Human activities are altering a range of environmental conditions, including atmospheric CO2 concentration, climate, and nutrient inputs. However, understanding and predicting their combined impacts on ecosystem structure and functioning and biogeochemical cycles is challenging. Divergent future projections of terrestrial ecosystem models reveal uncertainties about fundamental processes and missing observational constraints. Models are routinely tested and calibrated against data from ecosystem flux measurements, remote sensing, atmospheric inversions and ecosystem inventories. These model projections constrain the current mean state of the terrestrial biosphere, but they provide limited information on the sensitivity of ecophysiological, biogeochemical, and hydrological processes to environmental changes. Observational and ecosystem manipulation studies (e.g., Free-Air Carbon Dioxide Enrichment (FACE), nutrient addition or warming experiments) can complement modelling studies with unique insights and inform model development and evaluation. This session focuses on how ecosystem processes respond to changes in CO2 concentration, warming, altered precipitation patterns, water and nutrient availability. It aims at fostering the interaction between the experimental and modelling communities by advancing the use of observational and experimental data for model evaluation and calibration. We encourage contributions from syntheses of multiple experiments, model intercomparisons and evaluations against ecosystem manipulation experiments, pre-experimental modelling, or the use of observations from "natural experiments". Contributions may span a range of scales and scopes, including plant ecophysiology, soil organic matter dynamics, soil microbial activity, nutrient cycling, plant-soil interactions, or ecosystem dynamics.

Convener: Benjamin StockerECSECS | Co-conveners: Teresa GimenoECSECS, Karin Rebel, Sönke Zaehle
| Attendance Tue, 05 May, 08:30–10:15 (CEST)

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Chat time: Tuesday, 5 May 2020, 08:30–10:15

D748 |
| solicited
Katrin Fleischer, Carlos Alberto Quesada, David Lapola, Lucia Fuchslueger, Laynara Lugli, Tatiana Reichert, and Anja Rammig

The Amazon rainforest faces immense pressures from human-induced deforestation and climate change and its future existence is largely indeterminate. Accurately projecting the forest’s response to future conditions, and thus preparing for the best possible outcome, requires a sound process-based understanding of its ecological and biogeochemical functioning. The intact forest acts as a sink of atmospheric carbon dioxide (CO2), however, this invaluable function is slowing down for unclear reasons, according to long-term plot measurements of tree growth. Earth system models, on the other hand, assume a continuous sink of carbon into the 21st century, predominantly driven by CO2 fertilization, concurrently buffering against adverse effects by climate change. Advancing empirical and experimental evidence points to strong nutrient constraints on the Amazon carbon sink, foremostly by phosphorus and other cations, so that the projected strength of the future carbon sink is certainly unrealistic. It is highly uncertain, however, to which degree nutrients are and will diminish elevated CO2-induced productivity, and to which extent plant-based mechanisms may upregulate phosphorus supply or optimize phosphorus use to facilitate the increasing demand by elevated CO2. Site-scale ecosystem model ensemble analysis underscores the diverging hypotheses on phosphorus feedbacks we are currently facing. In addition, heterogeneous soil phosphorus availability across the Amazon basin, in combination with a hyperdiverse plant community, challenges current efforts to project phosphorus constraints on the future of the Amazon carbon sink. We here give an outlook of current progress and future research needs of model-experiment integration to tackle this pressing question.

How to cite: Fleischer, K., Quesada, C. A., Lapola, D., Fuchslueger, L., Lugli, L., Reichert, T., and Rammig, A.: Nutrient constraints on the Amazon carbon sink: from field measurements to model projections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8023, https://doi.org/10.5194/egusphere-egu2020-8023, 2020.

D749 |
Florian Hofhansl, Werner Huber, Anton Weissenhofer, Wolfgang Wanek, and Oskar Franklin

Currently applied dynamic vegetation models do not realistically represent forest ecosystem processes and thus are not able to reproduce in-situ observations of forest ecosystem responses to drought. This is due to the fact that models typically rely on plant functional types to forecast the functional response of vegetation to climate change and to anthropogenic disturbance. However, recent observations of divergent ecosystem responses between topographic forest sites, differing in the availability of water and nutrients, indicate that we should no longer rely on this outdated concept but rather should explore new avenues of representing vegetation dynamics and associated climate change response in next-generation approaches.

Global climate change scenarios forecast increasing severity of climate extremes in association with El Niño–Southern Oscillation (ENSO). Such climate anomalies have been shown to affect forest ecosystem processes such as net primary productivity, which is determined by climate (precipitation, temperature, and light) and soil fertility (geology and topography). However, more recently it has been suggested that the impact of such climate fluctuations on forest productivity was strongly related to local site characteristics, which determined the sensitivity of forest ecosystem processes to climate anomalies.

We propose a novel approach integrating in-situ observations with remotely sensed estimates of forest aboveground productivity for parameterization of next-generation vegetation models capable of forecasting realistic forest ecosystem responses under future scenarios. Our approach considers local site characteristics associated with topography and disturbance history, both of which determine the sensitivity of forest aboveground productivity to projected climate anomalies. Our results therefore should have crucial implications for management and restoration of forest ecosystems and could be used to refine estimates of forest C sink-strength under future scenarios.

How to cite: Hofhansl, F., Huber, W., Weissenhofer, A., Wanek, W., and Franklin, O.: Local-scale and spatially explicit response of tropical forests to climate change, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10396, https://doi.org/10.5194/egusphere-egu2020-10396, 2020.

D750 |
Raphael Manu, Marife D. Corre, Edzo Veldkamp, and Oliver van Straaten

Nutrient availability in tropical forest ecosystems plays a critical role in sustaining forest growth and productivity. Observational evidence for nutrient limitations on net primary productivity (NPP) in the tropics is rare yet crucial for predicting the impacts of human-induced changes on tropical forests, particularly for underrepresented tropical regions in Africa. In an ecosystem-scale nutrient manipulation experiment, we assessed the response of different components of above-ground net primary production (ANPP) to nutrient addition of nitrogen (N), phosphorus (P), potassium (K) and all possible combinations (NP, NK, PK, and NPK) at rates of 125 kg N ha-1yr-1, 50 kg P ha-1 yr-1 and 50 kg K ha-1yr-1.

We established 32 (8 treatments × 4 replicates) experimental plots of 40 × 40 m2 each and measured stem growth of over 15,000 trees with diameter at breast height (dbh) ≥ 1 cm as well as litter production and above-ground woody biomass production (AWBP), of a lower-montane tropical forest (1100 m a.s.l.) in northwestern Uganda.

