Common Mycorrhizae Network: A Review of the Theories and Mechanisms Behind Underground Interactions

Most terrestrial plants establish symbiotic associations with mycorrhizal fungi for accessing essential plant nutrients. Mycorrhizal fungi have been frequently reported to interconnect plants via a common mycelial network (CMN), in which nutrients and signaling compounds can be exchanged between the connected plants. Several studies have been performed to demonstrate the potential effects of the CMN mediating resource transfer and its importance for plant fitness. Due to several contrasting results, different theories have been developed to predict benefits or disadvantages for host plants involved in the network and how it might affect plant communities. However, the importance of the mycelium connections for resources translocation compared to other indirect pathways, such as leakage of fungi hyphae and subsequent uptake by neighboring plant roots, is hard to distinguish and quantify. If resources can be translocated via mycelial connections in significant amounts that could affect plant fitness, it would represent an important tactic for plants co-existence and it could shape community composition and dynamics. Here, we report and critically discuss the most recent findings on studies aiming to evaluate and quantify resources translocation between plants sharing a CMN and predict the pattern that drives the movement of such resources into the CMN. We aim to point gaps and define open questions to guide upcoming studies in the area for a prospect better understanding of possible plant-to-plant interactions via CMN and its effect in shaping plants communities. We also propose new experiment set-ups and technologies that could be used to improve previous experiments. For example, the use of mutant lines plants with manipulation of genes involved in the symbiotic associations, coupled with labeling techniques to track resources translocation between connected plants, could provide a more accurate idea about resource allocation and plant physiological responses that are truly accountable to CMN

Complex dissolved organic matter (DOM) on the roof of the world – Tibetan DOM molecular characteristics indicate sources, land use effects, and processing along the fluvial–limnic continuum

The Tibetan Plateau (TP) is the world's largest and highest plateau, comprising the earth's biggest alpine pasture system. It is sensitive to the impacts of climate change and anthropogenic pressure. Carbon cycling on the TP is influenced by glaciation and degradation of the pasture ecosystem. Dissolved organic matter (DOM) connects carbon reservoirs, following the hydrological continuum from glaciers and headwaters to lakes. Due to its complexity, DOM cycling along the aquatic continuum and the impact of land use and climate change on DOM characteristics are still not well understood. Here, we study solid phase extracted (SPE) DOM molecular characteristics using ultrahigh-resolution mass spectrometry (FT-ICR-MS) along the TP hydrological continuum from glaciers, groundwater springs, and wetlands, including pastures and alpine steppes, to the endorheic Lake Nam Co. Our study revealed that the SPE-DOM composition was largely influenced by local sources of glaciers, wetlands, and groundwater springs as well as pasture degradation. Glacial meltwater SPE-DOM contained more saturated compounds suggesting microbial sources together with aromatic compounds probably derived from aeolian deposition. In comparison, wetland and stream SPE-DOM were characterised by a higher percentage of highly unsaturated and aromatic molecular formulae. These were likely derived from inputs of vascular plants and soils. Groundwater spring SPE-DOM from degraded pastures differed from intact pasture samples. In degraded systems a strongly oxidised signature with the lowest counts of P heteroatoms, a lower ratio, and a higher aromaticity of SPE-DOM together with a high degradation index suggested a strong transformation of SPE-DOM. SPE-DOM of the endorheic lake was richer in unsaturated molecular formulae compared to the tributaries. This suggests algae and microbial sources and production in the lake. The SPE-DOM rich in aromatic and highly unsaturated formulae visible in the brackish zone of the lake shore contrasted sharply with that of the lake. Aromatic molecular formulae were strongly depleted in the lake deep water suggesting photooxidation of riverine SPE-DOM. This indicates that alpine SPE-DOM signatures are shaped by small-scale catchment properties, land degradation, and the influence of glaciers and wetlands. The close link of alpine SPE-DOM composition to landscape properties is indicative of a strong susceptibility of DOM characteristics to climatic and land use changes in High Asia

Stickstoff- und Wassermanagement auf Ackerböden : Zwischenfrüchte statt Winterbrache

Boden- und Düngungsmanagement ist Grundwassermanagement. Am Institut für Bodenkunde wird im Rahmen mehrerer Forschungsprojekte zur nachhaltigen Landwirtschaft geforscht. Ziel ist, durch neue Herangehensweisen und Lösungswege, die Produktivität der Böden zu erhalten und dabei gleichzeitig die anderen Funktionen und Leistungen von Böden und ganzen Ökosystemen zu sichern

Fast Spectrophotometric Method as Alternative for CuO Oxidation to Assess Lignin in Soils with Different Tree Cover

