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The high invasion success of Fallopia japonica in Europe and North America is related to its niche construction strategy. A hotly debated and prominent possibility is that F. japonica uses weapons for chemical niche construction, which could have considerable consequences for plant nutrition and ecosystem functioning. At least one of its phenolic compounds is capable of inhibiting nitrification and nitrification is actually lower in F. japonica invaded systems. It was assumed that F. japonica has a higher affinity for ammonium and can therefore outcompete native plants that prefer nitrate. However, the uptake of ammonium by F. japonica has only been minimally studied and it has been shown that nitrogen-use efficiency seems to be the main trait. In a lab study using stable isotope labelling we tested nitrogen and carbon uptake of F. japonica against the strongest native competitor in European riparian zones U. dioica. We hypothesized that F. japonica has a greater potential to take up ammonium and that U. dioica would take advantage of the nitrate supply, and that F. japonica would have a slightly better nitrogen-use efficiency than U. .We performed combined ¹³C-CO2 and ¹⁵N-NO3 and -NH4 labelling on young F. japonica and U. dioica plants. They were pulse labelled with ¹³CO₂ and fertilized with ¹⁵N enriched nitrate or ammonium (44 mg N kg -¹ dry soil). Atom excess of ¹⁵N and ¹³C, was measured after seven days in non-rooted soil, rhizosphere, fine roots, transport roots, and shoots. Contrary to our expectations, F. japonica always utilized less soil mineral N independent of the type of nitrogen.Overall, our data revealed that the ability of F. japonica to inhibit nitrification is not based on an affinity for ammonium. Therefore, it appears that F. japonica constructs its biogeochemical niche in a way to benefit from nitrogen-use efficiency, which we found to be higher, by supressing nitrification in nutrient rich habitats.

During decomposition of organic matter, soil microbes determine the fate of C. They partition C between anabolic biosynthesis of various new microbial metabolites (i.e. C reuse) and catabolic C emissions (i.e. C waste, mainly through respiration). This partitioning is commonly referred to as microbial carbon-use efficiency (CUE). The reuse of C during biosynthesis provides a potential for the accumulation of microbial metabolic residues in soil. The microbial metabolic performance is a key factor in soil C dynamics, because the vast majority of C inputs to soil will – sooner or later – be processed by soil microorganisms. Soil C inputs will thus be subjected to microbial allocation of C towards reuse or emitted waste, with the former leading to C remaining in soil. Recognized as a crucial control in C cycling, microbial CUE is implemented – implicitly or explicitly – in soil C models, which react highly sensitive to even small changes in CUE. Due to the models’ high sensitivity, reliable soil C projections demand accurate CUE quantifications, capturing unambiguously all metabolic C fluxes.The current concept of microbial CUE neglects microbial maintenance which could make up considerable parts of the microbially processed C. Commonly, CUE is quantified from C incorporated into biomass or used for growth and C released as CO2. Extracellular metabolites, such as polymeric substances (EPS), exoenzymes or nutrient mobilizing compounds, as well as intracellular maintenance metabolites, such as storage compounds or endoenzymes, are ignored although they represent microbial metabolic C reuse and thus C remaining in soil.Based on theoretical considerations and a case study for EPS production, we will demonstrate that neglecting microbial maintenance can have severe impact on estimation of terrestrial C storage. For instance, ignoring measured EPS production (of a quantity of C which equals 37 % of the C used for growth) causes a substantial underestimation of CUE. Here, current approaches of CUE provide an apparent CUE of 0.20 while disregard an actual CUE of 0.25 (i.e. CUE is 25 % higher when maintenance metabolism is considered). Based on our findings, we suggest an adjustment of how we conceptualize and calculate microbial CUE in soils.

