BG3.25

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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