Given the ongoing climate change, estimating the amount of less degradable plant compounds that can be stored in the soil, such as lignin, is a topic of primary importance. There are few methods applicable to soils for the determination of lignin, such as the copper oxide (CuO) oxidation method (CuOL). Acetyl bromide spectrophotometric lignin (ABSL) could be a valid alternative providing information that is less detailed compared to CuOL, but it offers data on the bulk amount of lignin and may offer a valid, fast, and cheap alternative to the CuO method. The aim of this work was to compare ABSL with the CuO method on several soils receiving plant residues from different trees. Mineral soil samples from 0 to 10 cm depth were obtained from a former agricultural site in northern Italy (Brusciana, Tuscany), where different tree plantations were established 22 years ago. The plantations were white poplar and common walnut, which were also intercropped with other species such as hazelnut, Italian alder, and autumn olive. Soil samples under these plantations were also compared to soil under an adjacent agricultural field. In general, the amount of lignin in the afforested stands was approximately double than in the agricultural field as determined by either method. The two methods returned a largely different scale of values due to their different mechanisms of action. The acid-to-aldehyde ratio of syringyl structural units highlights that forest plantation provides a plant input material that is more slowly oxidatively degraded compared to arable soil. A linear mixed model proved that ABSL performed well in relation to CuOL, especially when considering the random variation in the model given by the plantation field design. In conclusion, ABSL can be considered a valid proxy of soil C pool derived from structural plant component, although further analyses are needed. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Carbon investment into mobilization of mineral and organic phosphorus by arbuscular mycorrhiza

To overcome phosphorus (P) deficiency, about 80% of plant species establish symbiosis with arbuscular mycorrhizal fungi (AMF), which in return constitute a major sink of photosynthates. Information on whether plant carbon (C) allocation towards AMF increases with declining availability of the P source is limited. We offered orthophosphate (OP), apatite (AP), or phytic acid (PA) as the only P source available to arbuscular mycorrhiza (AM) (Solanum lycopersicum x Rhizophagus irregularis) in a mesocosm experiment, where the fungi had exclusive access to each P source. After exposure, we determined P contents in the plant, related these to the overall C budget of the system, including the organic C (OC) contents, the respired CO2, the phospholipid fatty acid (PLFA) 16:1ω5c (extraradical mycelium), and the neutral fatty acid (NLFA) 16:1ω5c (energy storage) at the fungal compartment. Arbuscular mycorrhizal (AM) plants incorporated P derived from the three P sources through the mycorrhizal pathway, but did this with differing C-P trading costs. The mobilization of PA and AP by the AM plant entailed larger mycelium infrastructure and significantly larger respiratory losses of CO2, in comparison with the utilization of the readily soluble OP. Our study thus suggests that AM plants invest larger C amounts into their fungal partners at lower P availability. This larger C flux to the AM fungi might also lead to larger soil organic C contents, in the course of forming larger AM biomass under P-limiting conditions. © 2020, The Author(s)

Temperature Sensitivity of CO2 and CH4 Fluxes from Coarse Woody Debris in Northern Boreal Forests

Carbon dioxide (CO2) and methane (CH4) are recognized as the main greenhouse gases causing climate warming. In forest ecosystems, the death of trees leads to the formation of coarse woody debris (CWD) that is one of the sources of greenhouse gas emissions due to wood decomposi-tion. We quantified the CO2 and CH4 fluxes from CWD of larch (Larix gmelinii (Rupr.)) and birch (Betula tortuosa Ledeb.) collected in the northern boreal forests of Central Siberia. The CWD samples were incubated at +5, +15 and +25◦ C. The CO2 and CH4 fluxes showed strong correlations with temperature, moisture, decomposition stage and the type of wood’s rot. The temperature coefficient Q10 indicated higher temperature sensitivity of CO2 flux within the temperature interval from +5 to +15◦ C than from +15 to +25◦ C. Methane flux had higher temperature sensitivity within the interval from +15 to +25◦ C. It was found that, in boreal forests, CWD of early decay stage can serve as a source of methane to the atmosphere when air temperatures increased above +15◦ C. Strong positive correlation between CH4 production and CO2 emission indicated a biological source and supported findings on aerobic origin of the main process contributing to the CH4 flux from decomposing CWD

The Kobresia pygmaea ecosystem of the Tibetan highlands – Origin, functioning and degradation of the world's largest pastoral alpine ecosystem: Kobresia pastures of Tibet

With 450,000 km2 Kobresia (syn. Carex) pygmaea dominated pastures in the eastern Tibetan highlands are the world's largest pastoral alpine ecosystem forming a durable turf cover at 3000–6000 m a.s.l. Kobresia's resilience and competitiveness is based on dwarf habit, predominantly below-ground allocation of photo assimilates, mixture of seed production and clonal growth, and high genetic diversity. Kobresia growth is co-limited by livestock-mediated nutrient withdrawal and, in the drier parts of the plateau, low rainfall during the short and cold growing season. Overstocking has caused pasture degradation and soil deterioration over most parts of the Tibetan highlands and is the basis for this man-made ecosystem. Natural autocyclic processes of turf destruction and soil erosion are initiated through polygonal turf cover cracking, and accelerated by soil-dwelling endemic small mammals in the absence of predators. The major consequences of vegetation cover deterioration include the release of large amounts of C, earlier diurnal formation of clouds, and decreased surface temperatures. These effects decrease the recovery potential of Kobresia pastures and make them more vulnerable to anthropogenic pressure and climate change. Traditional migratory rangeland management was sustainable over millennia, and possibly still offers the best strategy to conserve and possibly increase C stocks in the Kobresia turf. © 201

