Fine-root functional trait responses to experimental warming: a global meta-analysis

Plant nitrogen uptake preference and drivers in natural ecosystems at the global scale

Elucidating plant nitrogen (N) acquisition is crucial for understanding plant N strategies and ecosystem productivity. However, the variation in plant N uptake preference and its controlling factors on a global scale remain unclear. We conducted a global synthesis to explore plant N preference patterns and driving factors. Globally, the average contributions of ammonium (NH4 +), nitrate (NO3 -), and glycine N to the total plant N uptake were 41.6 ± 1.1%, 32.8 ± 1.2%, and 25.6 ± 0.9%, respectively. However, plant N uptake preferences differed significantly among climatic regions and vegetation types. Soil NH4 + was the most preferred N form by plants in (sub)tropical regions, whereas NO3 - preference was significantly higher in high-latitude than low-latitude regions. Plant functional type was one of the most important factors driving NO3 - preference, with significantly higher NO3 - preference of nonwoody species than broadleaf-evergreen, conifer, and shrub species. Organic N preference was lowest in (sub)tropics and significantly lower than that in temperate and alpine regions. This study shows clear climatic patterns and different influencing factors of plant NH4 + and NO3 - preference, which can contribute to the accurate prediction of N constraints on ecosystem productivity and soil carbon dynamics.

Hidden aging: the secret role of root senescence

Root age-dependent processes have remained poorly understood. Here, we define root age-related terms in their eco-/physiological context, provide a synthesis of read-outs and traits characterizing root senescence in different root types, and follow their modulation in the light of metabolic, hormonal, and genetic control. Evidence for an endogenously regulated senescence program in roots includes changes in root anatomy, metabolism, and color, decrease in root activity, increasing levels of stress-related hormones, and increasing expression of certain transcription factors (TFs) or genes involved in oxidative stress defense. Uncovering the genetic regulation of the developmental program steering root senescence is of great importance to establish a balanced view on whole-plant aging and improve resource efficiency in crops.

A Global Synthesis of How Plants Respond to Climate Warming From Traits to Fitness

Despite intensive research, our understanding of how plants respond to warming by coordinating their full arsenal of traits to adjust fitness is lacking. To fill this gap, we applied a trait-based framework with three clusters (two functional clusters: "carbon-fixation rate" and "carbon-fixation area"; a third cluster: "total carbon fixation") to a global dataset compiled from 572 studies of warming experiments with 677 species and a comprehensive list of traits and fitness components. The pairwise correlation analysis complemented with SEM and PCA showed that plants increased biomass (the core variable in the third cluster) under warming by coordinating satellite traits in two functional clusters to adjust their core traits, net photosynthesis rate and total leaf area, respectively. In particular, the trait coordination was characterised by the maintenance of net photosynthesis rate and the increase of total leaf area, which was robust across ecological contexts although warming responses of the variables per se displayed context-dependences. Moreover, the trade-offs between biomass and reproduction (itself bearing mass vs. number trade-offs) in their warming responses scaled the coordination to enhance fitness except in the contexts where reproduction was reduced. These findings could help explain and predict plant form and function in a warming world.

Individual Versus Combined Effects of Warming, Elevated CO2 and Drought on Grassland Water Uptake and Fine Root Traits

Increasing warming, atmospheric CO2 and drought are expected to change the water dynamics of terrestrial ecosystems. Yet, limited knowledge exists about how the interactive effects of these factors will affect grassland water uptake, and whether adaptations in fine root production and traits will alter water uptake capacity. In a managed C3 grassland, we tested the individual and combined effects of warming (+3°C), elevated CO2 (eCO2; +300 ppm) and drought on root water uptake (RWU) as well as on fine root production, trait adaptation, and fine root-to-shoot production ratios, and their relationships with RWU capacity. High temperatures, amplified by warming, exacerbated RWU reductions under drought, with negligible water-sparing effects from eCO2. Drought, both under current and future (warming, eCO2) climatic conditions, shifted RWU towards deeper soil layers. Overall, RWU capacity related positively to fine root production and specific root length (SRL), and negatively to mean root diameters. Warming effects on traits (reduced SRL, increased diameter) and the ratio of fine root-to-shoot production (increased) were offset by eCO2. We conclude that under warmer future conditions, irrespective of shifts in water sourcing, it is particularly hot droughts that will lead to increasingly severe restrictions of grassland water dynamics.

Empirical evidence and theoretical understanding of ecosystem carbon and nitrogen cycle interactions

Interactions between carbon (C) and nitrogen (N) cycles in terrestrial ecosystems are simulated in advanced vegetation models, yet methodologies vary widely, leading to divergent simulations of past land C balance trends. This underscores the need to reassess our understanding of ecosystem processes, given recent theoretical advancements and empirical data. We review current knowledge, emphasising evidence from experiments and trait data compilations for vegetation responses to CO2 and N input, alongside theoretical and ecological principles for modelling. N fertilisation increases leaf N content but inconsistently enhances leaf-level photosynthetic capacity. Whole-plant responses include increased leaf area and biomass, with reduced root allocation and increased aboveground biomass. Elevated atmospheric CO2 also boosts leaf area and biomass but intensifies belowground allocation, depleting soil N and likely reducing N losses. Global leaf traits data confirm these findings, indicating that soil N availability influences leaf N content more than photosynthetic capacity. A demonstration model based on the functional balance hypothesis accurately predicts responses to N and CO2 fertilisation on tissue allocation, growth and biomass, offering a path to reduce uncertainty in global C cycle projections.

Species mixtures enhance fine root biomass but inhibit root decay under throughfall manipulation in young natural boreal forests

Fine roots play crucial roles in terrestrial biogeochemical cycles. Although biodiversity loss and changes in precipitation are two major drivers of global change, our understanding of their effects on fine root biomass (FRB), root functional traits, and fine root decay (FRD) remains incomplete. We manipulated precipitation in young boreal forests dominated by Populus tremuloides, Pinus banksiana, and their relatively even mixtures using 25 % addition, ambient, and 25 % reduction in throughfall during the growing season. We collected root samples using soil core and trunk-traced methods to quantify FRB and root traits, and we simulated fine root decay using an in-situ experiment over 531 days. We found that compared to the average of single-species-dominated stands, species mixtures increased FRB by 41 % under ambient throughfall, by 89 % under throughfall reduction and by 71 % under throughfall addition. Root surface area, fine root volume, and root length density responded to species mixtures similarly to FRB. Meanwhile, species mixtures reduced FRD across all water treatments. There was a positive relationship between the effect of species mixtures on the FRD of absorptive roots and those on the FRB. Our results highlight that species mixtures could modify carbon cycling by enhancing fine root biomass accumulation and reducing its decomposition of young boreal forests under changing precipitation.

