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Soil microbial biomass and microbial stoichiometric ratios are important for understanding carbon and nutrient cycling in terrestrial ecosystems. Here, we compiled data from 12245 observations of soil microbial biomass from 1626 published studies to map global patterns of microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), microbial biomass phosphorus (MBP), and their stoichiometry using a random forest model. Concentrations of MBC, MBN, and MBP were most closely linked to soil organic carbon, while climatic factors were most important for stoichiometry in microbial biomass ratios. Modeled seasonal MBC concentrations peaked in summer in tundra and in boreal forests, but in autumn in subtropical and in tropical biomes. The global mean MBC/MBN, MBC/MBP, and MBN/MBP ratios were estimated to be 10, 48, and 6.7, respectively, at 0-30 cm soil depth. The highest concentrations, stocks, and microbial C/N/P ratios were found at high latitudes in tundra and boreal forests, probably due to the higher soil organic matter content, greater fungal abundance, and lower nutrient availability in colder than in warmer biomes. At 30-100 cm soil depth, concentrations of MBC, MBN, and MBP were highest in temperate forests. The MBC/MBP ratio showed greater flexibility at the global scale than did the MBC/MBN ratio, possibly reflecting physiological adaptations and microbial community shifts with latitude. The results of this study are important for understanding C, N, and P cycling at the global scale, as well as for developing soil C-cycling models including soil microbial C, N, and P as important parameters.

Soils are the largest carbon (C) reservoir in terrestrial ecosystems. There are still numerous uncertainties concerning the fate of soil organic carbon and its feedback on climate change. Radiocarbon is a useful approach to better understanding the carbon cycle. The nuclear weapon testing in the 1960s induced a peak in 14C atmospheric concentration – a signal that can be used to trace the incorporation and turnover of C in soil. By separating the soil in different fractions and measuring the 14C in them, we can quantify how much C and for how long is stored in soils, and where soil organic carbon is stabilised.Our study aimed at identifying the impact of climate and mineralogy on soil organic matter turnover on a regional scale. We analysed C pools and 14C contents in the organic layer, mineral soil (0-20cm) and its fractions from 54 sites across Switzerland. These 54 sites are systematically spread across natural climatic and geological gradients and were repeatedly sampled in the 1990s and 2014. The mineral soil was incubated for 181 days and 14C was measured in the respired CO2. The mineral soil was fractionated according to density into particulate organic matter (POM) and mineral-associated organic matter (MAOM). We then oxidised the mineral-associated organic matter with hydrogen peroxide to remove its labile fraction of carbon. Our 14C dataset was analysed together with ancillary data comprising soil properties and climatic variables from the studied sites.Our radiocarbon dataset showed that the carbon that was respired from the mineral soil originated predominantly from particulate organic matter. The bomb spike signal was incorporated in the organic layer and in the particulate organic matter, while the mineral-associated organic matter had turnover times on centennial to millennial time scales (from 94 to 3060 years). Further chemical oxidation of MAOM using hydrogen peroxide revealed a stronger depletion in radiocarbon of the residual fraction with Δ14C values ranging between -173 ‰ and -47 ‰. This indicates that the MAOM is a mixture of 14C-enriched organic matter and very old material.With respect to the controlling factors of soil organic matter turnover time, the radiocarbon signature of the POM is most strongly affected by climatic variables such as mean annual temperatures. In contrast to POM, the mineral-associated organic matter, comprising the greatest pool of soil organic carbon is driven by chemical soil properties. For instance, older 14C ages are found in acidic soils with low pH values ranging between 3 and 4. In these soils, Al and Fe oxides concentrations are high. We showed that the concentrations of pedogenic oxides in the soil correlate with soil organic carbon concentrations in the mineral-associated organic matter. In soils with higher pH (>7), we can also find old 14C ages. In these soils, C is stabilised by interactions with calcium ions and carbonates.Overall, our regional scale dataset shows that the net accumulation of labile soil organic matter seems to be climate sensitive, while mineralogy and weathering contribute most significantly to the stabilisation of organic carbon in the soil.

Climate warming is most pronounced in cold regions impacting plant-soil system with counteracting effects on ecosystem carbon storage. Whereas upward migration of plants (alpine greening) in a warmer climate potentially results in carbon uptake from the atmosphere and sequestration in the soil, increased decomposition at higher temperatures can enhance carbon release from the soil. Carbon losses from soils might be particularly high in soils where large amounts of carbon have accumulated under past climatic conditions. To infer soil organic carbon (SOC) changes of alpine soils in a warming climate, we assessed SOC stocks and their stability change along elevational gradients in the Swiss Central Alps. In our study, we excavated 21 soil pits to the parent material along elevational gradients (2000 to 3100 m a.s.l.) on three different bedrock types (calcareous, amphibolitic and siliceous) and analyzed their SOC stocks, stable isotope composition, and C stability by a soil incubation experiment. First results show a distinct elevation pattern with a strong decline in SOC storage with decreasing vegetation cover above 2700 m a.s.l.. However, soils on amphibolitic bedrock still contained substantial amounts of SOC even at elevations above 3000 m a.s.l. (0.8% SOC; >1.4 kg C/m2). As soils at this high elevation were buried under a few decimeter thick debris layer and no plants were present, it seems likely that these are remnants of fossil soils. In support, the buried soil organic matter (OM) had rather high δ13C values of -23.2‰ and narrow CN ratios of 10.7, indicating that they consist of strongly transformed OM. Analysis of their radiocarbon contents and CN analysis of stones will provide further information on the origin of these high elevation soils. Regardless of their origin, these soils represent a CO2 source. Soils released 0.2-0.5% of their SOC contents within one month upon incubation at 10 and 22°C in the laboratory. Moreover, in situ chamber measurements at the end of August 2022 showed a mean CO2 efflux of approximately 3.6 mg CO2-C/m2h to the atmosphere. Along the studied gradient, the δ13C values of surface soils strongly decreased with decreasing elevation. At the same time, the SOC mineralizability and soil C/N ratios showed a pronounced increase towards the soils at lower elevation having a dense grassland cover. This indicates that along with increasing SOC stocks, the contribution of relatively fresh OM with high turnover rates increases. Overall, our results show that there is a transition from the accumulation of SOC with alpine greening, reflected by alpine soils at lower elevations (< 2700 m a.s.l.), to buried SOC releasing C (possibly ancient C) at high elevation. Future measurements will provide further insight into the rapidity and magnitude of SOC stock changes in alpine terrain with ongoing climate warming.

