N-fertilized soils than in the control. The turnover time ranged between 80 and 113 days and was not significantly affected by fertilization. Net primary production (NPP) and the abundance of legumes differed strongly across the treatments, and the fungal:bacterial ratio was very low in all treatments. Structural equation modeling revealed that microbial CUE was exclusively controlled by N fertilization and that neither the abundance of legumes (as a proxy for the quality of the organic matter inputs) nor NPP (as a proxy for C inputs) had an effect on microbial CUE. Our results show that N fertilization did not only decrease microbial respiration, but also microbial C uptake, indicating that less C was intracellularly processed in the N fertilized soils. The reason for reduced C uptake and increased CUE in the N-fertilization treatments is likely an inhibition of oxidative enzymes involved in the degradation of aromatic compounds by N in combination with a reduced energy requirement for microbial N acquisition in the fertilized soils. In conclusion, the study shows that N availability can control soil C cycling by affecting microbial CUE, while plant community-mediated changes in organic matter inputs and P and K availability played no important role for C partitioning of the microbial community in this temperate grassland.
Functional leaf traits of vascular epiphytes: vertical trends within the forest, intra- and interspecific trait variability, and taxonomic signals2016 - Functional Ecology, 30: 188-198
Mycorrhizas across scales: a journey between genomics, global patterns of biodiversity and biogeochemistry2016 - New Phytologist, 209: 913-916
Abstract:Mycorrhizal fungi are found in almost all ecosystems of the planet.They interact with a majority of plant species, and it seems thatevery single aspect of the life history of a plant individual is affectedby the presence of mycorrhizal fungal symbion ts in its roots (van derHeijden et al., ). Mycorrhizal fungi are also known to affectplant population-level and community-level dynamics. Yet, classic800-page plant ecology textbooks typically devote only one to twopages to mycorrhizal symbioses. Is it time to put mycorrhizalecologists on the editorial boards of these textbooks? Meetings likethe International Conference on Mycorrhizas (ICOM) tend tosuggest that this might not be a bad idea.On August 3–7, 2015, mycorrhizal researchers from around theworld shared their thoughts and empirical results on these globallywidespread symbioses at a comfortable elevation of 2135 m inFlagstaff, Arizona, surrounded by beautiful landscapes, likewidespread Ponderosa Pine forests, the San Francisco Peaks area,and the impressive Grand Canyon. New Phytologist was presentas a sponsor, continuing its ongoing support of mycorrhizalresearch (Selosse & Martin, ; Dickie et al., ). Throughtalks and posters, mycorrhizal researchers literally took us on ajourney across all scales of observation of this symbiosis: from theintracellular environment to global patterns of mycorrhizalfungal diversity and biogeochemical cycles (Fig. 1).
Mycorrhizal fungi are found in almost all ecosystems of the planet.They interact with a majority of plant species, and it seems thatevery single aspect of the life history of a plant individual is affectedby the presence of mycorrhizal fungal symbion ts in its roots (van derHeijden et al., ). Mycorrhizal fungi are also known to affectplant population-level and community-level dynamics. Yet, classic800-page plant ecology textbooks typically devote only one to twopages to mycorrhizal symbioses. Is it time to put mycorrhizalecologists on the editorial boards of these textbooks? Meetings likethe International Conference on Mycorrhizas (ICOM) tend tosuggest that this might not be a bad idea.On August 3–7, 2015, mycorrhizal researchers from around theworld shared their thoughts and empirical results on these globallywidespread symbioses at a comfortable elevation of 2135 m inFlagstaff, Arizona, surrounded by beautiful landscapes, likewidespread Ponderosa Pine forests, the San Francisco Peaks area,and the impressive Grand Canyon. New Phytologist was presentas a sponsor, continuing its ongoing support of mycorrhizalresearch (Selosse & Martin, ; Dickie et al., ). Throughtalks and posters, mycorrhizal researchers literally took us on ajourney across all scales of observation of this symbiosis: from theintracellular environment to global patterns of mycorrhizalfungal diversity and biogeochemical cycles (Fig. 1).
