Elevated CO2 increases root exudation from loblolly pine (Pinus taeda) seedlings as an N-mediated response

Nitrogen addition affected the root competition in Cunninghamia lanceolata-Phoebe chekiangensis mixed plantation

Little is known about below-ground competition in mixed-species plantations under increasing nitrogen (N) deposition. This study aims to determine the effects of N addition on root competition in coniferous and broad-leaved species mixed plantations. A pot experiment was conducted using the coniferous species Cunninghamia lanceolata and the broad-leaved species Phoebe chekiangensis planted in mixed plantations with different competition intensities under N addition (0 or 45 kg N ha-1 yr-1). Biomass allocation, root morphology, root growth level, and competitive ability were determined after five months of treatment. Our findings indicated that root interactions in mixed plantations did not influence biomass allocation in either C. lanceolata or P. chekiangensis but promoted growth in C. lanceolata when no N was added. However, N addition decreased biomass accumulation in both species in the mixed plantation and had a negative effect on the root growth of C. lanceolata due to intensified competition. Addition of N increased the relative importance of root predatory competition in P. chekiangensis, and increased the allelopathic competitive advantage in C. lanceolata. This suggests that N addition causes a shift in the root competitive strategy from tolerance to competition. Overall, these findings highlight the significant impact that the addition of N can have on plant interactions in mixed plantations. Our results provide implications for the mechanisms of root competition in response to increasing atmospheric N deposition in mixed plantations.

Characteristics of bacterial communities in rhizosphere and bulk soil in Fe-deficient citrus growing in coastal saline-alkali land

Aims

Citruses often occur with imbalance in iron nutrition in coastal saline-alkali lands, which severely limits the yield and quality of the fruit. In the rhizosphere, the salt content plays a crucial role in reducing uptake of iron, as well as the activity and abundance of bacteria. However, few studies have explored how salt content affects the effectiveness of iron and the community structure of bacteria across different vertical spatial scales.

Methods

We investigated the citrus rhizosphere (0-30 cm) and bulk (0-60 cm) soil microenvironments of the coastal saline soil were analyzed using the 16S rRNA amplicon and inductively coupled plasma-optical emission spectroscopy.

Results

We found that the nutrient-related elements in the rhizosphere and bulk soil decreased with increasing soil depth, while the salinity-related elements showed the opposite trend. The nutrient-related element content in the rhizosphere was higher than that in the bulk, whereas the salinity-alkaline-related element content was lower than that in the bulk. The structure and diversity of bacterial communities are affected by the rhizosphere and soil depth. In the bulk, there are enriched bacteria such as WB1-A12, Nitrospiraceae and Anaerolineae that are tolerant to salt-alkali stress. In the rhizosphere, bacteria that promote plant nutrient absorption and secretion of iron carriers, such as Pseudomonas, Streptomyces, and Duganella, are prominent.

Conclusions

The soil depth and rhizosphere affect soil nutrients and saline alkali-related factors. Changes in soil depth and rhizosphere determine the structure and diversity of bacterial communities. Rhizosphere enhances iron absorption promoting bacteria to alleviate iron deficiency stress in saline-alkali soils. Our results indicate that citrus roots maybe can resist the stress of iron deficiency in saline-alkali soils by enhancing iron absorption promoting bacteria.

Mitigating the adverse effect of warming on rice canopy and rhizosphere microbial community by nitrogen application: An approach to counteract future climate change for rice

The adverse impact of climate change on crop production continues to increase, necessitating the development of suitable strategies to mitigate these effects and improve food security. Several studies have revealed how global warming negatively impacts the grain-filling stage of rice and that this effect could be mitigated by nitrogen; however, the impact of nitrogen application on rice canopy and rhizosphere microbial communities remains unclear. We conducted a study using an open-field warming system. Results showed that warming influenced rice canopy by decreasing aboveground biomass and harvest index, whereas nitrogen application had positive effect on rice production under warming conditions by increasing the plant nitrogen content, biomass, harvest index and soil fertilities. Moreover, soil ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) contents were significantly decreased under warming, which were higher after nitrogen application. Notably, warming and nitrogen fertilizer caused 19 % (P < 0.01) and 7 % (P < 0.05) variations, respectively, in the β diversity of the microbial community, respectively. The impact of warming was significant on NH4+-N-related microorganisms; however, this impact was weakened by nitrogen application for microbes in the rhizosphere. This study demonstrated that enhanced nitrogen fertilizer can alleviate the adverse impact of warming by weakening its effects on rhizosphere microbes, improving soil fertility, promoting rice nitrogen uptake, and increasing the aboveground biomass and harvest index. These findings provide an important theoretical basis for developing practical, responsive cultivation strategies.

Phosphorus deficiency is the main limiting factor for re-vegetation and soil microorganisms in Mu Us Sandy Land, Northwest China

Long-term drought induced by low rainfall leads to environmental degradation of land in arid and semi-arid regions. In past decades, re-vegetation of degraded sandy soils to prevent soil erosion has been widely employed, including in Mu Us Sandy Land, which suffers from severe soil erosion. However, it remains unclear how re-vegetation affects soil properties and soil microbes after long restoration periods. In this study, typical plots planting Artemisia ordosica and Salix psammophila were selected to investigate the influence of plant types on soil properties; an area of bare sandy land was used as a control. The results show that re-vegetation increased soil organic carbon (C), total nitrogen (N), soil microbial carbon, microbial nitrogen and soil organic acid, while decreasing soil total phosphorous (TP) content significantly, resulting in increased C/P and N/P ratios. Correlation analysis showed that TP was negatively correlated with oxalic acid (OA) and acetic acid (AA), indicating that increased AA and OA content could accelerate the active utilization of phosphorus and induced low TP in soil. Re-vegetation with A. ordosica significantly decreased the microbial diversity of topsoil. The redundancy analysis showed that TP was main index in affecting microbes. These results that lower P content, higher C/P and N/P ratio and influence of TP on microbes suggest that phosphorus is the main limiting factor for re-vegetation and growth of soil microorganisms. In the future, strategies for the development of sustainable ecosystems in regions suffers from severe soil erosion should consider phosphorus supplementation.

Elevated CO2 reduced antimony toxicity in wheat plants by improving photosynthesis, soil microbial content, minerals, and redox status

Introduction

Antimony (Sb), a common rare heavy metal, is naturally present in soils at low concentrations. However, it is increasingly used in industrial applications, which in turn, leads to an increased release into the environment, exerting a detrimental impact on plant growth. Thus, it is important to study Sb effects on plants under the current and future CO2 (eCO2).

Methods

To this end, high Sb concentrations (1500 mg/kg soil) effects under ambient (420 ppm) and eCO2 (710 ppm) on wheat growth, physiology (photosynthesis reactions) and biochemistry (minerals contents, redox state), were studied and soil microbial were evaluated.

Results and discussion

Our results showed that Sb uptake significantly decreased wheat growth by 42%. This reduction could be explained by the inhibition in photosynthesis rate, Rubisco activity, and photosynthetic pigments (Cha and Chb), by 35%, 44%, and 51%, respectively. Sb significantly reduced total bacterial and fungal count and increased phenolic and organic acids levels in the soil to decrease Sb uptake. Moreover, it induced oxidative markers, as indicated by the increased levels of H2O2 and MDA (1.96 and 2.8-fold compared to the control condition, respectively). To reduce this damage, antioxidant capacity (TAC), CAT, POX, and SOD enzymes activity were increased by 1.61, 2.2, 2.87, and 1.86-fold, respectively. In contrast, eCO2 mitigated growth inhibition in Sb-treated wheat. eCO2 and Sb coapplication mitigated the Sb harmful effect on growth by reducing Sb uptake and improving photosynthesis and Rubisco enzyme activity by 0.58, 1.57, and 1.4-fold compared to the corresponding Sb treatments, respectively. To reduce Sb uptake and improve mineral availability for plants, a high accumulation of phenolics level and organic acids in the soil was observed. eCO2 reduces Sb-induced oxidative damage by improving redox status. In conclusion, our study has provided valuable insights into the physiological and biochemical bases underlie the Sb-stress mitigating of eCO2 conditions. Furthermore, this is important step to define strategies to prevent its adverse effects of Sb on plants in the future.

