Drought Effects on Soil Carbon Stability Mediated by Rhizodeposition and Microbes

Drought will increase in frequency and intensity in many areas of the world and has the potential to turn entire ecosystems from a sink to a source of C. Soil represents one of the largest C pools on earth, and small changes in the balance between inputs and outputs may have extreme consequences for total atmospheric CO2 concentrations. Outputs are determined by microbial decomposition of soil organic matter (SOM) which can be divided in pools of different inherent stability and turn-over. A major stable pool of C is represented by organo-mineral complexes (C bound to silt and clay), which is primarily controlled by plant-derived inputs to soil and soil microbes. Drought effects on plants, microbes and their interactions could cause changes to the stable pool of C, however information on this topic is lacking. In this thesis I: (i) reviewed and quantified drought-induced effects on soil respiration and microbial communities by meta-analysis; (ii) quantified the effects of drying and rewetting on wheat-derived C stabilization and N cycling; (iii) quantified and qualified drought-induced effects on root exudation of soybean and sunflower; (iv) examined drought effects to C stabilization in the field. Results show that drought can induce intense losses of C by increasing soil respiration following rewetting. Highest losses were produced in combination of intense drought and C-rich soils. At the same time drying and rewetting can cause intense stress on plants, reducing biomass and C inputs to soil. However plants can adopt different strategies to drought-induced changes which are reflected in different rates and quality of root exudates. In field drought did not change the size of the mineral-associated or more stable soil C, highlighting resistance of grassland soils. Specific microbial groups were linked to stable soil C at different depths and legumes were shown to be a key functional group in mediating drought effects and increasing stable C in soil

Root Exudation of Primary Metabolites: Mechanisms and Their Roles in Plant Responses to Environmental Stimuli

Root exudation is an important process determining plant interactions with the soil environment. Many studies have linked this process to soil nutrient mobilization. Yet, it remains unresolved how exudation is controlled and how exactly and under what circumstances plants benefit from exudation. The majority of root exudates including primary metabolites (sugars, amino acids, and organic acids) are believed to be passively lost from the root and used by rhizosphere-dwelling microbes. In this review, we synthetize recent advances in ecology and plant biology to explain and propose mechanisms by which root exudation of primary metabolites is controlled, and what role their exudation plays in plant nutrient acquisition strategies. Specifically, we propose a novel conceptual framework for root exudates. This framework is built upon two main concepts: (1) root exudation of primary metabolites is driven by diffusion, with plants and microbes both modulating concentration gradients and therefore diffusion rates to soil depending on their nutritional status; (2) exuded metabolite concentrations can be sensed at the root tip and signals are translated to modify root architecture. The flux of primary metabolites through root exudation is mostly located at the root tip, where the lack of cell differentiation favors diffusion of metabolites to the soil. We show examples of how the root tip senses concentration changes of exuded metabolites and translates that into signals to modify root growth. Plants can modify the concentration of metabolites either by controlling source/sink processes or by expressing and regulating efflux carriers, therefore challenging the idea of root exudation as a purely unregulated passive process. Through root exudate flux, plants can locally enhance concentrations of many common metabolites, which can serve as sensors and integrators of the plant nutritional status and of the nutrient availability in the surrounding environment. Plant-associated micro-organisms also constitute a strong sink for plant carbon, thereby increasing concentration gradients of metabolites and affecting root exudation. Understanding the mechanisms of and the effects that environmental stimuli have on the magnitude and type of root exudation will ultimately improve our knowledge of processes determining soil CO2 emissions, ecosystem functioning, and how to improve the sustainability of agricultural production

Plant-microbial linkages underpin carbon sequestration in contrasting mountain tundra vegetation types

Tundra ecosystems hold large stocks of soil organic matter (SOM), likely due to low temperatures limiting rates of microbial SOM decomposition more than those of SOM accumulation from plant primary productivity and microbial necromass inputs. Here we test the hypotheses that distinct tundra vegetation types and their carbon supply to characteristic rhizosphere microbes determine SOM cycling independent of temperature. In the subarctic Scandes, we used a three-way factorial design with paired heath and meadow vegetation at each of two elevations, and with each combination of vegetation type and elevation subjected during one growing season to either ambient light (i.e., ambient plant productivity), or 95% shading (i.e., reduced plant productivity). We assessed potential above-and belowground ecosystem linkages by uni-and multivariate analyses of variance, and structural equation modelling. We observed direct coupling between tundra vegetation type and microbial community composition and function, which underpinned the ecosystem's potential for SOM storage. Greater primary productivity at low elevation and ambient light supported higher microbial biomass and nitrogen immobilisation, with lower microbial mass-specific enzymatic activity and SOM humification. Congruently, larger SOM at lower elevation and in heath sustained fungal-dominated microbial communities, which were less substrate-limited, and invested less into enzymatic SOM mineralisation, owing to a greater carbon-use efficiency (CUE). Our results highlight the importance of tundra plant community characteristics (i.e., productivity and vegetation type), via their effects on soil microbial community size, structure and physiology, as essential drivers of SOM turnover. The here documented concerted patterns in above-and belowground ecosystem functioning is strongly supportive of using plant community characteristics as surrogates for assessing tundra carbon storage potential and its evolution under climate and vegetation changes