After 18 months of nutrient addition, we found that different aspects of ANPP, including litter production and AWBP are controlled by multiple soil nutrients. Specifically, we measured higher total fine-litter production in the N (13.6 ± 1.4 Mg ha-1 yr-1) and K (13.3 ± 1.8 Mg ha-1 yr-1) addition plots than the control (11.1 ± 0.6 Mg ha-1 yr-1) plots. Both reproductive litter (flowers and fruits; 10% of total fine-litter fall) and leaf litter (62% of total fine-litter fall) significantly increased with K addition. In general, fine-litter production in our plots is higher than what has been reported so far for lower-montane tropical forests. Increased AWBP is associated with N addition plots. The response of trees to nutrient addition however, varied with tree sizes. Trees with dbh between 10 – 30 cm increased significantly in AWBP under PK addition. There was no effect of nutrient addition associated with either smaller (1 – 10 cm dbh) or larger trees (dbh > 30 cm). The medium-sized trees which may have experienced resource competition but have now transitioned into the canopy layer (exposed to sunlight) are able to use additional nutrient for active growth. In contrast, bigger trees may allocate extra nutrient for reproduction and leaf-vitality, while smaller trees remain shaded, co-limited by sunlight and therefore unable to utilize increased available nutrients for stem diameter growth. ANPP increased by 39% with N addition and marginally by 23% with K additions relative to the control. In conclusion, our experiment provides evidence of N and potentially K limitation of ANPP in this lower-montane tropical forest, and highlights that, in a highly diverse ecosystem different components of ANPP may be regulated by multiple nutrients. 

How to cite: Manu, R., Corre, M. D., Veldkamp, E., and van Straaten, O.: Early responses of elevated nutrient input on above-ground net primary production of a lower-montane tropical forest in Uganda, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13935, https://doi.org/10.5194/egusphere-egu2020-13935, 2020.

D751 |
Tarek EI-Madany, Markus Reichstein, Arnaud Carrara, M. Pilar Martin, Gerardo Moreno, Rosario Gonzalez-Cascon, Josep Penuelas, Vicente Burchard-Levin, Tiana Hammer, Jürgen Knauer, Olaf Kolle, Yunpeng Luo, Javier Pacheco-Labrador, Oscar Perez-Priego, Victor Rolo, Thomas Wutzler, and Mirco Migliavacca

Nitrogen (N) and phosphorus (P) are the two most important limiting soil nutrients reducing carbon sequestration globally. Through anthropogenic N-deposition, stoichiometric imbalances in plant-available N and P are expected in terrestrial ecosystems. This will result in increased P-limitation to plants and associated, but yet understudied, implications for ecosystem carbon sequestration, water-use efficiency (WUE), and biophysical properties. Here, we show results of a large-scale fertilization experiment designed to quantify effects of stoichiometric N:P ratio imbalances on WUE in a semi-arid tree-grass ecosystem. At the ecosystem-scale, the addition of N increased leaf area index, canopy chlorophyll content, and WUE. The addition of P, which relived the N:P imbalance, resulted in a further increase of WUE, more fixed carbon per transpired water. We conclude that increased N and combined N+P addition leads to shifts in many aspects of ecosystem functioning and biophysics, in particular related to water use strategies.

How to cite: EI-Madany, T., Reichstein, M., Carrara, A., Martin, M. P., Moreno, G., Gonzalez-Cascon, R., Penuelas, J., Burchard-Levin, V., Hammer, T., Knauer, J., Kolle, O., Luo, Y., Pacheco-Labrador, J., Perez-Priego, O., Rolo, V., Wutzler, T., and Migliavacca, M.: Variability of ecosystem scale water-use efficiency in a nutrient manipulation experiment , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13534, https://doi.org/10.5194/egusphere-egu2020-13534, 2020.

D752 |
Haiying Cui, Manuel Delgado-Baquerizo, Wei Sun, Jian-Ying Ma, Wenzheng Song, Keying Wang, and Xiaoli Ling

Plant phosphorus (P) resorption, mutualistic symbiosis with mycorrhizas, such as arbuscular mycorrhizal fungi (AMF) and soil organic P mineralization are crucial strategies for acquiring sufficient P to meet plant nutrient demand. Which is the main strategy, however, responding to elevated nitrogen (N) addition to alleviate P deficiency caused by N enrichment remains unclear in terrestrial ecosystems. We explored the responses of foliar P resorption of dominate species (Leymus chinensis), soil microbial properties and organic P mineralization to multi-level N addition in a temperate meadow steppe, Northeast China. We found the enhancements in plant biomass, microbial biomass C and N (MBC, MBN), alkaline phosphatase activities (ALP), and phoD gene abundance (main gene coded soil ALP), while the reductions in soil pH, available P, microbial biomass P, and AMF abundance, and no significant responses of foliar P content under simulative N deposition. When the rates exceeded the threshold 10 g N m-2yr-1, plants and microbes had little additional responses to N enrichment. Notably, N addition had distinct effects on three plant P acquisition strategies, that no conspicuous increase in P resorption efficiency, reduced dependence on mutualistic with AMF symbiosis and accelerated organic P mineralization. A positive correlation between ALP activity, phoD gene abundance and P mineralization rate suggested increases in phosphatase activities and its functional gene copies play crucial roles in organic P mineralization. Nitrogen addition aggravated P deficiency to the production of plant and microbial biomass, which further accelerated soil organic P mineralization and foliar P resorption. Due to lack of plasticity in P resorption efficiency and reduced dependence on mutualistic with AMF symbiosis, however, the organic P mineralization dominated in P acquisition to meet increased P demand. Furthermore, the increase in ALP activities, activation of phoD genes and decrease in soil pH were the main pathways to accelerate organic P mineralization and consequently alleviated P deficiency caused by anthropogenic N deposition, especially at conditions of N saturation. Our results provide strong evidences that N addition can accelerate the rate of P cycling and mobilize plant P uptake strategies such as soil organic P mineralization and leaf P resorption, which are important to better maintain sustainable ecosystem development in the more fertilized word.

Acknowledgments: This work was supported by the National Key Research and Development Program of China (2016YFC0500602), National Natural Science Foundation of China (31570470, 31870456), the Fundamental Research Funds for the Central Universities (2412018ZD010), and the Program of Introducing Talents of Discipline to Universities (B16011). H.C. acknowledges support from Chinese Scholarship Council (CSC).

How to cite: Cui, H., Delgado-Baquerizo, M., Sun, W., Ma, J.-Y., Song, W., Wang, K., and Ling, X.: Nitrogen addition accelerates ecosystem phosphorus cycling at multiple scales in a temperate grassland of Northeast China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1421, https://doi.org/10.5194/egusphere-egu2020-1421, 2020.