Plant Beneficial Bacteria and Their Potential Applications in Vertical Farming Systems

In this literature review, we discuss the various functions of beneficial plant bacteria in improving plant nutrition, the defense against biotic and abiotic stress, and hormonal regulation. We also review the recent research on rhizophagy, a nutrient scavenging mechanism in which bacteria enter and exit root cells on a cyclical basis. These concepts are covered in the contexts of soil agriculture and controlled environment agriculture, and they are also used in vertical farming systems. Vertical farming-its advantages and disadvantages over soil agriculture, and the various climatic factors in controlled environment agriculture-is also discussed in relation to plant-bacterial relationships. The different factors under grower control, such as choice of substrate, oxygenation rates, temperature, light, and CO₂ supplementation, may influence plant-bacterial interactions in unintended ways. Understanding the specific effects of these environmental factors may inform the best cultural practices and further elucidate the mechanisms by which beneficial bacteria promote plant growth.

Eutrophication reduced the release of dissolved organic carbon from tropical seagrass roots through exudation and decomposition

Seagrass bed ecosystem is one of the most effective carbon capture and storage systems on earth. Seagrass roots are the key link of carbon flow between leaf-root-sediment, and the release of dissolved organic carbon (DOC) from seagrass roots through exudation and decomposition are vital sources to the sediment organic carbon (SOC) in the seagrass beds. Unfortunately, human-induced eutrophication may change the release process of DOC from seagrass roots, thereby affecting the sediment carbon storage capacity. However, little is known about the effect of nutrient enrichment on the release of DOC from seagrass roots, hindering the development of seagrass underground ecology. Therefore, we selected Thalassia hemprichii, the tropical dominant seagrass species, as the research object, and made a comparison of the release of DOC from roots through exudation and decomposition under different nitrate treatments. We found that under control, 10 μmol L^-1, 20 μmol L^-1 and 40 μmol L^-1 nitrate treatments, soluble sugar of T. hemprichii roots were 71.37 ± 3.43 mg g^-1, 67.03 ± 5.33 mg g^-1, 49.14 ± 3.48 mg g^-1, and 18.51 ± 2.09 mg g^-1, respectively, while the corresponding root DOC exudation rates were 7.00 ± 0.97 mg g DW root^-1 h^-1, 5.11 ± 0.42 mg g DW root^-1 h^-1, 4.08 ± 0.23 mg g DW root^-1 h^-1, and 3.78 ± 0.74 mg g DW root^-1 h^-1, respectively. There was a significant positive correlation between root soluble sugar and DOC exudation rate. DOC concentration of sediment porewater and SOC content also decreased under nitrate enrichment (though not significantly), which were both significantly positively correlated with the rate of root exuded DOC. Meanwhile, nitrate enrichment also reduced the release rate of DOC from seagrass roots during initial decomposition, and the release flux of DOC from decomposition. Therefore, nutrient enrichment could decrease nonstructural carbohydrates of seagrass roots, reducing the rate of root exuded DOC, thereby lowered SOC, as well as the DOC release from seagrass root decomposition. In order to increase the release of DOC from seagrass roots and improve the carbon sequestration capacity of seagrass beds, effective measures should be taken to control the coastal nutrients input into seagrass beds.

The effect of elevated CO2 on aboveground and belowground carbon allocation and eco-physiology of four species of angiosperm and gymnosperm forest trees

Although atmospheric CO2 concentration ([CO2]) continues to rise, the question of how tree carbon (C) allocation is affected by this change remains. Studies show that C assimilation increases under elevated CO2 (eCO2). Yet, no detailed study has determined the fate of the surplus C, i.e., its compartment and physiological process allocation, nor in multiple species together. In this project, we grew 2-year-old saplings of four key Mediterranean tree species (the conifers Cupressus sempervirens L. and Pinus halepensis Mill., and the broadleaf Quercus calliprinos Webb. and Ceratonia siliqua L.) to [CO2] levels of 400 or 700 p.p.m. for 6 months. We measured the allocation of C to below and aboveground growth, respiration, root exudation, storage and leaf litter. In addition, we monitored intrinsic water-use efficiency (WUE), soil moisture, soil chemistry and nutrient uptake. Net assimilation, WUE and soil nitrogen uptake significantly increased at eCO2 across the four species. Broadleaf species showed soil water savings, which were absent in conifers. All other effects were species-specific: Cupressus had higher leaf respiration, Pinus had lower starch in branches and transiently higher exudation rate and Quercus had higher root respiration. Elevated CO2 did not affect growth or litter production. Our results are pivotal to understanding the sensitivity of tree C allocation to the change in [CO2] when water is abundant. Species-specific responses should be regarded cautiously when predicting future changes in forest function in a higher CO2 world.

Root exudation in a sloping Moso bamboo forest in relation to fine root biomass and traits

Exudation by fine roots generally varies with their morphological traits, but the effect of belowground resource availability on the root exudation via root morphological traits and biomass remains unknown. We aimed to determine the effects of morphological and physiological traits on root exudation rates and to estimate stand-scale exudation (E_stand) by measuring the mass, length, and surface area of fine roots in a Moso bamboo forest. We measured root exudation as well as morphological and physiological traits in upper and lower plots on a slope with different belowground resource availability. The mean (± S.D.) root exudation rates per mass in the upper and lower slope were 0.049 ± 0.047 and 0.040 ± 0.059 mg C g^-1 h^-1, respectively, which were in the range of exudation found in woody forest ecosystems. We observed significant relationships between root exudation per mass and root respiration, as well as specific root length and surface area. In contrast, exudation per length and area did not correlate with morphological traits. The morphological traits did not differ between slope positions, resulting in no significant difference in root exudation per mass. Fine root biomass, length, and surface area on a unit ground basis were much higher in the lower than those in the upper slope positions. E_stand was higher when estimated by mass than by length and area because the morphological effect on exudation was ignored when scaled using mass. E_stand was 1.4–2.0-fold higher in the lower than that in upper slope positions, suggesting that the scaling parameters of mass, length, and area determined the E_stand estimate more than the exudation rate per mass, length, and area. Regardless of scaling, E_stand was much higher in the Moso bamboo forest than in other forest ecosystems because of a large fine-root biomass.

Responses of fine root exudation, respiration and morphology in three early successional tree species to increased air humidity and different soil nitrogen sources

Global climate change scenarios predict an increase in air temperature, precipitation and air humidity for northern latitudes. Elevated air humidity may significantly reduce the water flux through forest canopies and affect interactions between water and nutrient uptake. However, we have limited understanding of how altered transpiration would affect root respiration and carbon (C) exudation as fine root morphology acclimates to different water flux. We investigated the effects of elevated air relative humidity (eRH) and different inorganic nitrogen sources (NO3- and NH4+) on above and belowground traits in hybrid aspen (Populus × wettsteinii Hämet-Ahti), silver birch (Betula pendula Roth.) and Scots pine (Pinus sylvestris L.) grown under controlled climate chamber conditions. The eRH significantly decreased the transpiration flux in all species, decreased root mass-specific exudation in pine, and increased root respiration in aspen. eRH also affected fine root morphology, with specific root area increasing for birch but decreasing in pine. The species comparison revealed that pine had the highest C exudation, whereas birch had the highest root respiration rate. Both humidity and nitrogen treatments affected the share of absorptive and pioneer roots within fine roots; however, the response was species-specific. The proportion of absorptive roots was highest in birch and aspen, the share of pioneer roots was greatest in aspen and the share of transport roots was greatest in pine. Fine roots with lower root tissue density were associated with pioneer root tips and had a higher C exudation rate. Our findings underline the importance of considering species-specific differences in relation to air humidity and soil nitrogen availability that interactively affect the C input-output balance. We highlight the role of changes in the fine root functional distribution as an important acclimation mechanism of trees in response to environmental change.

