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Naturally occuring pyrogenic carbon (PyC) is produced during wildfires under oxygen limiting conditions. After a fire event, PyC is fragmented, dissolved and transported at the soil surface1,2 and/or downward into the soil3,4⁠. PyC represents on average 15 % of organic carbon in soils and sediments5⁠.Its residence time in soil ranges from 50 to 1000 years6⁠,which makes it the most persistent form of organic carbon in soils. However, at the mouth of the world’s largest rivers, PyC is on average 16,000 years old. This difference is probably due to isolated measurements of turnover time in surface soil horizons which does not take into consideration transport and accumulation processes happening at the landscape scale. We make the following hypothesis : (i) PyC accumulates at depth in soil and in lowland and hill-foot positions, and (ii) PyC in accumulation zones is significantly older than PyC from other sites/depths.We studied the dynamics of PyC in a well characterized, 120 ha watershed in Brittany, France (ORE AgrHys). We collected soil cores at different topographic positions along three transects and quantified PyC using standard (chemo-thermal oxidation, hydrogen pyrolysis) and novel (Rock-Eval thermal analysis) analytical methods. We also measured the 14C ages of the PyC fraction. We show that relative to total SOC, PyC is preferentially redistributed to depth and that the subsoil (30 to 60 cm) represents about a third of the total soil PyC stock. We do not observe accumulation of PyC at the hill-foot except where superficial erosion products are retained before reaching the stream. We discuss the potential sources and redistribution mechanisms of PyC in the area over the last 10000 years.1. Bellè, S.-L. et al. Biogeosciences Discuss. 1–35 (2021)2. Rumpel, C., Ba, A., Darboux, F., Chaplot, V. & Planchon, O. Geoderma 154, 131–137 (2009).3. Soucémarianadin, L. et al. Soil Biol. Biochem. 133, 12–15 (2019).4. Schiedung, M., Bellè, S. L., Sigmund, G., Kalbitz, K. & Abiven, S. Biogeosciences 17, 6457–6474 (2020).5. Reisser, M., Purves, R. S., Schmidt, M. W. I. & Abiven, S. Front. Earth Sci. 4, 1–14 (2016).6. Singh, N., Abiven, S., Torn, M. S. & Schmidt, M. W. I. Biogeosciences 9, 2847–2857 (2012).

Assessing soil organic carbon biogeochemical stability is critical for estimating future changes in soil carbon stocks. Several methods for the assessment of soil organic carbon (SOC) biogeochemical stability have been proposed but very few can be implemented on large sample sets. Indeed, to date, only simple physical fractionation protocols (e.g. Lavallee et al., 2020) and Rock-Eval® thermal analysis techniques (Delahaie et al., 2022, SOIL discussion) have been implemented on data sets larger than a few hundred samples. Simple fractionation techniques allow separating a particulate organic carbon fraction (POC; considered labile) and an organic fraction associated with minerals (MaOC; considered more stable). Regarding thermal analyses, Rock-Eval® results associated to the PARTYsoc machine-learning model (Cécillon et al., 2021) provide a measure of the active (mean residence time of ca. 30 years) and centennially stable SOC fractions.In this study, we present the results of physical fractionations performed on ca. 1000 samples and thermal analyses performed on ca. 2000 samples from French mainland topsoils (RMQS program). We compare the amount and the drivers of each fraction. Our results show that most of the MaOC fraction is not stable at a centennial timescale. However, we show using a Random Forest model that the MaOC content and the centennially stable SOC content are similarly influenced by a common set of drivers: clay, pH and climatic conditions (mean annual temperature and mean annual precipitation). Finally, we discuss the complementarity of these two types of relatively high-throughput fractionation protocols. ReferencesCécillon, L., Baudin, F., Chenu, C., Christensen, B. T., Franko, U., Houot, S., Kanari, E., Kätterer, T., Merbach, I., van Oort, F., Poeplau, C., Quezada, J. C., Savignac, F., Soucémarianadin, L. N., & Barré, P. (2021). Partitioning soil organic carbon into its centennially stable and active fractions with machine-learning models based on Rock-Eval® thermal analysis (PARTY SOC v2. 0 and PARTY SOC v2. 0 EU). Geoscientific Model Development, 14(6), 3879-3898. Delahaie, A. A., Barré, P., Baudin, F., Arrouays, D., Bispo, A., Boulonne, L., Chenu, C., Jolivet, C., Martin, M. P., Ratié, C., Saby, N. P. A., Savignac, F., & Cécillon, L. (2022). Elemental stoichiometry and Rock-Eval® thermal stability of organic matter in French topsoils. EGUsphere, 1-31. Lavallee, J. M., Soong, J. L., & Cotrufo, M. F. (2020). Conceptualizing soil organic matter into particulate and mineral‐associated forms to address global change in the 21st century. Global Change Biology, 26(1), 261-273.

Biomass pyrolysis and a non-oxidative use of the produced biochar is widely acknowledged as a negative emission technology and part of Pyrogenic Carbon Capture and Storage (PyCCS). Biochar with a molar H/C ratio < 0.4, which is usually achieved by pyrolysis at 550°C or above, is highly persistent when applied to the soil. Still, the exact residence time remains subject to debate. Practical assessment tools and reliable models for carbon accounting are needed. Persistence of soil applied biochar is often assessed using soil incubation trials of rather short time horizons, lasting several month or years, with consecutive extrapolation of the observed degradation rate. Within such experiments, the decomposition rate of the biochar continues to decrease over time indicating that biochar consists of a broad range of carbonaceous compounds of different recalcitrance. Hydrogen pyrolysis, an analytical method used to determine the degree of aromatisation, suggests that 75% of the carbon in biochar with an overall H/C ratio 1000 years, (Bowring et al., 2022; Howell et al., 2022), independent of the soil type and climate. The remaining 25% of the biochar carbon (heteroaromatic, aliphatic, etc) are considered semi-persistent carbon (SPC), presenting a mean residence time (MRT) in soil of 50 to 100 years, depending on soil type and climate. Based on this, up to 99% of the PyC loss quantified in an incubation trial may be attributed to the spectrum of SPC compounds, while the occurring PAC decay is very small and a neglectable loss in the context of carbon sink accounting. To validate the PAC residence time, data on long-term dynamics of the global, natural PyC cycle provides new insights. Natural PyC is produced from incomplete combustion in e.g. forest fires and is introduced to ecosystems globally at a scale of 0.114-0.383 Gt year-1. Given the global deposits of natural PyC of 550-1,650 Gt a MRT of 1,440 to 14,500 years can be calculated (Bird et al., 2015; Santín et al., 2016; Bowring et al., 2022 ). Natural PyC is produced in an uncontrolled manner, thus achieving a lower degree of aromatisation, a smaller PAC pool and lower MRT compared to optimized PyC produced by controlled pyrolysis. Therefore, observations from the natural PyC cycle render the assumed PAC residence time conservative. Further research is necessary to enable empirical quantification of the PAC content of biochar across a broad range of feedstock material and pyrolysis conditions. Hydrogen pyrolysis is an elaborate, yet expensive tool not suitable for routine analysis e.g. in biochar and biochar C-sink certification. Thus, further methods for PAC quantification must be developed and standardized.Bird et al. (2015). Annual Review of Earth and Planetary Sciences, 43(1), 273–298. https://doi.org/10.1146/annurev-earth-060614-105038 Bowring et al. (2022). Nature Geoscience 15:2, 15(2), 135–142. https://doi.org/10.1038/S41561-021-00892-0 Howell et al. (2022). Science of The Total Environment, 849, 157610. https://doi.org/10.1016/J.SCITOTENV.2022.157610 Santín et al. (2016). Global Change Biology, 22(1), 76–91. https://doi.org/10.1111/gcb.12985