D753 |
Kathrin Rousk

Nitrogen (N2) fixation performed by moss-associated cyanobacteria is one of the main sources of new N in pristine, high latitude ecosystems like boreal forests and arctic tundra. Here, mosses and associated cyanobacteria can contribute more than 50% to total ecosystem N input. However, N2 fixation in mosses is strongly influenced by abiotic factors, in particular moisture and temperature. Hence, climate change will significantly affect this key ecosystem process in pristine ecosystems. Here, I will present a synthesis of several field and laboratory assessments of moss-associated N2 fixation in response to climate change by manipulating moisture and temperature in subarctic and arctic tundra.

Both in a long-term climate warming experiment in the arctic, and along a continental climate gradient, spanning arctic, subarctic and temperate ecosystems, increased temperatures (up to 30 °C) lead to either no effect or decreased N2 fixation rates in different moss species. Yet, N2 fixation rates were strongly dependent on moss-moisture, which seems to be a more important driver of N2 fixation in mosses than temperature.

In another set of studies, two dominant moss species (Hylocomium splendens, Pleurozium schreberi) were collected from a steep precipitation gradient (400-1200 mm mean annual precipitation, MAP) in the Subarctic close to Abisko, Northern Sweden, and were incubated at different moisture and temperature levels in the laboratory. Nitrogen fixation, cyanobacterial abundance (via qPCR) and cyanobacterial community composition (via sequencing) on the mosses were assessed. Moisture and temperature interacted strongly to control moss-associated N2 fixation rates, and the highest activity was found at the wet end of the precipitation gradient. Although cyanobacterial abundance was higher in one of the investigated mosses (H. splendens), translating into higher N2 fixation rates, cyanobacterial community composition did not differ between the two moss species. Nostoc was the most common cyanobacterial genera on both mosses, and hardly any methanotrophic N2 fixing bacteria were found on the mosses along the precipitation gradient. Increased temperatures lead to increased abundances of certain cyanobacterial genera (Cylindrospermum and Nostoc), while others declined in response to warming. Hence, cyanobacterial communities colonizing mosses will be dominated by a few cyanobacteria species in a warmer climate, and temperature and moisture interact strongly to affect their activity. Thus, these two major climate change factors should be considered in unison when estimating climate change effects on key ecosystem processes such as N2 fixation. Further, host identity determines cyanobacterial abundance, and thereby, N2 fixation rates.




How to cite: Rousk, K.: Nitrogen fixation beyond leguminous plants: characterising overlooked plant-bacteria associations in the light of climate change, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-103, https://doi.org/10.5194/egusphere-egu2020-103, 2020.

D754 |
Han Wang, Colin Prentice, Trevor Keenan, Ülo Niinemets, and Nils Stenseth

The distribution of leaf nitrogen (NL) within canopies has been discussed for decades in relation to the optimality hypothesis that predicts coordination of carboxylation capacity with absorbed light. Although an optimal (that is, proportional) response of both carboxylation capacity and NLto light is extensively supported by field observations of variation among sites, the observed saturation curve of NLwithin canopies seems to challenge the generality of that response. By considering dynamic light regimes, we propose an optimality-based theory that successfully reconciles the apparent conflict of observed NLdistribution within and between canopies. This theory proposes that due to the highly uneven temporal distribution of sun flecks, the light level to which understory leaves acclimate is much higher than the average light level. This proposition leads to a saturation curve for the vertical distribution of NL. Our within-canopy data analysis supports this theory. Understorey leaves require significantly less NLto achieve photosynthetic capacity as an acclimation to sun flecks. The contribution of structural and photosynthetic components to NLpredicted by the theory is quantitatively and consistently supported by global datasets of variation both within and between canopies.

How to cite: Wang, H., Prentice, C., Keenan, T., Niinemets, Ü., and Stenseth, N.: Leaf nitrogen distribution within canopies is (also) optimal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12097, https://doi.org/10.5194/egusphere-egu2020-12097, 2020.

D755 |
| Highlight
| Vladimir Ivanovich Vernadsky Medal Lecture
Pierre Friedlingstein

Human activities have an unprecedented impact on the global carbon cycle.  Atmospheric CO2 concentrations have been continuously monitored since 1958, and show a 30% increase, from 315 ppm in 1958 to 411 ppm in 2019. Anthropogenic emissions, primarily from fossil fuel combustion, but also from land-use changes, are the drivers of these changes, with global emissions almost tripling over that period, from 4GtC per year in 1958 to almost 12 GtC per year at present. Although the current rate of increase in fossil fuel emission of about 1% per year is lower than over the 2000s (about 3.0 % per year), there are no sign of global emissions peaking yet, despite climate policies being put in places in many countries.

The atmospheric CO2 increase induces land and ocean carbon uptake, respectively driven by enhanced photosynthesis, leading to larger land biomass and soil carbon; and by enhanced air-sea CO2 exchange, leading to larger carbon content in the surface ocean and export to the deep ocean. These mechanism are negative feedbacks in the Earth system and are removing about 50% of the CO2 emitted in the atmosphere. Without these land and ocean carbon sinks, atmospheric CO2 would already be about 600 ppm.

However, modelling studies show that climate change reduces land and ocean carbon sinks, hence amplifying the warming. Although there is agreement that such positive feedback will develop over the course of the century, there are not yet clear evidences of a major climate driven reduction of the carbon sinks.  So far, observations and modelling studies of the historical carbon cycle do not show any sign of a tipping point in the global carbon cycle.

How to cite: Friedlingstein, P.: Human induced changes on the global carbon cycle over the last 60 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4822, https://doi.org/10.5194/egusphere-egu2020-4822, 2020.

D756 |
Ana Bastos and the ICOS drought task-force

In the past 20 years, three major drought-heat events (DH) occurred in Europe: DH2003 in western Europe, DH2010 in western Russia and, more recently, DH2018 in central Europe and Scandinavia. These events were all preceded by warm and dry springs that contributed to the summer heatwaves and are comparable in magnitude. However, they varied in their geographical distribution and biomes affected, so they can be used as “natural experiments” to improve our understanding of ecosystems’ responses to climate conditions that will become more frequent in the coming decades.

We analyze anomalies in carbon, water and energy fluxes from 11 Dynamic Global Vegetation Models (DGVMs) forced with higher spatial and temporal resolution climate forcing than conventional global simulations, as well as an ensemble of estimates from the data driven FLUXCOM product. All three DH events were associated with decreases in summer gross primary productivity (GPP), but the relative strength at continental scale differed between events. The DGVMs and FLUXCOM show event-dependent agreement in estimated gross and net CO2 fluxes. We also find a progressively stronger negative effect of heat anomalies on ecosystem productivity between each event, which might indicate a transition towards more water-limited regime at continental scale by progressively warmer background conditions.

