SSS4.8

Woody plant encroachment is influencing many open, grassy ecosystems across the globe, such as savanna, tundra and temperate grassland ecosystems. Drivers of woody plant encroachment are local land use change and global climate change, with shifts in grazing and mowing regimes as important local drivers and elevated CO2 levels, higher temperature and altered precipitation amounts as global drivers. Encroachment of woody species into open, herbaceous ecosystems comes along with substantial shifts in soil conditions, a reduction light availability and ultimately vegetation shifts in the understorey towards species better adapted to the ambient conditions. While vegetation shifts in response to woody plant encroachment in grassy ecosystems have been frequently investigated, e.g. regarding altered plant composition and functional traits related to resource acquisition and dispersal, consequences for biotic interactions have been less studied.

The symbiosis of plant roots with mycorrhizal fungi is one of the most relevant biotic interaction for plants species, with over 90% of all plants forming mycorrhizal symbiosis and arbuscular mycorrhizal symbiosis as the most prominent mycorrhizal type among herbaceous species. Plants involved in the arbuscular mycorrhizal (AM) symbiosis trade photosynthetically derived carbon for fungal-provided soil nutrients. However, little is known about how plant light demand and ambient light conditions influence root-associating AM fungal communities, and thus their response to prominent climate change processes like shrub encroachment.

We conducted a manipulative field experiment to test whether plants’ shade tolerance influences their root AM fungal communities in open and shaded grassland sites. We found that light-dependent shifts in AM fungal community structure were similar for experimental bait plant roots and the surrounding soil. Yet, lower AM fungal beta and gamma diversity for shade-intolerant plants in shade likely reflected preferential carbon allocation to specific AM fungi due to the limited plant carbon available to support symbiotic fungi. We conclude that favourable environmental conditions, including optimal light availability, widen the plant biotic niche, i.e. selectivity for specific AM fungi is reduced, and compatibility with a larger number of AM fungal taxa is promoted. With respect to predicted stronger woody plant encroachment predicted under current climate change scenarios, these results indicate that we might be losing AM fungal diversity and the functions associated with these fungal taxa. This calls for continous investment into conservation efforts and management practices to counteract this trend and keep savanna, tundra and grassland ecosystems open. 

How to cite: Neuenkamp, L., Zobel, M., Koorem, K., Jairus, T., Davison, J., Öpik, M., Vasar, M., and Moora, M.: Light availability and light demand of plants shape the arbuscular mycorrhizal fungal communities in their roots, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12657, https://doi.org/10.5194/egusphere-egu21-12657, 2021.

More than one third of global soil organic matter (SOM) is stored in peatlands, despite them occupying less than 3% of the land surface. Increasing global temperatures have the potential to stimulate the decomposition of carbon stored in peatlands, contributing to the release of disproportionate amounts of greenhouse gases to the atmosphere but increasing atmospheric CO₂ concentrations may stimulate photosynthesis and return C into ecosystems.  Key questions remain about the magnitude and rate of these interacting and opposite processes to environmental change drivers.

We assessed the impact of a 0–9°C temperature gradient of deep peat warming (4 years of warming; 0-200 cm depth) in ambient or elevated CO₂ (2 years of +500 ppm CO₂ addition) on the quantity and quality of SOM at the climate change manipulation experiment SPRUCE (Spruce and Peatland Responses Under Changing Environments) in Minnesota USA. We assessed how warming and elevated CO₂ affect the degradation of plant and microbial residues as well as the incorporation of these compounds into SOM. Specifically, we combined the analyses of free extractable n-alkanes and fatty acids together with measurements of compound-specific stable carbon isotopes (δ¹³C).

We observed a 6‰ offset in δ¹³C between bulk SOM and n-alkanes, which were uniformly depleted in δ¹³C when compared to bulk organic matter. Such an offset between SOM and n-alkanes is common due to biosynthetic isotope fractionation processes and confirms previous findings. After 4 years of deep peat warming, and 2 years of elevated CO₂ addition a strong depth-specific response became visible with changes in SOM quantity and quality. In the upper 0-30 cm depth, individual n-alkanes and fatty acid concentrations declined with increasing temperatures with warming treatments, but not below 50 cm depth. In turn, the δ¹³C values of bulk organic matter and of individual n-alkanes and fatty acids increased in the upper 0-30 cm with increasing temperatures, but not below 50 cm depth. Thus n-alkanes, which typically turnover slower than bulk SOM, underwent a rapid transformation after a relatively short period of simulated warming in the acrotelm. Our results suggest that warming accelerated microbial decomposition of plant-derived lipids, leaving behind more degraded organic matter. The non-uniform, and depth dependent warming response implies that warming will have cascading effects on SOM decomposition in the acrotelm in peatlands. It remains to be seen how fast the catotelm will respond to rising temperatures and atmospheric CO₂ concentrations.

