The increase in frequency and severity of droughts endangers ecosystem functioning worldwide, however, the mechanisms determining ecosystem susceptibility to drought and legacy effects during recovery remain poorly understood. To disentangle complex ecosystem dynamics we imposed a 9.5-week drought on the Biosphere 2 tropical rainforest, a thirty-year old enclosed forest. To trace ecosystem scale interactions, we implemented a whole-ecosystem labelling approach in the world’s largest controlled growth facility: the Biosphere 2 Tropical Rainforest,  the B2 Water, Atmosphere, and Life Dynamics (B2WALD) experiment. We measured the dynamics and processes across scales analyzing total ecosystem exchange, soil, trunk and leaf fluxes of H₂O, CO₂ and volatile organic compounds (VOCs), and their stable isotopes over five months. To trace changes in soil-plant-atmosphere interactions we labelled the entire ecosystem with a ¹³CO₂-isotope pulse during pre-drought and drought and traced the carbon flow from the leaves to stems, roots, and soil. Subsequently, we introduced ²H-labelled deep-water label during severe drought, providing a unique opportunity to evaluate the importance of deep-water reserves, transit times and legacy effects during the recovery of ecosystem functioning.

The tropical rainforest displayed highly dynamic, non-linear responses during dry-down and rewetting. Drought sequentially propagated through the vertical forest strata, with a rapid increase in vapor pressure deficit, the driving force of tree water loss, in the top canopy layer and early dry-down of the upper soil layer but delayed depletion of deep soil moisture. This induced a two-phase response of ecosystem fluxes: gross primary production (GPP), ecosystem respiration (Reco), and evapotranspiration (ET) declined rapidly during early drought and moderately under severe drought. Atmospheric VOC composition was highly dynamic, with peak emissions of isoprene during early drought followed by monoterpenes and hexanal during severe drought. Thus, the dynamics of different VOCs in the atmosphere closely mirrored different drought stages, and point to distinct physiological processes underlying stages of the total ecosystem response.

Ecosystem ¹³CO₂-pulse-labeling showed that drought enhanced the mean residence times of freshly assimilated carbon- indicating down-regulation of carbon cycling velocity and delayed transport form leaves to trunk and roots. During the recovery significant legacy effects were observed. Interestingly, the majority of the deep-rooted canopy trees taped into deep-water reserves, but exhibited large differences in transit times until maximum d²H-labelled water was transpired. Drought-sensitive canopy trees, which dominated the ecosystem water flux, responded swiftly reaching ²H-enriched transpiration within 1-21 days and maximum values 14-days after the ²H-pulse. In contrast, drought tolerant canopy trees transpired maximum ²H-labeled deep-water with a delay of 4-weeks. Understory trees and shrubs showed no or minimal ²H₂O uptake, indicating limited access to deep water.

We found highly diverse responses of carbon and water fluxes, driven by the interplay two key factors: species-specific drought adaptations and heterogeneity in microclimate conditions within the mixed forest. These data highlight the importance of quantifying drought impacts on forest functioning beyond the intensity of (meteorological) drought, but also taking the structural and functional composition of the forest into account, as interactive effects between biotic and abiotic factors determine how drought cascades through the system.

How to cite: Werner, C., Meredith, L., Ladd, N., Ingrisch, J., Kübbert, A., and van Harem, J. and the B2WALD: Diverse functional responses drive ecosystem drought impact and recovery - insights from an ecosystem-scale drought experiment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3455, https://doi.org/10.5194/egusphere-egu21-3455, 2021.

How to cite: Hafner, B. D., Brunn, M., Zwetsloot, M. J., Hikino, K., Pritsch, K., Weikl, F., Rühr, N. K., Sayer, E. J., and Bauerle, T. L.: Fine root exudation rate increases in drier soils, but tree level carbon exudation does not change under drought in mature Fagus sylvatica - Picea abies trees, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13426, https://doi.org/10.5194/egusphere-egu21-13426, 2021.

How to cite: Peltier, D., Carbone, M., Ebert, C., Xu, X., Adams, H., McDowell, N., Pockman, W., Richardson, A., and Trowbridge, A.: Quantifying whole tree non-structural carbon dynamics under long-term experimental drought using radiocarbon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3330, https://doi.org/10.5194/egusphere-egu21-3330, 2021.

This contribution presents the result of a free-air ¹³C labeling experiment on mature Norway spruce (P. abies [L.] KARST.) upon watering after five years of recurrent summer drought in southern Germany, focusing on whole tree allocation processes. Mature spruce trees had been exposed to recurrent summer drought from 2014 to 2018 through complete exclusion of precipitation throughfall from spring to late fall (i.e., March to November).  In early summer 2019, the drought stressed spruce trees were watered to investigate their recovery processes. In parallel with the watering, we conducted a whole-tree ¹³C labeling in canopies and traced the signal in various C sinks, i.e. stem phloem and CO₂ efflux, tree rings at different heights, coarse roots, fine root tips, mycorrhiza, root exudates, and soil CO₂ efflux.