Soil organic matter (SOM) originates from various sources such as foliar litter, roots and microbial (e.g. fungal) components. The relative sources contribution represents one of the key unknowns in SOM dynamics. Our study aimed to explore whether stable isotope ratios of non-exchangeable hydrogen (Hn) bound to organic matter can be used to differentiate SOM sources, since natural 2Hn abundance can strongly differ between root and foliar tissues. We also investigated if long-term irrigation with 2H-depleted water in a pine forest can be used to track Hn incorporation into organic matter inputs and eventually in the soil pools.In a 17-year-long irrigation experiment in a dry pine forest, we assessed variations in natural abundance of 2Hn, 13C, and 15N in SOM sources (foliar litter, fine roots, fungal mycelia), decomposing litter, soil (organic layers and uppermost 5 cm-mineral soil) and particle-size fractions. We then applied a Bayesian mixing model (including δ2Hn,δ13C, and δ15N) to estimate the relative sources contribution to SOM.Natural 2Hn abundance was significantly higher in roots vs. foliar litter (up to +39‰), and in fungal mycelia vs. roots (up to +41‰). Results from Bayesian mixing model suggest that foliar litter contributed to approximately 68 ± 10% of SOM in organic layers and in coarse particulate organic matter (POM). Foliar litter and roots contributed similarly to upper 2 cm of mineral soil (46 ± 11%), while 2-5 cm of mineral soil were largely derived from roots (61 ± 13%). Fungal mycelia contributed to 18 ± 8% of mineral-associated organic matter (MOM), while only to 1-2% of coarse and fine POM. Bayesian mixing models provided only a general indication of the sources contribution to SOM, also considering that isotopic signatures shifted during decomposition. Measurements of isotope signatures in microbial necromass might allow a more accurate assessment of the different SOM sources contribution.The δ2Hn depletion of soil water under irrigation was paralleled by a comparable decrease in δ2Hn of roots (~12‰). In comparison, the natural 2Hn abundance in fresh needles and foliar litter decreased less strongly (~ 7‰ and 4‰, respectively), likely due to photosynthetic adjustments that may have counterbalanced the irrigation water 2H-depletion. Similar to soil water 2H-depletion, δ2Hn values in coarse POM were 11‰ lower in irrigated vs. dry plots, suggesting that nearly all organic Hn turned over or exchanged with soil water in less than two decades. In contrast, δ2Hn values in fine POM and MOM decreased only by 3‰ under irrigation, which indicate that these fractions comprise slower cycling Hn pools.Our study showed that the natural 2Hn abundance represents a promising tool to differentiate among SOM sources. While 13C and 15N did not clearly separate between roots and foliar litter, Hn isotopic signatures allowed a good discrimination between SOM sources. In addition, long-term irrigation can provide a potential in situ 2H-labelling of SOM, which may help to examine organic Hn turnover rates across SOM pools.

Increasing forest disturbance is among the most profound impacts of climate change on terrestrial ecosystems. Insect outbreaks, storms, or wildfires can destroy the whole tree layer, with serious consequences for biogeochemical cycles until succession returns the ecosystem back to a forested state. However, tree regeneration is often inhibited by ungulate herbivory and herbaceous competition, and disturbed ecosystems remain in non-forested states for decades. The impact of such vegetation changes on soil carbon (C) and nitrogen (N) cycles is highly unknown, because a multitude of plant-soil feedbacks are involved, and underlying processes have hardly been investigated. Here, we studied soil microbial community structure, gene abundance of bacteria, fungi, and N cycling microorganisms, soil enzymes, and C-N dynamics across a disturbed forest landscape in Central Europe, covering a range of successional stages after storm damage and bark-beetle attacks. We used a chronosequence-approach including disturbed sites regrown with Picea abies stands, and disturbed sites dominated by herbaceous pioneer plants, particularly Calamagrostis grasses. Soil C and N stocks increased under a prolonged herbaceous cover. Three decades after disturbance the stocks were ca. 45% higher than those of regrown forest stands. Beside C inputs from herbaceous fine roots, we link this increase to changes in the structure and functioning of the microbial community, which reduces the decomposition of organic matter. With a prolonged herbaceous cover, decreasing fungal abundances coincided with declining activities of phenol oxidase and of hydrolytic enzymes used to acquire nutrients. Since ectomycorrhizal fungi were almost absent compared to regrowing forest stands, this may be linked to reduced ectomycorrhizal mining for organic N. Moreover, ammonia-oxidising (amoA) gene abundances increased along with ammonium and nitrate concentrations, pointing towards an accelerated inorganic N cycle under a prolonged herbaceous cover. A surplus of inorganic N and grass-rhizodeposits renders it also likely that saprotrophs are less dependent on organic matter-bound C and N. Taken together, we found strong evidence for a linkage between above- and belowground communities following forest disturbance. We suggest a prolonged cover of herbaceous pioneer plants opens the nitrogen cycle through microbial communities which reduces mining for organic N and thus, increases soil C storage.

Climate change is causing negative effects on forests and their functioning through more frequent and intense periods of drought. Repeated conditions of water limitation not only affect the growth and vitality of trees but also feed back on the cycling of carbon (C) at the plant-soil interface. However, the impact of the intensity of drought on the transfer of assimilated C belowground remains quantitatively unresolved. We assessed how increasing levels of soil water limitation affect the growth of Scots pine (Pinus sylvestris L.) saplings and performed a 13C-CO2 pulse labelling experiment to trace the pathway of newly-assimilated C into needles, fine roots, soil pore CO2, and phospholipid fatty acids of soil microbial groups. We hypothesized that increased water stress would reduce tree C uptake, and the magnitude and velocity at which newly-assimilated C is allocated belowground and further metabolized. Moreover, we expected that severe levels of soil water deficit would lead to a build-up of newly-assimilated C in fine roots. Our data indicated that with more intense water limitation, trees reduced their growth despite initially partitioning more biomass to belowground tissues under severe water stress. Moderate levels of water limitation barely affected the uptake of 13C label and the magnitude and transit times of C being allocated from needles to the rhizosphere. In contrast, severe water limitation increased the fraction of 13C label allocated to roots and soil fungi while a lower fraction of 13CO2 was respired from the soil. We conclude that when soil water becomes largely unavailable, C cycling within trees becomes slower, and a major fraction of C allocated belowground is accumulated in roots or transferred to the soil and associated microorganisms without being metabolically used. Our experiment overall demonstrates the relevance of quantifying the level of water limitation at which C allocation dynamics within trees and soils are altered to inform about the trajectory of forests to the environmental pressures they face.