In recent years, important processes controlling ecosystem carbon dynamics have been connected to fine-root exudation of soluble carbon compounds. Root exudation patterns may change depending on plant interactions and plant susceptibility to and recovery from drought. Recent investigations suggest that root exudation tends to increase with stress events, but quantification of the amount of carbon released from roots across soil depths with differing water availability and species interactions are missing.We tested if root exudation rates were negatively correlated with soil water content across soil depths during and after drought. We further tested if species in mixture, often considered to be less stressed under drought, exuded less carbon than species in monospecific environments. Exudates were sampled in a mature Fagus sylvatica L. and Picea abies (L.) Karst. forest at the end of a five-year throughfall exclusion period and again one year after the drought ended. We quantified root exudates and their variation with soil depth for both tree species in monospecific and mixed species zones.Carbon exudation significantly increased in fine roots exposed to dry soils (

Forest soils are crucial for many ecosystem services that rely on soil organic matter (SOM) stability. Carbon allocated to roots and released as exudates to the rhizosphere plays a key role in SOM stabilization. Under periodic drought, elevated root exudation and SOM accumulation have been reported. Yet, whether root exudates control SOM formation and stability in mature forests once the drought ends is largely unknown. We examined whether root exudates from P. abies and F. sylvatica trees relate to SOM formation and stability in soil depth profiles one year following five years of experimental drought (Kroof experiment, Germany). We collected root exudates throughout the rooting zone and combined the data with thermogravimetric analysis of SOM in the rhizosphere and non-rooted soil. We found that the rhizosphere of both species was characterized by stable SOM fractions that did not decrease post-drought, suggesting potential protection of SOM due to rhizodeposition and root exudates. In contrast, stable SOM fractions decreased relative to controls in non-rooted topsoil below P. abies, indicating a loss of stabilized SOM from drought-affected and re-wetted soil. Our measurements provide valuable insights into post-drought SOM formation and mechanisms of SOM stabilization in forest ecosystems under climate change.

Growing evidence suggests that climate classification facilitates the identification of zones that either agree or disagree with processes explaining soil organic carbon (SOC) persistence. Already forty years ago, Post et al. (1982) posited that the strict temperature and precipitation-based classification defining the Holdridge Life Zones (HLZ) provides a descriptive tool to guide our understanding of the heterogeneous distribution of global SOC stocks. Here we argue that this classification has the potential for describing SOC persistence by linking top-down and bottom-up approaches from different scales, which allows selection of individual regional relevancies necessary to manage and track the fate of our largest terrestrial carbon (C) reservoir. © 2021 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH GmbH

Riparian zones, important hotspots for nitrogen retention, are at risk of losing ecosystem functioning by species invasion and chemical contaminants. Invasive Fallopia japonica is suspected of using polyphenolic compounds as a “novel weapon” to decrease nitrification which may be amplified by copper pollution. Inhibited nitrification results in lower nitrate availability for competing plants and is presumed to be part of Fallopia japonica´s competitive strategy. Polyphenols are known to enter the soil with leaf litter, but may also be exuded by roots. Yet, the entry pathway and the specific compounds hampering nitrification are not fully determined. Within the group of secondary metabolites produced by Fallopia japonica, emodin and resveratrol are frequently described, although their role in the invasion strategy via modification of nitrification has never been tested. As plants are likely to increase exudation and the production of polyphenols under stress, synergistic inhibition of nitrification may be expected under contaminant pollution. Hence, the following hypotheses were tested: (I) Resveratrol and emodin inhibit nitrification. (II) Under copper stress, Fallopia japonica increases the content of emodin and resveratrol. Therefore, both stressors act synergistically on nitrification inhibition. (III) As we assume polyphenols to enter the soil via root exudation, nitrification is more strongly inhibited in the rhizosphere compared to the non-rooted soil.We ran a mesocosm experiment with Fallopia japonica and copper additions (0, 90, 270, 810 mg Cu kg-1 soil) over two growing seasons. In September of the second year, we analyzed total polyphenol, resveratrol, and emodin concentration in roots, fresh leaves, and senescent leaves using LC-HRMS. Potential nitrification was measured in the rhizosphere and the non-rooted soil. All samples were analyzed in fivefold repetition across all copper concentrations. We further tested how the nitrification in soil responds to additions of resveratrol and emodin.Resveratrol inhibited nitrification while for emodin no significant effect was found. Under copper stress, concentrations of resveratrol in roots and emodin in senescent leaves were elevated, while total polyphenolic content was not influenced. Copper contamination had a strong concentration-dependent inhibitive effect on potential nitrification. Independent of the copper concentration, Fallopia japonica decreased the potential nitrification slightly more than the highest copper concentration (by 75 % compared to control). Despite the increase of resveratrol in roots, the stressors had neither a synergistic nor additive effect, because of the overwhelming influence of Fallopia japonica alone. In contrast to hypothesis (III), nitrification inhibition was lower in the rhizosphere compared to the non-rooted soil, suggesting that nitrification was not primarily controlled by active root exudation but possibly more by aboveground leachates or litter decomposition. We link this to Fallopia japonica´s competition strategy hampering nitrification more in the soil that provides nitrate for competing plants.Our data reveals that polyphenols produced by Fallopia japonica may act as a “novel weapon” to benefit the own nutrition and to outcompete other plants. By inhibiting nitrification outside the own rhizosphere, the nitrogen availability for the riparian plant community could be substantially reduced having potentially negative effects on the biodiversity of riparian ecosystems and their ecosystem functioning.