Impacts of Drying and Rewetting on the Radiocarbon Signature of Respired CO2 and Implications for Incubating Archived Soils

The radiocarbon signature of respired CO2 (∆14C-CO2) measured in laboratory soil incubations integrates contributions from soil carbon pools with a wide range of ages, making it a powerful model constraint. Incubating archived soils enriched by “bomb-C” from mid-20th century nuclear weapons testing would be even more powerful as it would enable us to trace this pulse over time. However, air-drying and subsequent rewetting of archived soils, as well as storage duration, may alter the relative contribution to respiration from soil carbon pools with different cycling rates. We designed three experiments to assess air-drying and rewetting effects on ∆14C-CO2 with constant storage duration (Experiment 1), without storage (Experiment 2), and with variable storage duration (Experiment 3). We found that air-drying and rewetting led to small but significant (α < 0.05) shifts in ∆14C-CO2 relative to undried controls in all experiments, with grassland soils responding more strongly than forest soils. Storage duration (4–14 y) did not have a substantial effect. Mean differences (95% CIs) for experiments 1, 2, and 3 were: 23.3‰ (±6.6), 19.6‰ (±10.3), and 29.3‰ (±29.1) for grassland soils, versus −11.6‰ (±4.1), 12.7‰ (±8.5), and −24.2‰ (±13.2) for forest soils. Our results indicate that air-drying and rewetting soils mobilizes a slightly older pool of carbon that would otherwise be inaccessible to microbes, an effect that persists throughout the incubation. However, as the bias in ∆14C-CO2 from air-drying and rewetting is small, measuring ∆14C-CO2 in incubations of archived soils appears to be a promising technique for constraining soil carbon models

Designing a Robust and Versatile System to Investigate Nutrient Exchange in, and Partitioning by, Mycorrhiza (Populus x canesces x Paxillus involutus) Under Axenic or Greenhouse Conditions

Phosphorus (P) bioavailability affects plant nutrition. P can be present in soils in different chemical forms that are not available for direct plant uptake and have to be acquired by different mechanisms, representing different resource niches. These mechanisms, of which many seem to be attributed to mycorrhiza, likely influence the diversity and stability of plant communities in natural ecosystems, as they also might help to overcome a future shortage of P supply in agro-ecosystems. In order to understand the mechanisms of P acquisition, the associated carbon costs, and the resource partitioning by mycorrhizal fungi, the ecosystem situation has to be mimicked in smaller scaled experiments. Here, different experimental setups are evaluated using plantlets of Populus x canescens and its functional ectomycorrhizal (ECM) fungus Paxillus involututs strain MAJ. To investigate resource partitioning involving mycorrhizae, the protocols of this study describe preparation of an in vitro and a rhizotrone culture systems for studies under axenic conditions as well as a mesocosm culture system for greenhouse conditions. We also describe the construction of separate compartments containing nutrients and excluding plant roots as well as the progress that has been made in in vitro propagation of plant and ECM fungal material. The practical experience made in our study shows that the in vitro culture system is prone to desiccation and its construction and maintenance are more time consuming and complicated. In contrast, with the axenic rhizotrone culture system and the mesocosms we have created more robust and very versatile systems that are also suitable for greenhouse conditions

Identification and validation of early genetic biomarkers for apple replant disease

Apple replant disease (ARD) is a serious threat to producers of apple trees and fruits worldwide. The ARD etiology is not unraveled and managing options are either economically not applicable or environmentally harmful. Thus, interest is given in biomarkers that allow to indicate ARD situations at early time points in order to classify soils according to ARD severity but also to analyze the effectiveness to potential countermeasures. This study aimed at (i) identifying ARD biomarkers on the transcriptional level in root tissue by analyzing the expression of previously identified candidate genes in ARD soils of different origin and texture and (ii) testing the specificity of these marker genes to ARD. In vitro propagated M26 plantlets were submitted to a bio-test with three ARD soils, either untreated or disinfected by γ-irradiation. Expression of seven candidate genes identified in a previous transcriptomic study was investigated by RT-qPCR in a time course experiment. Already three days after planting, a prominent upregulation of the phytoalexin biosynthesis genes biphenyl synthase 3 (BIS3) and biphenyl 4-hydroxylase (B4Hb) was observed in the untreated ARD variants of all three soils. The phytoalexin composition in roots was comparable for all three soils and the total phytoalexin content correlated with the expression of BIS3 and B4Hb. The third promising candidate gene that was upregulated under ARD conditions was the ethylene-responsive transcription factor 1B-like (ERF1B). In a second experiment M26 plantlets were exposed to different abiotic stressors, namely heat, salt and nutrient starvation, and candidate gene expression was determined in the roots. The expression levels of BIS3 and B4Hb were highly and specifically upregulated in ARD soil, but not upon the abiotic stress conditions, whereas ERF1B also showed higher expression under heat stress. In conclusion, BIS3 and B4Hb are recommended as early ARD biomarkers due to their high expression levels and their high specificity. Copyright: © 2020 Rohr et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited