Mechanisms of resistance and resilience in the plant-soil system of Mountain grassland communities

Overall, the findings of this thesis highlight the importance of land use and plant-microbial interactions for the resilience of terrestrial ecosystems to extreme climatic events. The results from stable isotope (13C and 15N) labelling experiments on mountain grasslands extend the existing knowledge about the link between plant and soil processes during drought and subsequent recovery. There is indication that the continuous release of carbon substrates from plants into the rhizosphere during drought plays a relevant role in the priming of soil microbial activity after rewetting. Furthermore, this thesis demonstrates that the mechanisms underlying ecosystem resistance and resilience can be altered by land use, in particular by management-related changes in plant functional composition. More conservative plant communities with low nutrient demands, as found in abandoned grassland, are more resistant and can profit from stronger interactions with mycorrhizal fungi during drought. On the contrary, more exploitative plant communities with high nutrient uptake, as found in managed grassland, are able to quickly recover and can benefit from stronger interactions with fast-growing bacteria after rewetting. In consequence the results of this work suggest a general trade-off between resistance and recovery, i.e. high resistance is followed by a slow recovery and vice versa. On the one hand, this is important for biogeochemical models dealing with the global carbon cycle and climate change feedbacks. On the other hand, this allows to control ecosystem resilience by adapting land use according to local risk scenarios, through promoting either the resistance to or the recovery from extreme climatic events

Soilless Tomato Production: Effects of Hemp Fiber and Rock Wool Growing Media on Yield, Secondary Metabolites, Substrate Characteristics and Greenhouse Gas Emissions

Replacement of rock wool by organic substrates is considered to reduce the environmental impact, e.g., through energy savings during production and waste prevention, caused by hydroponically produced crops. A suitable substrate for plant production is characterized by an optimal composition of air- and water-filled pores. In our study, we used hemp fibers as an organic alternative to rock wool in order to cultivate tomato plants in hydroponics for 36 weeks. The leaf area, plant length, and yields, as well as the quality of fruits including soluble solid contents, dry weight content, mineral composition, and contents of phenolic compounds caused by both substrates, were similar. Carotenoids were significantly increased in fruits from plants grown in hemp at some measuring dates. Nevertheless, higher emission rates of greenhouse gases such as N2O, CO2, and CH4 caused by hemp fiber compared to those emitted by rock wool during use are rather disadvantageous for the environment. While hemp proved to be a suitable substrate in terms of some physical properties (total pore volume, bulk density), a lower volume of air and easily available water as well as very rapid microbial decomposition and the associated high nitrogen immobilization must be considered as disadvantages.Peer Reviewe

Drought-Induced Accumulation of Root Exudates Supports Post-drought Recovery of Microbes in Mountain Grassland

Droughts strongly affect carbon and nitrogen cycling in grasslands, with consequences for ecosystem productivity. Therefore, we investigated how experimental grassland communities interact with groups of soil microorganisms. In particular, we explored the mechanisms of the drought-induced decoupling of plant photosynthesis and microbial carbon cycling and its recovery after rewetting. Our aim was to better understand how root exudation during drought is linked to pulses of soil microbial activity and changes in plant nitrogen uptake after rewetting. We set up a mesocosm experiment on a meadow site and used shelters to simulate drought. We performed two 13C-CO2 pulse labelings, the first at peak drought and the second in the recovery phase, and traced the flow of assimilates into the carbohydrates of plants and the water extractable organic carbon and microorganisms from the soil. Total microbial tracer uptake in the main metabolism was estimated by chloroform fumigation extraction, whereas the lipid biomarkers were used to assess differences between the microbial groups. Drought led to a reduction of aboveground versus belowground plant growth and to an increase of 13C tracer contents in the carbohydrates, particularly in the roots. Newly assimilated 13C tracer unexpectedly accumulated in the water-extractable soil organic carbon, indicating that root exudation continued during the drought. In contrast, drought strongly reduced the amount of 13C tracer assimilated into the soil microorganisms. This reduction was more severe in the growth-related lipid biomarkers than in the metabolic compounds, suggesting a slowdown of microbial processes at peak drought. Shortly after rewetting, the tracer accumulation in the belowground plant carbohydrates and in the water-extractable soil organic carbon disappeared. Interestingly, this disappearance was paralleled by a quick recovery of the carbon uptake into metabolic and growth-related compounds from the rhizospheric microorganisms, which was probably related to the higher nitrogen supply to the plant shoots. We conclude that the decoupling of plant photosynthesis and soil microbial carbon cycling during drought is due to reduced carbon uptake and metabolic turnover of rhizospheric soil microorganisms. Moreover, our study suggests that the maintenance of root exudation during drought is connected to a fast reinitiation of soil microbial activity after rewetting, supporting plant recovery through increased nitrogen availability