Reduced carbon assimilation by plants and increased net ecosystem exchange are often considered to reduce the overall carbon sink function of drought-stressed ecosystems. However, plants and soil may respond differently under drought, leading to imprecise predictions of carbon sequestration in soil. We determined the net carbon assimilation and related it to soil organic carbon (SOC) stocks as well as to root exudate production to measure belowground carbon investment in mature trees (F. sylvatica and P. abies) exposed to experimental drought for five growing seasons. Despite more than 50 % reduction in net carbon assimilation under drought, SOC stocks increased on average by more than 30 %. The proportion of carbon allocated as root exudates increased two- to threefold under drought. Increasing amounts of carbon in organo-mineral associations suggest increased carbon stability under water-limited P. abies but not under F. sylvatica. Our data indicate that the belowground sink strength increased rapidly for the ecological and economic most relevant tree species in Europe. However, evaluating the ecosystem´s carbon sink strength by using the net ecosystem exchange alone neglects belowground SOC accumulation under drought. Although belowground-invested carbon could contribute to reducing the soil carbon-climate feedback temporarily and may support ecosystem resilience, SOC accumulated primarily in dry mineral topsoil may be vulnerable upon exposure to rewetting events.

After drought events, tree recovery depends on sufficient carbon (C) allocation to the sink organs. The present study aimed to elucidate dynamics of tree-level C sink activity and allocation of recent photoassimilates (C$_{new}$) and stored C in c. 70-year-old Norway spruce (Picea abies) trees during a 4-week period after drought release. We conducted a continuous, whole-tree $^{13}$C labeling in parallel with controlled watering after 5 years of experimental summer drought. The fate of C$_{new}$ to growth and CO$_{2}$ efflux was tracked along branches, stems, coarse- and fine roots, ectomycorrhizae and root exudates to soil CO$_{2}$ efflux after drought release. Compared with control trees, drought recovering trees showed an overall 6% lower C sink activity and 19% less allocation of C$_{new}$ to aboveground sinks, indicating a low priority for aboveground sinks during recovery. In contrast, fine-root growth in recovering trees was seven times greater than that of controls. However, only half of the C used for new fine-root growth was comprised of C$_{new}$ while the other half was supplied by stored C. For drought recovery of mature spruce trees, in addition to C$_{new}$, stored C appears to be critical for the regeneration of the fine-root system and the associated water uptake capacity.

