The importance of phosphorus (P) in plant function and ecosystem biogeochemistry has led to the addition of a P cycle to a range of vegetation models, but the predictions of these P-enabled models have rarely been evaluated with ecosystem-scale data. Here, we confronted eight state-of-the-art, P-enabled models with data from EucFACE, a P-limited Eucalyptus forest subject to long-term Free-Air CO₂ Enrichment. We evaluated the capability of the models to capture the observed elevated CO₂ responses in this ecosystem. We show that the inclusion of phosphorus-cycle is necessary to more realistically simulate ecosystem function and biogeochemistry, but this enhanced capacity did not directly translate into improved prediction accuracy. Specifically, models diverged in capturing the observed CO₂ responses, with simulation accuracy depending upon model assumptions about plant physiology, allocation, plant-soil interactions and soil nutrient processes. Confronting models with experimental responses observed at EucFACE represents a valuable opportunity to improve our understanding of the carbon-phosphorus interaction under rising CO₂, and is an important step towards more accurate predictions of the future land carbon sink under climate change.

How to cite: Jiang, M., Medlyn, B., Wårlind, D., Knauer, J., Goll, D., Yu, L., Fleischer, K., Zhang, H., Yang, X., Zaehle, S., Ellsworth, D., and Smith, B.: Confronting models with data: carbon-phosphorus interaction under elevated CO2 in a mature forest ecosystem (EucFACE), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3712, https://doi.org/10.5194/egusphere-egu23-3712, 2023.

In a future world, ecosystems will be affected by a concomitant increase in atmospheric CO₂ concentrations, temperature and drought events. While the individual effects of elevated CO₂, warming and drought on plant and ecosystem productivity are comparatively well understood, there is a major lack of experimental studies testing for their interactive effects. In a multifactor experiment (ClimGrass) established in 2013 on a managed montane grassland in Central Austria we tested how elevated CO₂ (eCO₂), warming (eT) and drought individually and interactively affect productivity and tissue stoichiometry.

Treatment effects varied and amplified across the eight treatment years, partly related to shifts in species composition. Above-ground net primary productivity (ANPP) was generally increased when eT and eCO₂ were combined, while it was not consistently affected by the individual treatments. Drought and drought recovery effects on ANPP, gross primary productivity (GPP) and belowground carbon allocation were amplified when drought was combined with eT and eCO₂. Both under current and future (eT, eCO₂) scenarios drought altered tissue stoichiometry by decreasing phosphorus concentrations during drought and increasing nitrogen and potassium concentrations post-drought. Overall, our study suggests that in the temperate grassland studied drought had an overriding effect on productivity and tissue stoichiometry, which was amplified by warming, but only weakly altered by elevated CO₂. 

How to cite: Bahn, M., Reinthaler, D., Piepho, H.-P., Pötsch, E., Schaumberger, A., Herndl, M., Meeran, K., Kaufmann, R., Radolinski, J., and Tissink, M. and the ClimGrass-Team: Individual versus combined effects of elevated CO2, warming and drought on grassland productivity and stoichiometry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15918, https://doi.org/10.5194/egusphere-egu23-15918, 2023.

Both natural and managed ecosystems contribute to mitigating climate change through the process of CO₂ sequestration from the atmosphere. However, projections of future global climate indicate that extreme weather events will become more frequent and more intense in the coming years. And since heat waves and droughts can alter the ecosystems functioning, and increase the vulnerability of carbon sinks, this trend represents a potential risk for their contribution to reaching global climate change mitigation goals. Further efforts are therefore needed to assess the resistance and resilience of different ecosystems and land uses to climate change. In this context, Alpine mountain ecosystems face a double challenge, on the one hand, warming in the Alps is occurring twice as fast as in other regions of the planet and drought events are increasingly frequent, on the other, socio-economic changes have led to partial land abandonment, with effects on the composition and distribution of plant species.

The main objective of this study is to analyze the impacts of extreme heat and drought events on the functioning of two different ecosystems in the Alps, a European larch forest (Larix decidua Mill.) and an abandoned subalpine pasture dominated by Nardus stricta, both located in the Aosta Valley region (Italy) at about 2100 m asl and thus experiencing the same climate conditions. The eddy covariance method was used to measure the carbon and water fluxes between the ecosystem and the atmosphere. Radiometric vegetation indices (eg. NDVI), and field observations related to plant phenology were used to explain the role of timing in determining the carbon and water fluxes impacts. Finally, functional traits of plant species were used to interpret the divergent ecosystem responses from an adaptive perspective. The results show that the different heat and drought events observed during the ten-year study period (2012-2022) had a variable impact on the different ecosystems monitored, also based on the timing of the extreme event in relation to the phenology of the vegetation and the presence/absence of the snowpack, with impacts generally more severe for the grassland compared to the forest. The contrasting responses observed will be discussed by exploring the linkage between the functioning of the whole ecosystem and the adaptive strategies of individual plant species.