Data from: Land use in mountain grasslands alters drought response and recovery of carbon allocation and plant-microbial interactions

1. Mountain grasslands have recently been exposed to substantial changes in land-use and climate and in the near future will likely face an increased frequency of extreme droughts. To date is not known how the drought responses of carbon (C) allocation, a key process in the C cycle, are affected by land-use changes in mountain grassland. 2. We performed an experimental summer drought on an abandoned grassland and a traditionally managed hay meadow and traced the fate of recent assimilates through the plant-soil continuum. We applied two 13CO2 pulses, at peak drought and in the recovery phase shortly after rewetting. 3. Drought decreased total C uptake in both grassland types and led to a loss of aboveground carbohydrate storage pools. The belowground C allocation to root sucrose was enhanced by drought, especially in the meadow, which also held larger root carbohydrate storage pools. 4. The microbial community of the abandoned grassland comprised more saprotrophic fungal and Gram (+) bacterial markers compared to the meadow. Drought increased the newly introduced AM and saprotrophic fungi:bacteria ratio in both grassland types. At peak drought the 13C transfer into AM fungi, saprotrophic fungi and Gram (-) bacteria was more strongly reduced in the meadow than in the abandoned grassland, which contrasted the patterns of the root carbohydrate pools. 5. In both grassland types the C allocation largely recovered after rewetting. Slowest recovery was found for AM fungi and their 13C uptake. In contrast, all bacterial markers quickly recovered C uptake. In the meadow, where plant nitrate uptake was enhanced after drought, C uptake was even higher than in control plots. 6. Synthesis. Our results suggest that resistance and resilience (i.e. recovery) of plant C dynamics and plant-microbial interactions are negatively related, i.e. high resistance is followed by slow recovery and vice versa. The abandoned grassland was more resistant to drought than the meadow and possibly had a stronger link to AM fungi that could have provided better access to water through the hyphal network. In contrast, meadow communities strongly reduced C allocation to storage and C transfer to the microbial community in the drought phase, but in the recovery phase invested C resources in the bacterial communities to gain more nutrients for regrowth. We conclude that management of mountain grasslands increases their resilience to drought

Land use and drought effects on grassland C dynamics

Data were collected from a common garden drought experiment in the Austrian Central Alps using intact vegetation-soil monoliths from an extensively managed meadow and a nearby abandoned grassland. The monoliths were installed in summer 2013 on the meadow site and pre-incubated until drought simulation was started in early summer 2014. The effects of land use on the response of plant C allocation and plant-microbial C transfer to drought and rewetting were studied by conducting two 13C pulse-chase labelling campaigns. A description of all data and abbreviations used in the excel file can be found in tab "0-README"

Data_Sheet_1_Sources of nitrous oxide emissions from hydroponic tomato cultivation: Evidence from stable isotope analyses.docx