Carbon (C) exuded via roots is proposed to increase under drought and facilitate important ecosystem functions. However, it is unknown how exudate quantities relate to the total C budget of a drought-stressed tree, that is, how much of net-C assimilation is allocated to exudation at the tree level. - We calculated the proportion of daily C assimilation allocated to root exudation during early summer by collecting root exudates from mature Fagus sylvatica and Picea abies exposed to experimental drought, and combining above- and belowground C fluxes with leaf, stem and fine-root surface area. - Exudation from individual roots increased exponentially with decreasing soil moisture, with the highest increase at the wilting point. Despite c. 50% reduced C assimilation under drought, exudation from fine-root systems was maintained and trees exuded 1.0% (F. sylvatica) to 2.5% (P. abies) of net C into the rhizosphere, increasing the proportion of C allocation to exudates two- to three-fold. Water-limited P. abies released two-thirds of its exudate C into the surface soil, whereas in droughted F. sylvatica it was only one-third. - Across the entire root system, droughted trees maintained exudation similar to controls, suggesting drought-imposed belowground C investment, which could be beneficial for ecosystem resilience.

Drought is a severe natural risk that increases drying-rewetting frequencies of soils. Yet, it remains largely unknown how forest ecosystems respond to dry-wet cycles, hampering our ability to evaluate the overall sink and source functionality for this large carbon pool. Recent investigations suggest that the release of soluble carbon via root exudation increases under drought, influencing soil carbon stabilization and mineralization. However, an integration of root exudation into the carbon allocation dynamics of drought stressed trees is missing. We hypothesized that roots in dry soil layers have a higher exudation rate than roots in more moist layers across different soil depths. Further, we tested if higher exudation rates under drought are attenuated by reduced root abundance in dry soils and if the fraction of root exudation from total carbon allocation increases with decreasing photosynthesis rates under drought. At the KROOF experimental site in southern Germany, where mature beech (Fagus sylvatica L.) and spruce (Picea abies (L.) Karst.) trees were exposed to artificial drought stress for five consecutive growing seasons, we show that at the root level root exudation rate increases in drier soils. Especially roots in the upper soil profile and roots of spruce trees increased root exudation under drought. When scaled to whole tree level, we did not find differences in total exudation between drought stressed and control trees, indicating sustained root exudation at the tree level under drought. As photosynthesis rates and therefore total carbon assimilation was substantially reduced under drought (by 50 % in beech and almost 70 % in spruce), the fraction of root exudation from total assimilation slightly increased for drought stressed trees. Our results demonstrate that stimulation of root exudation rates with drought exists in natural temperate forest ecosystems but might be mitigated by reduced fine root abundance under drought. Nevertheless, increased exudation per root surface area will have localized impacts on rhizosphere microbial composition and activity especially in the topsoil exposed to more extreme dry-wet cycles. Finally, also the exudate composition can help to determine how priming of soil organic matter relates to belowground carbon allocation dynamics and to disclose processes of complementary species interaction and should be emphasised in future studies.

This contribution presents the result of a free-air 13C labeling experiment on mature Norway spruce (P. abies [L.] KARST.) upon watering after five years of recurrent summer drought in southern Germany, focusing on whole tree allocation processes. Mature spruce trees had been exposed to recurrent summer drought from 2014 to 2018 through complete exclusion of precipitation throughfall from spring to late fall (i.e., March to November). In early summer 2019, the drought stressed spruce trees were watered to investigate their recovery processes. In parallel with the watering, we conducted a whole-tree 13C labeling in canopies and traced the signal in various C sinks, i.e. stem phloem and CO2 efflux, tree rings at different heights, coarse roots, fine root tips, mycorrhiza, root exudates, and soil CO2 efflux.We hypothesize that drought stressed spruce preferentially allocates newly assimilated C to belowground sinks upon drought release. Conversely to our expectations, allocation to belowground C sinks was not stimulated in drought stressed compared to control spruce. Likewise, the relative amount of recently fixed C allocated to aboveground sinks did not differ between treatments. Our findings suggest that the belowground C sinks are not of higher priority for the allocation of newly assimilated C upon watering after long-term drought. The observed allocation pattern is discussed taking total above- and belowground biomass as well as C source/sink relations into account.