Plants allocate carbon to roots, shoots, respiration and rhizodeposition. The quantitative partitioning depends on genetic and environmental factors, as well as on the plant’s phenology. The first two or three pools represent most of the carbon input to the soil, depending on the ecosystem considered. Quantifying this partitioning is of major importance as roots, shoots, and rhizodeposition are constituted of several organic compounds differing in residence time. Moreover, besides being carbon inputs, these compounds modify in different ways their surroundings and might slow down or accelerate the cycling of carbon and nutrients.One frequently used method to quantify this partitioning is the 13C labelling of atmospheric CO2, which allows to trace organic carbon in the soil-plant system. 13C labelling of CO2 can be either continuous, or as single or multiple pulses. It can be combined with gas measurements that estimate the priming effect induced by the plant inputs. This poster aims at synthesizing the literature about experimental quantification of carbon allocation to different plant pools, and comparing the methods used. It allows to identify the plant traits that predict this allocation. Moreover, we show the preliminary results of a continuous 13C labelling experiment that compares plant traits with carbon content variations in soil columns.Whereas shoots and roots are relatively simple to quantify, rhizodeposition is not. It therefore represents a major incertitude when considering the carbon inputs to soil. Moreover, labile exudates represent an important part of rhizodeposition that induce an increase of older soil carbon mineralisation. Several experiments show that the loss of carbon induced by rhizodeposition priming effect is bigger than the input. These results are often associated to plants with high photosynthetic activity.A trade-off is to be found when considering 13C labelling. Continuous labelling allows a quantification of net rhizodeposition which is independent of the phenology and that integrates the whole plant growth. On the other side, pulse labelling is easier and less expensive. Regarding priming effect quantification, gas flux measurements are precise but do not integrate the whole plant growth. Mass quantification through carbon content change do, but it requires very accurate estimations of these changes.

Wildfires occur regularly in boreal forests of Northern Canada and are increasing in frequency and intensity due to the impacts of projected global climate change. A by-product of these forest fires is pyrogenic carbon (PyC) as a residue of incomplete combustion. The short-and long-term dynamics of this important soil organic carbon (SOC) pool in permafrost-affected mineral soils, however, is largely unknown. Here we studied eleven boreal forest soils at distinct landscape positions under continuous (northern sites) and discontinuous (southern sites) permafrost. In these we assessed the short-term fate of 13C-labeled PyC and its precursor grass organic matter over two year in-situ soil core incubations. Further, we isolated PyC by hydrogen pyrolysis (PyCHyPy) for quantification and radiocarbon measurements to investigate long-term pools across the landscape.Losses of PyC after two years were dominated by decomposition with up to three times more PyC losses at northern sites (36%) compared to the southern sites (11%). The losses of the grass organic matter were substantial (69-84%) but losses were larger in southern soils. The PyC persistence depended on site and soil specific properties and not solely on its chemical resistance. Fresh PyC was increasingly decomposition in nutrient limited mineral soils under continuous permafrost, indicating that polyaromatic compounds can act as a nutrient source. Mineral interactions were important and contributed to the stabilization of ~40% of recovered PyC. Mineral-associated PyC mainly remained in particulate forms as identified on the microscale using NanoSIMS. Beside large grass organic matter losses, remaining fractions were recovered predominantly as particles in northern soils but highly dispersed on mineral surfaces in the southern soils on the microscale. Our results highlight that permafrost-affected boreal forest soils are sensitive to fresh PyC and organic matter inputs with substantial losses by decomposition even under continuous permafrost conditions with unique stabilisation mechanisms at the organo-mineral interface.On the long-term, we identified that the native PyCHyPy represents a millennial age carbon pool with significantly higher ages in continuous (5.5-7.8 cal. ka BP; F14C=0.44-0.54) than in discontinuous (1.2-2.2 cal. ka BP; F14C=0.76-0.88) permafrost soils in 0-15 cm soil depth. The PyCHyPy was markedly older than the bulk SOC (modern with F14C=0.65-1.11). With soil depths, PyCHyPy ages increased to >18 cal. ka BP (F14C

Pyrogenic carbon (PyC), the product of incomplete biomass combustion, is a key component of soil organic carbon (SOC) because it can persist in soils for centuries to millennia. Quantifying PyC across large spatial scales remains a significant challenge in constraining the global carbon cycle. We measured PyC in topsoils across Europe using molecular marker (benzene polycarboxylic acids, BPCA) and spectroscopic techniques (Diffuse Reflectance Infrared Fourier Transform Spectroscopy, DRIFTS). We developed a calibration between BPCA and DRIFTS, but the calibration was less reliable (Y-variance explained = 0.62) than previous reports due to low soil PyC content and heterogeneity of soil matrices. Thus, we performed multiple regressions to identify drivers of PyC distribution using only the measured BPCA data. PyC content varied widely among soils, contributing 0–24% of SOC. Organic carbon was the strongest predictor of soil PyC content, but mean annual temperature, clay, and cation exchange capacity also emerged as predictors. PyC contributes a smaller proportion of SOC in European soils compared to other geographic regions. Comparing soil PyC measurements to PyC production rates in high latitude and Mediterranean regions suggests that transport, degradation, and recombustion are important mechanisms regulating soil PyC accumulation.