We hypothesize that drought stressed spruce preferentially allocates newly assimilated C to belowground sinks upon drought release. Conversely to our expectations, allocation to belowground C sinks was not stimulated in drought stressed compared to control spruce. Likewise, the relative amount of recently fixed C allocated to aboveground sinks did not differ between treatments. Our findings suggest that the belowground C sinks are not of higher priority for the allocation of newly assimilated C upon watering after long-term drought. The observed allocation pattern is discussed taking total above- and belowground biomass as well as C source/sink relations into account.

How to cite: Hikino, K., Danzberger, J., Riedel, V., Hafner, B. D., Hesse, B. D., Rehschuh, R., Ruehr, N. K., Brunn, M., Lehmann, M., Weikl, F., Pritsch, K., and Grams, T. E. E.: Carbon allocation of mature spruce upon drought release – results from a whole-tree 13C-labeling study , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11173, https://doi.org/10.5194/egusphere-egu21-11173, 2021.

In ecosystems where nitrogen (N) limits plant productivity, N deposition can stimulate plant growth, and consequently, promote carbon (C) sequestration by increasing input of detrital C and other forms of plant C to the soil. However, other forms of nutrient limitation such as phosphorus (P) limitation and N-P co-limitation are widespread and may increase in prevalence with N deposition. Our understanding of how terrestrial ecosystem C, N and P cycling may be affected by N deposition when N is not the sole limiting resource is fairly limited. In this work, we investigate the consequences of enhanced N addition on C, N and P cycling in grasslands that exhibit contrasting forms of nutrient limitation.

We do so by collecting data from a long-term nutrient manipulation experiment on two N-P co-limited grasslands; an acidic grassland of stronger N-limitation and a calcareous grassland of stronger P limitation, and integrating this into a mechanistic C, N and P cycling model (N14CP). To simulate the experimental grasslands and explore the role of P access mechanisms in determining ecosystem state, we allowed P access to vary, and compared the outputs to plant-soil C, N and P data. Combinations of organic P access and inorganic P availability most closely representing data were used to simulate the grasslands and quantify their temporal response to nutrient manipulation.

The modelled grasslands showed contrasting responses to simulated N deposition. In the acidic grassland, N addition greatly increased C stocks by stimulating biomass productivity, but the same N treatments reduced the organic C pool in the calcareous grassland. Nitrogen deposition exacerbated P limitation in the calcareous grassland by reducing the size of the bioavailable P pool to plants, reducing biomass input to the soil C pool. Plant acquisition of organic P played an important role in determining the nutrient conditions of the grasslands, as both simulated grasslands increased organic P uptake to meet enhanced P demand driven by N deposition. Greater access to organic P in the acidic grassland prevented a shift to P limitation under elevated levels of N deposition, but organic P access was too low in the calcareous grassland to prevent worsening P limitation.

We conclude that grasslands of differing limiting nutrients may respond to N deposition in contrasting ways, and stress that as N deposition shifts ecosystems toward P limitation, a globally important carbon sink risks degradation.

How to cite: Taylor, C., Janes-Bassett, V., Phoenix, G., Keane, B., Hartley, I., and Davies, J.: Carbon storage in phosphorus limited grasslands may decline in response to elevated nitrogen deposition: a long term field manipulation and modelling study , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-33, https://doi.org/10.5194/egusphere-egu21-33, 2020.

Potassium (K) is essential for a wide range of physiological functions in plants, and a limiting element for wood productivity in numerous forest ecosystems. However, the contribution of each of the K-sensitive physiological processes to the limitation of wood productivity is poorly known. In trees, K deficiency acts both on the source and the sinks of carbon making it difficult to disentangle its effects on wood productivity. The literature dealing with the influence of K-limitation on tree physiologywhile disparate, shows some converging results. Furthermore, K-limited tropical Eucalyptus plantations have been studied extensively over the last 2 decades. Large scale fertilization experiments, run over multiple rotations, allow us to gain insight into the ecosystem’s K-cycle as a whole and the physiological processes that are impacted the most by K deficiency. Mechanistic modeling of this system should allow us to quantify the relative contribution of each process when it comes to wood productivity limitation by K. We have thus adapted an eco-physiological model (CASTANEA-CNP), previously used in temperate forest settings, to use in tropical eucalypt plantations. This has led us to adapt existing nutrient (N and P) eco-physiological modeling frameworks specifically for K as well as focus on processes that are little impacted by N and P availability but greatly by K availability. The biological K-cycle model was calibrated using the comprehensive experimental data. Carbon and water fluxes were calibrated using data from a flux tower site (Eucflux) with the same environmental conditions as the experimental plots. The development of a new canopy generation model was mandated by both the continuous nature of leaf generation in Eucalyptus grandis and the major interaction between leaf ontogeny and the K-cycle. At first we focus mainly on carbon assimilation at the canopy level. Here we present the preliminary results obtained by this model.