Drought is considered a severe natural risk that increases drying-rewetting frequencies of soil. Yet, it remains largely unknown how forest ecosystems respond, hampering our ability to evaluate the overall sink and source functionality for this large carbon pool. Recent investigations present that the loss of soluble carbon via root exudation increases under drought, facilitating fundamental carbon stabilization and mineralization dynamics. However, information on the vertical variation of root exudation from interacting tree species is missing. Here we show that drought increases root exudation rates only in the upper soil profile, while exudation rates decrease in the deeper profile under drought. These trends occurred in both, monocultures and species mixtures. Surprisingly, beech (Fagus sylvatica) and spruce (Picea abies) trees showed opposing results depending on species mixture. While root exudation rates increased in beech growing together with spruce, drought-susceptible spruce had higher exudation rates when grown in monoculture, suggesting the benefit of spruce in mixed cultures via reduced belowground carbon loss. Our results demonstrate that stimulation of root exudation rates with drought exists in natural temperate forest ecosystems, but only in shallow soil depths. In contrast, decreased exudation rates in deeper soil during drought suggest carbon stabilization. The exudate composition can help to determine how priming of soil organic matter relates to microbial respiration and to disclose belowground processes of complementary species interaction.

Vertical gradients in stable carbon isotope ratios (δ13C) in soil profiles can serve to approximate decomposition of organic matter under field conditions. This study tested the time (0, 30, and ≥ 150 y) required for the development of vertical δ13C profiles in topsoil by comparing arable, afforested and continuously forested sites, and showed that three decades following afforestation of former cropland are sufficient to develop distinct δ13C depth profiles.

Enrichment of 13C in SOM with soil depth is related to interacting processes influenced by temperature and precipitation. Our objectives were to derive climate effects on patterns of vertical δ13C values of soil organic matter (SOM) while minimizing the effect of confounding variables. We investigated vertical changes in δ13C values of SOM in 1-cm depth intervals in silvicultural mature beech (Fagus sylvatica L.) forest ecosystems in northern Rhineland-Palatinate across gradients of MAT (7.9 to 9.7 °C mean annual temperature) and MAP (607 to 1085 mm mean annual precipitation) in winter 2011. Forest stands (n = 10) were chosen based on data sets provided by the Rhineland-Palatinate Forest Administration so that variations in these gradients occurred while other environmental factors like physico-chemical soil properties, tree species, stand age, exposition and precipitation (for the temperature gradient) or temperature (for the precipitation gradient) did not differ among study sites. From litter down to the mineral soil at 10 cm depth, soil organic carbon (SOC) content decreased (47.5 ± SE 0.1% to 2.5 ± 0.1%) while the δ13C values increased (− 29.4 ± 0.1‰ to − 26.1 ± 0.1‰). Litter of sites under higher MAP/lower MAT had lower δ13C values which was in line with literature data on climate driven plant physiological process. To compare the dimension of the vertical 13C enrichment, δ13C values were regressed linearly against log-transformed carbon contents yielding absolute values of these slopes (beta). Beta values ranged between 0.6 and 4.5 (range of r from − 0.7 to − 1.0; p < 0.01). Due to an assumed decay continuum and similar variations of δ13C values in litter and in 10 cm depth, we conclude that effects on isotope composition in the Oi layer continue vertically and therefore, δ13C values in litter do not solely control beta values. Beta values decreased with increasing MAT (r = − 0.83; p < 0.05). Reduced soil moisture and therefore both, reduced microbial activity and reduced downward transport of microbial cycled DOM (=13C enriched) might be responsible for less pronounced δ13C depth profiles in case of high temperatures. Greater C:N ratios (lower degradability) of the litter under higher temperatures likely contributed to these depth trends. Beta values increased with increasing MAP (r = 0.73; p < 0.05). We found decreasing C:N ratios in the mineral soil that possibly indicates higher decomposition under higher precipitation. Exclusion of the organic layers from linear regressions indicated a stronger impact of MAP on the development of δ13C depth profiles. Our results confirm temperature and precipitation effects on δ13C depth profiles and indicate stronger 13C enrichment under lower MAT/higher MAP. Therefore, time series of vertical δ13C depth profiles might provide insights into climate change effects.