Anoxic wetland soils are the biggest natural source of methane (CH4) globally. Climate-driven changes to soil moisture regimes are expected to alter wetland CH4 production, consumption, and transport in a variety of different ways. At the same time, moisture changes also influence plants and their traits, especially belowground. Wetland plants provide key carbon substrates for methanogenesis, and transport CH4 to the atmosphere but also oxygen into the soils, which can promote CH4 consumption. We tested hypotheses on these complex abiotic and biotic interactions to understand how belowground plant traits influence net CH4 emissions in wetlands. Specifically, we address the following questions 1) which root traits are most important for wetland CH4 processes? 2) how does water table manipulation influence the link between root biomass and CH4? 3) how does soil moisture influence the amount of root-derived carbon emitted as CH4? First, using a literature review, we developed a conceptual framework describing root traits that would be most related to CH4 processes, highlighting trait categories regulating CH4 substrate provision and transport. Second, using a field manipulation in an experimental rice system, we found that root biomass and CH4 emissions are positively linked, but only under low moisture conditions. Lastly, in a lab incubation, we found that the amount of root-derived C-CH4 increases with increased fresh root litter, in water saturated peat soils. Overall, our studies identify root traits and their moisture interactions that should be considered in wetland CH4 measurements and models.

Context Root rot is common for kiwifruit (Actinidia chinensis var. diliciosa) plants. It is not easily controlled by chemical methods, and biological agents are an alternative. Aims This study investigated the effects of biological agents on the activity and diversity of soil microorganisms, soil enzyme activity and physicochemical indices during the growth period of kiwifruit plants. Methods There were four treatments: (1) root-rot plants applied with traditional fertiliser (RP); (2) root-rot plants applied with biological agents CY (CY); (3) root-rot plants applied with biological agents CL (CL); and (4) healthy plants with traditional fertiliser (HP). Most samples were taken during the growth period of kiwifruit plants, while activities of defensive-related enzymes and quality of kiwifruits were tested in the fructescence period. Key results CY and CL treatments significantly improved microbial activity, changed microbial structure, increased the diversity, richness and uniformity of microbial species, and altered the relative utilisation ratio of six carbon sources. Soil nutrients of kiwifruit plants with root rot improved with CY and CL compared to RP. Health of kiwifruit plants with CY and CL also improved compared to RP. Activities of defensive-related enzymes in CY and CL treatments were significantly higher than in RP treatment (P

Context Soil organic matter (SOM) plays a vital role in carbon (C) storage and agricultural sustainability. Additions of bovine urine to soils can cause positive priming of soil C decomposition and represents a pathway for SOM loss. However, data is limited to a few soils. Aims We investigated the priming response to bovine urine of 27 dairy grazed pasture soils from the North Island of New Zealand. Methods Soils from Allophanic, Gley, Recent and Brown soil orders were collected. 14C-labelled dairy cow urine was applied (1000 kg N ha−1) to undisturbed soil cores and carbon dioxide (CO2) fluxes measured (25°C) for 21 days. Urine applications were repeated, and CO2 measured for a further 21 days (25°C). Water was the control treatment. Key results CO2 fluxes rapidly increased after both urine additions by 86 ± 1% 24 h after the first urine addition, and 68 ± 4% after the second. Positive, negative and no priming were observed, and the mean absolute deviation of priming ranged between 200 and 1000 μg C g−1, and variability was greater after the second urine addition. Urine induced changes in pH and electrical conductivity (EC) had no effect on priming, and soil C contents were correlated to cumulative CO2, but not priming, and varied over time. Conclusions Factors affecting soil priming remain elusive and priming was highly variable within and between soil types. Implications The impacts of bovine urine on C pools requires further investigation to determine if, or when, urine patches are potential pathways for soil C loss.

Ecosystems integrity and services are threatened by anthropogenic global changes. Mitigating and adapting to these changes require knowledge of ecosystem functioning in the expected novel environments, informed in large part through experimentation and modelling. This paper describes 13 advanced controlled environment facilities for experimental ecosystem studies, herein termed ecotrons, open to the international community. Ecotrons enable simulation of a wide range of natural environmental conditions in replicated and independent experimental units while measuring various ecosystem processes. This capacity to realistically control ecosystem environments is used to emulate a variety of climatic scenarios and soil conditions, in natural sunlight or through broad‐spectrum lighting. The use of large ecosystem samples, intact or reconstructed, minimizes border effects and increases biological and physical complexity. Measurements of concentrations of greenhouse trace gases as well as their net exchange between the ecosystem and the atmosphere are performed in most ecotrons, often quasi continuously. The flow of matter is often tracked with the use of stable isotope tracers of carbon and other elements. Equipment is available for measurements of soil water status as well as root and canopy growth. The experiments ran so far emphasize the diversity of the hosted research. Half of them concern global changes, often with a manipulation of more than one driver. About a quarter deal with the impact of biodiversity loss on ecosystem functioning and one quarter with ecosystem or plant physiology. We discuss how the methodology for environmental simulation and process measurements, especially in soil, can be improved and stress the need to establish stronger links with modelling in future projects. These developments will enable further improvements in mechanistic understanding and predictive capacity of ecotron research which will play, in complementarity with field experimentation and monitoring, a crucial role in exploring the ecosystem consequences of environmental changes., Experimentation and modelling are necessary to predict ecosystem functioning under future environments and to develop mitigating and adapting strategies. This paper describes 13 advanced controlled environment facilities, called ecotrons, open to the international community. An ecotron comprises a set of replicated enclosures designed to host ecosystems samples and enable realistic simulations of above‐ and belowground environmental conditions, while simultaneously and automatically measuring ecosystem processes. The characteristics of these infrastructures are given as well as examples of collaborative projects hosted so far.

