Asymmetric belowground carbon transfer in a diverse tree community

Mycorrhizal fungi can colonize multiple trees of a single or multiple taxa, facilitating bidirectional exchange of carbon between trees. Mycorrhiza-induced carbon transfer was shown in the forest, but it is unknown whether carbon is shared symmetrically among tree species, and if not, which tree species are better donors and which are better recipients. Here we test this question by investigating carbon transfer dynamics among five Mediterranean tree species in a microcosm system, including both ectomycorrhizal (EM) and arbuscular (AM) plants. Trees were planted together in 'community boxes' using natural soil from a mixed forest plot that serves as habitat for all five tree species and their native mycorrhizal fungi. In each box, only the trees of a single species were pulse-labeled with ¹³ CO₂ . We found that carbon transfer was asymmetric, with oak being a better donor, and pistacia and cypress better recipients. Shared mycorrhizal species may have facilitated carbon transfer, but their diversity did not affect the amount, nor timing, of the transfer. Overall, our findings in a microcosm system expose rich, but hidden, belowground interactions in a diverse population of trees and mycorrhizal fungi. The asymmetric carbon exchange among co-habiting tree species could potentially contribute to forest resilience in an uncertain future.

Integrative Study Supports the Role of Trehalose in Carbon Transfer From Fungi to Mycotrophic Orchid

Orchids rely on mycorrhizal symbiosis, especially in the stage of mycoheterotrophic protocorms, which depend on carbon and energy supply from fungi. The transfer of carbon from fungi to orchids is well-documented, but the identity of compounds ensuring this transfer remains elusive. Some evidence has been obtained for the role of amino acids, but there is also vague and neglected evidence for the role of soluble carbohydrates, probably trehalose, which is an abundant fungal carbohydrate. We therefore focused on the possible role of trehalose in carbon and energy transfer. We investigated the common marsh orchid (Dactylorhiza majalis) and its symbiotic fungus Ceratobasidium sp. using a combination of cultivation approaches, high-performance liquid chromatography, application of a specific inhibitor of the enzyme trehalase, and histochemical localization of trehalase activity. We found that axenically grown orchid protocorms possess an efficient, trehalase-dependent, metabolic pathway for utilizing exogenous trehalose, which can be as good a source of carbon and energy as their major endogenous soluble carbohydrates. This is in contrast to non-orchid plants that cannot utilize trehalose to such an extent. In symbiotically grown protocorms and roots of adult orchids, trehalase activity was tightly colocalized with mycorrhizal structures indicating its pronounced role in the mycorrhizal interface. Inhibition of trehalase activity arrested the growth of both symbiotically grown protocorms and trehalose-supported axenic protocorms. Since trehalose constitutes only an inconsiderable part of the endogenous saccharide spectrum of orchids, degradation of fungal trehalose likely takes place in orchid mycorrhiza. Our results strongly support the neglected view of the fungal trehalose, or the glucose produced by its cleavage as compounds transported from fungi to orchids to ensure carbon and energy flow. Therefore, we suggest that not only amino acids, but also soluble carbohydrates are transported. We may propose that the soluble carbohydrates would be a better source of energy for plant metabolism than amino acids, which is partially supported by our finding of the essential role of trehalase.

Can stable isotopes quantify soil carbon build-up from organic fertilizers?

The application of organic fertilizers (OF) can supply carbon (C) to the soil in crop fields. OF-derived C (OF-C) is often estimated using the differential method that can be biased due to indirect effects of OF on soil C. This study tested three methods to quantify OF-C: (i) the widespread differential method, (ii) the synchronic isotope method comparing plots with and without OF and (iii) the asynchronic isotope method mimicking a trial without a control plot. These methods were implemented on an Arenosol and an Andosol supplied during 13 years with slurry or compost. The results highlighted the relevance of using the synchronic isotope method, which focuses on the direct effect of OFs on the soil organic matter (without bias of vegetation change) and considers control soil's evolution. The higher the isotopic difference between soil and OF, the shorter the method implementation time needed: for an initial difference of 7.5 ‰ and 3.5 ‰, quantification is suitable after 4 and 9 years of fertilization respectively. Attention should be paid to OF-δ¹³C variability to guarantee the method validity. The method proved to be suitable to study the factors controlling the OF-C fate in tropical soils.