The different impacts of the three DH events on continental-scale summer GPP are in part related to regional asymmetries in climate anomalies that act to amplify or offset the impact of DH events on ecosystem productivity, depending on their geographical distribution and biomes affected. At the annual scale, both FLUXCOM and DGVMs indicate close to neutral or above-average CO2 uptake in the three years, when removing the long-term trend. This is in part because increased productivity in spring in response to warming and reduced respiration in autumn compensated for less photosynthetic uptake in summer.

All models show good skill in simulating the soil-moisture anomalies during DH events, but in general DGVMs show poor skill in simulating the increased sensitivity of GPP to soil-moisture and decreased sensitivity to temperature during extreme events. The relative skill of individual DGVMs in simulating these changes of GPP sensitivity to climate during extreme events explains the mismatch in simulated productivity anomalies with FLUXCOM estimates.

How to cite: Bastos, A. and the ICOS drought task-force: Diverging impacts of extreme summers on European C-cycling from different regional and seasonal compensation effects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3476, https://doi.org/10.5194/egusphere-egu2020-3476, 2020.

D757 |
Vincent Humphrey, Alexis Berg, Philippe Ciais, Christian Frankenberg, Pierre Gentine, Martin Jung, Markus Reichstein, and Seneviratne Sonia I.

Obtaining reliable estimates of the sensitivity of carbon fluxes to water availability, temperature and vapor pressure deficit is essential for constraining climate-carbon feedbacks in Earth system models. However, these variables often co-vary because of soil moisture – atmosphere feedbacks, especially in situations where they are most susceptible to strongly impact ecosystems (e.g. during droughts and heatwaves), leading to potentially conflicting results when sensitivities are assessed independently. In particular, there is conflicting evidence on the role of temperature versus water availability in explaining these variations at the global scale.

Here, we show that accounting for the effect of soil moisture – atmosphere coupling resolves much of this controversy. Using idealized climate model experiments, we find that variability in soil moisture accounts for 90% of the inter-annual variability in land carbon uptake, mainly through its impact on photosynthesis. Without SM variability, the inter-annual variability (IAV) of land carbon uptake is almost eliminated. We show that the effects of soil moisture can be decomposed into 1) a direct ecosystem response to soil water stress and 2) a dominant indirect response to extreme temperature and vapor pressure deficit triggered by land-atmosphere coupling and controlled by anomalous soil moisture conditions.  Importantly, these two mechanisms do not necessarily have the same spatial extent, and some regions can be more sensitive to indirect effects than to direct effects.

These two pathways explain why results from coupled climate models suggest a dominant role of soil moisture, while uncoupled simulations diagnose a strong temperature effect. These findings have strong implications for offline model sensitivity analyses as well as field scale manipulation experiments (i.e. rainfall exclusion studies) where the impact of drought on carbon exchange and vegetation activity is often studied by intervening solely on soil moisture content with little consideration of the physical feedbacks on temperature and air humidity occurring in natural conditions.

How to cite: Humphrey, V., Berg, A., Ciais, P., Frankenberg, C., Gentine, P., Jung, M., Reichstein, M., and Sonia I., S.: Conflicting drivers of land carbon uptake variability reconciled by land-atmosphere coupling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11172, https://doi.org/10.5194/egusphere-egu2020-11172, 2020.

D758 |
Manon Sabot, Martin De Kauwe, Belinda Medlyn, and Andy Pitman

Nearly 2/3 of the annual global evapotranspiration (ET) over land arises from the vegetation. Yet, coupled-climate models only attribute between 22% – 58% of the annual terrestrial ET to plants. In coupled-climate models, the exchange of carbon and water between the terrestrial biosphere and the atmosphere is simulated by land-surface models (LSMs). Within those LSMs, stomatal conductance (gs) models allow plants to regulate their transpiration and carbon uptake, but most are empirically linked to climate, soil moisture availabilty, and CO2. Therefore, how and which gs schemes are implemented within LSMs is a key source of model uncertainty. This uncertainty has led to considerable investment in theory development in the recent years; multiple alternative hypotheses of optimal leaf-level regulation of gas exchange have been proposed as solutions to reduce existing model biases. However, a systematic inter-model evaluation is lacking (i.e. inter-model comparison within a single framework is needed to understand how different mechanistic assumptions across these new gs models affect plant behaviour). Here, we asked how, and under what conditions, nine novel optimal gs models differ from one another. The models were trained to match under average conditions before being subjected to: (i) a dry-down, (ii) high vapour pressure deficit, and (iii) elevated CO2. These experiments allowed us to identify the models’ specific responses and sensitivities. To further assess whether the models’ responses were realistic, we tested them against photosynthetic and hydraulic field data measured along mesic-xeric gradients in Europe and Australia. Finally, we evaluated model performance versus model complexity and the amount of information taken in by each model, which enables us to make recommendations regarding the use of stomatal conductance schemes in global climate models.

How to cite: Sabot, M., De Kauwe, M., Medlyn, B., and Pitman, A.: One stomatal model to rule them all? Evaluating competing hypotheses to regulate the exchange of carbon and water against experimental data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-678, https://doi.org/10.5194/egusphere-egu2020-678, 2020.

D759 |
Wei Li and Cicheng Zhang

Desert environments are characterized by limited and highly variable rainfall, which is an intermittent source of water critical to the evolution of the structure and functioning of desert ecosystems. The present study was to assess the effects of different amounts of rainfall received through discrete rainfall events and of the ecophysiological responses for Reaumurica soongorica along multiple average precipitation (MAP) gradient. A field experiment was performed under seven simulated rainfall amounts (0 - 40 mm) with Reaumurica soongorica at respective High-P (120 mm), Middle-P (67 mm), and Low-P (35 mm) sites along middle and lower reach of Heihe River Basin in July, 2015. Pre-dawn plant water potential (ψpd), the rates of photosynthesis and stomatal conductance were measured synchronously. Results showed that: photosynthetic response of R.soongorica to rainfall pulse was significant different. The mean daily leaf gas exchange and maximum photosynthesis rate (Pn-max) of R.soongorica were decreased obviously with decreasing MAP. Vapour pressure deficit (VPD) was the predominant factor for gas exchange limiting. Under the control of VPD, stomatal conductance was pregressively reduced with decreasing ψpd, which was functioned as limiting Pn-max and further increasing water use efficiency (WUE). However, when MAP was declined below 35 mm, the response of stomatal conductance to ψpd was weakened, from which Pn-max began to increase again. 2 to 4 days hystereric response of R.soongorica ψpd to various rainfall events was found in High-P. Stomatal conductance was then increased linearly with increasing ψpd, from which Pn-max was also enhanced linearly. While weakly response of ψpd to similar rainfall events was observed in Low-P, where stomatal conductance and Pn-max was maintained stable after rain. Mentioned above, the effective rainfall pluse, induced by obvious physiological response of R. soongorica, was 3.63-6.73 mm and 6.73-10.09 mm for Linze and Ejina, respective. Our results provided comprehensively understanding in the consequences of long-term variability in rainfall for the physiology of desert plants and species dynamics in desert ecosystems.