Aims Non-structural carbohydrates (NSCs) are plant storage compounds used for metabolism, transport, osmoregulation and regrowth following the loss of plant tissue. Even in conditions suitable for optimal growth, plants continue to store NSCs. This storage may be due to passive accumulation from sink-inhibited growth or active reserves that come at the expense of growth. The former pathway implies that NSCs may be a by-product of sink limitation, while the latter suggests a functional role of NSCs for use during poor conditions. Methods Using 13C pulse labelling, we traced the source of soluble sugars in stem and root organs during drought and everwet conditions for seedlings of two tropical tree species that differ in drought tolerance to estimate the relative allocation of NSCs stored prior to drought versus NSCs assimilated during drought. We monitored growth, stomatal conductance, stem water potential and NSC storage to assess a broad carbon response to drought. Important Findings We found that the drought-sensitive species had reduced growth, conserved NSC concentrations in leaf, stem and root organs and had a larger proportion of soluble sugars in stem and root organs that originated from pre-drought storage relative to seedlings in control conditions. In contrast, the drought-tolerant species maintained growth and stem and root NSC concentrations but had reduced leaf NSCs concentrations with a larger proportion of stem and root soluble sugars originated from freshly assimilated photosynthates relative to control seedlings. These results suggest the drought-sensitive species passively accumulated NSCs during water deficit due to growth inhibition, while the drought-tolerant species actively responded to water deficit by allocating NSCs to stem and root organs. These strategies seem correlated with baseline maximum growth rates, which supports previous research suggesting a trade-off between growth and drought tolerance while providing new evidence for the importance of plasticity in NSC allocation during drought.

Wildfires burn 4 % of the Earth’s vegetated land surface annually and thus are key to global biochemical cycles such as the terrestrial organic carbon (OC) cycle. The incomplete combustion of biomass during fires produces a thermochemical-altered and environmentally more persistent form of OC than its unburnt precursor material, namely pyrogenic carbon (PyC). PyC accounts for one of the largest and oldest terrestrial OC pool and constitutes substantial parts of the total soil organic carbon (SOC) pool. Tropical ecosystems are a key component of the terrestrial OC cycle, with highest annual primary production and largest pool of OC in vegetation biomass; however, these ecosystems are prone to disturbances such as global warming, land use changes or wildfires. In addition, tropical soils store up to one third of the terrestrial OC, and substantial amounts are held in deeper soil horizons (>30cm). However, our understanding of the quantities, ages and dominant processes of PyC (and SOC) cycling in tropical soils and in the landscape (e.g. watersheds) are still limited.We sampled soils (0-60 cm depth) in tropical semi-arid watersheds under shrubland (Maddur and South Gundal watershed) and an experimental, sub-humid watershed under dry deciduous forest (Mule Hole Critical Zone observatory) in southwestern Karnataka, India. We included prevalent soil types (Lixisol, Vertisol and Ferralsol) under different geology in order to determine PyC (and SOC) stocks and radiocarbon ages. Along soil cores, we sampled bulk density cores to calculate accurate PyC (and SOC) stocks, and used the chemo-thermal oxidation (PyCCTO) and hydrogen pyrolysis (PyCHyPy) methods to separate the PyC. The bulk SOC and the PyCHyPy were further radiocarbon dated. Our aim was to quantify PyC (and SOC) stocks and identify major processes of PyC (and SOC) in prevalent soil types under shrubland and forest and in the landscape of in tropical watersheds.SOC stocks were lowest for Lixisols (58-98 Mg ha-1), intermediate for Ferralsols (85-129 Mg ha-1) and highest for Vertisols (118-169 Mg ha-1), and all soils stored substantial proportions of OC in subsoils (up to 40 % below 30cm depth). Radiocarbon data of bulk SOC indicates fast turnover of OC in these tropical topsoils (modern ages in 0-15cm for shrublands and forests: between 99.7 to 107.0 % modern 14C (± 0.5 %)); however, the 14C of bulk SOC shows much older ages in subsoils (>40cm), from 1530 to 8490 calibrated radiocarbon years BP in Ferralsols and from 1155 to 8125 calibrated radiocarbon years BP in Vertisols with increasing depth.PyCCTO stocks accounted for 0.3-0.9 Mg ha-1 (Lixisols), 0.4-0.7 Mg ha-1 (Ferralsol) and 0.9-1.9 Mg ha-1 (Vertisol). This means that PyCCTO represented approximately 1.1-3.4 % of SOC in the topsoil (0-15 cm) and increased with depth to 2.7-5.6 % in 45-60 cm depth on average over all sites. Conclusively, PyC can make up a substantial proportion of the SOC in tropical soils, and it may represent one of the oldest OC pools of bulk SOC.

Soil organic matter (SOM) is the largest organic carbon (OC) pool on Earth’s surface after sedimentary rocks. Soil carbon storage is a key process that can mitigate climate change through the sequestration of CO2 from the atmosphere. However, numerous uncertainties persist concerning how SOM reacts to changing environments due to the challenges in disentangling the effects and interplay between different climatic and physico-chemical controls on SOM stabilization. Radiocarbon has proven to be a useful tool to identify SOM sources and turnover times, yet comprehensive investigations of 14C dynamics of SOM across climatic and environmental gradients remain scarce. Our study aimed at better understanding the drivers of carbon dynamics across different ecoregions in a large suite of 54 Swiss soils (0-20 cm depth) that span a broad range of climate and geological conditions. We measure radiocarbon signatures of different SOM fractions separated on the basis of density and chemical reactivity from both recently sampled (2014) and archived soils (collected in the 1990s) in order to estimate the evolution of 14C in the different soil fractions over two decades. Results are interpreted in the context of a comprehensive soil database in order to assess the impact of different drivers, such as climatic conditions, bedrock, altitude, land-use, soil biogeochemical properties on 14C signatures and turnover times of different SOM pools. First results show a strong contrast between particulate organic matter (POM) and mineral associated organic matter (MAOM) fractions of the soils. The particulate organic matter 14C signature decreased between the two soil inventories, on average from 113 ‰ to 78 ‰, following the decline of 14C bomb spike in the atmosphere. This shows that thePOM is a fast cycling reactive pool. In contrast, MAOM finer than 20 µm showed an increase in Δ14C from -35‰ in the 1990s’ samples to 0.8 ‰ in 2014, indicating substantial C fluxes through MAOM cycling at decadal time scales. Further oxidation of MAOM using hydrogen peroxide, removing about 80 to 90% of its C, revealed that MAOM is composed of very old SOM with Δ14C values as low as -104.9 ± 0.8 ‰ and thus millennia old. By contrast, the removed SOC had high Δ14C values around 40 ‰. This finding implies that MAOM consists of a continuum from rather stable SOM to rather rapidly cycling components. First results also indicate a strong influence of pH on turnover times, suggesting slower OM processing in acidic soils. By linking our 14C data to auxiliary data, we will explore the factors driving turnover rates of fast and slower cycling OC pools and pinpoint their vulnerability to climate change., EGUsphere