How to cite: Galvagno, M., Oddi, L., Cremonese, E., Filippa, G., Migliavacca, M., and Wohlfahrt, G.: Divergent responses of mountain forests and grasslands to heat and drought events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3858, https://doi.org/10.5194/egusphere-egu23-3858, 2023.

In this research we consider the response of soil respiration under elevated CO₂ (eCO₂) in an oak-dominated temperate forest. We hypothesised that under elevated CO₂ (550 ppm) soil moisture would increase as a result of reduced stomatal conductance, which would in turn lead to higher soil respiration. Continuous measurements were performed on three pairs of plots near Stafford (United Kingdom). Respiration was measured diurnally for 2 minutes each time, using the LI-COR 8100A set-up, and the rate of respiration (flux rate) was calculated SoilFluxPro software. Next, an empirical model was fitted to the dataset based on hourly averages of the flux rates, soil temperature, and soil moisture. Three respiration collars per plot were averaged, thus accounting for spatial variability within the site. Model parameterization and gap filling were conducted on individual plots to calculate annual rates for 2019-2021. Cross-validation was performed by using 80% (randomly selected) of each dataset for training and the remaining 20% for testing the data against the parameters obtained by the empirical models. Preliminary results suggest that annual respiration rates were significantly higher for the eCO₂ across all pairs in 2019. However, 2 out of 3 pairs in 2020 and 2021 showed significantly higher respiration for the aCO₂ plots compared to eCO₂, which is not in line with our hypothesis. Relationships with soil moisture and temperature help to explain what drives the difference in these fluxes. Our findings show that the relationship between higher CO₂ concentrations in the atmosphere and soil respiration is not a straightforward one, which is of interest when considering the role of forest C-cycling on a global scale.

How to cite: Douwes Dekker, N., Pendall, E., Hamilton, L., Barba, J., Pihlblad, J., Mackenzie, R., Kourmouli, A., Yamulki, S., Gauci, V., and Ullah, S.: Inter-annual variability in the response of soil respiration to elevated CO2 concentrations in the atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11200, https://doi.org/10.5194/egusphere-egu23-11200, 2023.

The global carbon cycle is highly affected by peatlands as they accumulate up to 40% of the soil carbon (C) stored globally. Gross primary productivity (GPP) is a key driver of this accumulation, it determines the amount of atmospheric CO₂ sequestered into biomass. One of the most widespread techniques to measure CO₂ exchange with the atmosphere are closed-chamber method, however, this technique is limited to small-scale studies and is dependent on spatial and temporal interpolations. A multi-model approach combining a water table depth (WTD) based model, classic rectangular hyperbolic (RH) models, modified RH models with temperature factors and an exponential model is used to study the effect of climate manipulation in a temperate peatland, selecting in each timestep the best model based on the Akaike Information Criterion corrected, p-value, root mean square error (RMSE) and R² results.

Climatic conditions are changing rapidly, affecting the peatlands’ capability to sequester and store C, enhancing decomposition and changing vegetation cover and performance. Temperature and precipitation highly impact vegetation performance inducing stress, which is why climate manipulation experiments have increased over time following our concerns about climate change. The effect of temperature on vegetation productivity under warming and reduced precipitation conditions was observed after 3 years of climate manipulation in a temperate peatland. As shown by the partial least-squares regression while during the first year, the variance of GPP explained by temperature was 51% and 59% respectively, it increased to 73% for warming (W) and warming plus reduced precipitation (WP) sites and equally, the correlation between temperature and GPP rose from -0.81 (W) and -0.76 (WP) to -0.85. Additionally, when the annual average values of WTD increased from -19.3 ± 7.4 cm in 2018 to -16.7 ± 6.8 cm in 2021, the effect of 15-day average WTD oscillations in vegetation productivity became minor, from 63% of GPP variance explained by WTD to just 18% as more water was available. The effects are also visible in GPP annual cumulative values, increasing from -671 g CO₂-C m^-2y^-1 and -672 g CO₂-C m^-2 y^-1 in 2019 to -883 g CO₂-C m^-2 y^-1 and -963 g CO₂-C m^-2 y^-1 in 2021 respectively for W and WP sites. It is undeniable that the effect of climate manipulation has induced changes in vegetation composition which produce the changes in temperature and WTD response of GPP and at the same time, the cumulative GPP yearly. The changes in vegetation composition can be observed through the comparison with control plots where the WTD effect remained significant, explaining still in 2021 24.5% of the GPP variance while it is 16% and 13% for W and WP, probably with vegetation less adapted to climate extremes and more WTD-dependent.