How to cite: Cornut, I., Delpierre, N., Le Maire, G., Guillemot, J., Nouvellon, Y., Campoe, O., and Laclau, J.-P.: Use eco-physiological modelling to investigate Potassium limitation of wood productivity in tropical eucalypt plantations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15742, https://doi.org/10.5194/egusphere-egu21-15742, 2021.

How to cite: Rouhiainen, J., Neukam, D., Dechow, R., Rabah Nasser, R., and Kage, H.: Multi-site Sensitivity analysis of DNDCv.Can model to predict N2O emissions from German croplands with maize, rapeseed and wheat , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16306, https://doi.org/10.5194/egusphere-egu21-16306, 2021.

Terrestrial nitrogen cycling plays an important role in regulating the export of nitrogen to surface waters. Terrestrial nitrogen transformations, however, are sensitive to changes in climate and disturbances, including altered temperature and precipitation patterns, snowpack depth, or wildfires. Thus, understanding how changing climate impacts terrestrial nitrogen cycling and the corresponding export of N to surface waters is an important part of understanding the impact of climate change on Earth’s freshwater resources. In this study, we investigate the short-term impacts of changing temperature, water partitioning and wildfire in the mountainous East River Watershed (ERW) located in the Upper Colorado River Basin. We use the High Altitude Nitrogen Suite of Models (HAN-SoMo), a course resolution semi-distributed ensemble of process based models developed for the ERW, to model changes in N through the vadose zone, surface waters and groundwater. HAN-SoMo was developed and calibrated against over 1600 N concentration measurements in the ERW to explore equifinality in parameter combinations. Three calibration scenarios were developed which explore the importance of different N sources and sinks throughout the watershed. Results of the most probable calibration suggest that the N cycling in the ERW is dominated by instream transformations and recycling of cow and plant matter. Additionally, results suggest that geogenic N from weathering of Mancos shale accounts for about 12% of N sources in the watershed. Here we will use the most probable HAN-SoMo calibration to explore possible changes in nitrogen cycling (key sources and sinks, N fluxes, etc.) under perturbations in temperature, water sources and wildfire.

How to cite: Weierbach, H., Maavara, T., Woodburn, E., and Bouskill, N.: Modelling the Impact of Future Climate on Mountainous Watershed Nitrogen Cycling , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12505, https://doi.org/10.5194/egusphere-egu21-12505, 2021.

Boreal forests are among the most carbon (C) rich forest types in the world and store up to 80% of its total C in the soil. Forest soil C development under climate change has received increased scientific attention yet large uncertainties remain, not least in terms of magnitude and direction of soil C responses. As with climate change, large uncertainties remain in terms of the effects of forest management on soil C sequestration and storage. Nonetheless, it is clear that forest management measures can have far reaching effects on ecosystem functioning and soil conditions. For example, clear cutting is a widely undertaken felling method in Scandinavia which profoundly affects the forest ecosystem and its functioning, including the soil. Nitrogen (N) fertilization is another common practice in Scandinavia which, despite uncertainties regarding effects on soil C dynamics, is being promoted as a climate change mitigation tool. A more novel practice of biochar addition to soils has been shown to have positive effects on soil conditions, including soil C storage, but studies on biochar in the context of forests are few.

In the face of climate change, the ForBioFunCtioN project is dedicated to investigating the response of boreal forest soil CO₂ and CH₄ fluxes to experimentally increased temperatures and increased precipitation – climatic changes in line with projections over Norway – within a forest management context. The experiment is set in a Norwegian spruce-dominated bilberry chronosequence, including a clear-cut site, a middle-aged thinned stand, a mature stand and an old unmanaged stand. Warming, simulated increased precipitation, N fertilizer and biochar additions will be applied on experimental plots in an additive manner that allows for disentangling the effects of individual parameters from interaction effects. Flux measurements will be undertaken at high temporal resolution using the state-of-the-art LI-7810 Trace Gas Analyzer (©LI-COR Biosciences). The presentation will show the experimental setup and first measurements from the large-scale experiment.

How to cite: Johannesson, C.-F., Larsen, K. S., Malicki, B., and Nordén, J.: ForBioFunCtioN: Forest soil carbon and the effects of climate change and forest management, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5553, https://doi.org/10.5194/egusphere-egu21-5553, 2021.