How to cite: Li, W. and Zhang, C.: Ecophysiological Responses of desert shrub to rainfall addition for Reaumurica soongorica in desert ecosystem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5176, https://doi.org/10.5194/egusphere-egu2020-5176, 2020.

D760 |
Yunfeng Han and Biao Zhu

Arbuscular mycorrhizal (AM) fungi play many important roles in terrestrial ecosystems. The effects of increasing nitrogen (N) deposition on AM fungi will inevitably affect many important ecosystem processes. However, our quantitative understanding on the generalizable patterns of how N deposition affects AM fungi at the global scale remains unclear.

We conducted a meta-analysis of 431 observations from 111 publications to investigate the responses of AM fungi to N addition, including abundance, richness and diversity, and explored the mechanisms of N addition affecting AM fungi by trait-based guilds method.

Results showed that N addition had strong negative effects on AM fungal abundance and richness, and different AM fungal guilds showed different responses to N addition: the rhizophilic guild significantly decreased under N addition, while the edaphophilic guild increased (but with much variability) under N addition. Further analysis showed that N addition affects AM fungi mainly by causing soil acidification and increasing soil available N. Specifically, soil acidification had a negative effect on both the rhizophilic and edaphophilic AM fungi and increased soil available N mainly negatively affect the edaphophilic AM fungi. Moreover, the response of AM fungi to N addition was also affected by the shifts in plant carbon (C) allocation caused by soil phosphorus (P) availability.

This synthesis highlights that trait-based AM fungal guilds as well as taking soil P and C from host plants into consideration can improve our understanding of dynamics of AM fungal communities under increasing N deposition. This would further enable better predictions of the functional consequences of changes in AM fungal communities such as impacts on soil organic C dynamics, plant P uptake and plant diversity.

How to cite: Han, Y. and Zhu, B.: Responses of arbuscular mycorrhizal fungi to nitrogen addition: a meta-analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5406, https://doi.org/10.5194/egusphere-egu2020-5406, 2020.

D761 |
Rodolfo Nóbrega, David Sandoval, and Colin Prentice

Root zone storage capacity (Rz) is a parameter widely used in terrestrial ecosystem models that estimate the amount of soil moisture available for transpiration. However, Rz is subject to large uncertainty, due to the lack of data on the distribution of soil properties and the depth of plant roots that actively take up water. Our study makes use of a mass-balance approach to investigate Rz in different ecosystems, and changes in water fluxes caused by land-cover change. The method needs no land-cover or soil information, and uses precipitation (P) and evapotranspiration (ET) time series to estimate the seasonal water deficit. To account for some of the uncertainty in ET, we use different methods for ET estimation, including methods based on satellite estimates, and modelling approaches that back-calculate ET from other ecosystem fluxes. We show that reduced ET due to land-cover change reduces Rz, which in turn increases baseflow in regions with a strong rainfall seasonality. This finding allows us to analyse the trade-off between gross primary production and hydrological fluxes at river basin scales. We also consider some ideas on how to use mass-balance Rz in water-stress functions as incorporated in existing terrestrial ecosystem models.

How to cite: Nóbrega, R., Sandoval, D., and Prentice, C.: Changes in root zone storage capacity and their effects on river discharge and gross primary production, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12017, https://doi.org/10.5194/egusphere-egu2020-12017, 2020.

D762 |
Siya Shao, Nigel Roulet, and Jianghua Wu

Northern peatlands have sequestered a huge amount of carbon through exceptionally low microbial activity which is partly attributed to their nutrient-poor conditions. Evergreen shrubs, a dominant species in ombrotrophic bogs, adapt to this nutrient-poor condition by developing organic nutrient acquisition strategies, mediated by ericoid mycorrhizal association. However, the mycorrhizal symbionts together with nutrient cycling have been omitted in peatland models, precluding our ability to simulate the significance of nutrient limitation in peatlands following environmental changes. To address this issue, we further developed the well-established peatland model MWM by incorporating a mechanistic mycorrhizal fungi model and both nitrogen and phosphorus cycles. The new model was adopted to simulate the fertilization effect on peatlands and evaluated against measurements from the long-term fertilization experiments at Mer Bleue, a raised ombrotrophic bog located in southern Ontario, Canada. The model successfully reproduced the observed dramatic changes with fertilization in mycorrhizal performance, vegetation composition and carbon cycle. Greater availability of inorganic nutrients diminished the role of mycorrhizal fungi in plant nutrient uptake. More assimilated carbon was allocated to shrub growth, which then inhibited the growth of sphagnum moss and ultimately posed a threat to the carbon-sequestration capacity of peatlands. Therefore, mycorrhizal activities, which have been overlooked in past peatland studies, could play a significant role in understanding how peatlands respond to increased nutrient deposition in the future.

How to cite: Shao, S., Roulet, N., and Wu, J.: The potential significance of ericoid mycorrhizal fungi in ombrotrophic peatland biogeochemistry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12789, https://doi.org/10.5194/egusphere-egu2020-12789, 2020.

D763 |
Yongmei Huang

Effect of nitrogen deposition on terrestrial ecosystems are one of the hot spots in the study of global change, and the significantly different responses were reported widely among different ecosystems. In this study, field simulated nitrogen deposition experiment was carried out in a temperate steppe, norther China from 2011 to 2018. Treatments were designed as: CK (0 g N/m2), N2 level (2 g N/m2), N5 level (5 g N/m2), N10 level (10 g N/m2), N25 level (25 g N/m2) and N50 level (50 g N/m2). The results showed that the N addition did not cause a noticeable change in the net primary productivity and soil acidification. N addition caused a significant decline in community biodiversity with a major shift in species composition. N utilization strategy, photosynthetic capacity, and water use efficiency of three dominant species behaved differently under N deposition. Soil was the major sink for N deposition testified by the 15N isotope tracer experiment. N addition decreased soil microorganism and plant 15N recovery and increased soil of 30-40 cm layer 15N recovery. N saturation of the temperature steppe would occur when N deposition rate reached 5.4-8.4gN m-2a-1.

How to cite: Huang, Y.: Responses of temperate steppe to simulated nitrogen deposition: from community structure to ecosystem functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21048, https://doi.org/10.5194/egusphere-egu2020-21048, 2020.