The importance of carbon fluxes between soils and atmosphere and their storage capacities have made soils a key component of nature-based solutions to mitigate climate change. Consequently, the need to forecast soil carbon dynamics at the decadal time scales is becoming a key research avenue for soil scientists and biogeochemists. One of the most important tools to fill this objective is the use of models, which have been developed and implemented since the mid- 20th century. Presuming that integrating more mechanisms would improve these models, some models of increased complexity were recently developed. Indeed, since roughly two decades, several approaches have been proposed to better represent the effect of some key mechanisms in particular related to soil biology and soil physics. For instance, few models are now able to describe explicitly and reliably the importance of soil microorganisms on the soil carbon dynamics from plot to global scale. Here, we will discuss what is the rationale behind model complexity increase, what are the limitations associated and discuss the status of evaluation for model prediction. In particular, we will show that despite model of increased complexity may provide accurate predictions in some conditions, those complex models also came with their own implicit assumptions and limitations that must be well understood before using complex models to forecast soil carbon dynamics and feed policy decisions. We also consider that models of lower complexity, which have been generally developed earlier, have also their own assets and have often been better evaluated. As a consequence, models of lower complexity may be considered as more robust and more adapted to forecast soil carbon dynamic and the improvement of their parameterization should also be considered as a valuable alternative. We will also present why multi model approach is important to reduce uncertainties and explain why using model ensemble, when implemented a diversity of carefully evaluated models, is a key method to forecast soil carbon dynamics. Finally, we will show that the fabulous growth of model complexity and the societal needs associated to soil is an incredible opportunity for soil scientists to increase the understanding of soil carbon cycle, in particular in the context of global changes, and to improve our future predictions on soil carbon.

Permafrost-affected mineral soils store large amounts of the soil organic matter (SOM) in high-latitude regions. These regions are large terrestrial carbon reservoirs and highly vulnerable to the global climate change. Global warming will cause rapid permafrost thaw and potentially accelerate decomposition of SOM. High-latitude regions, such as boreal and arctic ecozones, are regularly affected by wildfires with increasing intensity and frequency caused by global climate change. Wildfires produce pyrogenic organic matter (PyOM) during incomplete combustion of the fuel biomass. Little is known about the cycling of SOM and especially PyOM in permafrost-affected mineral soils, which limits our understanding of potential shifts in cycling and interaction with the soil mineral phase over time.Here we study the fate of highly 13C-labelled (2-3 atm%) ryegrass organic matter and PyOM from the same feedstock (pyrolyzed at 400°C for 4h) during two years of in-situ incubation in boreal forest mineral soils. Soil cores (10 cm length and 6 cm diameter) were buried in the upper 10 cm of mineral soils under continuous and discontinuous to sporadic permafrost conditions at eleven forest locations (with six replicates) in Northern Canada. At the same locations, litter bags (green and rooibos teabags) were buried and soil temperatures were recorded. The soils cores were separated in three depth (0-3, 3-6 and 6-10 cm) to trace the vertical allocation of the applied organic matter. Density and particle fractionations are applied to identify mineral interactions of the ryegrass and pyrolyzed organic matter.Preliminary δ13C results from the soil cores show a more extensive vertical allocation of ryegrass organic matter and PyOM in continuous permafrost-affected soils within the cores. This can be associated to the importance of freeze and thaw cycles for the carbon dynamics of permafrost-affected mineral soils. Tracing the labelled ryegrass organic matter and PyOM offers not only the opportunity to quantify the translocated fraction but also the decomposed proportion of the freshly added organic matter and thus understand short-term carbon dynamics. Preliminary results from the litter bags indicate a larger mass loss of slow cycling woody organic matter (rooibos tea) in discontinuous to sporadic permafrost-affected mineral soils, while larger mass losses of fast cycling organic matter (green tea) were observed in continuous permafrost-affected soils. These initial results indicate a complex cycling of organic matter in soils under different permafrost conditions.

Recently identified post-fire carbon fluxes indicate that in order to understand if global fires represent a net carbon source or sink, one must consider both terrestrial carbon retention through pyrogenic carbon (PyC) production, and carbon losses via multiple pathways. Here, these legacy source and sink pathways are quantified using a CMIP6 land surface model to estimate Earth's fire carbon budget. Over 1901-2010, global PyC drives annual soil carbon accumulation of 337 TgCyr-1, offset by legacy carbon losses totalling -248 TgCyr-1. The residual of these values constrains maximum annual pyrogenic carbon mineralisation to 89 TgCyr-1, and PyC mean residence time to 5387 years, assuming steady state. However, paucity of observational constraints for representing PyC mineralisation mean that without assuming steady state, we are unable to determine the sign of the overall fire carbon balance. The residual is negative over forests and positive over grassland-savannahs (implying a potential sink), suggesting contrasting roles of vegetation in the fire carbon cycle. Without widespread tropical grassland-savannah coverage, the legacy effects of fires could not feasibly enhance terrestrial C storage -a result afforded by grasses’ capacity for fire recovery. The dependency of the fire C residual on vegetation composition suggests that the preservation/restoration of native grasslands may be an important vector for decreasing C losses from future fire activity. We call for significant investments in understanding of PyC degradation and its drivers, in addition to improved estimates of legacy fire C fluxes. Reliable quantification of PyC mineralisation and erosion, particularly over grasslands, remains the principal missing link in a holistic understanding of fire’s role in the Earth system.

Recently identified post-fire carbon fluxes indicate that, to understand whether global fires represent a net carbon source or sink, one must consider both terrestrial carbon retention through pyrogenic carbon production and carbon losses via multiple pathways. Here these legacy source and sink pathways are quantified using a CMIP6 land surface model to estimate Earth’s fire carbon budget. Over the period 1901–2010, global pyrogenic carbon has driven an annual soil carbon accumulation of 337 TgC yr−1, offset by legacy carbon losses totalling −248 TgC yr−1. The residual of these values constrains the maximum annual pyrogenic carbon mineralization to 89 TgC yr−1 and the pyrogenic carbon mean residence time to 5,387 years, assuming a steady state. The residual is negative over forests and positive over grassland-savannahs (implying a potential sink), suggesting contrasting roles of vegetation in the fire carbon cycle. The paucity of observational constraints for representing pyrogenic carbon mineralization means that, without assuming a steady state, we are unable to determine the sign of the overall fire carbon balance. Constraining pyrogenic carbon mineralization rates, particularly over grassland-savannahs, is a critical research frontier that would enable a fuller understanding of fire’s role in the Earth system and inform attendant land use and conservation policy.