The Lotus japonicus acyl-acyl carrier protein thioesterase FatM is required for mycorrhiza formation and lipid accumulation of Rhizophagus irregularis

Arbuscular mycorrhiza (AM) fungi establish symbiotic interactions with plants, providing the host plant with minerals, i.e. phosphate, in exchange for organic carbon. Arbuscular mycorrhiza fungi of the order Glomerales produce vesicles which store lipids as an energy and carbon source. Acyl-acyl carrier protein (ACP) thioesterases (Fat) are essential components of the plant plastid-localized fatty acid synthase and determine the chain length of de novo synthesized fatty acids. In addition to the ubiquitous FatA and FatB thioesterases, AM-competent plants contain an additional, AM-specific, FatM gene. Here, we characterize FatM from Lotus japonicus by phenotypically analyzing fatm mutant lines and by studying the biochemical function of the recombinant FatM protein. Reduced shoot phosphate content in fatm indicates compromised symbiotic phosphate uptake due to reduced arbuscule branching, and the fungus shows reduced lipid accumulation accompanied by the occurrence of smaller and less frequent vesicles. Lipid profiling reveals a decrease in mycorrhiza-specific phospholipid forms, AM fungal signature fatty acids (e.g. 16:1ω5, 18:1ω7 and 20:3) and storage lipids. Recombinant FatM shows preference for palmitoyl (16:0)-ACP, indicating that large amounts of 16:0 fatty acid are exported from the plastids of arbuscule-containing cells. Stable isotope labeling with [¹³ C₂ ]acetate showed reduced incorporation into mycorrhiza-specific fatty acids in the fatm mutant. Therefore, colonized cells reprogram plastidial de novo fatty acid synthesis towards the production of extra amounts of 16:0, which is in agreement with previous results that fatty acid-containing lipids are transported from the plant to the fungus.

In situ 13CO2 pulse labelling of field-grown eucalypt trees revealed the effects of potassium nutrition and throughfall exclusion on phloem transport of photosynthetic carbon

Potassium (K) is an important limiting factor of tree growth, but little is known of the effects of K supply on the long-distance transport of photosynthetic carbon (C) in the phloem and of the interaction between K fertilization and drought. We pulse-labelled 2-year-old Eucalyptus grandis L. trees grown in a field trial combining K fertilization (+K and -K) and throughfall exclusion (+W and -W), and we estimated the velocity of C transfer by comparing time lags between the uptake of (13)CO2 and its recovery in trunk CO2 efflux recorded at different heights. We also analysed the dynamics of the labelled photosynthates recovered in the foliage and in the phloem sap (inner bark extract). The mean residence time of labelled C in the foliage was short (21-31 h). The time series of (13)C in excess in the foliage was affected by the level of fertilization, whereas the effect of throughfall exclusion was not significant. The velocity of C transfer in the trunk (0.20-0.82 m h(-1)) was twice as high in +K trees than in -K trees, with no significant effect of throughfall exclusion except for one +K -W tree labelled in the middle of the drought season that was exposed to a more pronounced water stress (midday leaf water potential of -2.2 MPa). Our results suggest that besides reductions in photosynthetic C supply and in C demand by sink organs, the lower velocity under K deficiency is due to a lower cross-sectional area of the sieve tubes, whereas an increase in phloem sap viscosity is more likely limiting phloem transport under drought. In all treatments, 10 times less (13)C was recovered in inner bark extracts at the bottom of the trunk when compared with the base of the crown, suggesting that a large part of the labelled assimilates has been exported out of the phloem and replaced by unlabelled C. This supports the 'leakage-retrieval mechanism' that may play a role in maintaining the pressure gradient between source and sink organs required to sustain high velocity of phloem transport in tall trees.

Cost of reproduction in a spring ephemeral species, Adonis ramosa (Ranunculaceae): carbon budget for seed production

BACKGROUND AND AIMS

Spring ephemerals have a specific life-history trait, i.e. shoot growth and sexual reproduction occur simultaneously during a short period from snowmelt to canopy closure in deciduous forests. The aim of this study is to clarify how spring ephemerals invest resources for seed production within a restricted period.

METHODS

In order to evaluate the cost of reproduction of a typical spring ephemeral species, Adonis ramosa, an experiment was conducted comprising defoliation treatments (intact, one-third and two-thirds leaf-cutting) and fruit manipulations (control, shading and removal) over two growing seasons. In addition, measurements were made of the movements of carbon assimilated via (13)C tracing.

KEY RESULTS

Survival rate was high irrespective of treatments and manipulations. The proportion of flowering plants and plant size decreased as a result of the defoliation treatments over 2 years, but the fruit manipulations did not affect flowering activity or plant size. Seed set and seed number decreased as a result of fruit shading treatment, but the defoliation treatments did not affect current seed production. Individual seed weight also decreased in the second year due to fruit shading. The (13)C tracing experiment revealed that young fruits had photosynthetic ability and current photosynthetic products from leaves were mainly transferred to the below-ground parts, while translocation to fruit was very small even when fruit photosynthesis was restricted by the shading treatment.

CONCLUSIONS

Current foliage photosynthetic products are largely stored in the below-ground parts for survival and future growth, and about one-third of the resources for seed production may be attained by fruit photosynthesis. Therefore, the trade-off between current seed production and subsequent growth is weak. The cost of seed production may be buffered by sufficient storage in the below-ground organs, effective photosynthesis under high irradiation and self-assimilation ability of fruits.