Earthworms may act as double-edged swords for soil organic matter (SOM). While they can enhance organic matter (OM) mineralization via increased microbial activity they can also elevate OM stabilization in aggregates as particulate or mineral-associated OM. In this study, we will test this potentially opposing impact in beech (Fagus sylvatica L.) forests on limestone, a forest ecosystem with particularly high earthworm activity. A specific focus will be on OM transformation along the continuum from the forest floor (O horizons) to mineral soil (A horizons). The forest floor can represent a substantial OM-pool which is an important source for SOM formation via bioturbation or leaching but can be vulnerable to alterations due to climate change. In an extended lab mesocosm experiment, we will incubate local earthworm species in soil columns consisting of O and A horizons from four beech forest sites along an elevation gradient from 550 to 1250 m in the Swiss Jura Mountain range. Along this gradient, the dominating forest floor type is mull with its thickness increasing with altitude. We will establish the following three treatments (1) control with soil and unlabeled litter, (2) with soil and labeled litter and (3) with soil, labeled litter, and earthworms. For this setup, the Ol horizon will be replaced with beech litter highly enriched with 13C, 15N, and 2H. Soil respiration (CO2) and leaching (C, N, and H in dissolved OM) will be repeatedly measured. Our setup will allow for a separation of fluxes from the O horizons and the A Horizon. After approximately 4, 7, and 10 months each, a subset of mesocosms will be harvested to investigate isotope enrichment in earthworm biomass, earthworm casts, physical soil fractions, PLFAs, and microbial necromass. This will allow us to establish a mass balance of beech litter turnover as affected by earthworms for a time scale representative of one vegetation period. Fluxes of unlabeled OM will inform on the fate of inherent SOM. We expect that (1) following an initial colonization phase, earthworms will stimulate labeled litter mineralization and enhance litter transfer to aggregate fractions while not affecting the total SOM stock. (2) In the long term, less of the labeled material will be mineralized and more SOM stabilized in aggregate fractions will be recycled.

The formation of high elevation treelines is thought to result from direct low-temperature growth limitation, but the in-direct role of nitrogen (N) in modifying growth has rarely been evaluated. Slow N mineralization rates in cold soils may push trees to rely more on N supplied by symbiotic mycorrhizal fungi. Here, we investigated the carbon (C) and N exchange between trees and mycorrhizal fungi along an Alpine treeline ecotone using bomb 14C and natural abundance isotopes (13C and 15N). We collected fine roots, branches, and needles from two tree species (Larix decidua L. and Pinus mugo spp. uncinata Ramond) and sporocarps of mycorrhizal and free-living (saprotrophic) fungal genera. 14C measurements demonstrated that mycorrhizal fungi rely on new photo-assimilates derived from fine roots, while saprotrophic fungi feed on several years old C (4-10 yr). The C transfer root-fungi seems to have isotopic fractionation that enriches the fungi with 13C in 1-5‰. Mycorrhizal fungi had higher δ15N values and C:N ratio than saprotrophic fungi. Assuming that the two fungi types share the same N source in the soil, the 15N enrichment and the lower N concentration of the mycorrhizal fungi could be explained through preferential transfer of 14N to the hosting trees. The δ15N in the trees generally decreased with increasing elevation, suggesting a greater reliance on N supplied by mycorrhizal fungi in colder soils. However, abrupt increase in the Larix δ15N at the treeline suggests opening of the N cycle for this deciduous tree species, either by a decreasing N demand of slow-growing trees or a reduced competition for N with other plants. Overall, our results indicate that in cold treeline ecotones the sources and availabilities of soil N have a key influence on determing the N uptake pathways of trees and consequently plant growth.

Soil organic matter (SOM) plays an important role in the global carbon cycle, especially in alpine ecosystems. However, ongoing forest expansion in high elevation systems potentially alters SOM storage through changes in organic matter (OM) inputs and microclimate. In this study we investigated the effects of an Picea abies L. afforestation chrono-sequence (40–130 years) of a former subalpine pasture in Switzerland on soil organic carbon (SOC) stocks and SOM dynamics. We found that SOC stocks remained relatively constant throughout the chrono-sequence, with comparable SOC stocks in the mineral soils after afforestation and previous pasture (SOC40-year-old forest = 11.6 ± 1.1 kg m−2, SOC130-year-old forest = 11.0 ± 0.3 kg m−2, and SOC pasture = 11.5 ± 0.5 kg m−2). However, including the additional carbon of the organic horizons in the forest, reaching up to 1.7 kg m−2 in the 55-year-old forest, resulted in a slight in-crease in overall SOC stocks following afforestation. We found that the soil C:N ratio in the mineral soil increased in the topsoil (0–5cm) with increasing forest stand age, from 11.9 ± 1.3 in the grassland to 14.3 ± 1.8 in the 130-year-old forest. In turn, we observed a decrease in soil C:N ratio with increasing depth in all forest stand ages. This suggests that litter-derived organic matter (C:N from 35.1 ± 1.9 to 42.4 ± 10.8) is likely incorporated and translocated from the organic horizon to the mineral topsoil (0–10 cm) of the profiles. As roots had very high C:N ratios (pasture 63.5 ± 2.8 and forests between 54.7 ± 3.9 and 61.2 ± 2.9), particulate root-derived organic matter seems to have a minor influence on forest soil C:N ratio and thereby on SOC stock accumulation in the mineral soil. These results suggest that, although the afforestation only moderately affected the SOC stock, there is an apparent alteration in the SOC dynamics through changes of the litter composition caused by the vegetation shift. We conclude that spruce afforestation on a former subalpine pasture does not necessarily change the total SOC stock and that consequently there is no SOC sequestration on a decadal to centennial scale.

