BG3.6

The data requirements of vegetation models are changing. For more than a decade, the community has been developing “next-generation” models that should be globally applicable and at the same time incorporate great process detail. The individual tree emerges from this development as the finest scale at which carbon, water, and nutrient dynamics can be realistically simulated. As such, precise tree-level observations of the relevant processes would ideally be available from across all forested biomes to inform and evaluate tree-centered vegetation models. This is not the case. Instead, we note a growing discrepancy between the demand for and the availability of highly-resolved measurements of carbon allocation in trees and forests.

To exemplify this discrepancy, we conducted a survey at 90 flux-tower sites from around the world that revealed priorities and deficiencies in existing data collections. We found that forest structure and aboveground carbon stocks have been ubiquitously inventoried, and that tree growth and foliage turnover have also been measured at many sites. By contrast, detailed information on water cycling, volume increment, and wood formation processes (especially belowground) are less common, as are records of tree mortality or terrestrial and airborne LiDAR that could help scale local observations. In addition, we found that the temporal resolution and length of existing time-series vary substantially across the current flux-tower network. Weighing the strengths and limitations of this and many other ecological monitoring networks, we conclude that the present data basis is insufficient to support accelerating vegetation model development.

Looking forward, we anticipate that not only the amount of tree-level observations needs to be increased – especially in tropical and boreal systems – but that the consistency, scalability, and predictability of forest carbon cycle observations needs to be improved. We also propose that intensive long-term monitoring sites be strategically paired with manipulative experiments at comparable sites to better connect past, present, and expected future dynamics. For this, we propose a versatile experimental framework and call for a community-wide discussion on the “yield on cost” of various field observations. We also list a number of key questions on how to best build and maintain cross-scale data archives in support of tree-centered vegetation modelling.

How to cite: Babst, F., Friend, A. D., Wei, J., von Arx, G., Papale, D., and Peters, R. L.: Observations of carbon allocation in the world’s forests must match pace with vegetation model development, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5943, https://doi.org/10.5194/egusphere-egu22-5943, 2022.

The restoration of degraded land after mining with the establishment of forest plantations, contributes to climate change mitigation by enhancing carbon storage. For this purpose, around 2,570 hectares at the Lignite Center of the Hellenic Public Power Corporation in Western Greece were planted with black locust (Robinia pseudoacacia L.), since it is a fast-growing, drought-tolerant species with N-fixing capacity. The aim of this study, which was carried out within the COFORMIT project, was to estimate the C fluxes of litterfall, forest floor and fine roots (<2mm in diameter), and the C stocks of coarse roots (≥2mm in diameter) and soil in these plantations and to examine how they are affected by seasonal variability, canopy density, soil depth. Sampling was performed in 18 plots of higher and lower canopy density (36 plots in total). In each plot, litterfall and forest floor were sampled bimonthly, while soil samples were collected once at two depths (0-10cm, 10-30cm). To estimate carbon stocks in coarse roots, the root system of five black locust individuals, representative of the 5 classes of diameter at breast height (DBH) have been excavated. For the determination of carbon fluxes of fine roots, 12 cores were sampled around the perimeter of each selected tree. To assess fine root turnover, 48 in-growth cores had been placed around four trees with varying DBH. Carbon fluxes of litterfall and forest floor peaked from October till December in both pools. Total carbon sequestration in litterfall was 1.39 t ha^-1 yr^-1 and it significantly increased with increasing canopy density. Mean annual carbon in forest floor was 2.98 t ha^-1 yr^-1, which was not significantly affected by canopy density. The carbon sequestration in coarse roots was 12.89 t ha^-1. Fine roots stored c. 0.27 t ha^-1 of carbon, while no effect of soil depth (0-10 cm vs. 10-30 cm) was detected. Fine root turnover was 0.17 t ha^-1 yr^-1 and it also did not differ with soil depth. Soil organic carbon (SOC) increased greatly with depth (from 19.57 t ha^-1 in 0-10 cm to 33.19 t ha^-1 in 10-30 cm) and in total (52.75 t ha^-1) was higher than the SOC levels reported in literature for black locust plantations of the same age, possibly due to the presence of lignite mining residues. Our results support the significant carbon sequestration potential of the studied restoration plantations and are discussed in relation to findings from other black locust restoration schemes.