How to cite: Nicholas, O. O. E., Cyrill, Z. U., Emily, S. F., Paul, H. J., Guido, W. L. B., and Michael, S. W. I.: Warming and elevated CO2 promote incorporation of plant-derived lipids into soil organic matter in a spruce-dominated ombrotrophic bog, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1118, https://doi.org/10.5194/egusphere-egu21-1118, 2021.

Peatlands store about third of the terrestrial carbon (C) and exert long-term climate cooling. Dominant plant genera in acidic peatlands, Sphagnum mosses, are main contributors to net primary productivity. Through associative relationships with diverse microbial organisms (microbiome), Sphagnum mosses control major biogeochemical processes, namely uptake, storage and potential release of carbon and nitrogen. Climate warming is expected to negatively impact C accumulation in peatlands and alter nutrient cycling, however Sphagnum-dominated peatland resilience to climate warming may depend on Sphagnum-microbiome associations. The ability of the microbiome to rapidly acclimatize to warming may aid Sphagnum exposed to elevated temperatures through host-microbiome acquired thermotolerance. We investigated the role of the microbiome on Sphagnum’s ability to acclimate to elevated temperatures using a microbiome-transfer approach to test: a) whether the thermal origin of the microbiome influences acclimation of Sphagnum growth and b) if microbial benefits to Sphagnum growth depend on donor Sphagnum species.

            Using a full-factorial design, microbiomes were separated from Sphagnum “donor” species from four different peatlands across a wide range of thermal environments (11.4-27°C). The microbiomes were transferred onto germ-free “recipient” Sphagnum species in the laboratory and exposed to a range of experimental temperatures (8.5 – 26.5°C) for growth analysis over 4 weeks.

            Normalized growth rates were maximized for plants that received a microbiome from a matched “donor” and with a similar origin temperature (ΔT_{treatment-origin}: 0.3±0.9°C [±standard error], p = 0.73). For non-matched “donor-recipient” Sphagnum pairs, ΔT_{treatment-origin} was slightly negative with -4.1±2.1°C (p = 0.06). The largest growth rate of the “recipient” was measured when grown with a microbiome from a matching “donor” Sphagnum species and was 252% and 48% larger than the maximum growth rate of the germ-free Sphagnum and the non-matched “donor-recipient” Sphagnum pairs, respectively.

            Our results suggest that the composition of the Sphagnum microbiome plays a critical role in host plant temperature acclimation. We found that microbially-provided benefits to the host plant were most pronounced when: 1) the thermal origin of the microbiome is similar to experimental temperatures, and 2) when donor and recipient Sphagnum species are the same. Together, these results suggest that Sphagnum temperature acclimation can be modulated, in part, by microbial interactions and may potentially play a role in peatland resilience to climate warming.

How to cite: Živković, T., Carell, A. A., Granath, G., Nilsson, M. B., Helbig, M., Warshan, D., Klarenberg, I. J., Gilbert, D., Shaw, A. J., Kostka, J. E., and Weston, D. J.: Microbial community composition is linked to Sphagnum acclimation to warming, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13734, https://doi.org/10.5194/egusphere-egu21-13734, 2021.

Soils will warm in near synchrony with the air over the whole profiles following global climate change. It is largely unknown how subsoil (below 30 cm) microbial communities will respond to this warming and how plant-derived soil organic carbon (SOC) will be affected. Predictions how climate change will affect the large subsoil carbon pool (>50 % of SOC is below 30 cm soil depth) remain uncertain.

At Blodgett forest (California, USA) a field warming experiment was set up in 2013 warming whole soil profiles to 100 cm soil depth by +4°C compared to control plots. We took samples in 2018, after 4.5 years of continuous warming and investigated how warming has affected the abundance and community structure of microoganisms (using phospholipid fatty acids, PLFAs), and plant litter (using cutin and suberin).