Ectomycorrhizal (ECM) fungi can both accelerate and decelerate decomposition of organic matter in forest soils, but a mechanistic understanding of this differential influence is limited. Here, we tested how ECM fungi affect decomposition along a natural fertility gradient in a temperate forest of European beech. Trees were girdled to reduce belowground carbon supply to the soil. Girdling shifted soil fungal community composition and decreased hyphal biomass production and soil CO2 efflux, indicating a reduced ECM fungal activity. Girdling also affected decomposition processes, but the effects depended on fertility. Our results indicate that ECM fungi decelerate decomposition under conditions of low fertility while under conditions of high fertility ECM fungi and their host roots have an accelerating effect. We conclude that both acceleration and deceleration of decomposition of organic matter by ECM fungi can occur within a forest, with soil fertility determining the direction and magnitude of these effects. We suggest a positive feedback between fertility, stand productivity and soil carbon and nitrogen dynamics that is mediated to a large extent by ECM fungi., New Phytologist, 239 (1), ISSN:0028-646X, ISSN:1469-8137

Im Schweizer Wald traten im Trockensommer 2018 vorzeitige Blattverfärbung und Welken bereits im Juli auf. Der Frage, ob dieses Phänomen vor allem auf die hohe Trockenintensität im Jahr 2018 zurückzuführen ist, oder aber sich als Folge eines Bodenwasserdefizites des trockenen Jahres 2015 ergab, sind wir mit einem Modell nachgegangen, das den Wassertransport vom Boden durch die Pflanze in die Atmosphäre simuliert. Das Modell bestätigte, dass der physiologische Trockenstress, ausgedrückt als Transpirationsdefizit, im Jahr 2018 erheblich höher war als 2015. Zudem zeigte sich, dass nach Trockenjahren wie 2015 ein Defizit des pflanzenverfügbaren Wassers in der Wurzelzone kaum über einzelne Jahre hinaus bestehen bleibt. Somit wurden die Schäden 2018 vorwiegend durch die höhere Trockenintensität in diesem Jahr ausgelöst. Die Modellierung der Wasserverfügbarkeit für den Schweizer Wald könnte zur zeitnahen Identifikation von klimasensitiven Waldstandorten verwendet werden.

Der Boden ist eine zentrale Schnittstelle in Waldökosystemen. Er erfüllt wichtige Funktionen als Lebensraum, als Speicher für Kohlenstoff und Nährstoffe, und er reguliert die Wasser- und Stoffkreisläufe. Für alle Bodenfunktionen spielt die organische Bodensubstanz eine Schlüsselrolle. Aufgrund des feuchten und kühlen Klimas sowie der relativ nachhaltigen Waldnutzung speichern Schweizer Waldböden pro Flächeneinheit die grössten Vorräte organischer Bodensubstanz Europas. Dieser Vorrat hat sich über Tausende Jahre hinweg aufgebaut. Der Klimawandel - höhere Temperaturen und eine intensivere Trockenheit - sowie grossflächige Störungen, beispielsweise durch Windwürfe und Kahlschläge, gefährden die organische Bodensubstanz. Eine bodenschonende Waldbewirtschaftung sowie eine permanente Bestockung mit standortsangepassten Baumarten tragen zum Erhalt dieser wichtigen Ressource bei.

Wälder sind zunehmend von Sturmschäden betroffen. In einem Projekt der Eidgenössischen Forschungsanstalt WSL wurde untersucht, wie sich Windwürfe auf die Kohlenstoffspeicherung im Waldboden auswirken. Hoch gelegene Nadelwälder sind besonders anfällig für Kohlenstoffverluste aus dem Boden. Im Unterschied zu Wäldern tieferer Lagen werden dort grössere Mengen an Kohlenstoff in rein organischen, mächtigen Humusauflagen gespeichert. Eine Stabilisierung durch mineralische Interaktionen fehlt weitgehend, was die Humusauflagen empfindlich macht für Kohlenstoffverluste durch störungsbedingte Veränderungen des Bestandesklimas. Die langsame Baumverjüngung und die geringen Streueinträge in den Boden bremsen den Humusaufbau nach einer Störung zusätzlich. Eine weitere Zunahme von Windwürfen könnte die Bodenkohlenstoffvorräte in Bergwäldern drastisch reduzieren, was negative Auswirkungen auf das Klima hätte. Darüber hinaus könnten störungsbedingte Humusverluste die Bodenqualität nachhaltig verschlechtern.

The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial. We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2-h resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use. Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre-drought levels within 3 wk after rewetting. These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

Storms represent a major disturbance factor in forest ecosystems, but the effects of windthrows on soil organic carbon (SOC) stocks are poorly quantified. Here, we assessed the SOC stocks of windthrown forests at 19 sites across Switzerland spanning an elevation gradient from 420 to 1550 m, encompassing a strong climatic gradient. Results show that the effect size of disturbance on SOC stocks increases with the size of the initial SOC stocks. The largest windthrow-induced SOC losses of up to 29 t C ha−1 occurred in high-elevation forests with a harsh climate developing thick organic layers. In contrast, SOC stocks of low-elevation forests with thin organic layers were hardly affected. A mineralization study further revealed high elevation forests to store higher amounts of easily mineralizable C in thick organic layers that got lost following windthrow. These findings are supported by a meta-analysis of available windthrow studies, showing an increase of storm-induced SOC losses with the size of the initial SOC stocks. Modelling simulations further indicate longer-lasting SOC losses and a slower recovery of SOC stocks after windthrow at high compared to low elevations, due to a slower regeneration of mountain forests and associated lower C inputs into soils in a harsh climate. Upscaling the experimental findings/observed patterns by linking them to a data base of Swiss forest soils shows a total SOC loss of ∼0.4 Mt. C for the whole forested area of Switzerland after two major storm events, counteracting the forest net carbon sink of decades. Our study provides strong evidence that the vulnerability of SOC stocks to windthrow is particularly high in forests featuring thick and slowly forming organic layers, such as mountain soils. Thus, the risk of losing SOC to more frequent windthrows in mountain forests strongly limits their potential to mitigate climate change., Science of The Total Environment, 857, ISSN:0048-9697, ISSN:1879-1026