Composition and activity of nitrifier communities in soil are unresponsive to elevated temperature and CO2, but strongly affected by drought

Nitrification is a fundamental process in terrestrial nitrogen cycling. However, detailed information on how climate change affects the structure of nitrifier communities is lacking, specifically from experiments in which multiple climate change factors are manipulated simultaneously. Consequently, our ability to predict how soil nitrogen (N) cycling will change in a future climate is limited. We conducted a field experiment in a managed grassland and simultaneously tested the effects of elevated atmospheric CO2, temperature, and drought on the abundance of active ammonia-oxidizing bacteria (AOB) and archaea (AOA), comammox (CMX) Nitrospira, and nitrite-oxidizing bacteria (NOB), and on gross mineralization and nitrification rates. We found that N transformation processes, as well as gene and transcript abundances, and nitrifier community composition were remarkably resistant to individual and interactive effects of elevated CO2 and temperature. During drought however, process rates were increased or at least maintained. At the same time, the abundance of active AOB increased probably due to higher NH4+ availability. Both, AOA and comammox Nitrospira decreased in response to drought and the active community composition of AOA and NOB was also significantly affected. In summary, our findings suggest that warming and elevated CO2 have only minor effects on nitrifier communities and soil biogeochemical variables in managed grasslands, whereas drought favors AOB and increases nitrification rates. This highlights the overriding importance of drought as a global change driver impacting on soil microbial community structure and its consequences for N cycling

Microbial growth under drought is confined to distinct taxa and modified by potential future climate conditions

Climate change increases the frequency and intensity of drought events, affecting soil functions including carbon sequestration and nutrient cycling, which are driven by growing microorganisms. Yet we know little about microbial responses to drought due to methodological limitations. Here, we estimate microbial growth rates in montane grassland soils exposed to ambient conditions, drought, and potential future climate conditions (i.e., soils exposed to 6 years of elevated temperatures and elevated CO2 levels). For this purpose, we combined 18O-water vapor equilibration with quantitative stable isotope probing (termed 'vapor-qSIP') to measure taxon-specific microbial growth in dry soils. In our experiments, drought caused >90% of bacterial and archaeal taxa to stop dividing and reduced the growth rates of persisting ones. Under drought, growing taxa accounted for only 4% of the total community as compared to 35% in the controls. Drought-tolerant communities were dominated by specialized members of the Actinobacteriota, particularly the genus Streptomyces. Six years of pre-exposure to future climate conditions (3 °C warming and + 300 ppm atmospheric CO2) alleviated drought effects on microbial growth, through more drought-tolerant taxa across major phyla, accounting for 9% of the total community. Our results provide insights into the response of active microbes to drought today and in a future climate, and highlight the importance of studying drought in combination with future climate conditions to capture interactive effects and improve predictions of future soil-climate feedbacks

Crying out for help with root exudates : adaptive mechanisms by which stressed plants assemble health-promoting soil microbiomes

Plants employ immunological and ecological strategies to resist biotic stress. Recent evidence suggests that plants adapt to biotic stress by changing their root exudation chemistry to assemble health-promoting microbiomes. This so-called ‘cry-for-help’ hypothesis provides a mechanistic explanation for previously characterized soil feedback responses to plant disease, such as the development of disease-suppressing soils upon successive cultivations of take all-infected wheat. Here, we divide the hypothesis into individual stages and evaluate the evidence for each component. We review how plant immune responses modify root exudation chemistry, as well as what impact this has on microbial activities, and the subsequent plant responses to these activities. Finally, we review the ecological relevance of the interaction, along with its translational potential for future crop protection strategies