Keywords: CO₂ sequestration, black locust, post-mining rehabilitation, soil organic carbon, forest floor, litter, below-ground biomass.

How to cite: Xanthopoulos, G., Radoglou, K., Spyroglou, G., Derrien, D., Angeli, N., and Fotelli, M.: Carbon sequestration in litterfall, forest floor, roots and soil in Robinia pseudoacacia restoration plantations., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10109, https://doi.org/10.5194/egusphere-egu22-10109, 2022.

Mixed forests are typically more productive and are faster to recover from drought compared to monospecific forests. Disentangling the contribution of each species to the overall success of the forest requires observations at the individual tree level. In this study, we measured a complete set of carbon (C) pools and fluxes at the tree-level in five tree species, two conifers and three broadleaf, co-existing in a mature evergreen mixed Mediterranean forest. Our study period included a drought year, followed by an above-average year. Across species, C sinks of 38-91 kg tree^-1 year^-1 were 16-32% larger than C source of 27-77 kg tree^-1 year^-1 in the dry year, with larger belowground C investment of the shallow-rooted species. Overall, respiration was the largest sink across species and years, accounting for 26-62% of all assimilated C, followed by growth (16%) and root exudation (19%). Non-structural carbohydrates accumulation was similar between the wet and the dry year. These detailed tree-level observations expose large interspecific differences in C allocation among fluxes and tissues and specifically in response to varying water availability. These insights become useful for forest management under ongoing change.

How to cite: Rog, I., Fox, H., and Klein, T.: Tree-scale carbon allocation dynamics in a mature mixed forest using long-term mass balance approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11351, https://doi.org/10.5194/egusphere-egu22-11351, 2022.

Leaves in tree crowns experience different microclimates depending on their vertical positions, particularly in light availability. This leads to different rates of carbon assimilation. Additionally, leaf phenology potentially varies along the same gradient. Therefore, it can be assumed that the size and seasonal dynamics of the non-structural carbohydrate (NSC) pool are different in sunlit and shaded twigs of individual trees.

Here we test how microclimatic gradients and leaf phenology influence the NSC dynamics of young twigs along the vertical gradient of individual tree crowns. Throughout the year 2020, we measured the NSC concentration in twigs from the top and bottom crown parts of mature trees from 9 species in a temperate mixed forest at the Swiss Canopy Crane II facility near Basel, Switzerland. We recorded the timing of budbreak along the canopy depth of those trees and continuously measured the light environment with loggers in various canopy positions in three consecutive seasons (2019-2021).

The timing of budbreak showed barely any difference between top and bottom crown parts in broadleaved species. However, in the conifers Abies alba and Picea abies, buds opened ca. 7 days earlier in the bottom crowns than the top. Light availability throughout the growing season in the lower crown parts was around 30 % of that at the top. In most species, the NSC pools were strikingly similar in sunlit and shaded twigs, both quantitatively and in terms of their seasonal dynamics. Only the two ring-porous species Quercus petraea and Fraxinus excelsior showed differences: in both, the lower twigs reached their minimum starch levels after budbreak about a week later than the top twigs, and took longer for the subsequent refilling. However, even in species that showed slight differences in the seasonal NSC dynamics between upper and lower canopy, the end of season NSC concentrations in late autumn were identical between top and bottom twigs.

The very similar NSC dynamics and pool sizes between twigs from upper and lower crown parts, despite stark differences in light availability, are surprising. Further analyses of carbon assimilation and the ratio of carbon source to sink tissues along these vertical canopy gradients will allow to better interpret those results and to get an improved understanding of how carbon storage is controlled in mature trees.

How to cite: Zahnd, C., Zehnder, L., Kahmen, A., and Hoch, G.: How do leaf phenology and microclimate influence carbon storage dynamics along the vertical canopy gradient of mature trees?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2861, https://doi.org/10.5194/egusphere-egu22-2861, 2022.

Trees store most of the nonstructural carbon (NSC, mainly sugars and starch) in the stem wood to support metabolism and growth. For many species, NSC concentration decreases radially with sapwood depth. Spatial distribution of NCS also varies with wood anatomical traits, as NSC is dispersed in the wood living-fibers or concentrated  in parenchyma cells. These distributions may  influence temporal changes in NSC and are related to mortality and growth. Observed relations between NSC storage and wood traits thus are associated with differences in the time scales of NSC cycling in  trees and in the ability of trees to survive and recover from stressful conditions such as drought or mechanical damage.  