Central Europe has been experiencing unprecedented droughts during the last decades, stressing the decrease in tree water availability. However, the assessment of physiological drought stress is challenging, and feedback between soil and vegetation is often omitted because of scarce belowground data. Here we aimed to model Swiss forests' water availability during the 2015 and 2018 droughts by implementing the mechanistic soil-vegetation-atmosphere-transport (SVAT) model LWF-Brook90 taking advantage of regionalized depth-resolved soil information. We calibrated the model against soil matric potential data measured from 2014 to 2018 at 44 sites along a Swiss climatic and edaphic drought gradient. Swiss forest soils' storage capacity of plant-available water ranged from 53 mm to 341 mm, with a median of 137 ± 42 mm down to the mean potential rooting depth of 1.2 m. Topsoil was the primary water source. However, trees switched to deeper soil water sources during drought. This effect was less pronounced for coniferous trees with a shallower rooting system than for deciduous trees, which resulted in a higher reduction of actual transpiration (transpiration deficit) in coniferous trees. Across Switzerland, forest trees reduced the transpiration by 23% (compared to potential transpiration) in 2015 and 2018, maintaining annual actual transpiration comparable to other years. Together with lower evaporative fluxes, the Swiss forests did not amplify the blue water deficit. The 2018 drought, characterized by a higher and more persistent transpiration deficit than in 2015, triggered widespread early wilting across Swiss forests that was better predicted by the SVAT-derived mean soil matric potential in the rooting zone than by climatic predictors. Such feedback-driven quantification of ecosystem water fluxes in the soil–plant-atmosphere continuum will be crucial to predicting physiological drought stress under future climate extremes., Global Change Biology, 28 (20), ISSN:1354-1013, ISSN:1365-2486

Above and belowground compartments in ecosystems are closely coupled on daily to annual timescales. In mature forests, this interlinkage and how it is impacted by drought is still poorly understood. Here, we pulse‐labelled 100‐year‐old trees with 13CO2 within a 15‐year‐long irrigation experiment in a naturally dry pine forest to quantify how drought regime affects the transfer and use of assimilates from trees to the rhizosphere and associated microbial communities. It took 4 days until new 13C‐labelled assimilates were allocated to the rhizosphere. One year later, the 13C signal of the 3‐h long pulse labelling was still detectable in stem and soil respiration, which provides evidence that parts of the assimilates are stored in trees before they are used for metabolic processes in the rhizosphere. Irrigation removing the natural water stress reduced the mean C residence time from canopy uptake until soil respiration from 89 to 40 days. Moreover, irrigation increased the amount of assimilates transferred to and respired in the soil within the first 10 days by 370%. A small precipitation event rewetting surface soils altered this pattern rapidly and reduced the effect size to +35%. Microbial biomass incorporated 46 ± 5% and 31 ± 7% of the C used in the rhizosphere in the dry control and irrigation treatment respectively. Mapping the spatial distribution of soil‐respired 13CO2 around the 10 pulse‐labelled trees showed that tree rhizospheres extended laterally 2.8 times beyond tree canopies, implying that there is a strong overlap of the rhizosphere among adjacent trees. Irrigation increased the rhizosphere area by 60%, which gives evidence of a long‐term acclimation of trees and their rhizosphere to the drought regime. The moisture‐sensitive transfer and use of C in the rhizosphere has consequences for C allocation within trees, soil microbial communities and soil carbon storage., To quantify the transfer of carbon from tree canopies into the belowground, we conducted a pulse‐labelling with 100‐year old trees in a 15‐year‐long irrigation experiment within a naturally dry pine forest. It took four days until new assimilates were allocated from tree canopies to the rhizosphere. One year later, the 13C signal of the three‐hour long pulse labelling was still detectable in stem and soil respiration. Mapping the spatial distribution of soil‐respired 13CO2 around pulse‐labelled trees showed that rhizospheres extended laterally 2.8 times beyond tree canopies. Drought reduced the transfer rates and spatial distribution of assimilates allocated to the soil.

The role of carbon (C) and nutrient uptake, allocation, storage and especially their interactions in survival and recovery of trees under increased frequencies and intensities of drought events is not well understood. A full factorial experiment with four soil water content regimes ranging from extreme drought to well-watered conditions and two fertilization levels was carried out. We aimed to investigate whether nutrient addition mitigates drought effects on downy oak (Quercus pubescens Willd.) and whether storage pools of non-structural carbohydrates (NSC) are modified to enhance survival after 2.5 years of drought and recovery after drought relief. Physiological traits, such as photosynthesis, predawn leaf water potential as well as tissue biomass together with pools and dynamics of NSC and nutrients at the whole-tree level were investigated. Our results showed that fertilization played a minor role in saplings’ physiological processes to cope with drought and drought relief, but reduced sapling mortality during extreme drought. Irrespective of nutrient supply, Q. pubescens showed increased soluble sugar concentration in all tissues with increasing drought intensity, mostly because of starch degradation. After 28 days of drought relief, tissue sugar concentrations decreased, reaching comparable values to those of well-watered plants. Only during the recovery process from extreme drought, root NSC concentration strongly declined, leading to an almost complete NSC depletion after 28 days of rewetting, simultaneously with new leaves flushing. These findings suggest that extreme drought can lead to root C exhaustion. After drought relief, the repair and regrowth of organs can even exacerbate the root C depletion. We concluded that under future climate conditions with repeated drought events, the insufficient and lagged C replenishment in roots might eventually lead to C starvation and further mortality.

The bomb radiocarbon approach allows to estimate the time elapsed since carbon (C) fixation and thus to study the use of stored C. Previous studies show that fine roots (≤ 2 mm), a large and dynamic C pool in trees, are constructed from C that was fixed years to more than a decade previously. In comparison, aboveground tissues grow from C mostly < several years old. Our current understanding is that 14C ages of tree tissues mainly reflect the 14C content of the C substrate which supports their growth. We addressed the question – what explains the variations in 14C age of C supporting growth among tissues and between trees?We compared tissue chronological ages determined from annual-ring count (roots) or observations (needles, branches) with 14C-based C ages along an elevational gradient near the Alpine tree line (Stillberg, Davos, Switzerland). Previous studies have shown that decreasing temperatures along this gradient limit tree growth. Carbon assimilation via photosynthesis is less sensitive to low temperatures, resulting in accumulation of storage compounds in tree line trees. We therefore expected stored C in trees at the tree line to have the slowest turnover rates and oldest C; i.e. that the age difference between 14C and chronological age would increase with elevation.While needles and branch wood tissues were produced from 0-2 yr C, fine roots were produced from C fixed up to a decade ago. Contrary to our prediction, the C age in the roots decreased from 9.5 yr 150 m below the tree line to 4.5 yr at the tree line. Relatively unstressed trees in a site located 600 m lower used the youngest C to build roots (1.5 yr). But the old C ages cannot be explained by a complete decoupling from fresh C, as the roots contained a metabolically active pool with ages of 0-1.5 yr used for root respiration. Previous studies showed that root growth commences before shoot growth and mostly occurs earlier in the growing season. It is therefore plausible that during root growth the reliance on old stored C is greater than at the end of the summer when sampling took place. Yet, the timing of root growth alone cannot explain the elevation trend in the age of C used to build fine roots. The carbon allocation model we adopted suggested that roots are built from older C when the turnover time of the root storage declines. In agreement, the soluble storage compounds below the tree line had the oldest 14C ages and the slowest turnover times.Overall, the oldest belowground C reserves were found at intermediate elevations, where growth limitation was slightly eased, and perhaps a larger proportion of fresh C assimilates were used for the growth of aboveground tissues rather than fine roots. Larger amounts of fresh C are allocated to the belowground when C availability is in surplus either when conditions are favorable (at the lowest elevation) or when growth rates are small compared to C assimilation (at treeline).