This work was supported by the National Science Centre of Poland (NCN) under projects No. 2016/21/B/ST10/02271 and 2020/37/B/ST10/01213.

How to cite: Albert-Saiz, M., Strozecki, M., Rastogi, A., and Juszczak, R.: The increasing effect of temperature on vegetation productivity under climate manipulation and water table depth alteration in a temperate peatland., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14993, https://doi.org/10.5194/egusphere-egu23-14993, 2023.

Most Earth system models (ESMs) do not explicitly represent the carbon (C) costs of plant nutrient acquisition, which leads to uncertainty in predictions of the current and future constraints to the land C sink. While plants acquire nutrients through different uptake pathways, such as from mycorrhizae, direct root uptake, retranslocation from senescing tissues, and biological fixation in the case of nitrogen (N), they usually have different associated C costs. Determining the amount of nutrients acquired through each uptake pathway and the associated C cost could increase understanding of the global C and nutrient cycles, as well as the predictive skills of ESMs.

Here, we integrate a plant productivity-optimizing nutrient (N and phosphorus (P)) acquisition model (Fixation & Uptake of Nutrients, FUN) into the Energy Exascale Earth System (E3SM) Land Model (ELM) to simulate the global C and nutrient cycles (Braghiere et al., 2022). We benchmarked the model with observations (in-situ, remotely sensed, and integrated using artificial intelligence), and other ESMs from CMIP6; we found significant improvements in present C cycle variables estimates. We also examine the impact of mycorrhizal spatial distributions on the global C cycle, since most plant species predominantly associate with a single type of mycorrhizal fungi and uncertainties in mycorrhizal distributions are non-trivial, with current estimates disagreeing in up to 50% over 40% of the land area (Braghiere et al., 2021). Global Net Primary Productivity (NPP) is reduced by 20% with N costs and 50% with NP costs, while modeled and observed nutrient limitation agreement increases when N and P are considered together. Even though NPP has been growing globally in response to increasing CO₂, as soil nutrient progressively becomes more limiting, the costs to NPP for nutrient acquisition have increased at a faster rate. This suggests that nutrient acquisition will increasingly demand a higher portion of assimilated C to support the same productivity.

Braghiere, et al. (2022) doi.org/10.1029/2022MS003204

Braghiere, et al. (2021) doi.org/10.1029/2021GL094514

How to cite: Braghiere, R. K., Fisher, J., Allen, K., Brzostek, E., Shi, M., Yang, X., Ricciuto, D., Fisher, R., Zhu, Q., Phillips, R., Sulman, B., Steidinger, B., Soudzilovskaia, N., Liang, J., Peay, K., and Crowther, T.: ­­Carbon Costs of Plant Nutrient Acquisition Improve Present-Day Carbon Cycle Estimates and Limit CO2 Fertilization Effect, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2946, https://doi.org/10.5194/egusphere-egu23-2946, 2023.

Nitrous oxide (N₂O) is a greenhouse gas and ozone-depleting agent that is predominately emitted from the land biosphere, linking to the yet only poorly understood carbon-nitrogen cycle. The seasonal cycle of the tropospheric N₂O mixing ratio (aN₂O), measured at globally distributed air sampling sites, offers an observational model constraint. Recent studies attribute the aN₂O seasonality to exchange with N₂O-depleted stratospheric air. Yet, how land biosphere N₂O fluxes contribute to the seasonal amplitude, phasing, and amplitude growth of aN₂O has not been well understood at global scales.

Here we apply surface-atmosphere fluxes from global simulations of the Nitrogen/N₂O Model Inter-comparison Project (NMIP) and the Bern3D ocean model to atmospheric transport matrices to simulate aN₂O at air sampling sites. Land N₂O fluxes from eight NMIP models show broad agreement on seasonal phasing. In contrast, seasonal amplitudes of regionally averaged fluxes differ severalfold across models. For example, seasonal amplitudes range from 2.2 to 5.4 TgN yr^­-1 (median: 3.7) for 20^oN-40^oN and from 1.1 to 4.5 TgN yr­^-1 (2.3) for 40^oN-60^oN during 2001 to 2020. The amplitudes for these two regions increased on average by 155% and 84%, respectively, over the industrial period. The increase predominantly results from anthropogenic activities, e.g, fertilizer application. The seasonal amplitude of regional ocean-atmosphere fluxes (0.2 to 1.1 TgN yr^-1) and their changes are comparably small.