Soil arsenic toxicity differentially impacts C3 (barley) and C4 (maize) crops under future climate atmospheric CO2

Soil arsenic (As) contamination limits global agricultural productivity. Anthropogenic emissions are causing atmospheric CO₂ levels to rise. Elevated CO₂ (eCO₂) boosts plant growth both under optimal and suboptimal growth conditions. However, the crop-specific interaction between eCO₂ and soil arsenic exposure has not been investigated at the whole plant, physiological and biochemical level. Here, we tested the effects of eCO₂ (620 ppm) and soil As exposure (mild and severe treatments, 25 and 100 mg As/Kg soil) on growth, photosynthesis and redox homeostasis in barley (C3) and maize (C4). Compared to maize, barley was more susceptible to soil As exposure at ambient CO₂ levels. Barley plants accumulated more As, particularly in roots. As accumulation inhibited plant growth and induced oxidative damage in a species-specific manner. As-exposed barley experienced severe oxidative stress as illustrated by high H₂O₂ and protein oxidation levels. Interestingly, eCO₂ differentially mitigated As-induced stress in barley and maize. In barley, eCO₂ exposure reduced photorespiration, H₂O₂ production, and lipid/protein oxidation. In maize eCO₂ exposure led to an upregulation of the ascorbate-glutathione (ASC/GSH)-mediated antioxidative defense system. Combined, this work highlights how ambient and future eCO₂ levels differentially affect the growth, physiology and biochemistry of barley and maize crops exposed to soil As pollution.

Response of net primary productivity to grassland phenological changes in Xinjiang, China

Determining the relationship between net primary productivity (NPP) and grassland phenology is important for an in-depth understanding of the impact of climate change on ecosystems. In this study, the NPP of grassland in Xinjiang, China, was simulated using the Carnegie-Ames-Stanford approach (CASA) model with Moderate Resolution Imaging Spectroradiometer (MODIS) grassland phenological (MCD12Q2) data to study trends in phenological metrics, grassland NPP, and the relations between these factors from 2001-2014. The results revealed advancement of the start of the growing season (SOS) for grassland in most regions (55.2%) in Xinjiang. The percentage of grassland area in which the end of the growing season (EOS) was delayed (50.9%) was generally the same as that in which the EOS was advanced (49.1%). The percentage of grassland area with an increase in the length of the growing season (LOS) for the grassland area (54.6%) was greater than that with a decrease in the LOS (45.4%). The percentage of grassland area with an increase in NPP (61.6%) was greater than that with a decrease in NPP (38.4%). Warmer regions featured an earlier SOS and a later EOS and thus a longer LOS. Regions with higher precipitation exhibited a later SOS and an earlier EOS and thus a shorter LOS. In most regions, the SOS was earlier, and spring NPP was higher. A linear statistical analysis showed that at various humidity (K) levels, grassland NPP in all regions initially increased but then decreased with increasing LOS. At higher levels of K, when NPP gradually increased, the LOS gradually decreased.

A novel 13 C pulse-labelling method to quantify the contribution of rhizodeposits to soil respiration in a grassland exposed to drought and nitrogen addition

Rhizodeposition plays an important role in below-ground carbon (C) cycling. However, quantification of rhizodeposition in intact plant-soil systems has remained elusive due to methodological issues. We used a ¹³ C-CO₂ pulse-labelling method to quantify the contribution of rhizodeposition to below-ground respiration. Intact plant-soil cores were taken from a grassland field, and in half, shoots and roots were removed (unplanted cores). Both unplanted and planted cores were assigned to drought and nitrogen (N) treatments. Afterwards, shoots in planted cores were pulse labelled with ¹³ C-CO₂ and then clipped to determine total below-ground respiration and its δ¹³ C. Simultaneously, δ¹³ C was measured for the respiration of live roots, soils with rhizodeposits, and unplanted treatments, and used as endmembers with which to determine root respiration and rhizodeposit C decomposition using two-source mixing models. Rhizodeposit decomposition accounted for 7-31% of total below-ground respiration. Drought reduced decomposition of both rhizodeposits and soil organic carbon (SOC), while N addition increased root respiration but not the contribution of rhizodeposit C decomposition to below-ground respiration. This study provides a new approach for the partitioning of below-ground respiration into different sources, and indicates that decomposition of rhizodeposit C is an important component of below-ground respiration that is sensitive to drought and N addition in grassland ecosystems.

Impacts of elevated CO2 on plant resistance to nutrient deficiency and toxic ions via root exudates: A review

Elevated atmospheric CO₂ (eCO₂) concentration can increase root exudation into soils, which improves plant tolerance to abiotic stresses. This review used a meta-analysis to assess effect sizes of eCO₂ on both efflux rates and total amounts of some specific root exudates, and dissected whether eCO₂ enhances plant's resistance to nutrient deficiency and ion toxicity via root exudates. Elevated CO₂ did not affect efflux rates of total dissolved organic carbon, a measure of combined root exudates per unit of root biomass or length, but increased the efflux amount of root systems per plant by 31% which is likely attributed to increased root biomass (29%). Elevated CO₂ increased efflux rates of soluble-sugars, carboxylates, and citrate by 47%, 111%, and 16%, respectively, but did not affect those of amino acids and malate. The increased carbon allocation to roots, increased plant requirements of mineral nutrients, and heightened detoxification responses to toxic ions under eCO₂ collectively contribute to the increased efflux rates despite lacking molecular evidence. The increased efflux rates of root exudates under eCO₂ were closely associated with improved nutrient uptake whilst less studies have validated the associations between root exudates and resistance to toxic ions of plants when grown under eCO₂. Future studies are required to reveal how climate change (eCO₂) affect the efflux of specific root exudates, particularly organic anions, the corresponding nutrient uptake and toxic ion resistance from plant molecular biology and soil microbial ecology perspectives.

Microbial functional genes commonly respond to elevated carbon dioxide

Atmospheric CO₂ concentration is increasing, largely due to anthropogenic activities. Previous studies of individual free-air CO₂ enrichment (FACE) experimental sites have shown significant impacts of elevated CO₂ (eCO₂) on soil microbial communities; however, no common microbial response patterns have yet emerged, challenging our ability to predict ecosystem functioning and sustainability in the future eCO₂ environment. Here we analyzed 66 soil microbial communities from five FACE sites, and showed common microbial response patterns to eCO₂, especially for key functional genes involved in carbon and nitrogen fixation (e.g., pcc/acc for carbon fixation, nifH for nitrogen fixation), carbon decomposition (e.g., amyA and pulA for labile carbon decomposition, mnp and lcc for recalcitrant carbon decomposition), and greenhouse gas emissions (e.g., mcrA for methane production, norB for nitrous oxide production) across five FACE sites. Also, the relative abundance of those key genes was generally increased and directionally associated with increased biomass, soil carbon decomposition, and soil moisture. In addition, a further literature survey of more disparate FACE experimental sites indicated increased biomass, soil carbon decay, nitrogen fixation, methane and nitrous oxide emissions, plant and soil carbon and nitrogen under eCO₂. A conceptual framework was developed to link commonly responsive functional genes with ecosystem processes, such as pcc/acc vs. soil carbon storage, amyA/pulA/mnp/lcc vs. soil carbon decomposition, and nifH vs. nitrogen availability, suggesting that such common responses of microbial functional genes may have the potential to predict ecosystem functioning and sustainability in the future eCO₂ environment.

Plant species determine tidal wetland methane response to sea level rise

Blue carbon (C) ecosystems are among the most effective C sinks of the biosphere, but methane (CH₄) emissions can offset their climate cooling effect. Drivers of CH₄ emissions from blue C ecosystems and effects of global change are poorly understood. Here we test for the effects of sea level rise (SLR) and its interactions with elevated atmospheric CO₂, eutrophication, and plant community composition on CH₄ emissions from an estuarine tidal wetland. Changes in CH₄ emissions with SLR are primarily mediated by shifts in plant community composition and associated plant traits that determine both the direction and magnitude of SLR effects on CH₄ emissions. We furthermore show strong stimulation of CH₄ emissions by elevated atmospheric CO₂, whereas effects of eutrophication are not significant. Overall, our findings demonstrate a high sensitivity of CH₄ emissions to global change with important implications for modeling greenhouse-gas dynamics of blue C ecosystems.