Here we focus on the following questions: i) what are the radial mobilization rates of NSC across the sapwood for tropical trees with different NSC spatial distributions? and ii) how old is the NSC stored in these trees and the NSC that they access for respiration and wood growth?

To answer these questions we measured NSC content through a radial path in the sapwood (from bark to pith) in eight tropical tree species, four fiber and four parenchyma-storing species, in a seasonal dry forest in Brazil during 2019. We measured the ¹⁴C in the soluble carbon extracted from wood segments corresponding to two depth ranges of each radial path (0-2cm and 2-4cm) and in the respired CO₂ from 6cm wood core segments. We estimated the age of the wood by counting tree rings and measuring the ¹⁴C in the cellulose.  

We found significant seasonal changes in the starch content at different sapwood depths in all trees evaluated, indicating that NSC is metabolically active across all depths where starch is stored. Radiocarbon data indicate that fiber-storing trees retained NSC in the wood for decades. In some cases NSC was even older than the wood that contained it, indicating the mixing of old NSC coming from deeper layers of wood. Irrespective of the stored NSC ages, trees always used younger NSC for respiration, indicating that our water extraction includes both reserves being used for metabolism and older C that may be less available at the moment of sampling. However, trees accessed older NSC when they faced stressful conditions, e.g. when in negative C balance, requiring a larger contribution of the old stored NSC to support respiration. Thus, tree species with low mortality and slow growth such as fiber-storing species may remobilize older NSC from deeper layers of wood to survive stressful conditions for longer time than parenchyma-storing species.

These findings highlight the diversity of NSC storage and remobilization strategies in tropical trees. These strategies have important implications for our understanding not only of how trees will respond to future climatic changes but also about the mechanism of carbon cycling in tropical trees and ecosystems. 

How to cite: Herrera, D., Römermann, C., Hartmann, H., Trumbore, S., Muhr, J., Brando, P., Silvério, D., and Sierra, C. A.: Nonstructural carbon age and lateral mixing in the stem wood of tropical trees, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6539, https://doi.org/10.5194/egusphere-egu22-6539, 2022.

We lack a detailed understanding of tree carbon flux dynamics to quantify stem respiration correctly. Stem CO₂ radial diffusivity and vertical CO₂ transport with the xylem sap produce uncertainties in respiration estimates based on stem CO₂ efflux measurements. Two independent approaches have been applied for comparison to assess such uncertainties: (1) the mass balance approach accounts for CO₂ efflux from the stem to the atmosphere, CO₂ transport through the xylem and CO₂ stored within the stem to estimate total respiration rates, and (2) measurements of stem O₂ consumption as a more robust proxy for stem respiration than stem CO₂ efflux, because O₂ is less soluble than CO₂ in the sap solution and hence less  affected by vertical transport.

In this study, we compare these two approaches and study CO₂ and O₂ flux dynamics along the stem height to capture CO₂ transport. During summer 2019, we measured vertical CO₂ and radial CO₂ and O₂ fluxes, sap flow, stem temperature and xylem sap pH from twigs. Stem CO₂ and O₂ fluxes were calculated along 4-m stem segments on mature beech trees in a managed forest in Germany.

We found that xylem [CO₂] and CO₂ and O₂ fluxes did not vary with stem height. Interestingly, stem CO₂ efflux was a poor predictor of stem respiration in the monitored mature trees. O₂ influx was always higher than CO₂ efflux (on average CO₂-to-O₂ ratios was 0.72), resulting in an underestimation of stem respiration when using CO₂ measurements only, which is standard practice.

Preliminary results of the implementation of this dataset into a biophysical stem respiration model (TReSpire) suggest that CO₂ respired in the xylem of mature trees encounter a relatively long diffusive pathway to reach the bark tissues, so that a significant fraction of CO₂ dissolves in the sap and is transported upwards before being detected by traditional approaches. Our study provides insights into stem carbon flux dynamics in large trees, and thus helps to improve estimation of ecosystem carbon cycling.  

How to cite: Helm, J., Salomón, R., Muhr, J., Steppe, K., Hilman, B., and Hartmann, H.: Observed and modelled stem respiration CO2 and O2 fluxes in mature beech trees, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4213, https://doi.org/10.5194/egusphere-egu22-4213, 2022.