The NMIP land fluxes result in seasonal amplitudes of aN₂O on average ranging from 0.27 to 0.84 ppb (observed: 0.23 to 0.91 ppb) at six selected stations (Alert and Barrow, Ascension Island, Ragged Point, and Samoa, and Cape Grim). The model spread in aN₂O amplitude is up to 0.55 ppb and, thus, large in comparison with observations. The contributions from Bern3D ocean fluxes to aN₂O seasonality at the six stations (0.13 to 0.26 ppb) are generally smaller than from land. Substantial data-model mismatches in aN₂O seasonal amplitudes and phasing are likely due to neglecting stratospheric fluxes in our models.

Our results demonstrate significant contributions of land biosphere N₂O emissions to aN₂O seasonality. Model uncertainties in land biosphere fluxes translate into large uncertainties in aN₂O seasonality, calling for land biosphere model improvements. In situ aN₂O observations, in combination with atmospheric transport and chemistry models, potentially provide a novel top-down constraint for global land biosphere models towards improved projections of C-N coupling, the land carbon sink, and atmospheric CO₂ and N₂O.

How to cite: Sun, Q., Lienert, S., and Joos, F.: The seasonal cycles of land biosphere N2O fluxes and atmospheric N2O, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6649, https://doi.org/10.5194/egusphere-egu23-6649, 2023.

Terrestrial ecosystem acts as important carbon dioxide (CO₂) sinks and nitrous oxide (N₂O) sources. Ecosystem green-house-gas fluxes could further lead to a climate feedback, which are highly correlated with the C-N coupling. However, magnitudes of such composited feedbacks as well as contributions by individual ecological processes remained certain uncertainties. Here, we applied a terrestrial biosphere model QUINCY with fully C-N-coupling to comprehensively examine responses of CO₂ and N₂O fluxes to future climate changes and quantify contributions by individual processes. Our results showed that the trade-offs in CO₂ and N₂O still led a negative feedback (-3386.9 Tg CO₂ yr^-1) averaged over 2050-2100 under the SSP 5-8.5 scenario relative to SSP 1-2.6 scenario, however, which varies from -1761.7 Tg CO₂ yr^-1 to -5012.1 CO₂ yr^-1 with a high or low climate sensitivity to CO₂ increases.  Further process analysis showed that the CO₂ fertilization effects on ecosystem climate feedbacks were equivalent for high and low climate sensitivity, but the higher climate sensitivity led to less carbon sequestration on tropical plants as well as higher N₂O emissions. The climate feedbacks attributed to individual soil processes, including decomposition, nitrification, denitrification and nitrogen biological fixation, were also comprehensively quantified. This finding suggests that reducing the uncertainties in climate sensitivity estimates could be of great significance to better predict future terrestrial ecosystem green-house-gas fluxes as well as corresponding climate feedbacks.

How to cite: Gong, C., Caldararu, S., Engel, J., Nabel, J., and Zaehle, S.: Higher climate sensitivities weaken negative climate feedbacks of terrestrial ecosystem through carbon sequestration and nitrous oxide emissions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3822, https://doi.org/10.5194/egusphere-egu23-3822, 2023.

Predicting the responses of terrestrial ecosystems to future global change strongly relies on our ability to accurately model global scale vegetation dynamics and surface CO₂ fluxes. However, terrestrial biosphere model carbon cycle processes remain subject to large uncertainties, partly because of unknown or poorly calibrated parameters. 

We can use the unprecedented amount of in situ and Earth Observation data to optimize these model parameters, as well as the considerable advances in parameter estimation techniques. Most of these techniques use Bayesian data assimilation approaches, which allow for objective calibrations of key model processes and parameters against observations, reducing the associated uncertainty. However, calibrating against present-day observations does not necessarily give us confidence in the future projections of the model, given that they are likely to exceed historical and present-day conditions. The relatively shorter timescales of present-day observations mean these cannot be directly used to create constraints on changes in the Earth System over the next century. Instead, we need to develop methods to translate short-term constraints into reductions in long-term projection uncertainty, bridging the gap between contemporary model optimisations and future predictions.

In this presentation, we will discuss two experiments highlighting how we can use parameter estimation to reduce model uncertainty and translate this information into constraints on future climate.

