Search

As direct mediators between plants and soil, roots play an important role in metabolic responses to environmental stresses such as drought, yet these responses are vastly uncharacterized on a plant-specific level, especially for co-occurring species. Here, we aim to examine the effects of drought on root metabolic profiles and carbon allocation pathways of three tropical rainforest species by combining cutting-edge metabolomic and imaging technologies in an in situ position-specific 13 C-pyruvate root-labeling experiment. Further, washed (rhizosphere-depleted) and unwashed roots were examined to test the impact of microbial presence on root metabolic pathways. Drought had a species-specific impact on the metabolic profiles and spatial distribution in Piper sp. and Hibiscus rosa sinensis roots, signifying different defense mechanisms; Piper sp. enhanced root structural defense via recalcitrant compounds including lignin, while H. rosa sinensis enhanced biochemical defense via secretion of antioxidants and fatty acids. In contrast, Clitoria fairchildiana , a legume tree, was not influenced as much by drought but rather by rhizosphere presence where carbohydrate storage was enhanced, indicating a close association with symbiotic microbes. This study demonstrates how multiple techniques can be combined to identify how plants cope with drought through different drought-tolerance strategies and the consequences of such changes on below-ground organic matter composition.

Mucilage is a gelatinous high-molecular-weight substance produced by almost all plants, serving numerous functions for plant and soil. To date, research has mainly focused on hydraulic and physical functions of mucilage in the rhizosphere. Studies on the relevance of mucilage as a microbial habitat are scarce. Extracellular polymeric substances (EPS) are similarly gelatinous high-molecular-weight substances produced by microorganisms. EPS support the establishment of microbial assemblages in soils, mainly through providing a moist environment, a protective barrier, and serving as carbon and nutrient sources. We propose that mucilage shares physical and chemical properties with EPS, functioning similarly as a biofilm matrix covering a large extent of the rhizosphere. Our analyses found no evidence of consistent differences in viscosity and surface tension between EPS and mucilage, these being important physical properties. With regard to chemical composition, polysaccharide, protein, neutral monosaccharide, and uronic acid composition also showed no consistent differences between these biogels. Our analyses and literature review suggest that all major functions known for EPS and required for biofilm formation are also provided by mucilage, offering a protected habitat optimized for nutrient mobilization. Mucilage enables high rhizo-microbial abundance and activity by functioning as carbon and nutrient source. We suggest that the role of mucilage as a biofilm matrix has been underestimated, and should be considered in conceptual models of the rhizosphere., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Nazari, Bickel, Benard, Mason-Jones, Carminati and Dippold.)

Sustainable forest management requires understanding of ecosystem phosphorus (P) cycling. Lang et al. (2017) [ Biogeochemistry, https://doi.org/10.1007/s10533-017-0375-0] introduced the concept of P-acquiring vs. P-recycling nutrition strategies for European beech ( Fagus sylvatica L. ) forests on silicate parent material, and demonstrated a change from P-acquiring to P-recycling nutrition from P-rich to P-poor sites. The present study extends this silicate rock-based assessment to forest sites with soils formed from carbonate bedrock. For all sites, it presents a large set of general soil and bedrock chemistry data. It thoroughly describes the soil P status and generates a comprehensive concept on forest ecosystem P nutrition covering the majority of Central European forest soils. For this purpose, an Ecosystem P Nutrition Index ( ENI ) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-P P ) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-P org precipitates formed from phosphate released during SOM decomposition. In contrast to silicate sites, progressing pedogenesis and accumulation of P-enriched carbonate bedrock dissolution residue at the carbonate sites promote again P-acquiring mechanisms for ecosystem P nutrition., Supplementary Information: The online version contains supplementary material available at 10.1007/s10533-021-00884-7., Competing Interests: Conflict of interestThe authors have no conflicts of interest to declare that are relevant to the content of this article., (© The Author(s) 2022.)