Phosphorus (P) is a macro nutrient affecting the terrestrial ecosystem productivity and future carbon (C) balance, and is known as the main constraint for C sequestration in tropical ecosystems, such as Amazon rainforests and Eucalyptus forests. However, in temperate and boreal forests, nitrogen (N) is usually considered as the main limiting nutrient for plant growth and the role of P cycling is less highlighted. Through a comprehensive investigation of soil and biological properties of five European beech forest sites along a wide range of natural soil P stocks, Lang et al. (2017) have concluded that P availability can shape plant-microorganism-soil interactions in forest ecosystem and the P cycling strategy switches from an “acquiring strategy” to a “recycling strategy” as the soil P availability decreases. The switch from a P-rich “acquiring strategy” to a P-poor “recycling strategy” is significantly concurrent with decreasing forest floor turnover rates (from 1/5 to 1/40 per year), increasing organic C to P ratios (from 110 to 984; A horizons), and increasing proportions of fine-root biomass in the forest floor (from 10% to 80%), as well as some less significant ecosystem properties, such as the total soil organic C and N, resorption of leaf P before senescence, and P concentrations in leaf litter and fine roots. However, none of the vegetative traits (foliar N and P contents, tree height, basal area, and tree volume) changed systematically along the soil P gradient, indicating that the shift in soil P cycling strategies allows for satisfying plant P demand.

This observational evidence imposes great challenges on the modeling of forest ecosystems, especially on the modeling of soil processes and plant-soil interactions. In conventional models, several of these features cannot be reproduced, due to 1) fixed first-order kinetics of decomposition, 2) lack of mechanisms for soil C storage/stabilization, 3) lack of explicit microbial dynamics and processes, 4) fixed stoichiometry in plant and soil, 5) lack of dynamic plant C allocation to roots and root exudation, 6) ignorance of C cost for nutrient mobilization.

In this presentation, we would like to propose modeling solutions to these challenges based on a novel modeling framework of QUINCY (Thum et al. 2019) and JSM (Yu et al. 2020). The new model allows separation of the sink (photosynthesis) and source (nutrients and water availabilities) in plant growth and applies a dynamic C allocation to maximize assimilation of limiting resources. It is vertical- and microbial-explicit in the soil processes, and mechanistically describes the effects of microbial dynamics on soil C decomposition and stabilization as well as nutrient mobilization. The preliminary results have shown increasing C investments from plant-microbe to soil P mobilization with decreasing soil P availability. It allows us to reproduce the observed patterns in litter turnover, root allocation, soil C storage,  and soil stoichiometry, indicating the important role of C-nutrient interactions in the P cycling modeling. We hope with the advances of modeling P cycling strategies in temperate forest ecosystems, we could also better model P cycling in the tropics and better project future C sequestration globally.

How to cite: Yu, L., Akselsson, C., Caldararu, S., Fleischer, K., Schrumf, M., and Zaehle, S.: Understanding the Phosphorus Cycling Strategies of Beech Forest Ecosystems along a Natural Soil Phosphorus Gradient: Observational Evidence and Modeling Challenges, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11834, https://doi.org/10.5194/egusphere-egu21-11834, 2021.

Climate effects are projected to be largest in Arctic and Sub-Arctic regions of the world with these areas being affected fourfold when compared to other areas around the globe. Because these regions are so susceptible to the changing climate, research into how these ecosystems will be affected by rising temperatures is essential. A large majority of current research points to a mass greening of the northern latitudes with higher temperatures, leading to an enhanced uptake of CO₂. While this is already documented, these ecosystems also possess a significant soil organic carbon pool with a high CO₂ emission potential. The net climate feedback of Arctic ecosystems therefore remain highly uncertain. Utilizing high-frequency measurements of ecosystem-level carbon exchange in these regions could unearth a valuable understanding of just how rising temperatures will affect the soil-plant continuum in varying future climate scenarios.

For the newly established FutureArctic project, located at ForHot in Iceland, we installed four ECO₂flux automated chambers at one of the naturally heated grasslands sites in July of 2020. The automatic chambers were placed at different locations along a soil temperature gradient with treatments covering an average of 0, 3.5, 7.5, and 12.5 degree warming above the ambient temperature. The major aim is to investigate the underlying carbon balance processes in order to ascertain a better insight into how future climate change induced temperature increases could affect such ecosystems. We hypothesize that the detailed analysis of carbon uptake (gross primary production, GPP) and carbon release (ecosystem respiration, R_E) along the temperature gradient will expose a latent change from net carbon sink to net carbon source as temperatures increase.