D764 |
Chris R Taylor, Ben Keane, Iain Hartley, and Gareth Phoenix

Terrestrial ecosystems absorb 30% of anthropogenic carbon dioxide (CO2) emissions, slowing its rising atmospheric concentration and substantially inhibiting climate change. This uptake is believed to be due to elevated CO2 (eCO2) stimulating plant photosynthesis and growth, thus increasing carbon (C) storage in plants and soil organic matter. However, nitrogen (N) limitation can reduce ecosystem C uptake capacity under eCO2 by as much as 50%. Phosphorus (P) limitation in ecosystems is almost as common as N-limitation and is increasing due to ongoing deposition of N from anthropogenic activities. Despite this, we do not know how P-limited ecosystems will respond to eCO2, constituting a major gap in our understanding of how large areas of the biosphere will impact atmospheric CO2 over the coming decades.

In the first study conducted into the effect of eCO2 on P-limited ecosystems with manipulated nutrient availability, the Phosphorus Limitation And ecosystem responses to Carbon dioxide Enrichment project (PLACE), investigates the effects of eCO2 on C cycling in grasslands, which are a critical global C store. Turf mesocosms from P-limited acidic and limestone grasslands, where N and P inputs have been manipulated for 20 years (control, low N (3.5 g m-2 y-1), high N (14 g m-2 y-1), and P (3.5 g m-2 y-1)), have been exposed to either ambient or eCO2 (600 ppm) in a miniFACE (mini Free Air Carbon Enrichment) system. Long-term P addition has alleviated P limitation while N additions have exacerbated it. The two contrasting grasslands contain different amounts of organic versus mineral P in their soils and, thus, plants may have to use contrasting strategies to acquire the additional P they need to increase growth rates under elevated CO2.

We present data from the first two growing seasons, including above and below ground productivity, and C, N and P cycling through plant, soil and microbial pools. Aboveground harvest data from the second year have shown eCO2 has only increased biomass production in the limestone grassland (by 17%; p< 0.0001), and not in the acid grassland. There was also a significant effect of nutrient treatment (p< 0.001) with biomass increasing under P and HN, indicating some co-NP limitation. Stable isotope tracing, using the fumigation CO2 signal has shown the fate of newly assimilated C and its contribution to gaseous C flux to the atmosphere in the form of methane (CH4) and respired CO2.  In summary, our first two years of eCO2 treatment suggests that productivity of limestone and acidic grassland respond differently and that these responses depend on nutrient availability, indicating the complexity of predicting P-limited ecosystem responses as atmospheric CO2 continues to rise.

How to cite: Taylor, C. R., Keane, B., Hartley, I., and Phoenix, G.: Carbon flux response and recovery to drought years in a hemi-boreal peat bog between different vegetation types, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13891, https://doi.org/10.5194/egusphere-egu2020-13891, 2020.

D765 |
Martina Botter, Matthias Zeeman, Paolo Burlando, and Simone Fatichi

The pressure of climate change and increasing food demand on agricultural systems made management strategies crucial for matching the production goals without affecting water quantity and quality. This is, for instance, the case for managed grasslands in the Alpine and pre-alpine regions. This study combines a large suite of observations from the TERENO observatory and ScaleX campaigns with mechanistic modeling, in order to analyze the response of managed grasslands north of the Alps to different climatic conditions and management strategies, aimed at evaluating changes in the ecohydrological response, as well as carbon, water and nutrient fluxes.  

First, we used the data to evaluate the performance of the mechanistic Tethys-Chloris (T&C) model, which fully integrates the solution of surface energy balance and hydrological budget with vegetation dynamics and soil biogeochemistry, for the period 2012-2016. This is characterized by significant climatic inter-annual variability and including the extraordinarily warm year 2015. The observations cover three different grassland sites composed by flux towers, soil moisture and temperature probes, lysimeters, nutrient leaching and dedicated vegetation sampling campaigns, allowing an unprecedented validation opportunity of model skills for multiple variable and temporal scales. The observed system response, in terms of water, energy and nutrients dynamics, are successfully reproduced, which increases confidence on the model capability to reproduce the feedbacks among hydrology, vegetation growth and soil biogeochemistry. The results highlight the impact of an early begin of the growing season on the vegetation productivity and nutrients leaching in years with reduced snow cover,  as well as the effects of summer drought on vegetation productivity.

Second, numerical experiments are used to test the response of this ecosystem to different grassland fertilization and cutting scenarios in the present climate and in warmer and CO2 enriched conditions. Of particular interest are the number and timing of grass cuts and fertilizer applications that could optimize grassland productivity without compromising water quality in a warmer climate.

How to cite: Botter, M., Zeeman, M., Burlando, P., and Fatichi, S.: Heavily data-constrained mechanistic ecohydrological modeling can guide management of pre-Alpine grasslands in the present and future climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18266, https://doi.org/10.5194/egusphere-egu2020-18266, 2020.

D766 |
Integrating the carbon and nitrogen interactions in the Common Land Model (CoLM-CN): model description and benchmarking
Qian Zhang
D767 |
Katrin Schneider and Ralf Kiese

It is generally accepted that climate change likely alters the ratio of water balance components in mid-latitude environments. Higher temperatures and an elevated water vapour deficit may increase evapotranspiration rates and reduce groundwater recharge rates. At the same time, agricultural management may interfere these effects, e.g. through reduced plant transpiration rates due to a high cutting frequency.

The study analyses climate change and agricultural management effects on the water fluxes and coupled nitrogen export in a prealpine grassland. It makes use of the grassland lysimeters, which are part of the TERENO preAlpine observatory in southern Bavaria (Germany). In a “space-for-time” approach, soil cores with an area of 1 m² and a depth of 1.5 m have been excavated and translocated to lower elevations. Furthermore, soil cores from the same area (that have not been translocated to lower elevations) act as control plots in the lysimeter network. The elevation gradient between the highest (864 m a.s.l.) and lowest (695 m a.s.l.) lysimeter station accounts for a temperature increase of approx. 2°C, while precipitation decreases from approx. 1350 mm a-1 to approx. 960 mm a-1. Following local agricultural practice, intensive as well as extensive grassland management is applied at the lysimeters: intensive management refers to a higher frequency of cutting (up to five times per year) and manure application (approx.. 250 kg N ha-1 a-1) than extensive management (two cuts and approx. 80 kg N ha-1 a-1).

The study compares the effects of temperature and precipitation changes (i.e. elevated temperature and decrease in precipitation) and different agricultural management on water balance components (evapotranspiration, groundwater recharge, Ammonia and Nitrate fluxes) measured at the lysimeters. Preliminary result show that the ratio of evapotranspiration to precipitation increases in the climate change treatment. Water-bound nitrogen fluxes are comparably low on all sites, indicating that nitrogen uptake by plant plants is dominating over nitrogen leaching.

How to cite: Schneider, K. and Kiese, R.: Water fluxes and coupled nitrogen export in a managed prealpine grassland: identifying the effects of climate change and agricultural management, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5198, https://doi.org/10.5194/egusphere-egu2020-5198, 2020.