The first demonstrates how we can use manipulation experiments to increase our confidence in optimized parameters. We use data from two nitrogen-limited sites from the Free Air CO₂ Enrichment experiment to optimize model parameters. The optimization is performed against ambient and elevated CO₂ conditions simultaneously, giving us a better insight into nitrogen limitations on CO₂ fertilization at these sites.

The second demonstrates how we can combine local model calibration with the emergent constraint approach. Using a parameter perturbation ensemble, we identify an emergent relationship between the optimal temperature for photosynthesis in tropical forests, and the projected amount of atmospheric CO₂ at the end of the century. We combine this with a constraint on the optimum temperature for photosynthesis in tropical forests derived from eddy-covariance measurements and parameter estimation techniques to reduce the likely range of future atmospheric CO₂ in a coupled climate-carbon cycle model under a common emissions scenario.

How to cite: Raoult, N., Peylin, P., and Cox, P.: Using parameter estimation to reduce future climate-carbon cycle projection uncertainty, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6448, https://doi.org/10.5194/egusphere-egu23-6448, 2023.

The ability of forests to continue absorbing atmospheric CO₂, and hence mitigating climate change, is constrained by global change drivers, such as increasing nitrogen deposition induced by anthropogenic activities. N input from atmospheric deposition can positively affect forest productivity in N-limited temperate and boreal ecosystems. However, under N saturation conditions, a cascade of negative effects can be observed, leading to tree growth decline, increase in soil acidification and N loss pathways, and nutrient imbalance. Experimental simulations of N deposition have been extensively used to directly determine whether atmospheric N input contributes to alleviating N limitation and to understand ecosystem responses. However, the majority of these experiments have considered soil N applications, often applying N doses several order of magnitude higher than ambient deposition, thus not mimicking realistic changes in atmospheric deposition. Moreover, soil applications exclude a priori atmosphere-canopy exchange processes, including direct canopy N uptake. In this context, the experiment established in a mature and eutrophic Fagus sylvatica L. forest in Italy represents a unique resource for advancing understanding on forest responses to global change. At this site, four different treatments have been carried out since 2015: control, canopy (30 kg ha^-1 yr^-1) and soil (30 and 60 kg ha^-1 yr^-1) N applications, with doses reflecting more realistic changes in atmospheric deposition (about twice and three times as much as ambient deposition). We asked whether the two experimental approaches (canopy vs. soil applications) can lead to different responses in terms of i) tree growth and intrinsic water-use efficiency (the ratio between photosynthesis and stomatal conductance) and ii) ecosystem nitrogen processes (including plant-microbe interactions). For this purpose, we combined dendroecological analyses (the measure of ring widths and stable carbon isotope composition, δ¹³C) with the measure of foliar nutrient concentrations and stable nitrogen isotope composition (δ¹⁵N) in different forest samples, including foliar, litter, soil main root and ectomycorrhizal root tips. On-going δ¹³C analyses will provide insight regarding the physiological mechanisms underpinning growth changes in relation to different treatments. Whereas, δ¹⁵N in different forest samples will help to elucidate differences in ecosystem N processes between the two approaches, i.e., N retention within the system or increasing N loss pathways and other nutrient limitations due to N saturation. 

How to cite: Guerrieri, R., Teglia, A., Ravaioli, D., Bucci, M., Scisci, E., Muzzi, E., and Magnani, F.: Responses of mature forests to nitrogen deposition: insights from a nitrogen manipulation experiment in Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9973, https://doi.org/10.5194/egusphere-egu23-9973, 2023.

Anthropogenic nitrogen (N) deposition and fertilization in the past decades shifted the ecosystem stoichiometry with potentially profound impacts on vegetation activity and ecosystem functioning. Current N-addition experiments mostly focus on leaf-level or individual plants at the plot scale, whereas studies investigating the responses of vegetation dynamics to N-addition at the landscape level are lacking. It is especially unclear how ecosystems with different water availability (water-limited versus water-surplus) respond to elevated N input. We compared vegetation dynamics and ecosystem functioning in two unique ecosystem-scale N addition experiments – one Mediterranean tree-grass ecosystem and one boreal evergreen forest. At each site, one pair of landscape-scale N addition was set up by adding N onto the footprint area of one eddy covariance (EC) tower, with another EC tower without N addition used as a control. We hypothesize that their different water availability can exert different responses in vegetation phenology and gross primary productivity (GPP).  Since the start of the experiments, we found that the fertilized treatments in both experimental sites have had higher amplitudes of GPP and more rapid green-up and leaf senescence compared to the control. However, phenological transition dates (PTDs) defining the start and end of the growing seasons (SOS, EOS), derived from GPP at the two sites show different patterns. During the green-up period, SOS was similar between the fertilized and control treatments at the Mediterranean site since the vegetation green-up here was mainly controlled by the timing of precipitation. In the boreal forest, however, the fertilized treatment was greening slightly later than the control. In the leaf senescence period, the fertilized treatment senesced earlier in the Mediterranean ecosystem compared to the control. In contrast, the fertilized treatment delayed in EOS compared to the control in boreal forests.  We propose that increased leaf area index and canopy greenness in the fertilized (based on the observed increasing evapotranspiration) compared to the control treatment in both ecosystems, along with different vegetation composition, probably contributes to the divergent response of EOS at the two sites with different water availability. This study underscores the necessity to take different water availability into account when evaluating the effects of N addition on vegetation dynamics. 