The warmed subsoil (below 30 cm) contained significantly less microbial biomass (28%) compared to control plots, whereas the topsoil remained unchanged. Additionally below 50 cm, the microbial community was different in warmed as compared to control plots. Actinobacteria were relatively more abundant and Gram+ bacteria adapted their cell-membrane structure to warming. The decrease in microbial abundance might be related to lower SOC concentrations in warmed compared to control subsoils. In contrast to smaller SOC concentrations and less fine root mass in the warmed plots, the concentrations of the plant polymers suberin and cutin did not change. Overall our results demonstrate that already four seasons of simulated whole-soil warming caused distinct depth-specific responses of soil biogeochemistry: warming altered the subsoil microbial community, but not concentrations of plant-derived soil organic carbon.

How to cite: Zosso, C., Ofiti, N. O. E., Soong, J. L., Solly, E. F., Torn, M. S., Huguet, A., Wiesenberg, G. L. B., and Schmidt, M. W. I.: Whole-soil warming alters microbial community, but not concentrations of plant-derived soil organic carbon in subsoil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4921, https://doi.org/10.5194/egusphere-egu21-4921, 2021.

European forests are facing higher frequencies of extreme droughts, potentially impairing tree growth and ecosystem functioning. Drought limits the metabolic activity of plants and soil organisms, either directly or through reduced belowground carbon (C) allocation of recent assimilates, thus affecting C cycling in the plant-soil system. However, the net effect on belowground soil C storage is still unclear, as drought suppresses both C inputs from plants and outputs from soils. Moreover, understanding the underlying mechanisms is complicated due to long-term acclimation and adaptation of plant and soil organisms to water limitation.

We investigated the impact of repeated summer droughts in a Scots pine (Pinus sylvestris L.) forest on soil C storage and C cycling, taking advantage of a large-scale irrigation experiment  running since 2003 in a dry inner-Alpine valley in Switzerland (Pfynwald, Valais), which removed the “natural” water limitation. We assessed the responses of soil organic carbon (SOC) stocks and C fluxes by measuring litter fall and decomposition, fine root biomass and production, soil CO₂ effluxes, C-mineralization, and ¹³C-labelled glucose utilization by soil microorganisms. 

After 16 years of irrigation, the organic layers lost significant amounts of C (-1000 g m^-2), despite a 50% increase in litter fall. This C loss was almost compensated by a C gain in the mineral soil (+870 g m^-2) under irrigation. The decrease in C storage in the organic layers can be related to a three-fold increase in litter decomposition mainly through soil macrofauna as indicated by a litter-bag experiment. In parallel, the C gain in the mineral soil can be attributed mainly to increased incorporation of litter by soil fauna, together with greater C input from the rhizosphere (+70% fine root biomass for Scots pine in mineral soil). Furthermore, irrigation stimulated soil CO₂ efflux as well as microbial C-mineralization of organic and mineral soil, indicating enhanced soil C cycling. Addition of ¹³C-enriched glucose to mineral soils revealed a stronger utilization of this easily available C substrate in the drought than in the irrigated soils, together with a negative priming of soil organic matter (SOM) decomposition shortly after substrate addition. These results suggest that the altered quantity and quality of C inputs under irrigation has increased the availability of easily degradable C in soil.

This study reveals that long-term summer irrigation in a drought-prone pine forest has strong impacts on multiple interlinked processes of the soil C cycle. The removal of water limitation strongly altered vertical soil C distribution, accelerated soil C cycling and altered the substrate use by soil organisms, but had only a small net effect on the whole-profile SOC stocks.

How to cite: Guidi, C., Brunner, I., Imboden, J., Gavazov, K., Schaub, M., and Hagedorn, F.: Long-term irrigation in a drought-prone pine forest leads to vertical redistribution of C stocks and accelerates C cycling in the soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3380, https://doi.org/10.5194/egusphere-egu21-3380, 2021.