Tree uprooting may distinctly affect landscape dynamics and slope denudation. Little is known, however, about the corresponding soil redistribution rates (erosion and accumulation) on either a long‐term (millennia; 10Be) or a short‐term (decades; 239+240Pu) scale. We determined these rates in a well‐investigated forest reserve (Zofinsky primeval forest, Czech Republic) using complementary techniques: nuclides in soils and tors to derive short‐ to long‐term rates and monitoring data (43 years) of repeated tree censuses using tree uprooting data. Temporal trends of soil erosion rates were obtained by dating the timing of exhumation (10Be) of tors. The average long‐term denudation rates were about 30–40 t km−2 yr−1. It seems that these rates varied over time with probably a maximum during the Pleistocene/Holocene transition (58–91 t km−2 yr−1). 239+240Pu activities in the soils identified soil redistribution rates of 50 to >100 t km−2 yr−1 for the last decades and agree with results from the tree uprooting monitoring (

Pyrogenic carbon (PyC) may leave the soil surface where it is deposited, either through degradation, lateral transport or transfer within the profile. However, PyC has been seldom measured in the subsoil. We estimated the quantity and molecular composition of PyC in the topsoil and subsoil of 22 French forests with diverse soil types and vegetation cover. While the absolute PyC content decreased with depth, its proportion to the soil organic carbon remained constant or tended to increase. The benzene polycarboxylic acids pattern indicated that more condensed structures were found in the subsoil. Our results show that PyC transfers through the soil profile, probably as soluble fraction, and tends to accumulate in the subsoil, specifically in podzolic soils.

Wildfires occur regularly in the boreal forests of Northern Canada and an increasing frequency and intensity due to the global climate change is projected. A by-product of these forest fires is pyrogenic carbon (PyC) as a residue of incomplete combustion. The quantity and age of PyC in boreal forest soils, however, are largely unknown although boreal soils contribute to a large extent to the global soil organic carbon (SOC) stocks. The Mackenzie River is a major export pathway for PyC between terrestrial and marine environments, with exported PyC ages on geological timescales. This indicates that soil may play an important role as an intermediate pool prior to the PyC export. We sampled eleven forest soils (with nine replicates) in the Canadian Taiga Plains and Shield within the Mackenzie River basin. Our sample sites were located in regions with soils under continuous permafrost in the Inuvik region (northern sites) and under sporadic and discontinuous permafrost in the South Slave Lake regions (southern sites). All sites were unaffected by fire for at least four decades. We used the hydrogen pyrolysis (HyPy) method to separate the PyCHyPy from the non-fire-derived SOC in the upper 0-15 cm to determine PyCHyPy stocks and performed radiocarbon dating upon both bulk soil and isolated PyCHyPy. The total SOC stocks were lower in the soil from the southern sites with on average 26 ± 20 Mg ha-1 (10-153 Mg ha-1) compared to 57 ± 29 Mg ha-1 (16-188 Mg ha-1) in the northern sites. The radiocarbon dating revealed much older PyCHyPy compared to the bulk soil SOC radiocarbon age, supporting the persistent nature of PyC and stabilization in soils. The PyCHyPy found in the soil of the southern sites, however, was much younger with ages in the range of 495-3 275 radiocarbon years BP than in the northern sites with ages on the range of 2 083-10 407 radiocarbon years BP. The larger SOC stocks and higher ages of PyCHyPy in the soils of the northern sites indicate the importance of permafrost conditions for the whole carbon cycle of boreal forests soils.

Pyrogenic carbon (PyC) is produced by the incomplete combustion of vegetation during wildfires and is a major and persistent pool of the global carbon (C) cycle. However, its redistribution in the landscape after fires remains largely unknown. Therefore, we conducted rainfall simulation experiments on 0.25mplots with two distinct Swiss forest soils (Cambisol (clay loam) and Luvisol (sandy silt)). We applied PyC produced from wood (Picea abies) labeled under FACE conditions and C4 grass (Miscanthus sinensis) to the soil surface to study PyC redistribution by runoff and splash and the vertical mobility of PyC in a 10 cm unsaturated soil column based on the differences in δC of soils and PyC. We assessed the effect of soil texture, slope angle and PyC characteristics (feedstock and particle size) on the mobility of PyC during 30 min of intense rainfall (102mmh/. Our results highlight that PyC is highly mobile. Surface runoff transported between 0.2% and 36.0% of the total added PyC. Erosion by splash further redistributed 10.3% to 25.3% of the added PyC. Soil type had a substantial impact on the redistribution of PyC by both runoff and splash: on average, we recovered 10.5% of the added PyC in runoff and splashed material for the clay-rich Cambisol and 61.3%of the added PyC for the sandy silt Luvisol combined. PyC feedstock had a clear but contrasting effect on PyC redistribution: relocation in the runoff and splashed material was greater for wood PyC (43.4% of total added PyC) than grass PyC (28.4 %). However, more wood PyC (11.5 %; fraction of organic C derived from the PyC) remained where it was initially applied compared to grass PyC (7.4 %). The results further suggest that the effect of PyC characteristics on its mobility can be highly variable and depend not only on the material from which it was derived, but also on other factors (e.g., particle size, porosity, density). In particular, the mobility of PyC was almost twice as large for fine-grained PyC (, This research has been supported by the Swiss National Science Foundation (grant no. 200021_178768).

Wildfires generally result in biospheric recovery approximating the pre-disturbance state. However legacy carbon(C) gains and losses that have until now been overlooked in global-scale theory and modelling indicate that post-fire C gains through pyrogenic carbon (PyC) production, and losses via fire regime shifts, post-fire mortality, topsoil loss and inland water export, may be central to whether 20th century fires have imposed a net terrestrial C source or sink. Here, we integrate PyC production and soil accumulation into a global terrestrial model (ORCHIDEE-MICT) and estimate wildfire C-gains and losses over 1901-2010, quantifying the fire-C balance at global, regional and vegetation scales. Excluding the effect of PyC mineralisation, fires provide a land storage of +177 TgC yr-1 (63% PyC production), dominated by grasslands. The global balance is nuanced, with forest fires resulting in strong terrestrial net C loss:gain ratios (>-2:1) that are greatest in tropical regions (>-3:1). Frequent tropical grassland fires are responsible for the bulk of the land PyC sink and its environmental persistence, whose theoretical minimum mean residence time we quantify at 2760yrs. We highlight the dependency of the global fire-C balance on vegetation coverage and the potential role of preserving grasslands, particularly those in the tropics, in that regard.