How to cite: Luo, Y., D`Odorico, P., Lee, S.-C., Ma, X., Migglivacca, M., Peichl, M., Stocker, B., and Gessler, A.: Divergent phenology response to nitrogen addition between a Mediterranean and a boreal forest, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10303, https://doi.org/10.5194/egusphere-egu23-10303, 2023.

Understanding the interactions between atmosphere and vegetation in changing climatic conditions is important so that we can predict the carbon sequestration potential of ecosystems. Helpful tools here are the terrestrial biosphere models (TBMs), since they include detailed ecophysiological process descriptions, e.g. the manifold interactions between the carbon and nitrogen cycles. However, the modelling of the nitrogen cycle poses challenges and having observational constraints on nitrogen cycle is crucial. Current remote sensing products offer estimates of leaf chlorophyll (Cab), that is related to the nitrogen cycle. In this study we want to assess how useful Cab observations are at site scale to constrain a TBM.

In this work we are studying a temperate mixed forest, Borden, located in Canada. We use a TBM QUantifying Interactions between terrestrial Nutrient Cycles, QUINCY, to model this site. From the site we have long-term (20 years) flux tower and LAI (from PAR measurements) observations together with leaf level observations of leaf chlorophyll (Cab), leaf nitrogen, and photochemical parameters of maximum carboxylation rate (Vcmax) and maximum potential electron transport rate (Jmax). 

The QUINCY model was predicting too late leaf senescence, which we tuned using the site level data. The amount of leaf nitrogen was originally quite successfully simulated by QUINCY, but the amount of simulated Cab was too low. Matching the simulated Cab values with the observations did not have a pronounced effect on the GPP. Additionally, the development of LAI and Cab were originally fully coupled in QUINCY, whereas the observations showed a delayed development of Cab compared to LAI. When we implemented this decoupling between LAI and Cab, an improvement of simulated GPP compared to the observations was found. Also then the simulated Vcmax and Jmax showed better correspondence to the observations. 

Assessment of the long-term behaviour of the model at the site showed that the model was able to capture the drought-induced drawdown of carbon fluxes taking place in 2007. The observations showed an increase in the component fluxes of carbon during the time period, but this was not replicated by the model. The start of season (SOS) and end of season (EOS) were estimated from both the simulated and observed GPP and LAI using a simple threshold method. The model was more successful in capturing the changes in the growing season metrics estimated by LAI than by GPP. The model was predicting too late onset of GPP in many years, but captured largely the interannual variation of SOS in observed GPP. 

This study paves the way for work using remotely sensed leaf chlorophyll in evaluation and improvement of the QUINCY model.

How to cite: Thum, T., Seppälä, O., Croft, H., Caldararu, S., Ojasalo, A., Rogers, C., Staebler, R., and Zaehle, S.: Using leaf chlorophyll observations to improve carbon cycle modelling at a temperate mixed forest, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2202, https://doi.org/10.5194/egusphere-egu23-2202, 2023.

How to cite: Steller, C., Humphrey, V., Fischer, E., and Knutti, R.: The Rising Pulse of Land Carbon Uptake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14426, https://doi.org/10.5194/egusphere-egu23-14426, 2023.

Heat waves are predicted to increase in frequency and intensity, endangering the productivity of agroecosystems including grapevines. In this work, we analyzed the effects of the long and intense heat wave (HW) occurring in July 2022 on the eco-physiological performance of a vineyard located in northern Italy (Caldaro, Province of Bolzano). The vineyard hosts two white grape cultivars (Sauvignon Blanc and Chardonnay on SO4 rootstock) with an average planting density of 6,500 vines ha^-1 and is equipped with a drip irrigation system. The interrow alleys are covered by grasses or cover crops. Summer 2022 showed a continuous daily temperature increase from the second week of July and peaked in a heat wave lasting 8 days (DOYs 196-203) with maximal Tmax= 38 and average Tmax= 36°C. The HW period was then interrupted by summer storms and rain. 