D768 |
Tatiana Reichert, Anja Rammig, Carlos A. Quesada, Lucia Fuchslueger, and Katrin Fleischer

The Amazon rainforest is the biggest tropical rainforest in the world and provides significant global climate regulation services. However, the future of the Amazon rainforest carbon sink under elevated CO2 is uncertain. The potential fertilization effect of elevated CO2 on the Amazon rainforest carbon sink may be constrained by phosphorus availability. Phosphorus is an essential element involved in all major plant processes and is considered to be the primary limiting nutrient in the Amazon rainforest. To cope with phosphorus limitation, plants have developed different strategies to increase the use efficiency, uptake, and availability of phosphorus. Vegetation models have identified phosphorus use and acquisition strategies as crucial to the projections of the future of the Amazon rainforest. Although some of the strategies are explicitly or implicitly represented in vegetation models, the mechanistic representations diverge due to the lack of empirical knowledge. Here, we synthesized the current understanding of the main strategies and how they may play out in the Amazon rainforest, namely, root phosphorus foraging, arbuscular mycorrhizal symbiosis, phosphatase and organic acids exudation, and leaf phosphorus resorption. We focus on mechanisms, drivers, and plasticity along soil phosphorus gradients of the named strategies, aiming to inform models and highlight important knowledge gaps, offering an opportunity to bring modeling and experimental research together.

How to cite: Reichert, T., Rammig, A., A. Quesada, C., Fuchslueger, L., and Fleischer, K.: Plant phosphorus use and acquisition strategies in the Amazon rainforest with relevance to vegetation models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7090, https://doi.org/10.5194/egusphere-egu2020-7090, 2020.

D769 |
Jennifer A. Holm, David M. Medvigy, Benjamin Smith, Jeffrey S. Dukes, Claus Beier, Mikhail Mishurov, Xiangtao Xu, Jeremy W. Lichstein, Craig D. Allen, Klaus S. Larsen, Yiqi Luo, Cari Ficken, William T. Pockman, William R.L. Anderegg, and Anja Rammig

Climatic extreme events are expected to occur more frequently and potentially be stronger in the future, increasing the likelihood of unprecedented climate extremes (UCEs), or record-breaking events such as prolonged droughts, to occur. To prepare for UCEs and their impacts, we need to develop a better understanding of terrestrial ecosystem responses to events such as extreme drought. We know that intense, extreme droughts can substantially affect ecosystem stability and carbon cycling through increased plant mortality and delaying ecosystem recovery. Our ability to predict such effects is limited due to the lack of experiments focusing on climatic excursions beyond the range of historical experience.

We explore the response of forest ecosystems to UCEs using two dynamic vegetation demographic models (VDMs), ED2 and LPJ-GUESS, in which the abundances of different plant functional types, as well as tree size- and age-class structure, are emergent properties of resource competition. We investigate the hypothesis that ecosystem responses to UCEs (e.g., unprecedented droughts) cannot be extrapolated from ecosystem responses to milder extremes, as a result of non-linear ecosystem responses (e.g. due to plant plasticity, functional diversity, and trait combinations). We evaluate each model’s mechanisms and state variables prior, during, and after a continuum of drought intensities ultimately reaching very extreme drought scenarios (i.e., 0% to 100% reduction in precipitation for drought durations of 1-year, 2-year, and 4-year scenarios) at two dry forested sites: Palo Verde, Costa Rica (i.e. tropical) and EucFACE, Australia (i.e. temperate). Both models produce nonlinear responses to these UCEs. Due to differences in model structure and process representation, the model’s sensitivity of biomass loss diverged based on either duration or intensity of droughts, as well as different model responses at each site. Biomass losses in ED2 are sensitive to drought duration, while in LPJ-GUESS they are mainly driven by drought intensity. Elevated atmospheric CO2 concentrations alone did not buffer the ecosystems from carbon losses during UCEs in the majority of our simulations. Our findings highlight discrepancies in process formulations and uncertainties in models, notably related to availability in plant carbohydrate storage and the diversity of plant hydraulic schemes. This shows that different hypotheses of plant responses to UCEs exist in two similar models, reflecting knowledge gaps, which should be tested with gap-informed field experiments. This iterative modeling-experiment framework would help improve predictions of terrestrial ecosystem responses and climate feedbacks.

How to cite: Holm, J. A., Medvigy, D. M., Smith, B., Dukes, J. S., Beier, C., Mishurov, M., Xu, X., Lichstein, J. W., Allen, C. D., Larsen, K. S., Luo, Y., Ficken, C., Pockman, W. T., Anderegg, W. R. L., and Rammig, A.: Exploring the impacts of unprecedented climate extremes on forest ecosystems: hypotheses to guide modeling and experimental studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12111, https://doi.org/10.5194/egusphere-egu2020-12111, 2020.

D770 |
Shen Tan and Han Wang

Accurate estimation of terrestrial evapotranspiration (ET) is a basic request in researches about water cycle and the energy exchange at land-atmosphere interface. Modelling ET with water and carbon coupling theory has been proven to be a robust and effective strategy. However, there still remaining an assumption needs demonstration: if site-calibrated parameters in empirical models are universally accurate and could be generalized in different regions or future scenario. In this research, we present a prototype coupling ET and carbon assimilation based on a first-principle primary production model with only two parameters calibrated with independent datasets. The water vapor diffuses through leaf stomata, which is regulated by the ratio of leaf-internal to external CO2 (χ) and could be estimated by environmental factors using our universal model. We validated the prototype with three steps. In the first step, we prove that diffusion process is the key linkage of water cycle and carbon assimilation at canopy scale. In the second step, comparation was carried over different vegetation types between predicted gross primary production (GPP) and tower-based observation, where results displays good agreement was found. Thirdly, we use the predicted χ and GPP to estimate canopy ET. Due to the strict description of physical and physiological process, our ET model is free of further consideration about the variation of parameters, thus could be ideally used in non-site region or future scenario. Sensitivity analysis results show that GPP would increase following the rising of CO2 concentration, but exhibit a parabolic trend when faced with rising air temperature. On the other hand, simulated ET exhibits nearly linear trend against warmer environment, but nearly no obvious relation with rising CO2 concentration.

How to cite: Tan, S. and Wang, H.: Application of a priori primary production model to estimate water vapor flux, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12188, https://doi.org/10.5194/egusphere-egu2020-12188, 2020.