Summary Shallow thermokarst ponds are a conspicuous landscape element of the Arctic Siberian tundra with high biogeochemical variability. Little is known about how microbes from the regional species pool assemble into local pond communities and how the resulting patterns affect functional properties such as dissolved organic carbon (DOC) remineralization and greenhouse gas (GHG) turnover. We analysed the pelagic microbiomes of 20 ponds in north‐eastern Siberia in the context of their physico‐chemical properties. Ponds were categorized as polygonal or trough according to their geomorphological origin. The diversity of bacteria and eukaryotic microbes was assessed by ribosomal gene tag sequencing. Null model analysis revealed an important role of stochastic assembly processes within ponds of identical origin, in particular for genotypes only occurring in few systems. Nevertheless, the two pond types clearly represented distinct niches for both the bacterial and eukaryotic microbial communities. Carbon dioxide concentration, indicative of heterotrophic microbial processes, varied greatly, especially in the trough ponds. Methane concentrations were lower in polygonal ponds and were correlated with the estimated abundance of methanotrophs. Thus, the overall functional variability of Arctic ponds reflects the stochastic assembly of their microbial communities. Distinct functional subcommunities can, nevertheless, be related to GHG concentrations.

Evolution of organic carbon content in soils has the potential to be a major driver of atmospheric greenhouse gas concentrations over the next century. Understanding soil carbon dynamics is a challenge due to a wide range of residence time of soil organic matter and limited constraints on the mechanisms influencing its persistence. In particular, large uncertainties exist on the persistence of pyrogenic organic carbon in soils. In order to characterise organic matter with varying degrees of persistence and distinguish pyrogenic organic carbon, we combined Rock-Eval analysis, a thermo-chemical method, with the benzene polycarboxylic acid molecular marker method and Raman spectroscopy, to characterise samples from long-term bare fallow experiments, progressively depleted in the most labile organic carbon over time. Considering the heterogeneity of soil samples, size fractions have been separated to distinguish pools of organic carbon with distinct properties. We observe that organic carbon dynamics is dependent on granulometry. A pool of organic carbon with intermediate residence time, from years to a few decades, representing ca 65 % of the bulk soil organic carbon stock, is mainly associated to fine fractions ( 20 µm) are rich in centennially-persistent organic carbon, representing ca 20 % of the initial organic carbon stock, due to the chemical recalcitrance of organic matter in these fractions, dominated by pyrogenic organic carbon. A second pool of persistent organic carbon, representing ca 15 % of the initial organic carbon stock, is associated with the clay fraction, indicating mechanisms of protection occurring at the submicron scale (

Wildfires are a key component in the global carbon (C) cycle, releasing significant amount of CO2 to the atmosphere, but also producing one of the most persistent terrestrial organic C forms by the incomplete combustion of natural vegetation, namely fire-derived or pyrogenic carbon (PyC). Post-fire, PyC is deposited on the soil surface, but can then be laterally redistributed by wind or water erosion. Due to the lack of vegetation cover after a fire, primary factors that govern rate of PyC distribution post-fire will be changes in soil surface properties and physical characteristics of PyC. However, the drivers and quantities of transported PyC (and non-fire derived organic matter) by water erosion from its site of production remain largely unknown, which limits our understanding of PyC movement in the landscape.In this study, we tracked PyC erosion (and movement in soil) in a controlled experiment using a gravity-type rainfall simulator (at an intensity of 50mmh-1). We studied the quantity of PyC eroded from the soil surface (runoff and splash sediment) as well as its vertical movement within the soil (soil cores) for two Swiss temperate forest soils (initial soil moisture of 12-16%). We calculated the distribution of PyC using the isotopic 13C signature differences of grass-PyC (Miscanthus grass), wood-PyC (spruce wood grown under FACE conditions) and natural forest soil C in soil flume of 0.25m2. We studied the combination of the following factors: soil texture (sandy silt and clay loam), slope (10° and 25°), PyC particle size (63µm and 63µm-2mm) and PyC feedstock (grass and wood) to identify the major drivers of PyC redistribution.We hypothesize that: i) higher quantities of PyC can be found in the sediment of the sandy silt soil, with higher slopes and with bigger PyC particles, ii) higher quantities of PyC move vertically in the clay loam soil and with smaller PyC particles and iii) PyC is preferentially eroded compared to bulk SOC.Results suggest that we find 100 times more runoff sediment on sandy silt soil compared to clay loam soil, and two times more runoff sediment on 25° slope compared to 10°. We also find 1.5 times more splash erosion on sandy silt soil than on the clay loam soil. Regarding PyC erosion, observations suggest that > 50% of initial PyC is eroded on the sandy silt soil, whereas a majority of the PyC particles moves vertically through the soil for the clay loam.

Wildfires produce a fire-derived carbon (C) residue known as pyrogenic carbon (PyC). This residue is one of the largest and most persistent terrestrial organic C pools with residence times over 1000 years. Large quantities of PyC can remain on the site where the fire occurs or can be transferred along the landscape and eventually enter the soil where it represents a major component of the total soil organic carbon (SOC). The drivers explaining the vertical PyC mobility in soils and the quantities it represents remain largely unknown, which limits our understanding of the fate of PyC in the terrestrial systems. Here, we used a manipulative soil column experimental approach in order to study the proportion of transportable PyC and its interaction with the soil mineral phase and non-fire-derived SOC during its vertical transport. To do this, we applied highly 13C-labelled ryegrass biochar (as a PyC proxy) on the upper layer of small soil columns (7 cm length, 2.5 cm diameter), and traced the fire-derived C in the soil and in the soluble fractions sequentially extracted during a percolation of 600 pore volumes under saturated conditions (equivalent to 18 years continues rainfall). We studied a combination of ranging soil texture (sandy loam and sand), SOC content (0.3-3.0%) and weathering state (age) of the ryegrass biochar (artificial oxidation with H2O2) to identify drivers controlling the vertical mobility of fire-derived C. We hypothesized that: i) Significant proportions of fire-derived C are transported through the soil column over the whole experimental period but decrease with time, ii) fire-derived C can be retained and stabilized within the soil column and influence the mobility of non-fire-derived SOC and iii) weathered fire-derived C is more mobile than fresh fire-derived C which may control its long-term fate in soils.