Identifying areas at risk of drought-induced tree mortality across South-Eastern Australia

South-East Australia has recently been subjected to two of the worst droughts in the historical record (Millennium Drought, 2000-2009 and Big Dry, 2017-2019). Unfortunately, a lack of forest monitoring has made it difficult to determine whether widespread tree mortality has resulted from these droughts. Anecdotal observations suggest the Big Dry may have led to more significant tree mortality than the Millennium drought. Critically, to be able to robustly project future expected climate change effects on Australian vegetation, we need to assess the vulnerability of Australian trees to drought. Here we implemented a model of plant hydraulics into the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model. We parameterized the drought response behaviour of five broad vegetation types, based on a common garden dry-down experiment with species originating across a rainfall gradient (188-1,125 mm/year) across South-East Australia. The new hydraulics model significantly improved (~35%-45% reduction in root mean square error) CABLE's previous predictions of latent heat fluxes during periods of water stress at two eddy covariance sites in Australia. Landscape-scale predictions of the greatest percentage loss of hydraulic conductivity (PLC) of about 40%-60%, were broadly consistent with satellite estimates of regions of the greatest change in both droughts. In neither drought did CABLE predict that trees would have reached critical PLC in widespread areas (i.e. it projected a low mortality risk), although the model highlighted critical levels near the desert regions of South-East Australia where few trees live. Overall, our experimentally constrained model results imply significant resilience to drought conferred by hydraulic function, but also highlight critical data and scientific gaps. Our approach presents a promising avenue to integrate experimental data and make regional-scale predictions of potential drought-induced hydraulic failure.

Surplus Carbon Drives Allocation and Plant-Soil Interactions

Plant growth is usually constrained by the availability of nutrients, water, or temperature, rather than photosynthetic carbon (C) fixation. Under these conditions leaf growth is curtailed more than C fixation, and the surplus photosynthates are exported from the leaf. In plants limited by nitrogen (N) or phosphorus (P), photosynthates are converted into sugars and secondary metabolites. Some surplus C is translocated to roots and released as root exudates or transferred to root-associated microorganisms. Surplus C is also produced under low moisture availability, low temperature, and high atmospheric CO₂ concentrations, with similar below-ground effects. Many interactions among above- and below-ground ecosystem components can be parsimoniously explained by the production, distribution, and release of surplus C under conditions that limit plant growth.

Influence of fine root traits on in situ exudation rates in four conifers from different mycorrhizal associations

Plant roots can exude organic compounds into the soil that are useful for plant survival because they can degrade microorganisms around the roots and enhance allelopathy against other plant invasions. We developed a method to collect carbon (C) exudation on a small scale from tree fine roots by C-free filter traps. We quantified total C through root exudation in four conifers from different microbial symbiotic groups (ectomycorrhiza (ECM) and arbuscular mycorrhiza (AM)) in a cool-temperate forest in Japan. We determined the relationship of mass-based exudation rate from three diameter classes (<0.5, 0.5-1.0, and 1.0-2.5 mm) of the intact root system with root traits such as morphological traits including root diameter, specific root length (SRL), specific root area (SRA), root tissue density (RTD) and chemical traits including root nitrogen (N) content and C/N. Across species, the mass-based root exudation rate was found to correlate with diameter, SRA, RTD, N and C/N. When comparing mycorrhizal types, there were significant relationships between the exudation and diameter, SRL, SRA, root N and C/N in ECM species; however, these were not significant in AM species. Our results show that relationships between in situ root exudation and every measured trait of morphology and chemistry were strongly driven by ECM roots and not by AM roots. These differences might explain the fact that ECM roots in this study potentially covaried by optimizing the exudation and root morphology in forest trees, while exudation in AM roots did not change with changes in root morphology. In addition, the contrasting results may be attributable to the effect of degree and position of ECM and AM colonization in fine root system. Differences in fine root exudation relationships to root morphology for the two types of mycorrhizae will help us better understand the underlying mechanisms of belowground C allocation in forest ecosystems.

Carbon sink strength of nodules but not other organs modulates photosynthesis of faba bean (Vicia faba) grown under elevated [CO2 ] and different water supply

Photosynthetic stimulation by elevated [CO₂ ] (e[CO₂ ]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N₂ -fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO₂ ]. However, drought not only constrains photosynthesis but also the size and activity of sinks, and little is known about the interaction of e[CO₂ ] and drought on carbon sink strength of nodules and other organs. To compare carbon sink strength, faba bean was grown under ambient (400 ppm) or elevated (700 ppm) atmospheric [CO₂ ] and subjected to well-watered or drought treatments, and then exposed to ¹³ C pulse-labelling using custom-built chambers to track the fate of new photosynthates. Drought decreased ¹³ C uptake and nodule sink strength, and this effect was even greater under e[CO₂ ], and was associated with an accumulation of amino acids in nodules. This resulted in decreased N₂ fixation, and increased accumulation of new photosynthates (¹³ C/sugars) in leaves, which in turn can feed back on photosynthesis. Our study suggests that nodule C sink activity is key to avoid sink limitation in legumes under e[CO₂ ], and legumes may only be able to achieve greater C gain if nodule activity is maintained.

Long-Term Nutrient Enrichment of an Oligotroph-Dominated Wetland Increases Bacterial Diversity in Bulk Soils and Plant Rhizospheres

In nutrient-limited conditions, plants rely on rhizosphere microbial members to facilitate nutrient acquisition, and in return, plants provide carbon resources to these root-associated microorganisms. However, atmospheric nutrient deposition can affect plant-microbe relationships by changing soil bacterial composition and by reducing cooperation between microbial taxa and plants. To examine how long-term nutrient addition shapes rhizosphere community composition, we compared traits associated with bacterial (fast-growing copiotrophs, slow-growing oligotrophs) and plant (C3 forb, C4 grass) communities residing in a nutrient-poor wetland ecosystem. Results revealed that oligotrophic taxa dominated soil bacterial communities and that fertilization increased the presence of oligotrophs in bulk and rhizosphere communities. Additionally, bacterial species diversity was greatest in fertilized soils, particularly in bulk soils. Nutrient enrichment (fertilized versus unfertilized) and plant association (bulk versus rhizosphere) determined bacterial community composition; bacterial community structure associated with plant functional group (grass versus forb) was similar within treatments but differed between fertilization treatments. The core forb microbiome consisted of 602 unique taxa, and the core grass microbiome consisted of 372 unique taxa. Forb rhizospheres were enriched in potentially disease-suppressive bacterial taxa, and grass rhizospheres were enriched in bacterial taxa associated with complex carbon decomposition. Results from this study demonstrate that fertilization serves as a strong environmental filter on the soil microbiome, which leads to distinct rhizosphere communities and can shift plant effects on the rhizosphere microbiome. These taxonomic shifts within plant rhizospheres could have implications for plant health and ecosystem functions associated with carbon and nitrogen cycling.IMPORTANCE Over the last century, humans have substantially altered nitrogen and phosphorus cycling. Use of synthetic fertilizer and burning of fossil fuels and biomass have increased nitrogen and phosphorus deposition, which results in unintended fertilization of historically low-nutrient ecosystems. With increased nutrient availability, plant biodiversity is expected to decline, and the abundance of copiotrophic taxa is anticipated to increase in bacterial communities. Here, we address how bacterial communities associated with different plant functional types (forb, grass) shift due to long-term nutrient enrichment. Unlike other studies, results revealed an increase in bacterial diversity, particularly of oligotrophic bacteria in fertilized plots. We observed that nutrient addition strongly determines forb and grass rhizosphere composition, which could indicate different metabolic preferences in the bacterial communities. This study highlights how long-term fertilization of oligotroph-dominated wetlands could alter diversity and metabolism of rhizosphere bacterial communities in unexpected ways.

Microbial carbon limitation: The need for integrating microorganisms into our understanding of ecosystem carbon cycling

Numerous studies have demonstrated that fertilization with nutrients such as nitrogen, phosphorus, and potassium increases plant productivity in both natural and managed ecosystems, demonstrating that primary productivity is nutrient limited in most terrestrial ecosystems. In contrast, it has been demonstrated that heterotrophic microbial communities in soil are primarily limited by organic carbon or energy. While this concept of contrasting limitations, that is, microbial carbon and plant nutrient limitation, is based on strong evidence that we review in this paper, it is often ignored in discussions of ecosystem response to global environment changes. The plant-centric perspective has equated plant nutrient limitations with those of whole ecosystems, thereby ignoring the important role of the heterotrophs responsible for soil decomposition in driving ecosystem carbon storage. To truly integrate carbon and nutrient cycles in ecosystem science, we must account for the fact that while plant productivity may be nutrient limited, the secondary productivity by heterotrophic communities is inherently carbon limited. Ecosystem carbon cycling integrates the independent physiological responses of its individual components, as well as tightly coupled exchanges between autotrophs and heterotrophs. To the extent that the interacting autotrophic and heterotrophic processes are controlled by organisms that are limited by nutrient versus carbon accessibility, respectively, we propose that ecosystems by definition cannot be 'limited' by nutrients or carbon alone. Here, we outline how models aimed at predicting non-steady state ecosystem responses over time can benefit from dissecting ecosystems into the organismal components and their inherent limitations to better represent plant-microbe interactions in coupled carbon and nutrient models.

Five-year nitrogen addition affects fine root exudation and its correlation with root respiration in a dominant species, Quercus crispula, of a cool temperate forest, Japan

In forest ecosystems, fine root respiration directly contributes to belowground carbon (C) cycling. Exudation from fine roots indirectly affects C cycling via enhanced microbial decomposition of soil organic matter. Although these root-derived C fluxes are essential components of belowground C cycling, how nitrogen (N) addition affects these fluxes and their correlations remains unclear. In this study, fine root exudation, respiration and chemical/morphological traits were measured in a dominant canopy species, Quercus crispula Blume, found in a cool temperate forest, the Tomakomai Experimental Forest, Hokkaido University, which has undergone 5-year N addition. Soil-dissolved organic carbon (DOC) was also measured in both bulk and rhizosphere soils to evaluate the impact of fine root exudation on soil C cycling. Compared with a control plot with no N treatment, fine roots in the N addition plot exhibited larger diameters and higher N concentrations, but lower specific root lengths and areas. On a root-weight basis, respiration was not different between plots, but exudation was slightly higher under N addition. On a root-area basis, exudation was significantly higher in the N addition plot. Additionally, differences in DOC between rhizosphere and bulk soils were two times higher in the N addition plot than the control plot. Although fine root respiration was positively correlated with exudation in both the control and N addition plots, the ratio of exudation C to respiration C decreased after 5-year N addition. Nitrogen addition also affected absolute C allocation to fine root exudation and changed the C allocation strategy between exudation and respiration fluxes. These findings will help enhance predictions of belowground C allocation and C cycling under N-rich conditions in the future.

Robust Response of Terrestrial Plants to Rising CO2

Human-caused CO₂ emissions over the past century have caused the climate of the Earth to warm and have directly impacted on the functioning of terrestrial plants. We examine the global response of terrestrial gross primary production (GPP) to the historic change in atmospheric CO₂. The GPP of the terrestrial biosphere has increased steadily, keeping pace remarkably in proportion to the rise in atmospheric CO₂. Water-use efficiency, namely the ratio of CO₂ uptake by photosynthesis to water loss by transpiration, has increased as a direct leaf-level effect of rising CO₂. This has allowed an increase in global leaf area, which has conspired with stimulation of photosynthesis per unit leaf area to produce a maximal response of the terrestrial biosphere to rising atmospheric CO₂ and contemporary climate change.

Impacts of Atmospheric CO2 and Soil Nutritional Value on Plant Responses to Rhizosphere Colonization by Soil Bacteria

Concerns over rising atmospheric CO₂ concentrations have led to growing interest in the effects of global change on plant-microbe interactions. As a primary substrate of plant metabolism, atmospheric CO₂ influences below-ground carbon allocation and root exudation chemistry, potentially affecting rhizosphere interactions with beneficial soil microbes. In this study, we have examined the effects of different atmospheric CO₂ concentrations on Arabidopsis rhizosphere colonization by the rhizobacterial strain Pseudomonas simiae WCS417 and the saprophytic strain Pseudomonas putida KT2440. Rhizosphere colonization by saprophytic KT2440 was not influenced by sub-ambient (200 ppm) and elevated (1,200 ppm) concentrations of CO₂, irrespective of the carbon (C) and nitrogen (N) content of the soil. Conversely, rhizosphere colonization by WCS417 in soil with relatively low C and N content increased from sub-ambient to elevated CO₂. Examination of plant responses to WCS417 revealed that plant growth and systemic resistance varied according to atmospheric CO₂ concentration and soil-type, ranging from growth promotion with induced susceptibility at sub-ambient CO₂, to growth repression with induced resistance at elevated CO₂. Collectively, our results demonstrate that the interaction between atmospheric CO₂ and soil nutritional status has a profound impact on plant responses to rhizobacteria. We conclude that predictions about plant performance under past and future climate scenarios depend on interactive plant responses to soil nutritional status and rhizobacteria.

Explaining the doubling of N2 O emissions under elevated CO2 in the Giessen FACE via in-field 15 N tracing

Rising atmospheric CO₂ concentrations are expected to increase nitrous oxide (N₂ O) emissions from soils via changes in microbial nitrogen (N) transformations. Several studies have shown that N₂ O emission increases under elevated atmospheric CO₂ (eCO₂ ), but the underlying processes are not yet fully understood. Here, we present results showing changes in soil N transformation dynamics from the Giessen Free Air CO₂ Enrichment (GiFACE): a permanent grassland that has been exposed to eCO₂ , +20% relative to ambient concentrations (aCO₂ ), for 15 years. We applied in the field an ammonium-nitrate fertilizer solution, in which either ammonium ( NH4+ ) or nitrate ( NO3- ) was labelled with ¹⁵ N. The simultaneous gross N transformation rates were analysed with a ¹⁵ N tracing model and a solver method. The results confirmed that after 15 years of eCO₂ the N₂ O emissions under eCO₂ were still more than twofold higher than under aCO₂ . The tracing model results indicated that plant uptake of NH4+ did not differ between treatments, but uptake of NO3- was significantly reduced under eCO₂ . However, the NH4+ and NO3- availability increased slightly under eCO₂ . The N₂ O isotopic signature indicated that under eCO₂ the sources of the additional emissions, 8,407 μg N₂ O-N/m² during the first 58 days after labelling, were associated with NO3- reduction (+2.0%), NH4+ oxidation (+11.1%) and organic N oxidation (+86.9%). We presume that increased plant growth and root exudation under eCO₂ provided an additional source of bioavailable supply of energy that triggered as a priming effect the stimulation of microbial soil organic matter (SOM) mineralization and fostered the activity of the bacterial nitrite reductase. The resulting increase in incomplete denitrification and therefore an increased N₂ O:N₂ emission ratio, explains the doubling of N₂ O emissions. If this occurs over a wide area of grasslands in the future, this positive feedback reaction may significantly accelerate climate change.

Structural inflexibility of the rhizosphere microbiome in mangrove plant Kandelia obovata under elevated CO2

Rhizosphere microbial communities play an important role in mediating the decomposition of soil organic matter. Increased CO₂ concentration may increase plant growth by stimulating photosynthesis or improving water use efficiency. However, possible eco-physiological influences of this greenhouse gas in mangrove plants are not well understood, especially how rhizosphere microbial communities respond to CO₂ increase. We characterized the effect of elevated CO₂ (eCO₂) on rhizospheric microbial communities associated with the mangrove plant Kandelia candel for 20 weeks, eCO₂ increased plant chlorophyll a levels and root microbial biomass. Operational taxonomic unit analysis revealed no significant effects of eCO₂ on rhizospheric bacterial communities; however, some influence on archaeal community structure was observed, especially on the ammonia-oxidizing archaea. Principal component analysis showed that microbial biomass C, total nitrogen, C/N ratio, nitrate nitrogen, and salinity were the main factors structuring the microbial community. The relative contribution of environmental parameters to variability among samples was 31.0%. In addition, functional analysis by average well color development showed that carbon source utilization under eCO₂ occurred in the order amino acids > carbohydrates > polymers > carboxylic acids > amines > phenolic acids; whereas, sugars, amino acids, and carboxylic acids were the preferred carbon sources in control groups. Differences in utilization ability of carbohydrates and amino acids resulted in changes in carbon metabolism between the two groups. Rhizosphere microbial communities appear to have some buffering ability in response to short-term (20 weeks) CO₂ increase, during which the metabolic efficiency of carbon sources is changed. The results will help better understand the structural inflexibility and functional plasticity of the rhizosphere microbiome in mangrove plants facing a changing environment (such as global climate change).

The Response Patterns of Arbuscular Mycorrhizal and Ectomycorrhizal Symbionts Under Elevated CO2: A Meta-Analysis

Elevated carbon dioxide (eCO2), a much-discussed topic in global warming, influences development and functions of mycorrhizal fungi and plants. However, due to the inconsistent results reported in various publications, the response patterns of symbionts associated with the arbuscular mycorrhizal (AM) or with ectomycorrhizal (ECM) fungi to eCO2 remains still unclear. Therefore, we performed a meta-analysis to identify how eCO2 affected mycorrhizal fungi and if there is a significant different response between AM and ECM symbionts. Our results demonstrated that eCO2 increased mycorrhizal plants biomass (+26.20%), nutrient contents [+2.45% in nitrogen (N), and +10.66% in phosphorus (P)] and mycorrhizal fungal growth (+22.87% in extraradical hyphal length and +21.77% in mycorrhizal fungal biomass), whereas plant nutrient concentrations decreased (-11.86% in N and -12.01% in P) because the increase in plant biomass was greater than that in nutrient content. The AM plants exhibited larger increases in their biomass (+33.90%) and in their N (+21.99%) and P contents (+19.48%) than did the ECM plants (+20.57% in biomass, -4.28% in N content and -13.35% in P content). However, ECM fungi demonstrated increased responses of mycorrhizal fungal biomass (+29.98%) under eCO2 compared with AM fungi (+6.61%). These data indicate different patterns in the growth of AM and ECM symbionts under eCO2: AM symbionts contributed more to plant growth, while ECM symbionts were more favorable to mycorrhizal fungal growth. In addition, the responses of plant biomass to eCO2 showed no significant difference between short-term and long-term groups, whereas a significant difference in the responses of mycorrhizal fungal growth was found between the two groups. The addition of N increased plant growth but decreased mycorrhizal fungal abundance, and P addition increased total plant biomass and extraradical hyphal length, but shoot biomass largely increased in low P conditions. Mixtures of mycorrhizal fungi affected the total plant and root biomasses more than a single mycorrhizal fungus. Clarifying the different patterns in AM and ECM symbionts under eCO2 would contribute to a better understanding of the interactions between mycorrhizal fungi and plant symbionts under the conditions of global climate change as well as of the coevolution of flora with Earth's environment.

The mycorrhizal type governs root exudation and nitrogen uptake of temperate tree species

Even though the two dominant mycorrhizal associations of temperate tree species differentially couple carbon (C) and nitrogen (N) cycles in temperate forests, systematic differences between the biogeochemical cycles of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree species remain poorly described. A classification according to the mycorrhizal type offers the chance, though, to develop a global frame concept for the prediction of temperate ecosystem responses to environmental change. Focusing on the influence of mycorrhizal types on two key plant processes of biogeochemical cycling (root exudation and N acquisition), we investigated four temperate deciduous tree species per mycorrhizal type in a drought experiment in large mesocosms. We hypothesized that (H1) C loss by root exudation is higher in ECM than in AM trees, (H2) drought leads to higher reductions in root exudation of drought-sensitive ECM trees and (H3) inorganic N uptake is higher in AM than in ECM trees. In contradiction to H2, we found no systematic difference in root exudation between the mycorrhizal types at ample soil moisture, but almost twofold higher exudation in ECM trees when exposed to soil drought. In addition, photosynthetic C cost of root exudation strongly increased by ~10-fold in drought-treated ECM trees, while it only doubled in AM trees, which confirms H1. With respect to H3, we corroborated that AM trees had higher absolute and relative inorganic N acquisition rates than ECM trees, while the organic N uptake did not differ between mycorrhizal types. We conclude that ECM trees are less efficient in inorganic N uptake than AM trees, but ECM trees increase root C release as an adaptive response to dry soil to maintain hydraulic conductivity and/or nutrient availability. These systematic differences in key biogeochemical processes support hints on the key role of the mycorrhizal types in coupling C and N cycles in temperate forests.

Relationship between fine-root exudation and respiration of two Quercus species in a Japanese temperate forest

Plants allocate a considerable amount of carbon (C) to fine roots as respiration and exudation. Fine-root exudation could stimulate microbial activity, which further contributes to soil heterotrophic respiration. Although both root respiration and exudation are important components of belowground C cycling, how they relate to each other is less well known. In this study, we aimed to explore this relationship on mature trees growing in the field. The measurements were performed on two canopy species, Quercus serrata Thunb. and Quercus glauca, in a warm temperate forest. The respiration and exudation rates of the same fine-root segment were measured in parallel with a syringe-basis incubation and a closed static chamber, respectively. We also measured root traits and ectomycorrhizal colonization ratio because these indexes commonly relate to root respiration and reflect root physiology. The microbial activity enhanced by root exudation was investigated by comparing the dissolved organic carbon (DOC) and microbial biomass carbon (MBC) between rhizosphere soils and bulk soils. Mean DOC concentration and MBC were ca two times higher in the rhizosphere soils and positively related to exudation rates, indicating that exudation further relates to the C dynamics in the soils. Flux rates of exudation and respiration were positively correlated with each other. Both root exudation and respiration rates positively related to ectomycorrhizal colonization and root tissue nitrogen, and therefore the relationship between the two fluxes may be attributed to fine-root activity. The flux rates of root respiration were 8.7 and 10.5 times as much as those of exudation on a root-length basis and a root-weight basis, respectively. In spite of the fact that flux rates of respiration and exudation varied enormously among the fine-root segments of the two Quercus species, exudation was in proportion to respiration. This result gives new insight into the fine-root C-allocation strategy and the belowground C dynamics.

Release of resource constraints allows greater carbon allocation to secondary metabolites and storage in winter wheat

The atmospheric CO₂ concentration ([CO₂ ]) is rapidly increasing, and this may have substantial impact on how plants allocate metabolic resources. A thorough understanding of allocation priorities can be achieved by modifying [CO₂ ] over a large gradient, including low [CO₂ ], thereby altering plant carbon (C) availability. Such information is of critical importance for understanding plant responses to global environmental change. We quantified the percentage of daytime whole-plant net assimilation (A) allocated to night-time respiration (R), structural growth (SG), nonstructural carbohydrates (NSC) and secondary metabolites (SMs) during 8 weeks of vegetative growth in winter wheat (Triticum aestivum) growing at low, ambient and elevated [CO₂ ] (170, 390 and 680 ppm). R/A remained relatively constant over a large gradient of [CO₂ ]. However, with increasing C availability, the fraction of assimilation allocated to biomass (SG + NSC + SMs), in particular NSC and SMs, increased. At low [CO₂ ], biomass and NSC increased in leaves but decreased in stems and roots, which may help plants achieve a functional equilibrium, that is, overcome the most severe resource limitation. These results reveal that increasing C availability from rising [CO₂ ] releases allocation constraints, thereby allowing greater investment into long-term survival in the form of NSC and SMs.

Decoupling of soil carbon and nitrogen turnover partly explains increased net ecosystem production in response to nitrogen fertilization

During the last decade there has been an ongoing controversy regarding the extent to which nitrogen fertilization can increase carbon sequestration and net ecosystem production in forest ecosystems. The debate is complicated by the fact that increased nitrogen availability caused by nitrogen deposition has coincided with increasing atmospheric carbon dioxide concentrations. The latter could further stimulate primary production but also result in increased allocation of carbon to root exudates, which could potentially 'prime' the decomposition of soil organic matter. Here we show that increased input of labile carbon to forest soil caused a decoupling of soil carbon and nitrogen cycling, which was manifested as a reduction in respiration of soil organic matter that coincided with a substantial increase in gross nitrogen mineralization. An estimate of the magnitude of the effect demonstrates that the decoupling could potentially result in an increase in net ecosystem production by up to 51 kg C ha-1 day-1 in nitrogen fertilized stands during peak summer. Even if the effect is several times lower on an annual basis, the results still suggest that nitrogen fertilization can have a much stronger influence on net ecosystem production than can be expected from a direct stimulation of primary production alone.

Stress differentially causes roots of tree seedlings to exude carbon

How carbon (C) flows through plants into soils is poorly understood. Carbon exuded comes from a pool of non-structural carbohydrates (NSC) in roots. Simple models of diffusion across concentration gradients indicate that the more C in roots, the more C should be exuded from roots. However, the mechanisms underlying the accumulation and loss of C from roots may differ depending on the stress experienced by plants. Thus, stress type may influence exudation independent of NSC. We tested this hypothesis by examining the relationship between NSC in fine roots and exudation of organic C in aspen (Populus tremuloides Michx.) seedlings after exposure to shade, cold soils and drought in a controlled environment. Fine root concentrations of NSC varied by treatment. Mass-specific C exudation increased with increasing fine root sugar concentration in all treatments, but stress type affected exudation independently of sugar concentration. Seedlings exposed to cold soils exuded the most C on a per mass basis. Through 13C labeling, we also found that stressed seedlings allocated relatively more new C to exudates than roots compared with unstressed seedlings. Stress affects exudation of C via mechanisms other than changes in root carbohydrate availability.

Key knowledge and data gaps in modelling the influence of CO2 concentration on the terrestrial carbon sink

Primary productivity of terrestrial vegetation is expected to increase under the influence of increasing atmospheric carbon dioxide concentrations ([CO₂]). Depending on the fate of such additionally fixed carbon, this could lead to an increase in terrestrial carbon storage, and thus a net terrestrial sink of atmospheric carbon. Such a mechanism is generally believed to be the primary global driver behind the observed large net uptake of anthropogenic CO₂ emissions by the biosphere. Mechanisms driving CO₂ uptake in the Terrestrial Biosphere Models (TBMs) used to attribute and project terrestrial carbon sinks, including that from increased [CO₂], remain in large parts unchanged since those models were conceived two decades ago. However, there exists a large body of new data and understanding providing an opportunity to update these models, and directing towards important topics for further research. In this review we highlight recent developments in understanding of the effects of elevated [CO₂] on photosynthesis, and in particular on the fate of additionally fixed carbon within the plant with its implications for carbon turnover rates, on the regulation of photosynthesis in response to environmental limitations on in-plant carbon sinks, and on emergent ecosystem responses. We recommend possible avenues for model improvement and identify requirements for better data on core processes relevant to the understanding and modelling of the effect of increasing [CO₂] on the global terrestrial carbon sink.

Nutrient constraints on terrestrial carbon fixation: The role of nitrogen

Carbon dioxide (CO₂) concentrations in the earth's atmosphere are projected to rise from current levels near 400ppm to over 700ppm by the end of the 21st century. Projections over this time frame must take into account the increases in total net primary production (NPP) expected from terrestrial plants, which result from elevated CO₂ (eCO₂) and have the potential to mitigate the impact of anthropogenic CO₂ emissions. However, a growing body of evidence indicates that limitations in soil nutrients, particularly nitrogen (N), the soil nutrient most limiting to plant growth, may greatly constrain future carbon fixation. Here, we review recent studies about the relationships between soil N supply, plant N nutrition, and carbon fixation in higher plants under eCO₂, highlighting key discoveries made in the field, particularly from free-air CO₂ enrichment (FACE) technology, and relate these findings to physiological and ecological mechanisms.

The soil microbiome at the Gi-FACE experiment responds to a moisture gradient but not to CO2 enrichment

The soil bacterial community at the Giessen free-air CO2 enrichment (Gi-FACE) experiment was analysed by tag sequencing of the 16S rRNA gene. No substantial effects of CO2 levels on bacterial community composition were detected. However, the soil moisture gradient at Gi-FACE had a significant effect on bacterial community composition. Different groups within the Acidobacteria and Verrucomicrobia phyla were affected differently by soil moisture content. These results suggest that modest increases in atmospheric CO2 may cause only minor changes in soil bacterial community composition and indicate that the functional responses of the soil community to CO2 enrichment previously reported at Gi-FACE are due to factors other than changes in bacterial community composition. The effects of the moisture gradient revealed new information about the relationships between poorly known Acidobacteria and Verrucomicrobia and soil moisture content. This study contrasts with the relatively small number of other temperate grassland free-air CO2 enrichment microbiome studies in the use of moderate CO2 enrichment and the resulting minor changes in the soil microbiome. Thus, it will facilitate the development of further climate change mitigation studies. In addition, the moisture gradient found at Gi-FACE contributes new knowledge in soil microbial ecology, particularly regarding the abundance and moisture relationships of the soil Verrucomicrobia.

Light compensation points in shade-grown seedlings of deciduous broadleaf tree species with different successional traits raised under elevated CO2

We measured leaf photosynthetic traits in shade-grown seedlings of four tree species native to northern Japan, raised under an elevated CO2 condition, to investigate the effects of elevated CO2 on shade tolerance of deciduous broadleaf tree species with different successional traits. We considered Betula platyphylla var. japonica and Betula maximowicziana as pioneer species, Quercus mongolica var. crispula as a mid-successional species, and Acer mono as a climax species. The plants were grown under shade conditions (10% of full sunlight) in a CO2 -regulated phytotron. Light compensation points (LCPs) decreased in all tree species when grown under elevated CO2 (720 μmol·mol(-1) ), which were accompanied by higher apparent quantum yields but no photosynthetic down-regulation. LCPs in Q. mongolica and A. mono grown under elevated CO2 were lower than those in the two pioneer birch species. The LCP in Q. mongolica seedlings was not different from that of A. mono in each CO2 treatment. However, lower dark respiration rates were observed in A. mono than in Q. mongolica, suggesting higher shade tolerance in A. mono as a climax species in relation to carbon loss at night. Thus, elevated CO2 may have enhanced shade tolerance by lowering LCPs in all species, but the ranking of shade tolerance related to successional traits did not change among species under elevated CO2 , i.e. the highest shade tolerance was observed in the climax species (A. mono), followed by a gap-dependent species (Q. mongolica), while lower shade tolerance was observed in the pioneer species (B. platyphylla and B. maximowicziana).

Living roots magnify the response of soil organic carbon decomposition to temperature in temperate grassland

Increasing atmospheric carbon dioxide (CO2 ) concentration is both a strong driver of primary productivity and widely believed to be the principal cause of recent increases in global temperature. Soils are the largest store of the world's terrestrial C. Consequently, many investigations have attempted to mechanistically understand how microbial mineralisation of soil organic carbon (SOC) to CO2 will be affected by projected increases in temperature. Most have attempted this in the absence of plants as the flux of CO2 from root and rhizomicrobial respiration in intact plant-soil systems confounds interpretation of measurements. We compared the effect of a small increase in temperature on respiration from soils without recent plant C with the effect on intact grass swards. We found that for 48 weeks, before acclimation occurred, an experimental 3 °C increase in sward temperature gave rise to a 50% increase in below ground respiration (ca. 0.4 kg C m(-2) ; Q10  = 3.5), whereas mineralisation of older SOC without plants increased with a Q10 of only 1.7 when subject to increases in ambient soil temperature. Subsequent (14) C dating of respired CO2 indicated that the presence of plants in swards more than doubled the effect of warming on the rate of mineralisation of SOC with an estimated mean C age of ca. 8 years or older relative to incubated soils without recent plant inputs. These results not only illustrate the formidable complexity of mechanisms controlling C fluxes in soils but also suggest that the dual biological and physical effects of CO2 on primary productivity and global temperature have the potential to synergistically increase the mineralisation of existing soil C.

Fruit removal increases root-zone respiration in cucumber

BACKGROUND AND AIMS

Many attempts have been made to avoid the commonly observed fluctuations in fruit initiation and fruit growth in crop plants, particularly in cucumber (Cucumis sativus). Weak sinks of the fruit have been assumed to result in low sink/source ratios for carbohydrates, which may inhibit photosynthesis. This study focuses on the effects of low sink-source ratios on photosynthesis and respiration, and in particular root-zone respiration.

METHODS

Mature fruit-bearing cucumber plants were grown in an aerated nutrient solution. The root containers were designed as open chambers to allow measurement of CO2 gas exchange in the root zone. A similar arrangement in a gas-exchange cuvette enabled simultaneous measurements of CO2 exchange in the shoot and root zones.