Preliminary analysis for the first nine months was conducted. The fluxes of CO₂ showed evident heterogeneity between the conditions with a greater overall increase of R_E than GPP with increasing temperature. During the growing season GPP was highest in the third treatment with warming between +5 to 10 degrees, which supports the Arctic greening hypothesis. At the highest temperature treatment, GPP was much lower than in the ambient treatment indicating a drop off in ecosystem productivity and, quite possibly, a temperature threshold for this ecosystem. The observed temperature response appears non-linear with a threshold of about +10°C where both GPP and R_E decrease. Knowledge of these non-linear temperature responses for GPP and R_E will be of great importance when trying to predict future changes to the carbon balance in Arctic and Sub-Arctic ecosystems.

How to cite: Avila, L. M., Steenberg Larsen, K., Sigurdsson, B. D., and Sigurdsson, P.: In situ gas-exchange: automated, light-dark measurements of CO2 fluxes on a geothermal temperature gradient in a Sub-Arctic grassland ecosystem , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11818, https://doi.org/10.5194/egusphere-egu21-11818, 2021.

N limited ecosystems worldwide are shifting towards other limitations due to anthropogenic N deposition exceeding the rate of change in other drivers. Mediterranean ecosystems are among those thought to be shifting to P limitation and ‘N:P imbalances’ but this occurs alongside strong environmental limits on C uptake in both winter and summer in a distinct bimodal phenological cycle. At the ecosystem scale, carbon cycle components such as gross and net primary productivity, respiration or ecosystem carbon use efficiency, are a result of accumulation of physiological and community effects in both plants and soils. The functioning of such systems is commonly measured at plot or individual level, but scalable, inter-site comparisons are complicated because multiple interacting gradients of driving factors, weather and seasonal timing do not correlate through geographic space. 

Here, we explore the interaction of seasonal conditions and nutrient treatment on eddy-covariance derived C cycle properties in a ecosystem-scale ‘stoichiometry manipulation experiment in a Mediterranean tree-grass system (5 years after a large, one off fertilization, 'MANIP experiment') where the close location means sites can be compared with similar local weather and seasonality. 

During the 5 year study period, N:P ratios in soil pools regress towards pretreatment conditions in both N and NP treated footprints, and generally plant level effects decline through time. Nonetheless, holistic C uptake differences between nutrient treatments (e.g. in GPP) remain for the entire 5 year study period. Effects are particularly strong in spring (late in the phenological year) where the accumulation of biomass magnifies treatment differences and water is not limiting.  The effects of increased nutrient availability and altered N:P stoichiometry are also visible on resource use in winter, a relatively fallow period rarely studied in non-continuous measurements. Treatment effects in these winter periods differ from effects in spring, but both contribute to overall ecosystem-level responses to the nutrient treatments. The effect of nutrient balance on carbon cycling and carbon use efficiency is seasonally driven as fertilized treatments have both higher ecosystem-scale carbon use efficiency and light use efficiency, including in winter. However the fertilized treatments are also relatively less affected by years with warmer winter temperatures than the unfertilized treatments. N:P imbalanced treatments are also more water sensitive than those with balanced stoichiometry. Physiological and community change response studies in such systems must consider both year round and multi-year representation for scalable and transferable understanding. 

How to cite: Nair, R., El-Madany, T., Carrara, A., Luo, Y., Kolle, O., Moreno, G., Reichstein, M., Schrumpf, M., Wutzler, T., and Migliavacca, M.: N:P imbalance effects on the seasonal C cycle in a Mediterranean Tree-Grass Ecosystem, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3154, https://doi.org/10.5194/egusphere-egu21-3154, 2021.

The feedback of the terrestrial carbon cycle to global climate change is among the largest uncertainties in climate change research. To test the potential ecosystem effects of future climate scenarios, a field-scale FACE (Free Air CO₂ Enrichment) experiment combined with increased temperatures and extended summer drought was performed in the period 2005–2013 on a temperate heathland/grassland ecosystem in Denmark (the CLIMAITE project). A major finding from the original experiment was that the soil carbon pool increased by approximately 20% under elevated CO₂ over the 8 years of the study*.

The FACE treatment was in effect also an in situ labeling experiment because the added CO₂ was depleted for ¹³C （¹³CO_2FACE=-29‰）compared to ambient atmospheric CO₂（¹³CO_2AIR=-8‰）. Therefore, the isotopic signal of the remaining soil carbon can be used to investigate the turnover of soil carbon during the time since the end of the original study.