The global methane (CH 4 ) budget is based on a sensitive balance between methanogenesis and CH 4 oxidation (aerobic and anaerobic). The response of these processes to climate warming, however, is not quantified. This largely reflects our lack of knowledge about the temperature sensitivity (Q 10 ) of the anaerobic oxidation of CH 4 (AOM)-a ubiquitous process in soils. Based on a 13 CH 4 labeling experiment, we determined the rate, Q 10 and activation energy of AOM and of methanogenesis in a paddy soil at three temperatures (5, 20, 35°C). The rates of AOM and of methanogenesis increased exponentially with temperature, whereby the AOM rate was significantly lower than methanogenesis. Both the activation energy and Q 10 of AOM dropped significantly from 5-20 to 20-35°C, indicating that AOM is a highly temperature-dependent microbial process. Nonetheless, the Q 10 of AOM and of methanogenesis were similar at 5-35°C, implying a comparable temperature dependence of AOM and methanogenesis in paddy soil. The continuous increase of AOM Q 10 over the 28-day experiment reflects the successive utilization of electron acceptors according to their thermodynamic efficiency. The basic constant for Q 10 of AOM was calculated to be 0.1 units for each 3.2 kJ mol -1 increase of activation energy. We estimate the AOM in paddy soils to consume 2.2~5.5 Tg CH 4 per year on a global scale. Considering these results in conjunction with literature data, the terrestrial AOM in total consumes ~30% of overall CH 4 production. Our data corroborate a similar Q 10 of AOM and methanogenesis. As the rate of AOM in paddy soils is lower than methanogenesis, however, it will not fully compensate for an increased methane production under climate warming., (© 2021 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Severe droughts endanger ecosystem functioning worldwide. We investigated how drought affects carbon and water fluxes as well as soil-plant-atmosphere interactions by tracing 13 CO 2 and deep water 2 H 2 O label pulses and volatile organic compounds (VOCs) in an enclosed experimental rainforest. Ecosystem dynamics were driven by different plant functional group responses to drought. Drought-sensitive canopy trees dominated total fluxes but also exhibited the strongest response to topsoil drying. Although all canopy-forming trees had access to deep water, these reserves were spared until late in the drought. Belowground carbon transport was slowed, yet allocation of fresh carbon to VOCs remained high. Atmospheric VOC composition reflected increasing stress responses and dynamic soil-plant-atmosphere interactions, potentially affecting atmospheric chemistry and climate feedbacks. These interactions and distinct functional group strategies thus modulate drought impacts and ecosystem susceptibility to climate change.

Mucilage, a gelatinous substance comprising mostly polysaccharides, is exuded by maize nodal and underground root tips. Although mucilage provides several benefits for rhizosphere functions, studies on the variation in mucilage amounts and its polysaccharide composition between genotypes are still lacking. In this study, eight maize ( Zea mays L.) genotypes from different globally distributed agroecological zones were grown under identical abiotic conditions in a randomized field experiment. Mucilage exudation amount, neutral sugars and uronic acids were quantified. Galactose (∼39-42%), fucose (∼22-30%), mannose (∼11-14%), and arabinose (∼8-11%) were the major neutral sugars in nodal root mucilage. Xylose (∼1-4%), and glucose (∼1-4%) occurred only in minor proportions. Glucuronic acid (∼3-5%) was the only uronic acid detected. The polysaccharide composition differed significantly between maize genotypes. Mucilage exudation was 135 and 125% higher in the Indian (900 M Gold) and Kenyan (DH 02) genotypes than in the central European genotypes, respectively. Mucilage exudation was positively associated with the vapor pressure deficit of the genotypes' agroecological zone. The results indicate that selection for environments with high vapor pressure deficit may favor higher mucilage exudation, possibly because mucilage can delay the onset of hydraulic failure during periods of high vapor pressure deficit. Genotypes from semi-arid climates might offer sources of genetic material for beneficial mucilage traits., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2020 Nazari, Riebeling, Banfield, Akale, Crosta, Mason-Jones, Dippold and Ahmed.)

Rationale: Many bacteria synthesize carbon (C) and energy storage compounds, including water-insoluble polyester lipids composed mainly or entirely of poly(3-hydroxybutyrate) (PHB). Despite the potential significance of C and energy storage for microbial life and C cycling, few measurements of PHB in soil have been reported., Methods: A new protocol was implemented, based on an earlier sediment extraction and derivatization procedure, with quantification by gas chromatography/mass spectrometry (GC/MS) and 13 C-isotopic analysis by GC/combustion/isotope ratio mass spectrometry (GC/C/IRMS)., Results: The PHB content was 4.3 μg C g -1 in an agricultural soil and 1.2 μg C g -1 in a forest topsoil. This was an order of magnitude more PHB than obtained by the existing extraction method, suggesting that native PHB in soil has been previously underestimated. Addition of glucose increased the PHB content by 135% and 1,215% over 5 days, with the largest increase in the relatively nutrient-poor forest soil. In the agricultural soil, 68% of the increase was derived from added 13 C-labeled glucose, confirming synthesis of PHB from glucose for the first time in soil., Conclusions: The presence and responsiveness of PHB in both these contrasting soils show that PHB could provide a useful indicator of bacterial nutritional status and unbalanced growth. Microbial storage could be important to C and nutrient cycling and be a widespread strategy in the life of soil bacteria. The presented method offers new insight into the significance of this compound in soil., (© 2019 John Wiley & Sons, Ltd.)