Summer droughts strongly affect soil organic carbon (SOC) cycling, but net effects on SOC storage are unclear as drought affects both C inputs and outputs from soils. Here, we explored the overlooked role of soil fauna on SOC storage in forests, hypothesizing that soil fauna is particularly drought-sensitive, thereby reducing litter incorporation into the mineral soil and, eventually, long-term SOC storage.In a drought-prone pine forest (Switzerland), we performed a large-scale irrigation experiment for 17 years and assessed its impact on vertical SOC distribution and composition. We also examined litter decomposition of dominant tree species using litterbags of different mesh sizes and determined soil fauna abundance and community composition.Long-term irrigation resulted in a C loss in the organic layers (-1.0 kg C m-2) and a comparable C gain in the mineral soil (+0.8 kg C m-2) in the first decade of irrigation, and thus did not affect total SOC stocks. Irrigation increased the mass loss of Quercus pubescens and Viburnum lantana leaf litter more strongly when meso- and macrofauna were included (+215%) compared to excluded (+44%). The enhanced faunal-mediated litter decomposition was paralleled by a many-fold increase in the abundance of meso- and macrofauna during irrigation. Moreover, irrigation led to a shift in Acari and Collembola community composition, with irrigation characterized by the presence of drought-sensitive species. In comparison, microbial SOC mineralization was less responsive to lower soil moisture. Our results suggest that the vertical redistribution of SOC with irrigation was mainly driven by litter incorporation through meso- and macrofauna, here accelerated by irrigation and suppressed by naturally occurring summer droughts.Our study shows that soil fauna is highly sensitive to natural drought reducing the incorporation of C from organic layers to the mineral soil. In the longer term, this potentially affects SOC storage, decreasing the C stored in mineral soil. Therefore, soil fauna plays a key but so far largely overlooked role in shaping SOC responses to drought.

N2O is a stratospheric ozone depleting substance and a potent greenhouse gas which significantly contributes to global warming. Although soils are the largest source of N2O emissions, knowledge gaps in the understanding of N2O production and reduction processes in soils still exist. Here, we investigated N2O production and consumption processes along soil depth profiles in a mesocosm experiment using natural-abundance N2O and NO3- isotopic signatures as well as abundances of soil microbial genes associated with N2O production (nirK, nirS) and reduction (nosZ). Soil columns either displayed undisturbed soil stratification (control treatments), or contained an artificial clay layer at 35 cm depth (clay treatment), which acted as a diffusion barrier and thus induced O2-limited conditions in deeper strata. We collected soil pore gas, soil solution and soil samples at five depths of the soil columns over the course of four weeks. In addition, we continuously monitored N2O fluxes at the soil surface and soil environmental parameters (oxygen, moisture, temperature) along the soil depth profiles. Microbial gene analysis in soil samples revealed similar abundances of nirK, nirS and nosZ in the two treatments across the entire soil depth profiles. The distribution of the functional genes was thus not indicative of enhanced N2O production and/or reduction in O2-limited conditions. However, lowest O2 concentrations below the clay layer were associated with highest 15N and 18O enrichments in both NO3- and N2O, indicating N2O production by denitrification and fractional N2O reduction. In addition, we found higher N2O concentrations and surface fluxes for the clay treatment. Our observations imply a dominance of N2O production over N2O reduction, even under conditions most favorable for complete denitrification.

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.

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

Storms represent a major disturbance factor in forest ecosystems, but the effects of windthrows on soil organic carbon (SOC) stocks are quantitatively poorly known. Here we present a comprehensive analysis of windthrow-induced changes in SOC stocks in Swiss forests by combining field-based measurements and modelling simulations. We measured the SOC stocks of 19 windthrown forests across Switzerland, about 10 and 20 years after they were disturbed by the storms ‘Lothar’ and ‘Vivian’ and compared them to the stocks of adjacent intact forests. We also calibrated the process-based model Yasso07 for additional 77 windthrown forests. Our results show that the effect of windthrow on SOC is strongly related to the size of the initial SOC stocks in the organic layer. In absolute and relative terms, the largest SOC losses occurred in high-elevation forests with thick organic layers, where initial SOC stocks decreased by up to 90% (or 30 t C ha-1). In contrast, SOC stocks of low-elevation forests with thin organic layers were hardly affected. The likely reason for this pattern is the high stocks of easily mineralizable organic matter in thick organic layers of mountain forests, while at low elevations a greater SOC fraction is stabilized by mineral interactions. Modelling simulations further show longer-lasting SOC losses and a slower recovery of SOC stocks after windthrow at high-elevations compared to low-elevations, due to a slower regeneration of mountain forests and associated lower C inputs into soils. We also upscaled the SOC changes after windthrow to the whole forested area of Switzerland and estimated a total SOC loss of ~0.3 Mt C after the storms ‘Lothar’ and ‘Vivian’. Our results provide strong empirical evidence that windthrows can reduce the SOC stocks of forest ecosystems, with mountain forests being hotspots for SOC losses.

Drought alters carbon (C) allocation within trees, thereby impairing tree growth. Recovery of root and leaf functioning and prioritized C supply to sink tissues after drought may compensate for drought -induced reduction of assimilation and growth. It remains unclear if C allocation to sink tissues during and following drought is controlled by altered sink metabolic activities or by the availability of new assimilates. Understanding such mechanisms is required to predict forests' resilience to a changing climate. We investigated the impact of drought and drought release on C allocation in a 100-y-old Scots pine forest. We applied (CO2)-C-13 pulse labeling to naturally dry control and long-term irrigated trees and tracked the fate of the label in aboveand belowground C pools and fluxes. Allocation of new assimilates belowground was ca. 53% lower under nonirrigated conditions. A short rainfall event, which led to a temporary increase in the soil water content (SWC) in the topsoil, strongly increased the amounts of C transported belowground in the nonirrigated plots to values comparable to those in the irrigated plots. This switch in allocation patterns was congruent with a tipping point at around 15% SWC in the response of the respiratory activity of soil microbes. These results indicate that the metabolic sink activity in the rhizosphere and its modulation by soil moisture can drive C allocation within adult trees and ecosystems. Even a subtle increase in soil moisture can lead to a rapid recovery of belowground functions that in turn affects the direction of C transport in trees.