I Mediatori in Educazione Speciale: mezzi, strumenti e metodiche

Nella propria attivit\ue0 di insegnamento ogni docente utilizza tutta una serie di metodi e di strumenti per favorire l'apprendimento di contenuti, maturando specifiche abilit\ue0 capaci di renderli sicuri e operativi.I canali attraverso i quali avviene la comunicazione didattica, i mediatori, si dividono in quattro categorie: attivi, iconici, analogici e simbolici. Passando da una categoria all'altra aumenta il ruolo della simbolizzazione e ci si allontana dal contatto diretto con l'esperienza.Conoscere e saper utilizzare i vari mediatori significa adeguare consapevolmente la propria azione educativa (quella speciale soprattutto) in funzione delle potenzialit\ue0 insite nel soggetto in sviluppo.In questo volume vengono descritte alcune significative esperienze di laboratorio diversificate per categorie di mediatori - attivate per anni durante i corsi di specializzazione per gli insegnanti di sostegno presso la SSIS del Veneto.L'uso dei mediatori e soprattutto la loro sapiente composizione, studiata per far maturare precise abilit\ue0, si rivela la via maestra per aiutare i soggetti diversamente abili. Le maggiori difficolt\ue0 in educazione speciale si incontrano nell'attivare processi di simbolizzazione necessari all'inserimento nella cultura e nel contesto civile. Il passaggio continuo da mediatori pre-simbolici a quello simbolico attiva nei soggetti diversamente abili tutti quei pre-requisiti necessari all'integrazione scolastica e umana.Il volume \ue8 stato pensato per offrire a insegnanti di sostegno, agli specializzandi, ma pure a tutti gli insegnanti che si imbattono quotidianamente in soggetti con difficolt\ue0 di apprendimento, uno strumento agile e di facile lettura cui ispirarsi per impostare una didattica attiva rispondente ai bisogni culturali dei propri allievi. Alcune significative esperienze di laboratorio, attivate durante i corsi di specializzazione per gli insegnanti di sostegno presso la SSIS Veneto, che hanno messo in luce l\u2019importanza dei mediatori nel percorso educativo di soggetti diversamente abili. Un volume per insegnanti di sostegno, specializzandi e insegnanti che si imbattono quotidianamente in soggetti con difficolt\ue0 di apprendimento.Alcune significative esperienze di laboratorio, attivate durante i corsi di specializzazione per gli insegnanti di sostegno presso la SSIS Veneto, che hanno messo in luce l\u2019importanza dei mediatori nel percorso educativo di soggetti diversamente abili. Un volume per insegnanti di sostegno, specializzandi e insegnanti che si imbattono quotidianamente in soggetti con difficolt\ue0 di apprendimento

I Mediatori in Educazione Speciale

Nella propria attività di insegnamento ogni docente utilizza tutta una serie di metodi e di strumenti per favorire l'apprendimento di contenuti, maturando specifiche abilità capaci di renderli sicuri e operativi.I canali attraverso i quali avviene la comunicazione didattica, i mediatori, si dividono in quattro categorie: attivi, iconici, analogici e simbolici. Passando da una categoria all'altra aumenta il ruolo della simbolizzazione e ci si allontana dal contatto diretto con l'esperienza.Conoscere e saper utilizzare i vari mediatori significa adeguare consapevolmente la propria azione educativa (quella speciale soprattutto) in funzione delle potenzialità insite nel soggetto in sviluppo.In questo volume vengono descritte alcune significative esperienze di laboratorio diversificate per categorie di mediatori - attivate per anni durante i corsi di specializzazione per gli insegnanti di sostegno presso la SSIS del Veneto.L'uso dei mediatori e soprattutto la loro sapiente composizione, studiata per far maturare precise abilità, si rivela la via maestra per aiutare i soggetti diversamente abili. Le maggiori difficoltà in educazione speciale si incontrano nell'attivare processi di simbolizzazione necessari all'inserimento nella cultura e nel contesto civile. Il passaggio continuo da mediatori pre-simbolici a quello simbolico attiva nei soggetti diversamente abili tutti quei pre-requisiti necessari all'integrazione scolastica e umana.Il volume è stato pensato per offrire a insegnanti di sostegno, agli specializzandi, ma pure a tutti gli insegnanti che si imbattono quotidianamente in soggetti con difficoltà di apprendimento, uno strumento agile e di facile lettura cui ispirarsi per impostare una didattica attiva rispondente ai bisogni culturali dei propri allievi. Alcune significative esperienze di laboratorio, attivate durante i corsi di specializzazione per gli insegnanti di sostegno presso la SSIS Veneto, che hanno messo in luce l’importanza dei mediatori nel percorso educativo di soggetti diversamente abili. Un volume per insegnanti di sostegno, specializzandi e insegnanti che si imbattono quotidianamente in soggetti con difficoltà di apprendimento