IntroductionHydroponic vegetable cultivation is characterized by high intensity and frequent nitrogen fertilizer application, which is related to greenhouse gas emissions, especially in the form of nitrous oxide (N2O). So far, there is little knowledge about the sources of N2O emissions from hydroponic systems, with the few studies indicating that denitrification could play a major role.MethodsHere, we use evidence from an experiment with tomato plants (Solanum lycopersicum) grown in a hydroponic greenhouse setup to further shed light into the process of N2O production based on the N2O isotopocule method and the 15N tracing approach. Gas samples from the headspace of rock wool substrate were collected prior to and after 15N labeling at two occasions using the closed chamber method and analyzed by gas chromatography and stable isotope ratio mass spectrometry.ResultsThe isotopocule analyses revealed that either heterotrophic bacterial denitrification (bD) or nitrifier denitrification (nD) was the major source of N2O emissions, when a typical nutrient solution with a low ammonium concentration (1–6 mg L−1) was applied. Furthermore, the isotopic shift in 15N site preference and in δ18O values indicated that approximately 80–90% of the N2O produced were already reduced to N2 by denitrifiers inside the rock wool substrate. Despite higher concentrations of ammonium present during the 15N labeling (30–60 mg L−1), results from the 15N tracing approach showed that N2O mainly originated from bD. Both, 15N label supplied in the form of ammonium and 15N label supplied in the form of nitrate, increased the 15N enrichment of N2O. This pointed to the contribution of other processes than bD. Nitrification activity was indicated by the conversion of small amounts of 15N-labeled ammonium into nitrate.Discussion/ConclusionComparing the results from N2O isotopocule analyses and the 15N tracing approach, likely a combination of bD, nD, and coupled nitrification and denitrification (cND) was responsible for the vast part of N2O emissions observed in this study. Overall, our findings help to better understand the processes underlying N2O and N2 emissions from hydroponic tomato cultivation, and thereby facilitate the development of targeted N2O mitigation measures.</p

Data from: Land use in mountain grasslands alters drought response and recovery of carbon allocation and plant-microbial interactions

1. Mountain grasslands have recently been exposed to substantial changes in land-use and climate and in the near future will likely face an increased frequency of extreme droughts. To date is not known how the drought responses of carbon (C) allocation, a key process in the C cycle, are affected by land-use changes in mountain grassland. 2. We performed an experimental summer drought on an abandoned grassland and a traditionally managed hay meadow and traced the fate of recent assimilates through the plant-soil continuum. We applied two 13CO2 pulses, at peak drought and in the recovery phase shortly after rewetting. 3. Drought decreased total C uptake in both grassland types and led to a loss of aboveground carbohydrate storage pools. The belowground C allocation to root sucrose was enhanced by drought, especially in the meadow, which also held larger root carbohydrate storage pools. 4. The microbial community of the abandoned grassland comprised more saprotrophic fungal and Gram (+) bacterial markers compared to the meadow. Drought increased the newly introduced AM and saprotrophic fungi:bacteria ratio in both grassland types. At peak drought the 13C transfer into AM fungi, saprotrophic fungi and Gram (-) bacteria was more strongly reduced in the meadow than in the abandoned grassland, which contrasted the patterns of the root carbohydrate pools. 5. In both grassland types the C allocation largely recovered after rewetting. Slowest recovery was found for AM fungi and their 13C uptake. In contrast, all bacterial markers quickly recovered C uptake. In the meadow, where plant nitrate uptake was enhanced after drought, C uptake was even higher than in control plots. 6. Synthesis. Our results suggest that resistance and resilience (i.e. recovery) of plant C dynamics and plant-microbial interactions are negatively related, i.e. high resistance is followed by slow recovery and vice versa. The abandoned grassland was more resistant to drought than the meadow and possibly had a stronger link to AM fungi that could have provided better access to water through the hyphal network. In contrast, meadow communities strongly reduced C allocation to storage and C transfer to the microbial community in the drought phase, but in the recovery phase invested C resources in the bacterial communities to gain more nutrients for regrowth. We conclude that management of mountain grasslands increases their resilience to drought

Functional composition has stronger impact than species richness on carbon gain and allocation in experimental grasslands

International audienc

Infective Endocarditis in Patients on Chronic Hemodialysis

International audienceInfective endocarditis (IE) is a common and serious complication in patients receiving chronic hemodialysis (HD)