Complex analyses of plankton structure and function

This paper critically evaluates some complex methods that have been used to characterize the structure and function of freshwater plankton communities. The focus is on methods related to plankton size structure and carbon transfer. The specific methods reviewed are 1) size spectrum analysis, 2) size-fractionated phytoplankton productivity, 3) size-fractionated zooplankton grazing, 4) plankton ecological transfer efficiency, and 5) grazer effects on phytoplankton community structure. Taken together, these methods can provide information on community ecological properties that are directly related to practical issues including water quality and fisheries productivity. However, caution is warranted since application without a complete understanding of assumptions and context of the manipulations could lead to erroneous conclusions. As an example, experimental studies involving the addition or removal of zooplankton, especially when coupled with nutrient addition treatments, could provide information on the degree of consumer vs. resource control of phytoplankton. Resource managers subsequently could use this information in developing effective measures for controlling nuisance algal biomass. However, the experiments must be done critically and with sufficient safeguards and other measurements to ensure that treatments (e.g., zooplankton exclosure by screening of water) actually are successful and do not introduce other changes in the community (e.g., removal of large algae). In all of the methods described here, the investigator must take care when generalizing results and, in particular, carry out a sufficient number of replications to encompass both the major seasonal and spatial variation that occurs in the ecosystem.

Reciprocal transfer of carbon isotopes between ectomycorrhizal Betula papyrifera and Pseudotsuga menziesii

Interspecific C transfer was studied in laboratory microcosms containing pairs of 6-month-old Betula papyrifera Marsh, and Pseudotsuga menziesii (Mirb.) Franca seedlings growing in individual, root-restrictive (28μm pore size) pouches filled with field soil. Interspecific transfer was examined by reciprocal labelling of seedlings with ¹³ CO_2(gas) and ¹⁴ CO_2(gas) . At the time of labelling, the root zones of ectomycorrhizal (EM) B. papyrifera and P. menziesii were interconnected by an extensive network of EM mycelium. Carbon transferred through EM connections was distinguished from that through soil pathways by comparing microcosms where interconnecting hyphae were left intact vs. those where they were severed immediately before labelling. Transfer was bidirectional, and represented 5 % of total isotope uptake by both B. papyrifera and P. menziesii together. P. menziesii received on average 50% more ¹⁴ C and 66% more ¹³ C from paper birch than vice versa, however, differences between species were not statistically significant. Neither net nor bidirectional transfer differed between severing treatments, leaving in question the relative importance of EM hyphae versus soil transfer pathways. The tendency for P. menziesii to receive more isotope than B. papyrifera corresponded with a 10-fold greater net photosynthetic rate per seedling and two-fold greater foliar N concentration of B. papyrifera than P. menziesii.

Reciprocal transfer of carbon isotopes between ectomycorrhizal Betula papyrifera and Pseudotsuga menziesii

Interspecific C transfer was studied in laboratory microcosms containing pairs of 6-month-old Betula papyrifera Marsh, and Pseudotsuga menziesii (Mirb.) Franca seedlings growing in individual, root-restrictive (28μm pore size) pouches filled with field soil. Interspecific transfer was examined by reciprocal labelling of seedlings with ¹³ CO_2(gas) and ¹⁴ CO_2(gas) . At the time of labelling, the root zones of ectomycorrhizal (EM) B. papyrifera and P. menziesii were interconnected by an extensive network of EM mycelium. Carbon transferred through EM connections was distinguished from that through soil pathways by comparing microcosms where interconnecting hyphae were left intact vs. those where they were severed immediately before labelling. Transfer was bidirectional, and represented 5 % of total isotope uptake by both B. papyrifera and P. menziesii together. P. menziesii received on average 50% more ¹⁴ C and 66% more ¹³ C from paper birch than vice versa, however, differences between species were not statistically significant. Neither net nor bidirectional transfer differed between severing treatments, leaving in question the relative importance of EM hyphae versus soil transfer pathways. The tendency for P. menziesii to receive more isotope than B. papyrifera corresponded with a 10-fold greater net photosynthetic rate per seedling and two-fold greater foliar N concentration of B. papyrifera than P. menziesii.

Distribution of photosynthetically fixed carbon within root systems of Eucalyptus pilularis plants ectomycorrhizal with Pisolithus tinctorius

In intact seedlings of Eucalyptus pilularis Stn. ectomycorrhizal with Pisolithus tinctorius (Pers.) Coker & Couch in growth pouches, and labelled photosynthetically with ¹⁴ C₄ more radioactivity was translocated into mycorrhizal than to non-mycorrhizal roots. Results from autoradiography of intact root systems were supported by data from scintillation counting of individual root tips. Greater ¹⁴ C accumulation occurred in young than in older mycorrhizas. It is proposed that individual mycorrhizal roots act most strongly as carbon sinks early in their life and that there is a progressive reduction in the amount of photosynthate translocated to them as they age.