Amplified photosynthetic responses to drought events offset the positive effects of warming on arid desert plants

Ongoing warming will influence plant photosynthesis via thermal effects and by enhancing water deficit. As the primary limiting factor for the growth and development of plants in arid deserts, water may alter the potential warming effects on plant photosynthesis and lead to increased uncertainty in plant dynamics. Here, we used open-top chambers (OTCs) to evaluate the impacts of in situ warming (+0.5 and +1.5 °C) on the photosynthesis and growth of two representative desert plants, Artemisia ordosica and Grubovia dasyphylla, from wet to dry spells. The plant traits associated with photosynthetic diffusive and biochemical processes were also measured to explore the underlying mechanisms involved. We found that warming significantly increased the net photosynthetic rate (Anet) during wet spells under 1.5 °C warming in both plants, while only increased that of A. ordosica under 0.5 °C warming. During dry spells, Anet decreased both in A. ordosica and G. dasyphylla, with the rates of declining being 48 % and 41 %, respectively, higher than control under warming. Consequently, warming significantly amplified photosynthetic responses to drought events, which offset the positive warming effects during wet spells and led to unchanged plant biomass in both species. Besides, alterations in plant traits tended to be associated with positive warming effects during wet spells, and the negative effects of drought were mainly due to stomatal limitation. Our results emphasised that the potential benefits of warming during wet spells may be reversed during drought events. Thus, the adverse effects of ongoing warming on desert productivity may increase during dry spells in growing seasons and during dry years.

Thinning-induced decrease in fine root biomass, but not other fine root traits in global forests

In forest ecosystems, changes in the expression of tree absorptive root traits following management interventions are expected to influence post-thinning forest structure and function. Fine root traits are expected to be especially responsive to forest thinning-one of the most common forest management interventions and the focus of our research here-influencing tree-level responses to environmental change, and thereby contributing to post-thinning stand-level dynamics and ecosystem processes. However, there remains limited understanding surrounding whether or not forest thinning influences the expression of root morphological, chemical, and physiological traits associated with belowground resource acquisition. We conducted a global meta-analysis to evaluate the response of 13 fine root traits to forest thinning. Our study included analysis of 769 paired observations of root traits values pre- and post-thinning, derived from 89 peer-reviewed publications. Our meta-analysis found that forest thinning leads to a decrease in fine root biomass by 11.7% on average, while other root traits including fine root length, root C and N concentrations, root lifespan, and root respiration rates, are largely unresponsive to thinning treatments. Thinning tended to reduce fine root biomass at early stand recovery stages, with increases in fine root biomass being detected at later seral stages, especially in heavy thinning experiments. The effect of thinning on fine root biomass was most pronounced in deeper soil horizons. The influence of thinning on fine root trait expression was not affected by ecosystem or stand type, with the exception of biomass which decreased in temperate and coniferous forests. Our results demonstrate variations of fine root traits to forest management, as well as the importance of stand recovery time and thinning intensity in regulating fine root trait expression in retention trees. These patterns may have strong implications for governing soil carbon stocks in managed forests associated with decreased root inputs into deeper soils. Overall, our findings can enhance our comprehension of how forest management affects fine root trait expression, and relationship between managed forests and belowground ecosystem structure and function.

Long-Term Soil Warming Drives Different Belowground Responses in Arbuscular Mycorrhizal and Ectomycorrhizal Trees

The ability of trees to acquire soil nutrients under future climate conditions will influence forest composition and function in a warmer world. Rarely are multiple belowground carbon allocation pathways measured simultaneously in large global change experiments, restricting our understanding of how trees may shift their allocation of resources to different nutrient acquisition mechanisms under future climates. Leveraging a 20-year soil warming experiment, we show that ectomycorrhizal (EM) trees reduce mycorrhizal colonization and root exudation while increasing fine root biomass, while arbuscular mycorrhizal (AM) trees largely maintained their belowground carbon allocation patterns in warmer soils. We suggest that AM trees may be better adapted to thrive under global warming due to higher rates of nitrogen mineralization in warmer soils and the ability of their mycorrhizal symbiont to acquire mineralized inorganic nutrients, whereas EM trees may need to alter their belowground carbon allocation patterns to remain competitive as global temperatures rise.

Response of Root Respiration to Warming and Nitrogen Addition Depends on Tree Species

Roots contribute a large fraction of CO2 efflux from soils, yet the extent to which global change factors affect root-derived fluxes is poorly understood. We investigated how red maple (Acer rubrum) and red oak (Quercus rubra) root biomass and respiration respond to long-term (15 years) soil warming, nitrogen addition, or their combination in a temperate forest. We found that ecosystem root respiration was decreased by 40% under both single-factor treatments (nitrogen addition or warming) but not under their combination (heated × nitrogen). This response was driven by the reduction of mass-specific root respiration under warming and a reduction in maple root biomass in both single-factor treatments. Mass-specific root respiration rates for both species acclimated to soil warming, resulting in a 43% reduction, but were not affected by N addition or the combined heated × N treatment. Notably, the addition of nitrogen to warmed soils alleviated thermal acclimation and returned mass-specific respiration rates to control levels. Oak roots contributed disproportionately to ecosystem root respiration despite the decrease in respiration rates as their biomass was maintained or enhanced under warming and nitrogen addition. In contrast, maple root respiration rates were consistently higher than oak, and this difference became critical in the heated × nitrogen treatment, where maple root biomass increased, contributing significantly more CO2 relative to single-factor treatments. Our findings highlight the importance of accounting for the root component of respiration when assessing soil carbon loss in response to global change and demonstrate that combining warming and N addition produces effects that cannot be predicted by studying these factors in isolation.

Resolving the Intricate Effects of Multiple Global Change Drivers on Root Litter Decomposition

Plant roots represent about a quarter of global plant biomass and constitute a primary source of soil organic carbon (C). Yet, considerable uncertainty persists regarding root litter decomposition and their responses to global change factors (GCFs). Much of this uncertainty stems from a limited understanding of the multifactorial effects of GCFs and it remains unclear how these effects are mediated by litter quality, soil conditions and microbial functionality. Using complementary field decomposition and laboratory incubation approaches, we assessed the relative controls of GCF-mediated changes in root litter traits and soil and microbial properties on fine-root decomposition under warming, nitrogen (N) enrichment, and precipitation alteration. We found that warming and N enrichment accelerated fine-root decomposition by over 10%, and their combination showed an additive effect, while precipitation reduction suppressed decomposition overall by 12%, with the suppressive effect being most significant under warming-alone and N enrichment-alone conditions. Significantly, changes in litter quality played a dominant role and accelerated fine-root decomposition by 15% ~ 18% under warming and N enrichment, while changes in soil and microbial properties were predominant and reduced decomposition by 7% ~ 10% under precipitation reduction and the combined warming and N enrichment. Examining only the decomposition environment or litter properties in isolation can distort global change effects on root decomposition, underestimating precipitation reduction impacts by 38% and overstating warming and N effects by up to 73%. These findings highlight that the net impact of GCFs on root litter decomposition hinges on the interplay between GCF-modulated root decomposability and decomposition environment, as well as on the synergistic or antagonistic relationships among GCFs themselves. Our study emphasizes that integrating the legacy effects of multiple GCFs on root traits, soil conditions and microbial functionality would improve our prediction of C and nutrient cycling under interactive global change scenarios.

Melatonin mitigates cadmium toxicity by promoting root development, delaying root senescence, and regulating cadmium transport in cotton

Cd ions are absorbed and transported from the soil by crop roots, which are the first organ to be exposed to Cd. This results in an increase in cadmium ions in crops, significantly affecting crop growth and yield. Exogenous melatonin (MT) can help reduce cadmium (Cd) stress in cotton, but the specific contribution of roots to this process remains unclear. In order to address this knowledge gap, an in-situ root phenotyping study was conducted to investigate the the phenotype and lifespan of roots under cadmium stress (Cd) and melatonin treatment (Cd + MT). The results showed that MT alleviated the decreases in plant height, leaf area, SPAD value, stem diameter, stomatal conductance and net photosynthetic rate under Cd stress, which further promoted the biomass accumulation in various cotton organs. What is more, the Cd + MT treatment increased root volume, surface area, and length under Cd stress by 25.63 %, 10.58 %, and 21.89 %, respectively, compared with Cd treatment. Interestingly, compared to Cd treatment, Cd + MT treatment also significantly extended the lifespan of roots and root hairs by 6.68 days and 2.18 days, respectively. In addition, Cd + MT treatment reduced the transport of Cd from roots to shoots, particularly to bolls, and decreased the Cd bioconcentration factor in bolls by 61.17 %, compared to Cd treatment. In conclusion, these findings show that applying MT externally helps reduce Cd stress by delaying root senescence, promoting root development and regulating Cd transport. This method can be an effective approach to managing Cd stress in cotton.

Functional type mediates the responses of root litter-driven priming effect and new carbon formation to warming

Input of root litter can alter soil organic carbon (SOC) dynamics via causing priming effect (PE) on native SOC decomposition and forming new SOC. However, it is unknown how functional type mediates the root litter-driven PE and new C formation as well as their response to warming, which are of pivotal for soil C budget. We mixed litter segments of absorptive roots and transport roots from a Chinese fir (Cunninghamia lanceolata) plantation into isotopically distinct soil and incubated at 19°C (local mean annual temperature) and 23°C (warming by 4°C) for 210 days. Cumulative PE was calculated via integrating the instantaneous PE rates during the incubation. And the newly formed root litter-derived SOC (SOCrl) was calculated by measuring the δ13C value of soil at the end of incubation using a two-source mixed model. We found that absorptive roots with faster decomposition rates, caused significantly higher cumulative PE and SOCrl than transport roots. The microbial biomass and enzyme activities involved in C, N and P acquisition were significantly higher in the absorptive- than the transport roots addition treatment, indicating a higher level of microbial activation caused by absorptive roots. Although warming significantly increased the litter decomposition for both of functional types, while just significantly increased the PE of transport roots, indicating a root functional type dependent sensitivity of PE to warming. However, warming had no significant effect on SOCrl either for absorptive roots or for transport roots. As a consequence, warming relatively decreased the net SOC balance (difference between PE and SOCrl) in the transport roots addition treatment. Overall, our study highlights, for the first time, that functional type primarily mediates the response of root litter-driven PE to climate warming but not the new C formation, which may advance our understanding of SOC dynamics in Chinese fir plantation under climate change.

Plant height as an indicator for alpine carbon sequestration and ecosystem response to warming

Growing evidence indicates that plant community structure and traits have changed under climate warming, especially in cold or high-elevation regions. However, the impact of these warming-induced changes on ecosystem carbon sequestration remains unclear. Using a warming experiment on the high-elevation Qinghai-Tibetan Plateau, we found that warming not only increased plant species height but also altered species composition, collectively resulting in a taller plant community associated with increased net ecosystem productivity (NEP). Along a 1,500 km transect on the Plateau, taller plant community promoted NEP and soil carbon through associated chlorophyll content and other photosynthetic traits at the community level. Overall, plant community height as a dominant trait is associated with species composition and regulates ecosystem C sequestration in the high-elevation biome. This trait-based association provides new insights into predicting the direction, magnitude and sensitivity of ecosystem C fluxes in response to climate warming.

Root-associated fungi and acquisitive root traits facilitate permafrost nitrogen uptake from long-term experimentally warmed tundra

Root-associated fungi (RAF) and root traits regulate plant acquisition of nitrogen (N), which is limiting to growth in Arctic ecosystems. With anthropogenic warming, a new N source from thawing permafrost has the potential to change vegetation composition and increase productivity, influencing climate feedbacks. Yet, the impact of warming on tundra plant root traits, RAF, and access to permafrost N is uncertain. We investigated the relationships between RAF, species-specific root traits, and uptake of N from the permafrost boundary by tundra plants experimentally warmed for nearly three decades at Toolik Lake, Alaska. Warming increased acquisitive root traits of nonmycorrhizal and mycorrhizal plants. RAF community composition of ericoid (ERM) but not ectomycorrhizal (ECM) shrubs was impacted by warming and correlated with root traits. RAF taxa in the dark septate endophyte, ERM, and ECM guilds strongly correlated with permafrost N uptake for ECM and ERM shrubs. Overall, a greater proportion of variation in permafrost N uptake was related to root traits than RAF. Our findings suggest that warming Arctic ecosystems will result in interactions between roots, RAF, and newly thawed permafrost that may strongly impact feedbacks to the climate system through mechanisms of carbon and N cycling.

Metabolic niches in the rhizosphere microbiome: dependence on soil horizons, root traits and climate variables in forest ecosystems

Understanding belowground plant-microbial interactions is important for biodiversity maintenance, community assembly and ecosystem functioning of forest ecosystems. Consequently, a large number of studies were conducted on root and microbial interactions, especially in the context of precipitation and temperature gradients under global climate change scenarios. Forests ecosystems have high biodiversity of plants and associated microbes, and contribute to major primary productivity of terrestrial ecosystems. However, the impact of root metabolites/exudates and root traits on soil microbial functional groups along these climate gradients is poorly described in these forest ecosystems. The plant root system exhibits differentiated exudation profiles and considerable trait plasticity in terms of root morphological/phenotypic traits, which can cause shifts in microbial abundance and diversity. The root metabolites composed of primary and secondary metabolites and volatile organic compounds that have diverse roles in appealing to and preventing distinct microbial strains, thus benefit plant fitness and growth, and tolerance to abiotic stresses such as drought. Climatic factors significantly alter the quantity and quality of metabolites that forest trees secrete into the soil. Thus, the heterogeneities in the rhizosphere due to different climate drivers generate ecological niches for various microbial assemblages to foster beneficial rhizospheric interactions in the forest ecosystems. However, the root exudations and microbial diversity in forest trees vary across different soil layers due to alterations in root system architecture, soil moisture, temperature, and nutrient stoichiometry. Changes in root system architecture or traits, e.g. root tissue density (RTD), specific root length (SRL), and specific root area (SRA), impact the root exudation profile and amount released into the soil and thus influence the abundance and diversity of different functional guilds of microbes. Here, we review the current knowledge about root morphological and functional (root exudation) trait changes that affect microbial interactions along drought and temperature gradients. This review aims to clarify how forest trees adapt to challenging environments by leveraging their root traits to interact beneficially with microbes. Understanding these strategies is vital for comprehending plant adaptation under global climate change, with significant implications for future research in plant biodiversity conservation, particularly within forest ecosystems.

Experimental warming altered plant functional traits and their coordination in a permafrost ecosystem

Knowledge about changes in plant functional traits is valuable for the mechanistic understanding of warming effects on ecosystem functions. However, observations have tended to focus on aboveground plant traits, and there is little information about changes in belowground plant traits or the coordination of above- and belowground traits under climate warming, particularly in permafrost ecosystems. Based on a 7-yr field warming experiment, we measured 26 above- and belowground plant traits of four dominant species, and explored community functional composition and trait networks in response to experimental warming in a permafrost ecosystem on the Tibetan Plateau. Experimental warming shifted community-level functional traits toward more acquisitive values, with earlier green-up, greater plant height, larger leaves, higher photosynthetic resource-use efficiency, thinner roots, and greater specific root length and root nutrient concentrations. However, warming had a negligible effect in terms of functional diversity. In addition, warming shifted hub traits which have the highest centrality in the network from specific root area to leaf area. These results demonstrate that above- and belowground traits exhibit consistent adaptive strategies, with more acquisitive traits in warmer environments. Such changes could provide an adaptive advantage for plants in response to environmental change.

Climate Warming-Driven Changes in the Molecular Composition of Soil Dissolved Organic Matter Across Depth: A Case Study on the Tibetan Plateau

Dissolved organic matter (DOM) is critical for soil carbon sequestration in terrestrial ecosystems. DOM molecular composition varies with soil depth. However, the spatial heterogeneity of depth-dependent DOM in response to climate warming remains unclear, especially in alpine ecosystems. In this study, the DOM of alpine meadow soil samples was characterized comprehensively by using spectroscopy and mass spectrometry, and open-top chambers (OTCs) were employed to simulate warming. It was found that climate warming had the greatest impact on the upper layer (0-30 cm), followed by the lower layer (60-80 cm), while the middle layer (30-60 cm) was the most stable among the three soil layers. The reasons for the obvious changes in DOM in the upper and lower layers of soil were further explained based on biotic and abiotic factors. Specifically, soil nutrients (NH4+-N, NO3--N, TC, and TP) affected the molecular composition of DOM in layer L1 (0-15 cm), while pH affected layer L5 (60-80 cm). Gemmatimonadetes, Proteobacteria, and Actinobacteria played important roles in the composition of DOM in the L5 layer (60-80 cm), while the dominant fungal groups affecting the DOM composition increased in the L1 layer (0-15 cm) under warming. In summary, this research has contributed to a deeper understanding of depth-dependent changes in DOM molecular composition in alpine ecosystems.

Fine root decomposition in forest ecosystems: an ecological perspective

Fine root decomposition is a physio-biochemical activity that is critical to the global carbon cycle (C) in forest ecosystems. It is crucial to investigate the mechanisms and factors that control fine root decomposition in forest ecosystems to understand their system-level carbon balance. This process can be influenced by several abiotic (e.g., mean annual temperature, mean annual precipitation, site elevation, stand age, salinity, soil pH) and biotic (e.g., microorganism, substrate quality) variables. Comparing decomposition rates within sites reveals positive impacts of nitrogen and phosphorus concentrations and negative effects of lignin concentration. Nevertheless, estimating the actual fine root breakdown is difficult due to inadequate methods, anthropogenic activities, and the impact of climate change. Herein, we propose that how fine root substrate and soil physiochemical characteristics interact with soil microorganisms to influence fine root decomposition. This review summarized the elements that influence this process, as well as the research methods used to investigate it. There is also need to study the influence of annual and seasonal changes affecting fine root decomposition. This cumulative evidence will provide information on temporal and spatial dynamics of forest ecosystems, and will determine how logging and reforestation affect fine root decomposition.

Soil warming duration and magnitude affect the dynamics of fine roots and rhizomes and associated C and N pools in subarctic grasslands

BACKGROUND AND AIMS

The response of subarctic grassland's below-ground to soil warming is key to understanding this ecosystem's adaptation to future climate. Functionally different below-ground plant organs can respond differently to changes in soil temperature (Ts). We aimed to understand the below-ground adaptation mechanisms by analysing the dynamics and chemistry of fine roots and rhizomes in relation to plant community composition and soil chemistry, along with the duration and magnitude of soil warming.

METHODS

We investigated the effects of the duration [medium-term warming (MTW; 11 years) and long-term warming (LTW; > 60 years)] and magnitude (0-8.4 °C) of soil warming on below-ground plant biomass (BPB), fine root biomass (FRB) and rhizome biomass (RHB) in geothermally warmed subarctic grasslands. We evaluated the changes in BPB, FRB and RHB and the corresponding carbon (C) and nitrogen (N) pools in the context of ambient, Ts < +2 °C and Ts > +2 °C scenarios.

KEY RESULTS

BPB decreased exponentially in response to an increase in Ts under MTW, whereas FRB declined under both MTW and LTW. The proportion of rhizomes increased and the C-N ratio in rhizomes decreased under LTW. The C and N pools in BPB in highly warmed plots under MTW were 50 % less than in the ambient plots, whereas under LTW, C and N pools in warmed plots were similar to those in non-warmed plots. Approximately 78 % of the variation in FRB, RHB, and C and N concentration and pools in fine roots and rhizomes was explained by the duration and magnitude of soil warming, soil chemistry, plant community functional composition, and above-ground biomass. Plant's below-ground biomass, chemistry and pools were related to a shift in the grassland's plant community composition - the abundance of ferns increased and BPB decreased towards higher Ts under MTW, while the recovery of below-ground C and N pools under LTW was related to a higher plant diversity.

CONCLUSION

Our results indicate that plant community-level adaptation of below ground to soil warming occurs over long periods. We provide insight into the potential adaptation phases of subarctic grasslands.

Individual and interactive effects of warming and nitrogen supply on CO2 fluxes and carbon allocation in subarctic grassland

Climate warming has been suggested to impact high latitude grasslands severely, potentially causing considerable carbon (C) losses from soil. Warming can also stimulate nitrogen (N) turnover, but it is largely unclear whether and how altered N availability impacts belowground C dynamics. Even less is known about the individual and interactive effects of warming and N availability on the fate of recently photosynthesized C in soil. On a 10-year geothermal warming gradient in Iceland, we studied the effects of soil warming and N addition on CO2 fluxes and the fate of recently photosynthesized C through CO2 flux measurements and a 13 CO2 pulse-labeling experiment. Under warming, ecosystem respiration exceeded maximum gross primary productivity, causing increased net CO2 emissions. N addition treatments revealed that, surprisingly, the plants in the warmed soil were N limited, which constrained primary productivity and decreased recently assimilated C in shoots and roots. In soil, microbes were increasingly C limited under warming and increased microbial uptake of recent C. Soil respiration was increased by warming and was fueled by increased belowground inputs and turnover of recently photosynthesized C. Our findings suggest that a decade of warming seemed to have induced a N limitation in plants and a C limitation by soil microbes. This caused a decrease in net ecosystem CO2 uptake and accelerated the respiratory release of photosynthesized C, which decreased the C sequestration potential of the grassland. Our study highlights the importance of belowground C allocation and C-N interactions in the C dynamics of subarctic ecosystems in a warmer world.

Nitrogen availability mediates soil carbon cycling response to climate warming: A meta-analysis

Global climate warming may induce a positive feedback through increasing soil carbon (C) release to the atmosphere. Although warming can affect both C input to and output from soil, direct and convincing evidence illustrating that warming induces a net change in soil C is still lacking. We synthesized the results from field warming experiments at 165 sites across the globe and found that climate warming had no significant effect on soil C stock. On average, warming significantly increased root biomass and soil respiration, but warming effects on root biomass and soil respiration strongly depended on soil nitrogen (N) availability. Under high N availability (soil C:N ratio < 15), warming had no significant effect on root biomass, but promoted the coupling between effect sizes of root biomass and soil C stock. Under relative N limitation (soil C:N ratio > 15), warming significantly enhanced root biomass. However, the enhancement of root biomass did not induce a corresponding C accumulation in soil, possibly because warming promoted microbial CO₂ release that offset the increased root C input. Also, reactive N input alleviated warming-induced C loss from soil, but elevated atmospheric CO₂ or precipitation increase/reduction did not. Together, our findings indicate that the relative availability of soil C to N (i.e., soil C:N ratio) critically mediates warming effects on soil C dynamics, suggesting that its incorporation into C-climate models may improve the prediction of soil C cycling under future global warming scenarios.

Belowground carbon allocation, root trait plasticity, and productivity during drought and warming in a pasture grass

Sustaining grassland production in a changing climate requires an understanding of plant adaptation strategies, including trait plasticity under warmer and drier conditions. However, our knowledge to date disproportionately relies on aboveground responses, despite the importance of belowground traits in maintaining aboveground growth, especially in grazed systems. We subjected a perennial pasture grass, Festuca arundinacea, to year-round warming (+3 °C) and cool-season drought (60% rainfall reduction) in a factorial field experiment to test the hypotheses that: (i) drought and warming increase carbon allocation belowground and shift root traits towards greater resource acquisition and (ii) increased belowground carbon reserves support post-drought aboveground recovery. Drought and warming reduced plant production and biomass allocation belowground. Drought increased specific root length and reduced root diameter in warmed plots but increased root starch concentrations under ambient temperature. Higher diameter and soluble sugar concentrations of roots and starch storage in crowns explained aboveground production under climate extremes. However, the lack of association between post-drought aboveground biomass and belowground carbon and nitrogen reserves contrasted with our predictions. These findings demonstrate that root trait plasticity and belowground carbon reserves play a key role in aboveground production during climate stress, helping predict pasture responses and inform management decisions under future climates.

Whole-soil warming shifts species composition without affecting diversity, biomass and productivity of the plant community in an alpine meadow

The structure and function of plant communities in alpine meadow ecosystems are potentially susceptible to climate warming. Here, we utilized a unique field manipulation experiment in an alpine meadow on the Qinghai-Tibetan Plateau and investigated the responses of plant species diversity, composition, biomass, and net primary productivity (NPP) at both community and functional group levels to whole-soil-profile warming (3-4 °C across 0-100 cm) during 2018-2021. Plant species diversity, biomass and NPP (both above- and belowground) at the community level showed remarkable resistance to warming. However, plant community composition gradually shifted over time. Over the whole experimental warming period, aboveground biomass of legumes significantly decreased by 45%. Conversely, warming significantly stimulated aboveground biomass of forbs by 84%, likely because of better growth and competitive advantages from the warming-induced stimulation of soil water and other variables. However, warming showed minor effects on aboveground biomass of grasses and sedges. Overall, we emphasize that experimental warming may significantly affect plant community composition in a short term by triggering adjustments in plant interspecific competition or survival strategies, which may cause potential changes in plant productivity over a more extended period and lead to changes in carbon source-sink dynamics in the alpine meadow ecosystem.

Increase in carbon input by enhanced fine root turnover in a long-term warmed forest soil

Fine root litter represents an important carbon input to soils, but the effect of global warming on fine root turnover (FRT) is hardly explored in forest ecosystems. Understanding tree fine roots' response to warming is crucial for predicting soil carbon dynamics and the functioning of forests as a sink for atmospheric carbon dioxide (CO₂). We studied fine root production (FRP) with ingrowth cores and used radiocarbon signatures of first-order, second- to third-order, and bulk fine roots to estimate fine root turnover times after 8 and 14 years of soil warming (+4 °C) in a temperate forest. Fine root turnover times of the individual root fractions were estimated with a one-pool model. Soil warming strongly increased fine root production by up to 128 % within one year, but after two years, the production was less pronounced (+35 %). The first-year production was likely very high due to the rapid exploitation of the root-free ingrowth cores. The radiocarbon signatures of fine roots were overall variable among treatments and plots. Soil warming tended to decrease fine root turnover times of all the measured root fractions after 8 and 14 years of warming, and there was a tendency for trees to use older carbon reserves for fine root production in warmed plots. Furthermore, soil warming increased fine root turnover from 50 to 106 g C m^-2 yr^-1 (based on two different approaches). Our findings suggest that future climate warming may increase carbon input into soils by enhancing fine root turnover. If this increase may partly offset carbon losses by increased mineralization of soil organic matter in temperate forest soils is still unclear and should guide future research.

Effects of land clearing for agriculture on soil organic carbon stocks in drylands: A meta-analysis

Agricultural activities have been expanding globally with the pressure to provide food security to the earth's growing population. These agricultural activities have profoundly impacted soil organic carbon (SOC) stocks in global drylands. However, the effects of clearing natural ecosystems for cropland (CNEC) on SOC are uncertain. To improve our understanding of carbon emissions and sequestration under different land uses, it is necessary to characterize the response patterns of SOC stocks to different types of CNEC. We conducted a meta-analysis with mixed-effect model based on 873 paired observations of SOC in croplands and adjacent natural ecosystems from 159 individual studies in global drylands. Our results indicate that CNEC significantly (p < .05) affects SOC stocks, resulting from a combination of natural land clearing, cropland management practices (fertilizer application, crop species, cultivation duration) and the significant negative effects of initial SOC stocks. Increases in SOC stocks (in 1 m depth) were found in croplands which previously natural land (deserts and shrublands) had low SOC stocks, and the increases were 278.86% (95% confidence interval, 196.43%-361.29%) and 45.38% (26.53%-62.23%), respectively. In contrast, SOC stocks (in 1 m depth) decreased by 24.11% (18.38%-29.85%) and 10.70% (1.80%-19.59%) in clearing forests and grasslands for cropland, respectively. We also established the general response curves of SOC stocks change to increasing cultivation duration, which is crucial for accurately estimating regional carbon dynamics following CNEC. SOC stocks increased significantly (p < .05) with high long-term fertilizer consumption in cleared grasslands with low initial SOC stocks (about 27.2 Mg ha^-1 ). The results derived from our meta-analysis could be used for refining the estimation of dryland carbon dynamics and developing SOC sequestration strategies to achieve the removal of CO₂ from the atmosphere.

Non-Additive Effects of Environmental Factors on Growth and Physiology of Invasive Solidago canadensis and a Co-Occurring Native Species (Artemisia argyi)

Changes in environmental factors, such as temperature and UV, have significant impacts on the growth and development of both native and invasive plant species. However, few studies examine the combined effects of warming and enhanced UV on plant growth and performance in invasive species. Here, we investigated single and combined effects of warming and UV radiation on growth, leaf functional and photosynthesis traits, and nutrient content (i.e., total organic carbon, nitrogen and phosphorous) of invasive Solidago canadensis and its co-occurring native species, Artemisia argyi, when grown in culture racks in the greenhouse. The species were grown in monoculture and together in a mixed community, with and without warming, and with and without increased UV in a full factorial design. We found that growth in S. canadensis and A. argyi were inhibited and more affected by warming than UV-B radiation. Additionally, there were both antagonistic and synergistic interactions between warming and UV-B on growth and performance in both species. Overall, our results suggested that S. canadensis was more tolerant to elevated temperatures and high UV radiation compared to the native species. Therefore, substantial increases in temperature and UV-B may favour invasive S. canadensis over native A. argyi. Research focusing on the effects of a wider range of temperatures and UV levels is required to improve our understanding of the responses of these two species to greater environmental variability and the impacts of climate change.

Bioenergetic control of soil carbon dynamics across depth

Soil carbon dynamics is strongly controlled by depth globally, with increasingly slow dynamics found at depth. The mechanistic basis remains however controversial, limiting our ability to predict carbon cycle-climate feedbacks. Here we combine radiocarbon and thermal analyses with long-term incubations in absence/presence of continuously ¹³C/¹⁴C-labelled plants to show that bioenergetic constraints of decomposers consistently drive the depth-dependency of soil carbon dynamics over a range of mineral reactivity contexts. The slow dynamics of subsoil carbon is tightly related to both its low energy density and high activation energy of decomposition, leading to an unfavourable 'return-on-energy-investment' for decomposers. We also observe strong acceleration of millennia-old subsoil carbon decomposition induced by roots ('rhizosphere priming'), showing that sufficient supply of energy by roots is able to alleviate the strong energy limitation of decomposition. These findings demonstrate that subsoil carbon persistence results from its poor energy quality together with the lack of energy supply by roots due to their low density at depth.

Changes in root chemical diversity along an elevation gradient of Changbai Mountain, China

Root chemical traits play a critical role in plant resource use strategies and ecosystem nutrient cycling; however, the chemical diversity of multiple elements of fine root and community chemical assembly belowground are poorly understood. Here, we measured 13 elements (C, N, K, Ca, Mg, S, P, Al, Fe, Na, Mn, Zn, and Cu) in the fine roots of 204 plant species along elevational transect from 540 to 2357 m of Changbai Mountain, China to explore the variation, diversity, and community assembly of root chemical traits. At the species level, the concentrations of macronutrients (N, K, Ca, Mg, S, and P) decreased, whereas the trace metals (Fe, Mn, and Zn) increased with elevation. Root chemical traits at the community level systematically shifted along elevational gradients showing a pattern similar to that at the species level, which were mainly influenced by climate and soil rather than species diversity. In general, the interactions of climate and soil were the main drivers of root chemical assembly for woody layers, whereas soil factors played significant role for root chemical assembly for herb layer. The chemical assembly of rock-derived element P was mainly driven by soil factors. Meanwhile, root chemical diversities were mainly regulated by species diversity, the interactions of climate and soil, and soil factors in the tree, shrub, and herb layers, respectively. A better understanding of plant root chemical diversity and community chemical assembly will help to reveal the role of chemical traits in ecosystem functioning.

Fine-root functional trait response to nitrogen deposition across forest ecosystems: A meta-analysis

Nitrogen (N) deposition has complex effects on vegetation dynamics and nutrient cycling in terrestrial ecosystems. However, how N deposition alters fine root traits remains unclear in forest ecosystems. Here, we carried out a synthesis based on 890 paired observations of 14 fine root traits from 79 articles to assess the effects of N deposition on fine root traits. The results showed that N deposition mainly affected root nutrient content and stoichiometry. Specifically, N deposition increased the root N content, root carbon: phosphorus (C:P) and root nitrogen: phosphorus (N:P) ratio, but decreased the root P content and root C:N ratio. Moreover, N deposition increased fine root respiration, but had no significant effect on other root morphological and physiological traits. N deposition effects on fine root biomass, root tissue density and fungal colonization decreased with N deposition duration. Compared to fine root P content, N deposition effects on fine root C content and C:P ratio increased with N deposition level. Moreover, the interaction between N deposition level and duration significantly affected fine root biomass. N deposition effects on fine-root biomass decreased with greater N deposition duration, especially in high N deposition experiments. Moreover, the effect of N deposition on root diameter decreased with mean annual temperature and mean annual precipitation. N form, forest type and soil depth significantly affect the effect of N deposition on fine root C:P. Therefore, the effects of N deposition on fine root traits were not only determined by N deposition level, duration and their interactions, but also regulated by abiotic factors. These findings highlight the diverse responses of fine root traits to N deposition have strong implications for forest ecosystems soil carbon stocks in a world of increasing N deposition associated with decreased root-derived carbon inputs and increases in fine-root respiration.

Anatomical structure interpretation of the effect of soil environment on fine root function

Fine root anatomy plays an important role in understanding the relationship between fine root function and soil environment. However, in different soil environments, the variation of fine root anatomical structure in different root sequences is not well studied. We measured the soil conditions and anatomical structure characteristics (root diameter, cortical tissue, vascular tissue and xylem) of fine roots of Cupressus funebris in four experimental sites, and analyzed each level of fine roots separately. We link these data to understand the relationship between fine root anatomy and soil conditions. We found that the anatomical structure of fine roots is closely related to soil environmental factors. The fine roots of lower root order are mainly affected by soil nutrients. Among them, the cortical tissue of first-order fine roots was positively correlated with potassium and phosphorus, but negatively correlated with nitrogen, while second- and third-order fine roots was positively correlated with soil total potassium and negatively correlated with nitrogen and phosphorus. For the fine roots of high root order, the cortical tissue disappeared, and the secondary vascular tissue was mainly affected by soil moisture. In addition, we also found that the division of fine root functional groups is not fixed. On the one hand, the function of third-order fine roots will slip. For example, the decrease of soil moisture will promote the transformation of third-order fine roots into transport roots, and the reduction of nitrogen will promote the transformation of third-order fine roots into absorption roots to fix nitrogen. This transformation strategy can effectively prevent the restriction of soil nutrients on plant growth. On the other hand, with the change of habitat, the first- and second-order fine roots are still the absorbing root, and the fourth- and fifth-order fine roots are still the transport root, but the efficiency of absorption and transport will be affected. In conclusion, our findings emphasize the fine roots in different soil environment to show high levels of plasticity, shows that fine root anatomical structure changes may make plants, and reveals that the fine is just order of reaction and its mechanism in the soil environment.

Response of plant functional traits to nitrogen enrichment under climate change: A meta-analysis

Soil nitrogen (N) supply is essential in influencing plant functional traits and regulating plant morphological and physiological performances. The effects of N on plants can be altered by complex environmental changes. However, conflicting results have been reported on the co-effects of N and climatic variables on plant performance, which may be attributed to differences in experiment setting and approach, e.g., ecosystem, duration, plant type, and fertilizer form. To elucidate the general response of plant performance to increasing soil N availability under climate change, a global meta-analysis was conducted to synthesize 380 publications studying interactions of N enrichment and four climatic variables (e.g., elevated atmospheric CO₂ (eCO₂), drought, precipitation, and warming) on performance-related traits (e.g., size, nutrient, and fitness). Results showed that N enrichment increased shoot and root size, nutrient, and fitness of terrestrial plants. The synergistic interactions of N × eCO₂ and antagonistic interactions of N × drought were found on plant overall performance (mainly on plant size), indicating that the N effects can be aggregated by eCO₂ and mitigated by drought. The co-effects of N and climatic variables on plant overall performance rely on experiment approach, duration, ecosystem type, or plant functional type. Synergistic interactions of N × eCO₂ and antagonistic interactions of N × drought, N × precipitation, and N × warming on plant overall performance were found mainly in greenhouse experiments and short-term experiments (duration ≤ one year), but not in the field or longer-term experiments. The results highlighted that N effects on plant performance were not isolated, but can be modified by climate changes. These findings can improve the future modeling predictions of plant performance under complex climate change and provide a fundamental basis for N management strategies to optimize plant performance in production, N nutrient, and reproduction while enabling sustainability of plant production systems.

Root age-related response of fine root respiration of Chinese fir seedlings to soil warming

The variation in fine root respiration with root age provides insight into root adaptation to climate warming, but the mechanism is poorly understood. In this study, we investigated the respiratory response of fine roots (<1 mm and 1-2 mm) of different ages (2-, 4- and 6-month old) of Chinese fir (Cunninghamia lanceolata (Lamb.)) seedlings to soil warming (4 °C above the control using cable heating). Fine roots were excised to measure the specific respiration rate at a reference temperature of 20 °C (SRR20), and root morphological and chemical traits were measured. Soil warming significantly increased SRR20 by 40% compared with the control, potentially indicating limited acclimation on a short time scale (6 months). However, soil warming increased SRR20 significantly in 2-month-old roots (by 72%) compared with 4- and 6-month-old roots, leading to a steeper decline in SRR20 with root age. This result suggests possible increased nutrient uptake efficiency in young fine roots under warmer temperatures. Soil warming significantly increased specific root length (SRL) but not root tissue nitrogen concentration (RTN). The variation in SRR20 between warming treatments, but not across root ages, was predicted by SRL and RTN individually or together. Our findings conclusively indicate that soil warming increased the respiration cost of young fine roots, which was predicted by adjusting for SRL and RTN, indicating that Chinese fir may adopt a faster fine root turnover strategy to enhance nutrient uptake and soil exploitation under warmer temperatures. Future studies should simultaneously investigate age-related root respiration and nutrient uptake in warming experiments to better understand the effects of warming on root metabolic activity.

Key Strategies Underlying the Adaptation of Mongolian Scots Pine (Pinussylvestris var. mongolica) in Sandy Land under Climate Change: A Review

Forest degradation and mortality have been widely reported in the context of increasingly significant global climate change. As the country with the largest total tree plantation area globally, China has a great responsibility in forestry management to cope with climate change effectively. Mongolian Scots pine (Pinus sylvestris var. mongolica) was widely introduced from its natural sites in China into several other sandy land areas for establishing shelterbelt in the Three-North Shelter Forest Program, scoring outstanding achievements in terms of wind-breaking and sand-fixing. Mongolian Scots pine plantations in China cover a total area of ~800,000 hectares, with the eldest trees having >60 years. However, plantation trees have been affected by premature senescence in their middle-age stages (i.e., dieback, growth decline, and death) since the 1990s. This phenomenon has raised concerns about the suitability of Mongolian Scots pine to sandy habitats and the rationality for further afforestation, especially under the global climate change scenario. Fortunately, dieback has occurred only sporadically at specific sites and in certain years and has not spread to other regions in northern China; nevertheless, global climate change has become increasingly significant in that region. These observations reflect the strong drought resistance and adaptability of Mongolian Scots pines. In this review, we summarized the most recent findings on the ecohydrological attributes of Mongolian Scots pine during its adaptation to both fragile habitats and climate change. Five main species-specific strategies (i.e., opportunistic water absorb strategy, hydraulic failure risk avoidance strategy, water conservation strategy, functional traits adjustment strategy, rapid regeneration strategy) were summarized, providing deep insights into the tree–water relationship. Overall, the findings of this study can be applied to improve plantation management and better cope with climate-change-related drought stress.

Long-term soil warming alters fine root dynamics and morphology, and their ectomycorrhizal fungal community in a temperate forest soil

Climate warming is predicted to affect temperate forests severely, but the response of fine roots, key to plant nutrition, water uptake, soil carbon, and nutrient cycling is unclear. Understanding how fine roots will respond to increasing temperature is a prerequisite for predicting the functioning of forests in a warmer climate. We studied the response of fine roots and their ectomycorrhizal (EcM) fungal and root-associated bacterial communities to soil warming by 4°C in a mixed spruce-beech forest in the Austrian Limestone Alps after 8 and 14 years of soil warming, respectively. Fine root biomass (FRB) and fine root production were 17% and 128% higher in the warmed plots, respectively, after 14 years. The increase in FRB (13%) was not significant after 8 years of treatment, whereas specific root length, specific root area, and root tip density were significantly higher in warmed plots at both sampling occasions. Soil warming did not affect EcM exploration types and diversity, but changed their community composition, with an increase in the relative abundance of Cenoccocum at 0-10 cm soil depth, a drought-stress-tolerant genus, and an increase in short- and long-distance exploration types like Sebacina and Boletus at 10-20 cm soil depth. Warming increased the root-associated bacterial diversity but did not affect their community composition. Soil warming did not affect nutrient concentrations of fine roots, though we found indications of limited soil phosphorus (P) and potassium (K) availability. Our findings suggest that, in the studied ecosystem, global warming could persistently increase soil carbon inputs due to accelerated fine root growth and turnover, and could simultaneously alter fine root morphology and EcM fungal community composition toward improved nutrient foraging.

Experimental warming and its legacy effects on root dynamics following two hurricane disturbances in a wet tropical forest

Tropical forests are expected to experience unprecedented warming and increases in hurricane disturbances in the coming decades; yet, our understanding of how these productive systems, especially their belowground component, will respond to the combined effects of varied environmental changes remains empirically limited. Here we evaluated the responses of root dynamics (production, mortality, and biomass) to soil and understory warming (+4°C) and after two consecutive tropical hurricanes in our in situ warming experiment in a tropical forest of Puerto Rico: Tropical Responses to Altered Climate Experiment (TRACE). We collected minirhizotron images from three warmed plots and three control plots of 12 m2 . Following Hurricanes Irma and María in September 2017, the infrared heater warming treatment was suspended for repairs, which allowed us to explore potential legacy effects of prior warming on forest recovery. We found that warming significantly reduced root production and root biomass over time. Following hurricane disturbance, both root biomass and production increased substantially across all plots; the root biomass increased 2.8-fold in controls but only 1.6-fold in previously warmed plots. This pattern held true for both herbaceous and woody roots, suggesting that the consistent antecedent warming conditions reduced root capacity to recover following hurricane disturbance. Root production and mortality were both related to soil ammonium nitrogen and microbial biomass nitrogen before and after the hurricanes. This experiment has provided an unprecedented look at the complex interactive effects of disturbance and climate change on the root component of a tropical forested ecosystem. A decrease in root production in a warmer world and slower root recovery after a major hurricane disturbance, as observed here, are likely to have longer-term consequences for tropical forest responses to future global change.

The physiological acclimation and growth response of Populus trichocarpa to warming

Plant metabolic acclimation to thermal stress remains underrepresented in current global climate models. Gaps exist in our understanding of how metabolic processes (i.e., photosynthesis, respiration) acclimate over time and how aboveground versus belowground acclimation differs. We measured the thermal acclimation of Populus trichocarpa, comparing aboveground versus belowground physiology over time. Ninety genetically identical ramets were propagated in mesocosms that separated root and microbial components. After establishment at 25°C for 6 weeks, 60 clones were warmed +4 or +8°C and monitored for 10 weeks, measuring photosynthesis (A), leaf respiration (R), soil respiration (R_s ), root plus soil respiration (R_s+r ), and root respiration (R_r ). We observed thermal acclimation in both A and R, with rates initially increasing, then declining as the thermal photosynthetic optimum (T_opt ) and the temperature-sensitivity (Q₁₀ ) of respiration adjusted to warmer conditions. Photosynthetic acclimation was constructive, based on an increase in both T_opt and peak A. Belowground, R_s+r decreased linearly with warming, while R_s rates declined abruptly, then remained constant with additional warming. Plant biomass was greatest at +4°C, with 30% allocated belowground. Rates of mass-based R_r were similar among treatments; however, root nitrogen declined at +8°C leading to less mass nitrogen-based R_r in that treatment. The Q₁₀ -temperature relationship of R_r was affected by warming, leading to differing values among treatments. Aboveground acclimation exceeded belowground acclimation, and plant nitrogen-use mediated the acclimatory response. Results suggest that moderate climate warming (+4°C) may lead to acclimation and increased plant biomass production but increases in production could be limited with severe warming (+8°C).

Plant and microbial regulations of soil carbon dynamics under warming in two alpine swamp meadow ecosystems on the Tibetan Plateau

Increasing temperature plays important roles in affecting plant and soil microbial communities as well as ecological processes and functions in terrestrial ecosystems. However, mechanisms of warming influencing soil carbon dynamics associated with plant-microbe interactions remain unclear. In this study, open-top chambers (OTCs) experiments were carried out to detect the responses of plants, soil microbes, and SOC contents, physical fractions (by particle-size fractionation) and chemical composition (by solid-state ¹³C NMR spectroscopy) to warming in two alpine swamp meadows (Kobresia humilis vs K. tibetica) on the Tibetan Plateau. Our results showed that four years of warming had significant influences on plant belowground biomass, microbial community and SOC contents in the K. humilis swamp meadow, but had much weaker or minor effects in the K. tibetica swamp meadow with water-logged status and lower level of warming. In the K. humilis swamp meadow, warming increased microbial biomass, C-hydrolysis gene abundance and N-acetylglucosaminidase enzyme activity. These positive effects of warming on microbial biomass and functions further increased soil dissolved inorganic nitrogen and alleviated the nitrogen limitation for plant growth, potentially leading to higher plant biomass. Therefore, increases in SOC and particulate organic carbon (POC) under warming were likely attributed to the higher C input with promoted plant biomass overweighting the simultaneous higher C degradation and release in the K. humilis swamp meadow. Conversely, warming marginally reduced soil alkyl C, which was likely associated with enhanced decomposition by fungi and gram-positive bacteria. Overall, the increases in unprotected POC and decreases in recalcitrant alkyl C demonstrate the sensitivity of SOC physical fractions as well as chemical composition to climate warming in the K. humilis alpine swamp meadow, and suggest that the overall stability of SOC might be lower despite the gain in the content of SOC after climate warming in this alpine swamp meadow.