Geothermal ecosystems as natural climate change experiments: The ForHot research site in Iceland as a case study2016 - Icelandic Agricultural Sciences (IAS), 29: 53-71
This article describes how natural geothermal soil temperature gradients in Iceland have been used to study terrestrial ecosystem responses to soil warming. The experimental approach was evaluated at three study sites in southern Iceland; one grassland site that has been warm for at least 50 years (GO), and another comparable grassland site (GN) and a Sitka spruce plantation (FN) site that have both been warmed since an earthquake took place in 2008. Within each site type, five ca. 50 m long transects, with six permanent study plots each, were established across the soil warming gradients, spanning from unwarmed control conditions to gradually warmer soils. It was attempted to select the plots so the annual warming levels would be ca. +1, +3, +5, +10 and +20 °C within each transect. Results of continuous measurements of soil temperature (Ts) from 2013-2015 revealed that the soil warming was relatively constant and followed the seasonal Ts cycle of the unwarmed control plots. Volumetric water content in the top 5 cm of soil was repeatedly surveyed during 2013-2016. The grassland soils were wetter than the FN soils, but they had sometimes some significant warming-induced drying in the surface layer of the warmest plots, in contrast to FN. Soil chemistry did not show any indications that geothermal water had reached the root zone, but soil pH did increase somewhat with warming, which was probably linked to vegetation changes. As expected, the potential decomposition rate of organic matter increased significantly with warming. It was concluded that the natural geothermal gradients at the ForHot sites in Iceland offered realistic conditions for studying terrestrial ecosystem responses to warming with minimal artefacts.
Nitrogen isotope fractionation during N uptake via arbuscular mycorrhizal and ectomycorrhizal fungi into grey alder2016 - Journal of Plant Physiology, 205: 84-92
Arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi affect plant nitrogen (N) dynamics. Plant
N isotope patterns have been used to characterise the contribution of ECM fungi to plant N uptake. By
quantifying and comparing the effects of an AM and an ECM fungus on growth, N uptake and isotopic
composition of one host plant grown at different relative N supply levels, the aim of this study was to
improve the mechanistic understanding of natural 15N abundance patterns in mycorrhizal plants and
their underlying causes.
Grey alders were inoculated with one ECM fungus or one AM fungus or left non-mycorrhizal. Plants
were grown under semi-hydroponic conditions and were supplied with three rates of relative N supply
ranging from deficient to luxurious.
Neither mycorrhizal fungus increased plant growth or N uptake. AM root colonisation had no effect
on whole plant 15N and decreased foliar 15N only under N deficiency. The roots of these plants were
15N-enriched. ECM root colonisation consistently decreased foliar and whole plant 15N.
It is concluded, that both mycorrhizal fungi contributed to plant N uptake into the shoot. Nitrogen
isotope fractionation during N assimilation and transformations in fungal mycelia is suggested to have
resulted in plants receiving 15N-depleted N via the mycorrhizal uptake pathways. Negative mycorrhizal
growth effects are explained by symbiotic resource trade on carbon and N and decreased direct plant N
© 2016 Elsevier GmbH. All rights reserved.
Carbon isotope composition of carbohydrates and polyols in leaf and phloem sap of Phaseolus vulgaris L. influences predictions of plant water use efficiency2016 - Plant and Cell Physiology, 57: 1756-1766
The use of carbon isotope abundance (δ13C) to assess plant carbon acquisition and water use has significant potential for use in crop management and plant improvement programs. Utilising Phaseolus vulgaris L. as a model system, this study demonstrates the occurrence and sensitivity of carbon isotope fractionation during the onset of abiotic stresses between leaf and phloem carbon pools. In addition to gas exchange data; compound-specific measures of carbon isotope abundance and concentrations of soluble components of phloem sap were compared to major carbohydrate and sugar alcohol pools in leaf tissue. Differences in both δ13C and concentration of metabolites were found in leaf and phloem tissues, the magnitude of which responded to changing environmental conditions. These changes have inplications for the modelling of leaf level gas exchange based upon δ13C natural abundance. While estimates of δ13C of low molecular weight carbohydrates and polyols increased the precision of predictions of water use efficiency compared to those based on bulk soluble carbon. The use of this technique requires consideration of the dynamics of the δ13C pool under investigation. Understanding the dynamics of changes in δ13C during movement and incorporation into heterotrophic tissues is vital for the continued development of tools that provide information on plant physiological performance relating to water use.