Plant and soil communities are intimately connected. Plants shape soil microbial community composition through their resource acquisition strategies and via root carbon (C) inputs, which has cascading effects on biogeochemical cycles. Drought has been shown to disrupt the connection between plants and soil microorganisms. However, the effects of drought intensity on soil microbial community functioning, including the uptake of recent plant-derived C, are largely unknown. Here, we determined how two plant communities with contrasting resource acquisition strategies (acquisitive versus conservative) responded to a gradient of drought (control, and eight drought intensities). Using a ¹³C pulse labelling approach, we tracked C allocated from plants to soil and its uptake by the microbial community. We measured potential extracellular enzyme activity as a proxy of microbial community functioning. We hypothesized that (1) drought responses are non-linear, and (2) acquisitive plant communities have lower drought resistance but recover faster than conservative plant communities, which is reflected in lower ¹³C transfer and reduced microbial functioning during drought and increases after drought. In general, we found that the responses we measured were non-linearly related to drought intensity. After three weeks of drought, drought intensity decreased aboveground net primary productivity (ANPP) of both plant communities. Soil extractable organic ¹³C decreased with increasing drought intensity, indicating that less recently assimilated C was allocated to root exudation. Although microbial biomass remained stable over the drought intensity gradient, ¹³C uptake into microbial biomass decreased at peak drought, and was lower in the conservative vs. acquisitive plant community at mild drought levels. Potential enzyme activity of β-1,4-glucosidase, involved in cellulose breakdown, and  β-N-acetyl-glucosaminidase, involved in chitin breakdown, decreased with increasing drought intensity. Urease activity was higher in conservative than acquisitive plant communities exposed to drought. Seven days after re-wetting, we found that microbial uptake of ¹³C increased along the drought gradient and was higher than the control in communities previously subjected to high drought intensities. This fast microbial recovery could affect nutrient mobilisation, which could underlie longer-term plant community recovery. Two months after re-wetting, we indeed found that plant communities that had previously experienced high drought intensity (> 75% soil water deficit) had higher ANPP than the control. We conclude that drought intensity has significant non-linear effects on microbial uptake of recent plant C and on potential extracellular enzyme activities both during drought and recovery, with consequences for plant community recovery dynamics.  

How to cite: Oram, N., Ingrisch, J., Gleixner, G., Praeg, N., Illmer, P., Brennan, F., Bardgett, R., and Bahn, M.: Drought intensity effects on grassland plant communities and soil microbial community function, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8870, https://doi.org/10.5194/egusphere-egu21-8870, 2021.

The raise of atmospheric CO₂ concentrations, with consequent increase in global warming and the likelihood of severe droughts, is altering the terrestrial biogeochemical carbon (C) cycle, with potential feedback to climate change.  Microbial physiology, i.e. growth, turnover and carbon use efficiency, control soil carbon fluxes to the atmosphere. Thus, improving our ability to accurately quantify microbial physiology, and how it is affected by climate change, is essential. Recent advances in the field have allowed the quantification of community-level microbial growth and carbon use efficiency in dry conditions via an ¹⁸O water vapor equilibration technique, allowing for the first time to evaluate microbial growth rates under drought conditions.

We modified the water vapor equilibration method using ²H-labelled water to estimate microbial community growth via deuterium incorporation into fatty acids. First, we verified that a rapid equilibration of ²H with soil water is possible. Then, we applied this approach to soil samples collected from a long-term climate change experiment (https://www.climgrass.at/) where warming, elevated atmospheric CO₂ (eCO₂) and drought are manipulated in a full factorial combination. Samples were taken in the field during peak drought and one week after rewetting. We used a high-throughput method to extract phospho- and neutral- lipid fatty acids (PLFA and NLFA) and we measured ²H enrichment in these compounds via GC-IRMS.

Our results show that within 48 h, ²H in water vapor was in equilibrium with soil water and was detectable in microbial PLFA and NLFAs. We were able to quantify growth rates for different groups of microorganisms (Gram-positive, Gram-negative, Fungi and Actinobacteria) and calculate community level carbon use efficiency. We showed that a reduction of carbon use efficiency in the combined warming + eCO₂ treatment was caused by a reduced growth of fungi and overall higher respiration rates. During drought, all groups showed a reduction in growth rates, albeit the reduction was stronger in bacteria than in fungi. Moreover, fungi accumulated high amounts of ²H into NLFAs, representing up to one third of the amount in PLFAs and indicating enhanced investment into storage compounds. This investment was still higher than in control plots two days after rewetting and returned to control levels within a week.