The methodology included continuous monitoring of NEE (-NEP), Reco, GPP, ET and energy fluxes by eddy covariance method. Continuous measurements at the plant scale included sun-induced chlorophyll fluorescence, active chlorophyll fluorescence, and sap-flow rates. Periodically, leaf gas exchange, leaf water potential and chlorophyll content were also recorded. The period considered for the analysis stretched from July 1 to August 11.

There was a general mild decreasing trend of GPP when the air temperature increased in July, reaching the minimum values during the HW (< 10 g C m^-2d^-1). Leaf photosynthesis also slightly decreased during the HW in both varieties down to values of 6-7 µmol CO₂ m^-2s^-1. Interestingly, even during the HW, GPP always markedly increased the day after irrigation water was supplied (5 irrigation events were performed in July with a total of approximately 45 L/vine).

Reco decreased when the air temperature increased, but a few days before the end of the HW it increased again, a process that was even more pronounced following the rainy period after the HW when negative values of NEP were recorded.

Vineyard ET and vine transpiration did not show a clear response to the HW, while they both increased each time vines were irrigated, suggesting that most ET in that period derived from vine transpiration and not from the vineyard alleys. Interestingly, after the end of the HW, vineyard ET did not increase despite the increased soil moisture occurring also in the alleys. The energy partitioning was unaffected by the HW, with the Bowen ratio dropping below 0.4 after the irrigations. Continuous monitoring of active chlorophyll fluorescence showed a slight increase in NPQ parameter (measured only on Chardonnay leaves) in the last and hotter days of the HW, while Fv/Fm was not affected by the higher temperatures.

In conclusion, despite the exceptional (for the cultivation area) intensity of the 2022  HW, the vines showed good tolerance to heat stress. We speculate that irrigation played an important role in the vines’ performance. The NEP, however, decreased during the HW and especially after its end due to soil-moisture triggered increasing Reco, which shifted the vineyard from sink to source of atmospheric CO₂.

How to cite: Zanotelli, D., Asensio, D., Schwarz, M., Benyahia, F., Hammerle, A., Ben Abdelkader, A., Bastos Campos, F., Callesen, T., Andreotti, C., Montagnani, L., Tagliavini, M., and Wohlfahrt, G.: Vineyard water and carbon dioxide exchange during a heat wave, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10124, https://doi.org/10.5194/egusphere-egu23-10124, 2023.

The influence of dry-wet cycles (DWC) on soil organic carbon (SOC) decomposition is still debated given the somehow controversial results observed in the literature. The objective of this study was to evaluate the effects of DWC on SOC mineralization relative to various moisture controls in 7 treatments from two long-term French field experiments presenting contrasted SOC concentrations. We conducted a laboratory incubation to quantify CO₂ emissions upon four soil moisture scenarios: continuously wet (WET), continuously moderately wet (MWET), continuously dry (DRY) and dry-wet cycles (DWC). We also calculated the SOC mineralization that would correspond to the average water content in DWC (mean_DWC). Our results showed that across all treatments the daily carbon mineralization rate increased with soil moisture (WET>MWET>DRY). In DWC scenario, mineralization rates fluctuated with the changes in soil moisture. As soils dried, daily mineralization rates decreased and the subsequent soil rewetting, to pF 1.5, caused a rapid mineralization flush or "Birch effect". However, these flushes did not compensate for the low mineralization rates in the drying phase as the cumulative mineralization was not higher in the DWC scenario compared to the mean_DWC which was the scenario with equivalent water content as the DWC. We also observed that not accounting the CO₂ emissions in the drying phase could lead to an overestimation of the effect of DWC. We recommend to measure continuously the soil respiration during dry-wet experiments and to compare the CO₂ emitted in DWC with a control that has a water content equivalent to the average water content in DWC. In addition, we questioned the importance of the effect of dry-wet cycles on overall soil respiration.

How to cite: Kpemoua, T. I., Barré, P., Houot, S., and Chenu, C.: Accurate evaluation of the Birch effect requires continuous CO2 measurements and relevant controls, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8575, https://doi.org/10.5194/egusphere-egu23-8575, 2023.