During the growing season in 2020, seven years after the CO₂ fumigation experiment was terminated, soil samples were extracted in all plots using the same sampling strategy as in previous samplings. Interestingly, the direct soil C pool measurements showed that the extra soil carbon, which was stored during the eight years with elevated CO₂ had been lost again over the course of the following seven years. The isotopic composition of the different soil layers had also changed back towards the values measured in control plots, although still being slightly more depleted for ¹³C. Still, the convergence of the isotopic composition in the different treatments confirms the trend observed from the direct C pool measurements and also hints that a part of the more recalcitrant carbon taken up during the elevated CO2 experiment is still there while most of the labile/less recalcitrant carbon has been decomposed and reemitted to the atmosphere. The results show that the soil carbon pool in the ecosystem is extremely dynamic and may change fast in response to changes in major ecosystem drivers, and in particular is highly sensitive to the atmospheric CO₂ concentration.

*Dietzen CA, Larsen KS, Ambus P, Michelsen A, Arndal MF, Beier C, Reinsch S, Schmidt IK (2019) Accumulation of soil carbon under elevated CO₂ unaffected by warming and drought. Global Change Biology, 25: 2970–2977. doi: 10.1111/gcb.14699.

How to cite: Li, Q., Larsen, K. S., and Gundersen, P.: Re-visiting a long-term Free Air CO2 Enrichment (FACE) experiment in a Danish heathland/grassland ecosystem (CLIMAITE) reveals highly dynamic soil carbon , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12059, https://doi.org/10.5194/egusphere-egu21-12059, 2021.

Contextualization: In 2011, it was published a curious conundrum, which forms the basis of the present study: why, when organic matter is thermodynamically unstable, does it persist in soils, sometimes for thousands of years? The question challenges the idea that the recalcitrant or labile character of soil organic matter (SOM) is a sufficient argument to ensure SOM persistence. Temperature could play an important role in SOM decomposition, especially in tropics. Particularly, tropical dry forest (TDF) represents an important ecosystem with unique biodiversity and fertile soils in Colombia. At present, the increase in population density and consequently, in the demands of energy and arable land, have led to its degradation.

Knowledge gap: Although the mentioned question was formulated several years ago, it has still to be answered, hence limiting the development of new soil organic carbon (SOC) models or the quantification of its ecosystem services. A key point, in terms of soil carbon storage, is to determine the maximum rate of CO₂ emissions from soils (Rmax). Traditionally, it is considered that Rmax occurs at the 50% of field capacity. Unfortunately, information about the environmental conditions under which this maximum occurs is scarce.

Purpose: The main objectives of this study were: (a) determine the maximum rate of soil respiration or CO₂ emissions from soil in TDF soils and (b) to estimate the main environmental drivers of maximum SOM decomposition along a temperature gradient (20°, 30°, 40°C) in incubated soils.

Methodology: Soils pertained to permanent plots were sampled in six different TDF of Colombia. The evolution of CO₂ emissions (monitored by an infrared gas analyser), relative humidity and soil temperature were recorded in time on incubated soils samples. Temperature was maintained constant at 20°C, 30°C and 40°C during soil incubations under soil drying conditions. Additionally, elemental composition (Fe, Ca, O, Al, Si, K, Mg, Na) of SOM and chemical composition of soil organic carbon (SOC: aromatic-C, O-alkyl-C, Aliphatic-C, Phenolic and Ketonic-C) were determined by X-ray photoelectron spectroscopy (XPS).

Results and conclusions: The majority of TDF soil samples (90.7%) presented that its peak of CO₂ emissions occurs at soil-water contents higher than saturation (0 MPa), at 20°, 30° and 40°C. Clearly, to consider that the maximum soil respiration rate could be observed at the 50% of field capacity, underestimated the real maximum value of carbon mineralization (48-68%.) Globally, increases in the Rmax values corresponded to increases in electrical conductivity, soil desorption rates, total carbon and nitrogen contents, and decreases in bulk density (BD) and aggregate stability. Taking into account the temperature gradient, increments in calcium and aromatic carbon contents corresponded to decrements in Rmax values but only at 30°C and 40°C, respectively. Some authors indicated that at high soil moisture contents, iron reduction could be release protected carbon. However, no significant relation between Fe and Rmax was observed. Consequently, physical and chemical properties related to SOM accessibility and decomposability by microbial activity, were the main drivers and controls of maximum SOM decomposition rates.

How to cite: Ojeda, G., García, H., Woche, S., Bachmann, J., Guggenberger, G., Pizano, C., Ceballos, F., and Sanchéz, M.: Maximum soil organic matter decomposition along temperature gradient in Colombian topsoils: Dry Forests, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5938, https://doi.org/10.5194/egusphere-egu21-5938, 2021.