Below ground carbon (BGC) inputs to soil, i.e. root biomass and rhizodeposition carbon (C), are among the most important variables driving soil C dynamics in agroecosystems. Hence, increasing BGC inputs to deep soil is a proposed strategy to sequester C in the long term. As BGC inputs are inherently difficult to measure in the field, they are usually estimated from yield in order to supply soil C models with input data. While fertilization intensity considerably affects above ground biomass, its influence on BGC inputs is largely unclear, especially with respect to the subsoil. Therefore, we determined net root biomass and rhizodeposition C of field-grown maize and wheat at harvest in different farming systems (bio-organic, conventional) and fertilization treatments (zero, manure, mineral) along an intensity gradient in two Swiss long-term field trials. Plants in microplots were repeatedly pulse-labelled with 13C-CO2 throughout the growing seasons and shoots, roots, and soil to 0.75 m depth were sampled at harvest. Despite a strong increase of above ground biomass with increasing fertilization intensity, BGC inputs were similar among treatments on both sites irrespective of soil depth. However, the proportions of rhizodeposition C of BGC inputs averaged 54 to 63% and were, therefore, much larger than the widely adopted 40% for field-grown cereals. They increased with soil depth and were highest under sole organic fertilization. The shift in whole-plant C allocation towards above ground biomass with increasing fertilization intensity entailed 10% higher C allocation below ground in organic than conventional farming for both maize and wheat. Our findings imply that yield-independent values provide closer estimates for BGC inputs to soil of cereals in different farming systems than yield-based functions. We further conclude that fertilization has only little potential to alter absolute amounts of BGC inputs to deep soil in order to sequester C in the long term.

Root biomass is the most commonly studied root parameter to investigate below ground crop response to environmental conditions and carbon cycling in agroecosystems. Root growth is strongly regulated by site-specific growth conditions and resource availability, but only little is known about the extent to which root biomass, vertical distribution, and size class respond to fertilization intensity as compared to site. We determined coarse (> 2 mm) and fine (> 0.5 mm and ≤2 mm) root biomass of maize and wheat in three soil layers to 0.75 m depth in different farming systems (half and full organic, full conventional) and fertilization treatments (zero, manure, full mineral N plus half mineral PK, full mineral NPK) of the Swiss long-term field trials DOK and ZOFE, respectively, and evaluated the effects of fertilization intensity and site on root biomass, vertical distribution, and size class. In DOK, total root biomass was similar in organic and conventional farming systems. In ZOFE, wheat root biomass was 1.7-times higher under full mineral N plus half mineral PK fertilization than under zero or manure fertilization and intermediate under full NPK fertilization. Vertical root distribution and size class were only marginally affected by fertilization intensity on both sites. By contrast, total root biomass of maize and topsoil root biomass of both maize and wheat were higher but subsoil root biomass of wheat and fine root proportions of both maize and wheat were lower in DOK than in ZOFE. We conclude that roots respond more to site than to fertilization intensity and that absolute inputs of root biomass carbon to soil are similar in low- and high-intensity systems. Further, root-shoot ratios were inversely related to fertilization intensity, implying that estimations of below ground carbon inputs to soil from shoot biomass need to be differentiated by fertilization intensity. Deep (below 0.5 m) root biomass was 3-times higher for wheat than for maize, suggesting that crop choice is more important than fertilization intensity for carbon sequestration in deep soil.

Global environmental change and biodiversity loss are closely linked through different feedback mechanisms. The University of Zurich Research Priority Programme on ‘Global Change and Biodiversity’ approach is to work with interdisciplinarity and transdisciplinarity to integrate mechanisms of interactions, feedback and scale and improve our understanding of the feedbacks between global change and biodiversity effects. Such work across research disciplines is not without its challenges. Here we share some of the questions that arose from our research approach over the last five years and how we addressed these challenges. First, our transdisciplinary approach allows combining different disciplines into a more holistic perspective towards integrative research, but demands collaborative work to establish common terminology, concepts, and metrics. Second, the research theme's common perspective (biodiversity is desirable, global change is not) may also induce a confirmation bias from preconceived ideas. Third, new challenges emerge from scaling mechanisms and feedbacks at different spatial and temporal scales. Fourth, we investigate how to relate biodiversity, global change, ecosystem services and functions using interdisciplinary approaches. Fifth, we identify gaps between existing experiments and data requirements, and propose the definition of new experimental setups by linking processes and performing experiments at typical experimental scales as well as at larger scales. We conclude by emphasising the necessity to integrate theory, experiments, modelling and simulation, high performance computing and big data to understand feedbacks between biodiversity loss and processes of global change.

Humans have interacted with fire for thousands of years, yet the utilization of fossil fuels marked the beginning of a new era. Ubiquitous in the environment, pyrogenic carbon (PyC) arises from incomplete combustion of biomass and fossil fuels, forming a continuum of condensed aromatic structures. Here, we develop and evaluate 14C records for two complementary PyC molecular markers, benzene polycarboxylic acids (BPCAs) and polycyclic aromatic hydrocarbons (PAHs), preserved in aquatic sediments from a suburban and a remote catchment in the United States (U.S.) from the mid-1700s to 1998. Results show that the majority of PyC stems from local sources and is transferred to aquatic sedimentary archives on subdecadal to millennial time scales. Whereas a small portion stems from near-contemporaneous production and sedimentation, the majority of PyC (∼90%) experiences delayed transmission due to “preaging” on millennial time scales in catchment soils prior to its ultimate deposition. BPCAs (soot) and PAHs (precu...

Over the past decades, average global wheat yields have increased by about 250 %, mainly due to the cultivation of high-yielding wheat cultivars. This selection process not only affected aboveground parts of plants, but in some cases also reduced the root biomass, with potentially large consequences for the amount of organic carbon (OC) transferred to the soil. To study the effect of wheat breeding for high-yielding cultivars on subsoil OC dynamics, two old and two new wheat cultivars from the Swiss wheat breeding program were grown for one growing season in 1.5 m-deep lysimeters and pulse-labelled with 13CO2, to quantify the amount of assimilated carbon that was transferred belowground and potentially stabilized in the soil. The results show that although the old wheat cultivars with higher root biomass transferred more assimilated carbon belowground compared to more recent cultivars, no significant differences in net soil organic carbon (SOC) stabilization were found between the different cultivars. As a consequence, the long-term effect of wheat cultivar selection on SOC stocks will depend on the amount of root biomass that is stabilized in the soil. Our results suggest that the process of wheat selection for high-yielding cultivars resulted in lower amounts of belowground carbon translocation, with potentially important effects on SOC stocks. Further research is necessary to quantify the long-term importance of this effect.