Projections of future climate change indicate that extreme events will be larger in frequency and intensity, with an increased risk of ecosystem transition from carbon sinks to carbon sources. In particular, warming is occurring at a higher rate in the Alps, with important impacts for tree species acclimated to a strong climate seasonality and a short growing season.

In this study, we investigated the ecosystem responses to heatwave and drought at a high-altitude Larix decidua (Mill.) forest in the western Italian Alps (IT-Trf, 2050 m asl), by coupling direct measurements of ecosystem-scale surface-atmosphere fluxes and tree-based observations. Ecosystem fluxes were monitored by means of the eddy covariance technique, measuring water and carbon fluxes (i.e., gross primary production, net ecosystem exchange, and evapotranspiration). From 2015 to 2017 additional observations were carried out at tree level, including stem growth and its duration, direct phenological observations, sap flow, and tree water deficit.

Results showed that the warm spells observed in 2015 and 2017, caused the advance of the larch phenological development and, thus, of the seasonal trajectories of many processes. However, we did not observe significant quantitative changes in the C sequestration at the ecosystem level, whereas in 2017 we found a reduction of 18% in larch stem growth and a contraction of 45% of the stem growth period. The growing season in 2017 was indeed characterized by different drought events and by the highest water deficit during the study years. By combining tree- and ecosystem-based observations, we demonstrated that larch growth decrease was not driven by a reduction of the photosynthetic activity.

We formulate two contrasting hypotheses to explain our results: i) a shift in C allocation within the plants towards the prioritization of NSC storage within leaves and roots over growth processes, which question the C-source limitation hypothesis, usually applied in vegetation modeling; ii) the ‘Insurance Hypothesis’, which can be used to explain the stability of the whole ecosystem gas exchanges, where the negative effects of climatic fluctuations on larch growth might have been buffered by the asynchrony responses of the understory species that can benefit from the higher temperatures.

How to cite: Galvagno, M., Migliavacca, M., Cremonese, E., Filippa, G., Vacchiano, G., Siniscalco, C., Oggioni, S., Morra di Cella, U., and Oddi, L.: Contrasting responses of forest growth and carbon sequestration to heat and drought in the Alps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11269, https://doi.org/10.5194/egusphere-egu22-11269, 2022.

Drought exerts a major control on the carbon (C) cycle of terrestrial ecosystems worldwide. However, the mechanisms and processes underpinning ecosystem responses remain uncertain, in particular in diverse, tall-growing ecosystems like tropical rainforests. In such ecosystems, trees are a predominant driver of ecosystem C cycling, as they link the major ecosystem fluxes, photosynthesis and ecosystem respiration through allocation and utilisation of recent assimilated C. Trees respond dynamically to drought, generally by reducing C assimilation and altering investments of recent C into metabolism, defence, growth and storage, which has consequences for the fate of C in the system. However, to date most of our understanding is derived from experiments on small trees and we lack an understanding of how whole-tree C allocation responds in diverse, stratified forest ecosystems.

To address this knowledge gap, we implemented a 9.5-week experimental drought in the world’s largest controlled growth facility, the Biosphere 2 Tropical Rainforest in Arizona, US. We continuously measured isotopic CO₂ fluxes of leaves, stem and soil as well as leaf and phloem non-structural carbohydrates across a range of canopy and understory forming trees during pre-drought and drought conditions. To study drought effects on the fate of recent photoassimilates, we labelled the entire ecosystem with a ¹³CO₂ pulse during pre-drought and drought conditions and traced the carbon flow in leaf, stem and soil fluxes and non-structural carbohydrates of leaves and phloem.

Across all studied trees, drought generally reduced CO₂ uptake and metabolic activity in leaves, stems and soil. The phloem transport rates slowed down and the turnover of recent photoassimilates declined. As drought progressed respiration was increasingly fuelled by C reserves, as indicated by isotopic flux dynamics and a depletion of starch pools, particularly in leaves. Drought response patterns of fluxes, carbohydrate pools and C allocation dynamics were highly variable among trees. Interestingly, response diversity was not primarily explained by species identity, but likely related to a combination of functional and structural traits and the trees’ microenvironment within the forest. We conclude that the structural and functional composition of a forest is an important driver for tree C allocation and needs to be considered for understanding the mechanisms underpinning forest C dynamics in response to drought.