Summer droughts strongly affect soil organic carbon (SOC) cycling, but net effects on SOC storage are unclear as drought affects both C inputs and outputs from soils. Here, we explored the overlooked role of soil fauna on SOC storage in forests, hypothesizing that soil faunal activity is particularly drought-sensitive, thereby reducing litter incorporation into the mineral soil and, eventually, long-term SOC storage. In a drought-prone pine forest (Switzerland), we performed a large-scale irrigation experiment for 17 years and assessed its impact on vertical SOC distribution and composition. We also examined litter mass loss of dominant tree species using different mesh-size litterbags and determined soil fauna abundance and community composition. The 17-year-long irrigation resulted in a C loss in the organic layers (−1.0 kg C m−2) and a comparable C gain in the mineral soil (+0.8 kg C m−2) and thus did not affect total SOC stocks. Irrigation increased the mass loss of Quercus pubescens and Viburnum lantana leaf litter, with greater effect sizes when meso- and macrofauna were included (+215%) than when excluded (+44%). The enhanced faunal-mediated litter mass loss was paralleled by a many-fold increase in the abundance of meso- and macrofauna during irrigation. Moreover, Acari and Collembola community composition shifted, with a higher presence of drought-sensitive species in irrigated soils. In comparison, microbial SOC mineralization was less sensitive to soil moisture. Our results suggest that the vertical redistribution of SOC with irrigation was mainly driven by faunal-mediated litter incorporation, together with increased root C inputs. Our study shows that soil fauna is highly sensitive to natural drought, which leads to a reduced C transfer from organic layers to the mineral soil. In the longer term, this potentially affects SOC storage and, therefore, soil fauna plays a key but so far largely overlooked role in shaping SOC responses to drought., Global Change Biology, 28 (9), ISSN:1354-1013, ISSN:1365-2486

Soil organic matter (SOM) is sensitive to land use and to physico-chemical soil properties as well soil microbial communities controlling SOM stabilization. Our study aimed at exploring how carbonates of Mediterranean soils – known to stabilize SOM by enhancing soil aggregation and binding SOM by calcium – are affecting SOM and microbial communities under different land uses. In soils with and without carbonates sampled in forests, cultivated and abandoned fields, we have fractionated SOM-pools according to particle sizes and determined C mineralization rates and polyvalent cations in these fractions. In agreement with larger scale assessments in the Mediterranean, our results show that SOM contents are greater in forest than in agricultural soils, but that the differences among land use types depend on carbonate contents. While SOM contents are greater in carbonated soils of forests, cultivated soils have low SOM levels independent of the carbonate content, presumably due to a reduced protection of particulate SOM by century-long intense tillage. Forest soils contain particularly large amounts of SOM associated with the coarser soil fraction. In this fraction, C mineralization was smallest in the carbonated forest soil, possibly due to a low SOM quality indicated by high C/N ratios and low δ15N values. Greater contents of Ca, Fe and Al in carbonated soils also suggests that stabilization through Ca and phyllosilicates was important. Mineralization of sand-associated SOM was highest in cultivated and abandoned soils, which is indicative for a small SOM stabilization, which in turn might restrict SOM accumulation after land abandonment. Microbial communities assessed by phospho-lipid fatty acid signatures and microbial carbon use efficiency were more closely associated with land use than with carbonate content – a pattern that reflects the distribution of C pools among size fractions which differed more strongly among land use types than between soils with and without carbonates.

The short-term dynamics of carbon and water fluxes across the soil–plant–atmosphere continuum are still not fully understood. One important constraint is the lack of methodologies that enable simultaneous measurements of soil CO2 concentration and respective isotopic composition at a high temporal resolution for longer periods of time. δ13C of soil CO2 can be used to derive information on the origin and physiological history of carbon, and δ18O in soil CO2 aids in inferring the interaction between CO2 and soil water. We established a real-time method for measuring soil CO2 concentration, δ13C and δ18O values across a soil profile at higher temporal resolutions (0.05–0.1 Hz) using an off-axis integrated cavity output spectroscopy (OA-ICOS). We also developed a calibration method correcting for the sensitivity of the device against concentration-dependent shifts in δ13C and δ18O values under highly varying CO2 concentration. The deviations of measured data were modelled, and a mathematical correction model was developed and applied for correcting the shift. By coupling an OA-ICOS with hydrophobic but gas-permeable membranes placed at different depths in acidic and calcareous soils, we investigated the contribution of abiotic and biotic components to total soil CO2 release. We found that in the calcareous Gleysol, CO2 originating from carbonate dissolution contributed to the total soil CO2 concentration at detectable degrees, potentially due to CO2 evasion from groundwater. The 13C - CO2 of topsoil at the calcareous soil site was found reflect δ13C values of atmospheric CO2 , and the δ13C of topsoil CO2 at the acidic soil site was representative of the biological respiratory processes. δ18O values of CO2 in both sites reflected the δ18O of soil water across most of the depth profile, except for the 80 cm depth at the calcareous site where a relative enrichment in 18O was observed.

Phosphorus (P) is a key nutrient but still we have a limited knowledge on the controls of mobilization and fluxes of P in forest soils. Our study explored the linkages between P mobilization in organic horizons and mineral soils and the P status of soils, as affected by two consecutive drying and rewetting (D/W) cycles. We sampled litter layers (Oi), mixed Oe-Oa horizons, and A horizons in three beech forests along a P availability gradient in Germany. Carbon mineralization and release of dissolved organic matter (DOC, DOP) and dissolved inorganic P (DIP) were studied in microcosms exposed to an initial harsh drying (40 °C for 72 h) and a moderate dry spell (1 month at 20 °C). In Oi horizons, net P mineralization decreased with decreasing P status despite a similar C mineralization at all sites. This supports the general concept that the stoichiometric difference between substrate and microbial biomass primarily drives P release from decomposing organic matter. Counterintuitively, P mobilization per unit soil P increased towards P-poor sites in the mineral soil, likely due to decreasing contents of reactive secondary minerals and the consequently smaller P sorption. Drying and rewetting caused stronger mobilization of DIP and DOP (+108% on average) than of DOC (+51%). The parallel decline in specific UV absorptivity of DOM suggests that lysis of microbial cells drove the drought-induced P release. The D/W effects on P mobilization were particularly strong in P-poor soils, where greater portions of P are bound to microbial biomass, which are prone to become released upon rewetting. Since mobilized P can potentially be leached from soils, our findings indicate, that drought-induced P mobilization fosters the progressive P depletion of already P-poor soils. The possible P leaching losses from mineral soils seem rather be driven by soil mineralogy than by P status.