Plant-derived compounds stimulate the decomposition of organic matter in arctic permafrost soils2016 - Scientific Reports, 6: 11
Arctic ecosystems are warming rapidly, which is expected to promote soil organic matter (SOM) decomposition. In addition to the direct warming effect, decomposition can also be indirectly stimulated via increased plant productivity and plant-soil C allocation, and this so called “priming effect” might significantly alter the ecosystem C balance. In this study, we provide first mechanistic insights into the susceptibility of SOM decomposition in arctic permafrost soils to priming. By comparing 119 soils from four locations across the Siberian Arctic that cover all horizons of active layer and upper permafrost, we found that an increased availability of plant-derived organic C particularly stimulated decomposition in subsoil horizons where most of the arctic soil carbon is located. Considering the 1,035 Pg of arctic soil carbon, such an additional stimulation of decomposition beyond the direct temperature effect can accelerate net ecosystem C losses, and amplify the positive feedback to global warming.
L-System model for the growth of arbuscular mycorrhizal fungi, both within and outside of their host roots2016 - Journal of the Royal Society Interface, 117: 11
Development of arbuscular mycorrhizal fungal colonization of roots and the surrounding soil is the central process of mycorrhizal symbiosis, important for ecosystem functioning and commercial inoculum applications. To improve mechanistic understanding of this highly spatially and temporarily dynamic process, we developed a three-dimensional model taking into account growth of the roots and hyphae. It is for the first time that infection within the root system is simulated dynamically and in a spatially resolved way. Comparison between data measured in a calibration experiment and simulated results showed a good fit. Our simulations showed that the position of the fungal inoculum affects the sensitivity of hyphal growth parameters. Variation in speed of secondary infection and hyphal lifetime had a different effect on root infection and hyphal length, respectively, depending on whether the inoculum was concentrated or dispersed. For other parameters (branching rate, distance between entry points), the relative effect was the same independent of inoculum placement. The model also indicated that maximum root colonization levels well below 100%, often observed experimentally, may be a result of differential spread of roots and hyphae, besides intrinsic plant control, particularly upon localized placement of inoculum and slow secondary infection.
Microbes as engines of ecosystem function: when does community structure enhance predictions of ecosystem processes?2016 - Frontiers in microbiology, 7: 214
Microorganisms are vital in mediating the earth's biogeochemical cycles; yet, despite our rapidly increasing ability to explore complex environmental microbial communities, the relationship between microbial community structure and ecosystem processes remains poorly understood. Here, we address a fundamental and unanswered question in microbial ecology: 'When do we need to understand microbial community structure to accurately predict function?' We present a statistical analysis investigating the value of environmental data and microbial community structure independently and in combination for explaining rates of carbon and nitrogen cycling processes within 82 global datasets. Environmental variables were the strongest predictors of process rates but left 44% of variation unexplained on average, suggesting the potential for microbial data to increase model accuracy. Although only 29% of our datasets were significantly improved by adding information on microbial community structure, we observed improvement in models of processes mediated by narrow phylogenetic guilds via functional gene data, and conversely, improvement in models of facultative microbial processes via community diversity metrics. Our results also suggest that microbial diversity can strengthen predictions of respiration rates beyond microbial biomass parameters, as 53% of models were improved by incorporating both sets of predictors compared to 35% by microbial biomass alone. Our analysis represents the first comprehensive analysis of research examining links between microbial communitystructure and ecosystem function. Taken together, our results indicate that a greater understanding of microbial communities informed by ecological principles may enhance our ability to predict ecosystem process rates relative to assessments based on environmental variables and microbial physiology.
Carbon and Nitrogen Uptake of Calcareous Benthic Foraminifera along a Depth-Related Oxygen Gradient in the OMZ of the Arabian Sea2016 - Frontiers in microbiology, 7: 71