How to cite: Ingrisch, J., Kübert, A., Huang, J., Meeran, K., van Haren, J., Shi, L., Bamberger, I., Kreuzwieser, J., Lehmann, M., Dippold, M., Meredith, L., Ladd, N., Bahn, M., and Werner, C.: Drought effects on whole-tree C dynamics in an enclosed tropical rainforest, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11285, https://doi.org/10.5194/egusphere-egu22-11285, 2022.

It is getting more and more clear that the sink activity in trees strongly determines carbon (C) uptake and within-plant C distribution. Here we show that short- and long-term changes in water availability impacted the belowground (mycorrhizosphere) C sink strength, which however, not only affected C transport to the roots but also nitrogen (N) uptake by tree roots and the N allocation to aboveground tissues. The negative drought impact on N uptake might be a result of reduced root growth and the lower availability of recent assimilates for mobilizing inorganic N in the soil as the physiological N uptake capacity of the roots was not clearly affected. On the other hand, we show that increased N availability in the soil can have positive effects on C allocation to the rooting system of trees under drought and consequently can reduce drought induced growth impairment and mortality. Our results show that the strong interaction between nutrients and carbon needs to be taken into account to understand the resilience of trees and forests towards drought events projected to occur more frequently in future.

How to cite: Gessler, A., Joseph, J., Hagedorn, F., Ouyang, S., and Schönbeck, L.: The interaction of carbon and nutrient relations in trees exposed to drought, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1638, https://doi.org/10.5194/egusphere-egu22-1638, 2022.

Severe drought acutely impairs plant hydraulic functioning and impedes processes of carbon (C) and nutrient as well as their allocation. However, how fertilization would modify the allocation of C and nutrient between sink and source organs during drought stress remains largely unknown.

We used three-year old potted seedlings of sessile oak and Scots pine in a greenhouse experiment where they were subjected to three different fertilization treatments (non-fertilized, moderate and high fertilization) and two water regimes (well-watered and severe drought) across two consecutive growing seasons. ¹³C and ¹⁵N labeling were labeled to trace the C and nitrogen (N) allocation. Leaf gas exchanges and predawn water potential, biomass of the different plant organs and NSC concentration, as well as relative ¹³C and ¹⁵N allocation to root, stems and leaves were assessed in the two growing seasons.

Our results showed that sessile oak grown under fertilization suffered faster from drought and showed earlier death than unfertilized seedlings. Fertilization significantly improved aboveground growth, increased shoot: root ratio and reduced NSC storage in sessile oak. This leads to drought-induced C depletion and increased mortality under severe drought. Progressing drought altered C and N translocation strategies in sessile oak by prioritizing C allocation to and N retention in the roots under moderate fertilization, but not in high fertilization. In sessile oak, seasonal dynamics of C and N allocation is coupled and independent of drought and fertilization. By contrast, fertilization and drought, both had only minor impacts on Scots pine C allocation and the tradeoff of C allocation between growth and reserves, as well as the uptake of added N by root. Severe drought strongly decreased NSC in stem and root of Scots pine, while NSC concentrations in leaf and fine root kept stable and high and at the status of mortality.

We conclude that sessile oak shows a more plastic response to environment changes than Scots pine by adjusting its C and N relations on the whole plant level. The impact of fertilization on tree seedlings drought responses seems to be species-specific and is also modified by the degree of drought and fertilization.

How to cite: Ouyang, S., Shen, W., Saurer, M., Duan, H., Ding, G., Tie, L., Li, M., and Arthur, G.: Fertilization effects on drought responses in sessile oak and Scots pine seedlings, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2100, https://doi.org/10.5194/egusphere-egu22-2100, 2022.