Global climate change scenarios predict increasing air temperature, enhanced precipitation and air humidity for Northern latitudes. We investigated the effects of elevated air relative humidity (RH) and different inorganic nitrogen sources (NO₃^-, NH₄⁺) on above- and belowground traits in different tree species, with particular emphasis on rhizodeposition rates. Silver birch, hybrid aspen and Scots pine saplings were grown in PERCIVAL growth chambers with stabile temperature, light intensity and two different air humidity conditions: moderate (mRH, 65% at day and 80% at night) and elevated (eRH, 80% at day and night). The collection of fine root exudates was conducted by a culture-based cuvette method and total organic carbon content was determined by Vario TOC analyser. Fine root respiration was measured with an infra-red gas analyser CIRAS 2.  

We analysed species-specific biomass allocation, water and rhizodeposition fluxes, foliar and fine root traits in response to changing environmental conditions. The eRH significantly decreased the transpiration flux in all species. In birch the transpiration flux was also affected by the nitrogen source. The average carbon exudation rate for aspen, birch and pine varied from 2 to 3  μg C g^-1 day ^-1. The exudation rates for deciduous tree species tended to increase at eRH, while conversely decreased for coniferous trees (p=0.045), coinciding with the changes in biomass allocation. C flux released by fine root respiration varied more than the fine root exudation, whereas the highest root respiration was found in silver birch and lowest in aspen. At eRH the above and belowground biomass ratio in aspen increased, at the expense of decreased root biomass and root respiration.  

Moreover, eRH significantly affected fine root morphology, whereas the response of specific root area was reverse for deciduous and coniferous tree species. However, fine roots with lower root tissue density had higher C exudation rate. Our findings underline the importance of considering species-specific differences by elucidating tree’s acclimation to environmental factors and their interactions.   

How to cite: Sell, M., Ostonen, I., Rohula-Okunev, G., Rezapour, A., and Kupper, P.: Carbon allocation in early successional tree species at elevated air humidity and different soil nitrogen sources, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8217, https://doi.org/10.5194/egusphere-egu21-8217, 2021.

Terrestrial climate-carbon feedbacks are the leading-order uncertainties in climate projections, hindering the full assessment of climate mitigation scenarios. Since year-to-year variations of atmospheric carbon dioxide growth rate (CGR) are mostly driven by fluctuations of tropical land carbon fluxes, it provides a “natural experiment” to explore the climate drivers of terrestrial carbon cycle. Recently, direct observations of terrestrial water storage confirmed the tight coupling between the water and carbon cycles, in addition to the well-documented temperature effects. Here we show that the strength of this relationship between CGR and the interannual variability of tropical water has increased substantially from 1960 to 2018 and has even recently become stronger than CGR-temperature correlations. We find that this increment may be relevant to local drying trends in a warming climate and that above-ground carbon uptake might be a critical underlying ecological process. We also demonstrate that most state-of-the-art Earth System models and land surface models do not capture this increasing carbon-water coupling over time. Our results suggest that tropical water availability could increasingly dominates the interannual variability of the terrestrial carbon cycle in the future and that current models may not be able to capture this feature.

How to cite: Liu, L. and Seneviratne, S.: Increasing tropical water control on interannual CO2 growth rate over the past decades, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16179, https://doi.org/10.5194/egusphere-egu21-16179, 2021.

Variability in the photosynthetic uptake of CO₂ by plants due to climate variability plays an essential role in modulating the growth rate of atmospheric CO₂. In particular water stress induced by the compounding effect of vapor pressure deficit (VPD) and soil moisture anomalies has a large bearing on photosynthetic CO₂ uptake, especially in semi-arid areas. The ongoing rise in atmospheric CO₂ concentration can influence the water-use efficiency of plants, their carbon assimilation rate, and consequently the global cycles of carbon and water.  However, the extent to which physiological effects of increasing CO₂ influence the coupling of VPD, soil moisture, and land-atmosphere CO₂ fluxes is currently poorly understood.

In this study we use the terrestrial ecosystem model QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system, Thum et al. 2019, GMD) to study CO₂-induced changes in the interaction of plant productivity and both soil moisture and VPD. With the sensitivity of stomatal conductance to atmospheric CO₂ concentration implemented in the model, we investigate century-long simulations across different climate regimes and biomes. In semi-arid regions we find a positive relationship between anomalies of VPD and gross primary productivity (GPP) at both start and end of summer months, and a negative relationship in the dry period (usually from June to August in boreal summer). This suggests there is a transition in the limiting factor of GPP from energy (compound effect of temperature and radiation) to water and back in the course of one year. The negative correlation between VPD and GPP during the dry period weakens over time with rising CO₂, while a stronger positive correlation between soil moisture and GPP becomes apparent. After quantifying and understanding the CO₂ effects in these model simulations, we apply our analysis framework to observational data from the FLUXNET site collection to analyze whether we can confirm the model-based findings despite shorter records.