Soil carbon (C) and nitrogen (N) cycles and their complex responses to environmental changes have received increasing attention. However, large uncertainties in model predictions remain, partially due to the lack of explicit representation and parameterization of microbial processes. One great challenge is to effectively integrate rich microbial functional traits into ecosystem modeling for better predictions. Here, using soil enzymes as indicators of soil function, we developed a competitive dynamic enzyme allocation scheme and detailed enzyme-mediated soil inorganic N processes in the Microbial-ENzyme Decomposition (MEND) model. We conducted a rigorous calibration and validation of MEND with diverse soil C-N fluxes, microbial C:N ratios, and functional gene abundances from a 12-year CO₂×N grassland experiment (BioCON) in Minnesota, USA. In addition to accurately simulating soil CO₂ fluxes and multiple N variables, the model correctly predicted microbial C:N ratios and their negative response to enriched N supply. Model validation further showed that, compared to the changes in simulated enzyme concentrations and decomposition rates, the changes in simulated activities of eight C-N associated enzymes were better explained by the measured gene abundances in responses to elevated atmospheric CO₂ concentration. Our results demonstrated that using enzymes as indicators of soil function and validating model predictions with functional gene abundances in ecosystem modeling can provide a basis for testing hypotheses about microbially-mediated biogeochemical processes in response to environmental changes. Further development and applications of the modeling framework presented here will enable microbial ecologists to address ecosystem-level questions beyond empirical observations, toward more predictive understanding, an ultimate goal of microbial ecology.

How to cite: Wang, G.: Soil enzymes as indicators of soil function: a step toward greater realism in microbial ecological modeling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1576, https://doi.org/10.5194/egusphere-egu23-1576, 2023.

Current models of soil biogeochemistry are facing difficulties to match the observed amount and composition of soil organic carbon (SOC). An important omission is that nitrogen (N) can directly and interactively affect microbial decomposition processes, rather than merely being a nutrient for plant production and a passive subordinate of C flow based on stoichiometric coupling. In the past decades, many litter manipulation studies have shown that SOC stocks do not increase significantly in response to experimental increases of litter C input. Moreover, many N fertilization studies showed that SOC accrual is predominantly driven by changes in heterotrophic respiration instead of litter production; i.e., soluble N directly affects multiple processes such as the activities of various C-cycling enzymes, microbial growth (e.g., via carbon use efficiency, CUE), and necromass production.

In order to reconcile empirical insights with simulated patterns of SOC and N dynamics, we developed variants of the Century soil submodel coupled with the stand-scale forest gap model ForClim. We implemented equations to capture neglected processes, including 1) the effect of available N on lignin decomposition; 2) the effect of available N on low C:N substrates (e.g., protein) degradation; 3) the effect of available N on microbial CUE dynamics. We tested the full combination of model variants comprising new or alternative equations against multi-level observational patterns, over large gradients of climate and fertility in Swiss forests.

The models reconcile that factors affecting C output (decomposition) predominantly control the site-to-site variation of equilibrium SOC stocks, instead of factors affecting litter C input. We further identified the individual processes essential for shaping the geographical pattern (across different climates, forests, and soil types) of the amount and composition of SOC. We conclude that taking into account the direct effects of available N on microbial processes is essential to improve the realism and accuracy of soil models under global change (i.e., reconciling the mainstream theory of plant-derived vs. microbial-derived SOC accumulation pathways), thus avoiding the need of frequent parameter tuning inherent in “C-centric” models.

How to cite: Yeung, C. C., Diaz Yanez, O., and Bugmann, H.: Nitrogen availability as a master control of plant-derived vs. microbial-derived soil carbon accumulation – insights from a novel model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10109, https://doi.org/10.5194/egusphere-egu23-10109, 2023.

Currently, few scalable, cost-effective CO₂ removal (CDR) strategies exist to mitigate anthropogenic climate change. Enhanced rock weathering (ERW) is a strategy in which finely ground silicate rock reacts with atmospheric CO₂ and produces weathering products that are transported to the ocean for long-term storage. Despite detailed knowledge of chemical weathering and its role in the carbon cycles at geologic timescales, few data display the efficacy of ERW for CDR at timescales appropriate for climate mitigation. To address this, we use the first large-scale field trial of ERW combined with tree planting at an afforestation experiment in mid-Wales. A factorial experimental design will enable us to determine the influence of basalt application and tree functional type on rock weathering and nutrient cycling parameters, such as soil pore water pH, alkalinity, cation concentrations and soil carbon. Here, we focus on the description and installation of the experiment, its monitoring protocol, and data analysis from the first two years of the site observations. We also outline how the data can be introduced into a mechanistic eco-hydrological model. Ultimately, we aim to synthesize these findings to inform predictions of global regions where ERW could be most effective for CDR.

How to cite: Jones, G., Paschalis, A., and Waring, B.: Harnessing enhanced rock weathering in a forestry context, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5716, https://doi.org/10.5194/egusphere-egu23-5716, 2023.