Drought is an ever-increasing threat; its negative effects on ecosystems and their functioning directly impact our food security. It is therefore critical to understand the mechanisms that affect drought resilience of ecosystems. Many ecosystem functions depend on plant-soil interactions and are mediated by dissolved organic molecules, which are then recorded in the dissolved organic carbon (DOC) that leaches from plants and soils. In particular, DOC properties during and after rewetting can reveal if and how ecosystems are affected by drought. We therefore investigated the concentration of DOC in four different plant communities on sandy soils in Germany over three years that differed in drought intensity, including the extreme 2018 drought. We also analysed the molecular composition of DOC using ultrahigh-resolution mass spectrometry to identify the carbon sources during the rewetting period. A linear mixed effect model revealed that drought intensity significantly affected DOC concentration. DOC concentration in soil leachate was slightly increased following medium drought intensity, but was significantly reduced following high drought intensity. This suggests that medium intensity drought may stimulate DOC release, however, high intensity drought reduces DOC release. Molecular composition analysis of the DOC present during the rewetting period revealed an initial release of plant-derived carbon followed by an increase in soil organic matter-like compounds. Our findings indicate that the initial release of plant-derived carbon into soil leachate might be crucial for the ability of ecosystems to quickly recover from drought. High intensity drought may interrupt plant functioning to the point of preventing the accumulation and subsequent release of plant-derived carbon during drought, and therefore hamper ecosystem recovery. This suggests the presence of tipping points with respect to the ability of ecosystems to recover from drought. As such, monitoring DOC concentrations could lead to better assessements of the drought resilience of ecosystems.

How to cite: Orme, A., Lange, M., Schroeter, S. A., Wicke, M., Pohnert, G., and Gleixner, G.: Increased Drought Intensity Reduces Release of Plant Carbon into Dissolved Organic Carbon Pool, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-509, https://doi.org/10.5194/egusphere-egu22-509, 2022.

Root dynamics and allocation belowground are a major uncertainty in ecosystem studies because roots are hard to measure in comparison to leaves, which can be assessed at fine timescales using a variety of remote sensing approaches. We built an automated minirhizotron system capable of taking root images on a sub-daily scale and analyzed all high frequency images from this with a neural network approach. We pair this with a daily series of above-ground vegetation indexes of the same plants from standardized ‘phenocam’ digital camera methods. Here we will demonstrate, from a mesocosm experiment that 1) in a mixed species mesocosm, root and shoot production (i.e. rate of change of indexes) as not synchronized on short (multi-day) timescales and 2) root growth rate was more important than overall biomass and leaf growth rate in determining variability in soil CO₂ efflux. Hence this efflux was likely driven by root growth respiration rather than maintenance respiration. We also show the first results from applying similar principles to ecosystem measurements. 

How to cite: Nair, R., Strube, M., Schrumpf, M., El-Madany, T., and Migliavacca, M.: Short-term asynchrony of root and shoot growth links to soil carbon flux dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5349, https://doi.org/10.5194/egusphere-egu22-5349, 2022.

Forest stand density has been shown to have different albeit small effects on soil carbon. We hypothesized that the absence of density effect on soil carbon (C) storage can be explained by a loss of old soil C. This replacement of old by fresh C results in zero net C sequestration by soils but could alter the quality of soil organic matter.   We used one afforestation experiment in Siberia, in which three tree species (spruce, larch and Scots pine) were grown for the last 30 years at 18 levels of stand density, ranging originally from 500 to 125,000 stems per ha. We selected five density levels and studied C and nitrogen (N) contents in mineral soils at 0-5 cm depth. The age of soil C was measured under larch and spruce for three levels of density by radiocarbon (¹⁴C) dating. In all soil samples we determined stability of soil organic matter (SOM) to mineralization (decomposability) and to elevated input of readily decomposable C – glucose (primability).  Stand density affected soil C and N contents differently depending on the tree species. Only under spruce both C and N contents were increasing with density, under larch and pine the covariation was insignificant, while N tended even to decline with density increase.  With the ¹⁴C data we were able to show strong dilution of old SOM by fresh C derived from the trees, the effect was stronger with higher density. This provides first evidence that density increase increases the fractions of new C versus old C and this can happen without altering the total C contents like under larch. While stand density altered soil C and N contents only under spruce, it altered C decomposability under all tree species: with density increase the C decomposability (per unit of C) declined under spruce but increased under larch and pine. This is relevant to predicting C losses from forest soils with different tree species and densities.  Higher losses would occur under larch and pine with higher densities, but increase of density under spruce would reduce the C losses from soil. Furthermore, while no significant covariation of stand density with C primability was detected, we first observed strong tree species effects on C primability. Twice as much C is lost from soil under larch than under spruce and pine by equal addition of C-glucose.  This indicates that elevated C deposition from roots and exudates to soil as predicted due to elevated CO₂ concentration would most strongly accelerate soil C turnover and C losses under larch than under spruce and pine. Overall, tree species altered the susceptibility of soil C to elevated C input and stand density had strong effect on the decomposability of SOM, which is important parameter of C stability. The effect of stand density is thus important to consider even if stand density does not affect total soil C content. 