How to cite: Zhan, C., Orth, R., Reichstein, M., Migliavacca, M., Zaehle, S., and Winkler, A.: How does increasing CO2 influence the land-atmosphere exchange of carbon and water in response to soil and air dryness?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13494, https://doi.org/10.5194/egusphere-egu21-13494, 2021.

Increasing atmospheric CO2 levels, temperature, and atmospheric drought as well as changes in rainfall structure (event frequency, storm intensity) are expected to jointly alter ecosystem responses in the future. High-resolution convection-permitting models have been recently employed at continental scales to develop robust projections of climate, and particularly precipitation, at fine spatiotemporal scales (~1km, ~1hour) overcoming the documented limitations of larger scale general circulation models.

Climate projections at fine spatiotemporal scale can support robust quantification of ecosystem responses under a changing climate when used in conjunction with state-of-the-art terrestrial biosphere models resolving the soil – vegetation – atmosphere continuum processes at those scales. In this study, we assess the changes in ecosystem functioning for multiple biomes in North America. We use the 4km, 1-h future WRF continental-wide simulation over the US together with a state-of-the-art stochastic weather generator and the Tethys & Chloris ecohydrological model to investigate ecosystem responses for 33 sites where eddy covariance data exist (i.e., FLUXNET sites).

We designed a series of numerical experiments tuned to disentangle the roles of CO2, temperature and the structure of precipitation, while considering the effect of natural weather variability.

Our results reveal that the impact of mean annual rainfall is dominant in more arid sites, while sites of intermediate wetness are more sensitive to the temporal structure of precipitation at fine scales. Wet sites, which are energy limited, are more sensitive to temperature increase instead. The impact of rainfall is partly offset by increases in atmospheric drought. The fertilization effect of elevated CO2 levels is strong in this high-end (RCP 8.5) scenario across all sites. Fertilization is more pronounced for sites of low and intermediate wetness, where stomatal closure allows for positive feedbacks through water savings.

How to cite: Moustakis, Y., Fatichi, S., Onof, C. J., and Paschalis, A.: Assessing the impact of rainfall intensification on ecosystem productivity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11391, https://doi.org/10.5194/egusphere-egu21-11391, 2021.

There are large uncertainties in the estimation of greenhouse-gas feedbacks: model-based estimates vary considerably; recent observations are too short provide strong constraints. Rapid climate changes during the last glacial period (Dansgaard-Oeschger, D-O, events) are potentially valuable because they are comparable in rate and magnitude to projected future climate warming, and are registered near-globally. Here we use D-O events to quantify the centennial-scale feedback strength of feedbacks involving CO₂, CH₄ and N₂O. We use climate model simulations of the D-O events to estimate the relationship between global mean and Greenland temperature. We then relate global mean temperature changes to changes in greenhouse-gas concentrations derived from ice-core records, and then estimate the associated radiative forcing. We found the magnitude of the feedbacks (expressed in gain, with 95 % confidence interval) to be 0.07 ± 0.02 for CO_2, 0.04 ± 0.01 for CH₄, 0.04 ± 0.01 for N₂O. These estimates are more constrained than previous model-based estimates but comparable to estimates based on recent observations.

How to cite: Liu, M., Menviel, L., Prentice, I. C., and Harrison, S. P.: Greenhouse-gas feedbacks estimated from Dansgaard-Oeschger events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3660, https://doi.org/10.5194/egusphere-egu21-3660, 2021.

In a young Norway spruce stand (planted in 2012) at Hoxmark, Southeast Norway, Net Ecosystem Exchange (NEE) was measured using Eddy Covariance. The data were carefully processed with time-dependent stand parameters (i.e. canopy height), a detailed footprint analysis and calculated at 30 min temporal resolution. Photosynthetic Active Radiation (PAR) as the primary driver for carbon uptake was also available at the site.

Despite its young age, the plantation already acted as a net carbon sink according to the annual NEE budget, e.g. by ca. 300 g C m^-2 in 2019. However, the response of the system depended strongly on hydrometeorological conditions. We demonstrate this by investigating the relationship between NEE and PAR for this system in a temporally local fashion (30 days moving windows), using a Michaelis-Menten approach involving three parameters. Although the regression captured up to ca. 80% of the variance, the parameter estimates differed substantially throughout the season, and were contrasting between the very dry year 2018 and the close to normal year 2019.

Comparison with other EC-equipped sites in a future study will clarify whether this variable sensitivity is due to the young age or is a pattern pertaining also to mature spruce stands.

How to cite: Lange, H., Zhao, J., and Bethke, R.: The NEE-PAR relationship for a young spruce plantation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5028, https://doi.org/10.5194/egusphere-egu21-5028, 2021.