Plant growth responses to environmental changes may be linked to xylem anatomical adjustments. The study of such links is essential for improving our understanding of plant functioning under global change. We investigated the xylem anatomy and above-ground growth of the dwarf shrub Vaccinium myrtillus in the understorey of Larix decidua and Pinus uncinata at the Swiss treeline after 9 years of free-air CO2 enrichment (+200 ppm) and 6 years of soil warming (+4 °C). We aimed to determine the responses of xylem anatomical traits and growth to these treatments, and to analyse xylem anatomy–growth relationships. We quantified anatomical characteristics of vessels and ray parenchyma and measured xylem ring width (RW), above-ground biomass and shoot elongation as growth parameters. Our results showed strong positive correlations between theoretical hydraulic conductivity (Kh) and shoot increment length or total biomass across all treatments. However, while soil warming stimulated shoot elongation and RW, it reduced vessel size (Dh) by 14%. Elevated CO2 had smaller effects than soil warming: it increased Dh (5%) in the last experimental years and only influenced growth by increasing basal stem size. The abundance of ray parenchyma, representing storage capacity, did not change under any treatment. Our results demonstrate a link between growth and stem Kh in V. myrtillus, but its growth responses to warming were not explained by the observed xylem anatomical changes. Smaller Dh under warming may increase resistance to freezing events frequently occurring at treeline and suggests that hydraulic efficiency is not limiting for V. myrtillus growing on moist soils at treeline. Our findings suggest that future higher atmospheric CO2 concentrations will have smaller effects on V. myrtillus growth and functioning than rising temperatures at high elevations; further, growth stimulation of this species under future warmer conditions may not be synchronized with xylem adjustments favouring hydraulic efficiency.

In forest ecosystems, microorganisms hold key functions as nutrient cyclers, decomposers, plant symbionts or pathogens and thereby regulate biogeochemical processes and forest health. These microbial dynamics are controlled by water availability in three fundamental ways: as resource, as solvent, and as transport medium. For one of the dominant tree species in Swiss forests - Scots pine (Pinus sylvestris L.) - high mortality rates have been observed in recent decades. In the Rhone valley of Switzerland, forest dieback appears to be primarily caused by direct effects of drought and an increasing susceptibility of trees to further constraints, such as pathogen attacks. Nonetheless, water limitation does not affect soil microbes and trees separately but rather induces a series of interconnected effects between trees and the associated soil microbiome, which could strongly alter carbon and nutrient cycling in forests. We conduct a study to investigate the effects of drought on the biological interplay between Scots pine trees and soil microbial communities. We aim to estimate how shifts in microbial community composition and functional capacity under drought may affect nutrient cycling and tree vitality potentially contributing to tree mortality. In order to understand these mechanisms, we perform greenhouse experiments with tree-soil mesocosms under controlled conditions. State-of-the art molecular methods such as metabarcoding of ribosomal markers, shotgun metagenome sequencing, and qPCR of key functional genes are used to unravel alterations in the soil microbiome and in the underlying functional metabolic potential related to drought and associated tree-mortality. Furthermore, to elucidate the impact of drought on microbial carbon dynamics, stable isotope labelling techniques have been applied to trace 13C labeled plant photosynthates into the soil microbial communities by analyzing 13C signatures of phospholipid fatty acids. Investigation of soil physicochemical properties and tree-vitality is done in parallel with the microbial assessments to understand the feedbacks on nutrient-cycling and the soil-tree continuum. The overarching aim of this study is to gain new insights into the complex relationships between soil, trees and microbes under drought.

European forests are facing higher frequencies of extreme droughts, potentially impairing tree growth and ecosystem functioning. Drought limits the metabolic activity of plants and soil organisms, either directly or through reduced belowground carbon (C) allocation of recent assimilates, thus affecting C cycling in the plant-soil system. However, the net effect on belowground soil C storage is still unclear, as drought suppresses both C inputs from plants and outputs from soils. Moreover, understanding the underlying mechanisms is complicated due to long-term acclimation and adaptation of plant and soil organisms to water limitation.We investigated the impact of repeated summer droughts in a Scots pine (Pinus sylvestris L.) forest on soil C storage and C cycling, taking advantage of a large-scale irrigation experiment running since 2003 in a dry inner-Alpine valley in Switzerland (Pfynwald, Valais), which removed the “natural” water limitation. We assessed the responses of soil organic carbon (SOC) stocks and C fluxes by measuring litter fall and decomposition, fine root biomass and production, soil CO2 effluxes, C-mineralization, and 13C-labelled glucose utilization by soil microorganisms. After 16 years of irrigation, the organic layers lost significant amounts of C (-1000 g m-2), despite a 50% increase in litter fall. This C loss was almost compensated by a C gain in the mineral soil (+870 g m-2) under irrigation. The decrease in C storage in the organic layers can be related to a three-fold increase in litter decomposition mainly through soil macrofauna as indicated by a litter-bag experiment. In parallel, the C gain in the mineral soil can be attributed mainly to increased incorporation of litter by soil fauna, together with greater C input from the rhizosphere (+70% fine root biomass for Scots pine in mineral soil). Furthermore, irrigation stimulated soil CO2 efflux as well as microbial C-mineralization of organic and mineral soil, indicating enhanced soil C cycling. Addition of 13C-enriched glucose to mineral soils revealed a stronger utilization of this easily available C substrate in the drought than in the irrigated soils, together with a negative priming of soil organic matter (SOM) decomposition shortly after substrate addition. These results suggest that the altered quantity and quality of C inputs under irrigation has increased the availability of easily degradable C in soil.This study reveals that long-term summer irrigation in a drought-prone pine forest has strong impacts on multiple interlinked processes of the soil C cycle. The removal of water limitation strongly altered vertical soil C distribution, accelerated soil C cycling and altered the substrate use by soil organisms, but had only a small net effect on the whole-profile SOC stocks.