How to cite: Menyailo, O., Sobachkin, R., Makarov, M., and Cheng, C.-H.: Tree species and stand density: Effects on soil organic matter content, decomposability and susceptibility to microbial priming, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1634, https://doi.org/10.5194/egusphere-egu22-1634, 2022.

Eco-engineering Fe ore tailings into technosols (i.e., soil-like growth substrates) has been advocated to be a promising technology for sustainable rehabilitation of tailings with native plant communities ^1-3. Arbuscular mycorrhizal (AM) symbiosis has been found to be able to colonize tailing technosol eco-engineered through exogenous plant biomass input, and contributed to aggregate development and organic matter stabilization in the tailings⁴. However, the AM performance and their eco-functionality usually varies depending on water conditions and tailing technosols developed from different plant biomass residue (PBR) input, which has yet been addressed in previous studies. Therefore, the present study aimed to investigate the role of AM symbiosis in aggregate development and association of organic carbon (C) and nitrogen (N) with mineral phase of aggregates in the developing technosols eco-engineered from Fe ore tailings, in relation to low water supply and plant biomass residues of contrasting nutrient quality (e.g., C:N ratios). The results showed that AM symbiosis did not influence aggregate development, but stimulated organic carbon and nitrogen stabilization  in tailings-technosol. In particular, AM symbiosis enriched organic C (rather than N) sequestration in minerals of tailings-technosol amended with Lucerne hay containing high N and low C:N ratio. Comparatively, AM symbiosis seemed to have enriched significantly N (rather than organic C) in aggregate minerals in tailings-technosols amended with Sugarcane mulch (with low N and high C:N ratio). This increased N sequestration may have resulted from N-rich AM fungal exudates or fungal biomass. AM symbiosis enhanced organic matter sequestration through enhancing associations between carboxyl-rich organics and key Fe-rich phyllosilicates and/or Fe(oxy)hydroxides. Drought stress limited AM symbiosis role in organic C and N sequestration in the tailing-technosol. In summary, the study indicated that plant biomass of different C:N ratio could influence AM role in organic matter stabilization in Fe ore tailings-technosol, and further studies are required to unravel implications of different organic C and N sequestration in aggregate minerals of tailings-technosols, in relation to long-term pedological development and sustainability of soil functions.

• Wu, S.; Liu, Y.; Bougoure, J. J.; Southam, G.; Chan, T. S.; Lu, Y. R.; Haw, S. C.; Nguyen, T. A. H.; You, F.; Huang, L., Organic Matter Amendment and Plant Colonization Drive Mineral Weathering, Organic Carbon Sequestration, and Water-Stable Aggregation in Magnetite Fe Ore Tailings. Environ Sci Technol 2019, 53, (23), 13720-13731.
• Huang, L.; Baumgartl, T.; Zhou, L.; Mulligan, R. In The new paradigm for phytostabilising mine wastes–ecologically engineered pedogenesis and functional root zones, Life-of-Mine Conference, 2014; 2014; pp 16-18.
• Huang, L.; Baumgartl, T.; Mulligan, D., Is rhizosphere remediation sufficient for sustainable revegetation of mine tailings? Ann Bot 2012, 110, (2), 223-38.
• Li, Z.; Wu, S.; Liu, Y.; Yi, Q.; You, F.; Ma, Y.; Thomsen, L.; Chan, T.-S.; Lu, Y.-R.; Hall, M.; Saha, N.; Huang, Y.; Huang, L., Arbuscular mycorrhizal symbiosis enhances water stable aggregate formation and organic matter stabilization in Fe ore tailings. Geoderma 2022, 406.

How to cite: Li, Z., Wu, S., Huang, L., and Huang, Y.: Arbuscular mycorrhizal colonization enhanced organic carbon and nitrogen sequestration in technosols eco-engineered from Fe ore tailings with different plant biomass residues, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6667, https://doi.org/10.5194/egusphere-egu22-6667, 2022.