CO 2 release from soil is commonly used to estimate toxicity of various substances on microorganisms. However, the mechanisms underlying persistent CO 2 release from soil exposed to toxicants inhibiting microbial respiration, for example, sodium azide (NaN 3 ) or heavy metals (Cd, Hg, Cu), remain unclear. To unravel these mechanisms, NaN 3 -amended soil was incubated with position-specifically 13 C-labeled glucose and 13 C was quantified in CO 2 , bulk soil, microbial biomass and phospholipid fatty acids (PLFAs). High 13 C recovery from C-1 in CO 2 indicates that glucose was predominantly metabolized via the pentose phosphate pathway irrespective of inhibition. Although NaN 3 prevented 13 C incorporation into PLFA and decreased total CO 2 release, 13 C in CO 2 increased by 12% compared with control soils due to an increased use of glucose for energy production. The allocation of glucose-derived carbon towards extracellular compounds, demonstrated by a fivefold higher 13 C recovery in bulk soil than in microbial biomass, suggests the synthesis of redox active substances for extracellular disposal of electrons to bypass inhibited electron transport chains within the cells. PLFA content doubled within 10 days of inhibition, demonstrating recovery of the microbial community. This growth was largely based on recycling of cost-intensive biomass compounds, for example, alkyl chains, from microbial necromass. The bypass of intracellular toxicity by extracellular electron transport permits the fast recovery of the microbial community. Such efficient strategies to overcome exposure to respiration-inhibiting toxicants may be exclusive to habitats containing redox-sensitive substances. Therefore, the toxic effects of respiration inhibitors on microorganisms are much less intensive in soils than in pure cultures.

Plants allocate carbon (C) to sink tissues depending on phenological, physiological or environmental factors. We still have little knowledge on C partitioning into various cellular compounds and metabolic pathways at various ecophysiological stages. We used compound-specific stable isotope analysis to investigate C partitioning of freshly assimilated C into tree compartments (needles, branches and stem) as well as into needle water-soluble organic C (WSOC), non-hydrolysable structural organic C (stOC) and individual chemical compound classes (amino acids, hemicellulose sugars, fatty acids and alkanes) of Norway spruce (Picea abies) following in situ (13)C pulse labelling 15 days after bud break. The (13)C allocation within the above-ground tree biomass demonstrated needles as a major C sink, accounting for 86% of the freshly assimilated C 6 h after labelling. In needles, the highest allocation occurred not only into the WSOC pool (44.1% of recovered needle (13)C) but also into stOC (33.9%). Needle growth, however, also caused high (13)C allocation into pathways not involved in the formation of structural compounds: (i) pathways in secondary metabolism, (ii) C-1 metabolism and (iii) amino acid synthesis from photorespiration. These pathways could be identified by a high (13)C enrichment of their key amino acids. In addition, (13)C was strongly allocated into the n-alkyl lipid fraction (0.3% of recovered (13)C), whereby (13)C allocation into cellular and cuticular exceeded that of epicuticular fatty acids. (13)C allocation decreased along the lipid transformation and translocation pathways: the allocation was highest for precursor fatty acids, lower for elongated fatty acids and lowest for the decarbonylated n-alkanes. The combination of (13)C pulse labelling with compound-specific (13)C analysis of key metabolites enabled tracing relevant C allocation pathways under field conditions. Besides the primary metabolism synthesizing structural cell compounds, a complex network of pathways consumed the assimilated (13)C and kept most of the assimilated C in the growing needles., (© The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Rationale: Amino sugars build up microbial cell walls and are important components of soil organic matter. To evaluate their sources and turnover, δ(13)C analysis of soil-derived amino sugars by liquid chromatography was recently suggested. However, amino sugar δ(13)C determination remains challenging due to (1) a strong matrix effect, (2) CO2 -binding by alkaline eluents, and (3) strongly different chromatographic behavior and concentrations of basic and acidic amino sugars. To overcome these difficulties we established an ion chromatography-oxidation-isotope ratio mass spectrometry method to improve and facilitate soil amino sugar analysis., Methods: After acid hydrolysis of soil samples, the extract was purified from salts and other components impeding chromatographic resolution. The amino sugar concentrations and δ(13)C values were determined by coupling an ion chromatograph to an isotope ratio mass spectrometer. The accuracy and precision of quantification and δ(13)C determination were assessed., Results: Internal standards enabled correction for losses during analysis, with a relative standard deviation <6%. The higher magnitude peaks of basic than of acidic amino sugars required an amount-dependent correction of δ(13)C values. This correction improved the accuracy of the determination of δ(13)C values to <1.5‰ and the precision to <0.5‰ for basic and acidic amino sugars in a single run., Conclusions: This method enables parallel quantification and δ(13)C determination of basic and acidic amino sugars in a single chromatogram due to the advantages of coupling an ion chromatograph to the isotope ratio mass spectrometer. Small adjustments of sample amount and injection volume are necessary to optimize precision and accuracy for individual soils., (Copyright © 2014 John Wiley & Sons, Ltd.)