Our study demonstrates that climate change can have strong effects on microbial physiology, with group-specific responses to different climate change factors. Our approach has the benefit of using fatty acid biomarkers to improve resolution into community level growth responses to climate change. This allowed a quantification of group-specific growth rates and concomitantly a measurement of investment into reserve compounds.

How to cite: Canarini, A., Fuchslueger, L., Schnecker, J., Watzka, M., Pötsch, E. M., Schaumberger, A., Bahn, M., and Richter, A.: Deuterium stable isotope probing of fatty acids reveals climate change effects on soil microbial physiology., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6770, https://doi.org/10.5194/egusphere-egu21-6770, 2021.

Elucidating the mechanisms underlying the changes in microbial physiology under anthropogenic nitrogen (N) input is of fundamental importance for understanding the carbon-N interaction under global environmental change. Carbon use efficiency (CUE), the ratio of microbial growth to assimilation, represents a critical microbial metabolic parameter that controls the fate of soil C. Despite the recognized importance of mineral protection as a driver of soil C cycling in terrestrial ecosystems, little is known on how mineral-organic association will modulate the response of microbial CUE to increasing N availability. Here, by combining a 6-year N‐manipulation experiment and ¹⁸O isotope incubation, mineral analysis and a two-pool C decomposition model, we evaluate how N-induced modification in mineral protection affect the changes in microbial growth, respiration and CUE. Our results showed that microbial CUE increased under N enrichment due to the enhanced microbial growth and decreased respiration. Such changes in microbial physiology further led to a significant decrease in CO₂-C release from the slow C pool under high N input. More importantly, the disruption in mineral-organic association induced by elevated root exudates is the foremost reason for the enhanced microbial growth and CUE under high N input. Taken together, these findings provide an empirical evidence for the linkage between soil mineral protection and microbial physiology, and highlight the need to consider the plant-mineralogy-microbial interactions in Earth system models to improve the prediction of soil C fate under global N deposition.

How to cite: Feng, X., Hu, J., Yang, Y., and Chen, L.: Increasing microbial carbon use efficiency with nitrogen addition resulting from plant-mineral interaction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-845, https://doi.org/10.5194/egusphere-egu21-845, 2021.

Climate change predicts an increase in temperature and an intensification of the hydrological cycles resulting in more extreme drought and rainfall events. When dry soils experience a rainfall event, there is a big CO₂ release from soil to the atmosphere which is regulated by soil microorganisms. In the present study, we set out to investigate how drought and warming affects the soil microbial responses to drying and rewetting (DRW); and how those responses are affected by differences in land use. Previous work has shown that exposure DRW cycles in the laboratory and in the field can induce faster recovery (more ‘resilient’) of the microbial responses after a DRW cycle. In addition, a history of drought has been suggested to result in microbial communities with higher carbon use efficiency (CUE) during DRW in a wet heathland in Northern Europe and in semi-arid grasslands in Texas. We wanted to extend these observations to subtropical environments.

With the aim of simulating drought and warming, rain shelters and open top chambers (OTC) were installed in Northern Ethiopia in 2 contrasting land-uses (a degraded cropland and a pristine forest) for 1.5 years. Soils were then sampled and exposed to a DRW cycle in the laboratory. Microbial growth and respiration responses were followed with high temporal resolution over 3 weeks, as well as, changes in microbial community structure.  

Microbial functions universally showed a resilient response after a DRW cycle, with bacterial growth and fungal growth increasing immediately upon rewetting linked with the expected respiration response. The field treatments and land-use differences, therefore, did not have an effect on the resilience of soil microbial communities to DRW cycles. There were differences between the two main decomposer groups: fungi were more resilient than bacteria, as they showed a faster recovery rate. Microbial CUE upon rewetting responded differently in the different field treatments and land-uses. CUE was generally higher in croplands than in forests. Besides, while simulated drought reduced CUE, simulated drought increased CUE. These differences might be explained by either (i) the selection or more efficient microbial communities due to a higher exposure to DRW events or (ii) differences in resource availability (i.e. plant input).  

How to cite: Leizeaga, A., Hicks, L., Brangarí, A., Cruz-Paredes, C., Wondie, M., Sanden, H., and Rousk, J.: Effects of land use, short-term drought and warming on microbial responses to drying and rewetting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16543, https://doi.org/10.5194/egusphere-egu21-16543, 2021.