D771 |
David Wårlind, Lars Nieradzik, Paul Miller, Mats Lindeskog, Peter Anthoni, Almut Arneth, and Ben Smith

With human land-use activities expected to increase in the future, it is important to understand how LULCC (Land-Use Land-Cover Change) activities affect the Earth’s surface, climate and biogeochemical cycles. Here we use the CMIP6 version of the EC-Earth3 Earth System Model (ESM) to assess the impacts of LULCC on surface fluxes of carbon (C) and nitrogen (N). EC-Earth is one of the first ESMs to interactively couple a 2nd generation dynamical vegetation model (LPJ-GUESS) with mechanistic C-N dynamics in soil and vegetation to an atmospheric model. The size, age structure, temporal dynamics and spatial heterogeneity of the vegetated landscape are represented and simulated dynamically in LPJ-GUESS. Such functionality has been argued to be essential to correctly capture biogeochemical and biophysical land-atmosphere interactions on longer timescales and has been shown to improve their representation compared to more common area-based vegetation schemes. The patch-based structure of LPJ-GUESS also makes it possible to represent the history (soil, litter status) of a single patch as it might have been involved in several land-use transitions. We examine the effects on surface fluxes of carbon and nitrogen in three LUMIP simulations, for both offline land-only and fully coupled ESM runs. We focus on the effects of gross land-use transitions (“land-hist”), net land-use transitions (“land-noShiftcultivate”) and fixing land-use at 1850 levels (“land-noLu”).

In general, EC-Earth shows a higher historical C loss due to LULCC than other ESMs, but our results are still in line with LULCC emissions constrained by biomass observations. Gross transitions result in a higher historical C loss compared to net transitions, while runs without LULCC (noLu) show a constant gain in C. LULCC also affects the total N content of the system and hence soil nutrient content. Gross (net) LULCC leads to a loss of 1.5 (1.0) PgN over the historical period whereas 1.5 PgN is gained in noLu runs. As increases in global fertilization and harvest fluxes more or less offset one another for both gross and net LULCC, the differences in total N pools derive from biological nitrogen fixation (BNF), soil fluxes, leaching and land-use associated fluxes of N. Changes in soil fertility result in a higher productivity for net compared to gross transitions, mainly in the Tropics. Net transitions also results in less N lost through land-use change and hence a higher net mineralisation rate. This is mainly notable in the Tropics where the initially organic matter content is lower compared to temperate and boreal regions. The productivity and harvested biomass from crops are similar for gross and net transitions as their N source mainly comes from N fertilization, with the exception of some developing countries where N fertilisation is not as high as in industrialised countries. Based on these examples of how the N cycle and productivity are affected by LULCC, we argue that the full complexity of gross transitions is required to accurately predict how LULCC affects the N cycle, productivity and biogeophysical feedbacks to the climate.

How to cite: Wårlind, D., Nieradzik, L., Miller, P., Lindeskog, M., Anthoni, P., Arneth, A., and Smith, B.: Importance of complex LULCC representation for N cycling and productivity in the EC-Earth framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19721, https://doi.org/10.5194/egusphere-egu2020-19721, 2020.

D772 |
Huanyuan Zhang, Iain Colin Prentice, César Terrer, Trevor Keenan, and Oskar Franklin

Based on Free Air Carbon Dioxide Enrichment (FACE) and other raised-CO2 experiments (eCO2), new hypotheses have been proposed to explain how the magnitude of the CO2 fertilization effect on biomass and biomass production depends primarily on soil nitrogen and phosphorus availability [1,2]. To test whether these hypotheses and measurements from eCO2 could explain the land carbon sink as independently determined from data and models, we combined a CO2 response curve for biomass production with a simple two-box model of biomass and soil to simulate the evolution of the land carbon sink during the past century. Results were compared to Dynamic Global Vegetation Model (DGVM) results, as reported by the Global Carbon Project, and to results from inversion studies based on atmospheric CO2 measurements. The interannual variability of the modelled land sink was realistic, dominated by the temperature dependence of heterotrophic respiration, and similar to DGVMs results. However, the magnitude of the derived land sink based on eCO2 results was smaller, and its geographical distribution was different to DGVMs average. Sensitivity tests showed that these findings were robust to reasonable variations of parameter values. The smaller sink is due to the smaller amount of vegetation biomass increment documented by eCO2 experiments in comparison with the mean predictions of DGVMs. A land sink closer to the observed one could be produced, however, when incorporating the hypothesis that nutrient-stressed plants export “excess” carbon (generated by increased photosynthesis, but unable to be used for growth) to the soil and that only a fraction of this excess carbon returns to the atmosphere.  This hypothesis requires further exploration but hints at a reconciliation between DGVMs that explain the land carbon sink without nutrient limitations, with experimental findings of (sometimes severe) restrictions on CO2 fertilization due to nutrient stress.

[1] Terrer et al. 2016, Science, doi.org/10.1126/science.aaf4610

[2] Terrer et al. 2019, Nature Climate Change, doi.org/10.1038/s41558-019-0545-2

How to cite: Zhang, H., Prentice, I. C., Terrer, C., Keenan, T., and Franklin, O.: Can elevated CO2 experiments explain the magnitude of the land carbon sink?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11589, https://doi.org/10.5194/egusphere-egu2020-11589, 2020.

D773 |
Juan-Pablo Fuentes, Daniela Celedón, Horacio Bown, Amanda Martínez, and Marina Vega

Increases in available soil N under forests can affect several ecosystem functions including soil CO2 fluxes and microbial activity. Most of the studies, concerning reactive N deposition in natural ecosystems, are located in the northern hemisphere, meanwhile little is known in southern forest ecosystems, including deciduous Nothofagus forests. We evaluated the effect of reactive N additions (N-NO3 broadcasted under 20 trees at a rate of 400 Kg ha-1 yr-1) in soil CO2 fluxes (Rs, μmol CO2 m-2 s-1), microbial biomass C (MBC), and related ecophysiological indexes, in the most septentrional N. glauca forest of Chile (19S 310464 S, 6213139 W). Twenty additional N. glauca trees, with no N additions, were used as controls. After seven months of N-application, soil respiration was significantly higher (p<0.001) under the N-fertilized trees than in the control trees. Soil MBC, the ratio between MBC and soil organic C (qmic), and the ratio between cummulative basal respiration and SOC (qmin) decreased significantly (p<0.05) in the N-enriched trees, but only after two years of N application. Basal respiration and the metabolic quotient qCO2 were not affected by N enrichment. Even though soil respiration increased, N enrichment inhibited microbial activity. Temporal variations in MBC, basal respiration, and qmic were also found and were associated to water availability.

How to cite: Fuentes, J.-P., Celedón, D., Bown, H., Martínez, A., and Vega, M.: Reactive N additions and their effects in microbial activity and soil respiration under a Nothofagus glauca (Phil.) Krasser forest of Chile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21843, https://doi.org/10.5194/egusphere-egu2020-21843, 2020.