KEY RESULTS

Reducing the sinks for carbohydrates by removing all fruit from the plants always resulted in a doubling of CO2 exchange in the root zone within a few hours. However, respiration of the shoot remained unaffected and photosynthesis was only marginally reduced, if at all.

CONCLUSIONS

The results suggest that the increased level of CO2 gas exchange in the root zone after removing the carbon sinks in the shoot is due primarily to the exudation of organic compounds by the roots and their decomposition by micro-organisms. This hypothesis must be tested in further experiments, but if proved correct it would make sense to include carbon leakage by root exudation in cucumber production models. In contrast, inhibition of photosynthesis was measurable only at zero fruit load, a situation that does not occur in cucumber production systems, and models that estimate production can therefore ignore (end-product) inhibition of photosynthesis.

The influence of phosphorus availability and Laccaria bicolor symbiosis on phosphate acquisition, antioxidant enzyme activity, and rhizospheric carbon flux in Populus tremuloides

Many forest tree species are dependent on their symbiotic interaction with ectomycorrhizal (ECM) fungi for phosphorus (P) uptake from forest soils where P availability is often limited. The ECM fungal association benefits the host plant under P limitation through enhanced soil exploration and increased P acquisition by mycorrhizas. To study the P starvation response (PSR) and its modification by ECM fungi in Populus tremuloides, a comparison was made between nonmycorrhizal (NM) and mycorrhizal with Laccaria bicolor (Myc) seedlings grown under different concentrations of phosphate (Pi) in sand culture. Although differences in growth between NM and Myc plants were small, Myc plants were more effective at acquiring P from low Pi treatments, with significantly lower k m values for root and leaf P accumulation. Pi limitation significantly increased the activity of catalase, ascorbate peroxidase, and guaiacol-dependent peroxidase in leaves and roots to greater extents in NM than Myc P. tremuloides. Phosphoenolpyruvate carboxylase activity also increased in NM plants under P limitation, but was unchanged in Myc plants. Formate, citrate, malonate, lactate, malate, and oxalate and total organic carbon exudation by roots was stimulated by P limitation to a greater extent in NM than Myc plants. Colonization by L. bicolor reduced the solution Pi concentration thresholds where PSR physiological changes occurred, indicating that enhanced Pi acquisition by P. tremuloides colonized by L. bicolor altered host P homeostasis and plant stress responses to P limitation. Understanding these plant-symbiont interactions facilitates the selection of more P-efficient forest trees and strategies for tree plantation production on marginal soils.

Labile carbon retention compensates for CO2 released by priming in forest soils

Increase of belowground C allocation by plants under global warming or elevated CO2 may promote decomposition of soil organic carbon (SOC) by priming and strongly affects SOC dynamics. The specific effects by priming of SOC depend on the amount and frequency of C inputs. Most previous priming studies have investigated single C additions, but they are not very representative for litterfall and root exudation in many terrestrial ecosystems. We evaluated effects of (13)C-labeled glucose added to soil in three temporal patterns: single, repeated, and continuous on dynamics of CO2 and priming of SOC decomposition over 6 months. Total and (13)C labeled CO2 were monitored to analyze priming dynamics and net C balance between SOC loss caused by priming and the retention of added glucose-C. Cumulative priming ranged from 1.3 to 5.5 mg C g(-1) SOC in the subtropical, and from -0.6 to 5.5 mg C g(-1) SOC in the tropical soils. Single addition induced more priming than repeated and continuous inputs. Therefore, single additions of high substrate amounts may overestimate priming effects over the short term. The amount of added glucose C remaining in soil after 6 months (subtropical: 8.1-11.2 mg C g(-1) SOC or 41-56% of added glucose; tropical: 8.7-15.0 mg C g(-1) SOC or 43-75% of glucose) was substantially higher than the net C loss due to SOC decomposition including priming effect. This overcompensation of C losses was highest with continuous inputs and lowest with single inputs. Therefore, raised labile organic C input to soils by higher plant productivity will increase SOC content even though priming accelerates decomposition of native SOC. Consequently, higher continuous input of C belowground by plants under warming or elevated CO2 can increase C stocks in soil despite accelerated C cycling by priming in soils.

Effects of elevated CO2 on the extractable amino acids of leaf litter and fine roots

Elevated atmospheric CO2 concentrations can change chemistry and input rate of plant tissue to soil, potentially influencing above- and below-ground biogeochemical cycles. Given the important role played by leaf and root litter chemistry in controlling ecosystem function and vulnerability to environmental stresses, we investigated the hydrolyzable amino acid distribution and concentration in leaf and fine root litter among control and elevated CO2 treatments at the Rhinelander free air CO2 enrichment (FACE) experiment (WI, USA). We extracted hydrolyzable amino acids from leaf litter and fine (< 2 mm) roots at three depths for both control and elevated CO2 plots. We found that elevated CO2 decreased the proportion of total leaf amino acid carbon (C), but had no effect on total leaf amino acid nitrogen (N). There was no treatment effect for total root amino acid N or amino acid C for any depth. The decrease in leaf amino acids is probably a result of the shift of protein compounds to more structural compounds. Despite the decrease in leaf amino acid C concentrations, the overall increase in annual plant production under elevated CO2 would result in an increase in plant amino acids to the soil.

Elevated CO2 concentration increase the mobility of Cd and Zn in the rhizosphere of hyperaccumulator Sedum alfredii

The effects of elevated CO2 on metal species and mobility in the rhizosphere of hyperaccumulator are not well understood. We report an experiment designed to compare the effects of elevated CO2 on Cd/Zn speciation and mobility in the rhizosphere of hyperaccumulating ecotype (HE) and a non-hyperaccumulating ecotype (NHE) of Sedum alfredii grown under ambient (350 μl l(-1)) or elevated (800 μl l(-1)) CO2 conditions. No difference in solution pH of NHE was observed between ambient and elevated CO2 treatments. For HE, however, elevated CO2 reduced soil solution pH by 0.22 unit, as compared to ambient CO2 conditions. Elevated CO2 increased dissolved organic carbon (DOC) and organic acid levels in soil solution of both ecotypes, but the increase in HE solution was much greater than in NHE solution. After the growth of HE, the concentrations of Cd and Zn in soil solution decreased significantly regardless of CO2 level. The visual MINTEQ speciation model predicted that Cd/Zn-DOM complexes were the dominant species in soil solutions, followed by free Cd(2+) and Zn(2+) species for both ecotypes. However, Cd/Zn-DOM complexes fraction in soil solution of HE was increased by the elevated CO2 treatment (by 8.01 % for Cd and 8.47 % for Zn, respectively). Resin equilibration experiment results indicated that DOM derived from the rhizosphere of HE under elevated CO2 (HE-DOM-E) (90 % for Cd and 73 % for Zn, respectively) showed greater ability to form complexes with Cd and Zn than those under ambient CO2 (HE-DOM-A) (82 % for Cd and 61 % for Zn, respectively) in the undiluted sample. HE-DOM-E showed greater ability to extract Cd and Zn from soil than HE-DOM-A. It was concluded that elevated CO2 could increase the mobility of Cd and Zn due to the enhanced formation of DOM-metal complexes in the rhizosphere of HE S. alfredii.

Long-term dynamics of mycorrhizal root tips in a loblolly pine forest grown with free-air CO2 enrichment and soil N fertilization for 6 years

Large-scale, long-term FACE (Free-Air CO2 enrichment) experiments indicate that increases in atmospheric CO2 concentrations will influence forest C cycling in unpredictable ways. It has been recently suggested that responses of mycorrhizal fungi could determine whether forest net primary productivity (NPP) is increased by elevated CO2 over long time periods and if forests soils will function as sources or sinks of C in the future. We studied the dynamic responses of ectomycorrhizae to N fertilization and atmospheric CO2 enrichment at the Duke FACE experiment using minirhizotrons over a 6 year period (2005-2010). Stimulation of mycorrhizal production by elevated CO2 was observed during only 1 (2007) of 6 years. This increased the standing crop of mycorrhizal tips during 2007 and 2008; during 2008, significantly higher mortality returned standing crop to ambient levels for the remainder of the experiment. It is therefore unlikely that increased production of mycorrhizal tips can explain the lack of progressive nitrogen limitations and associated increases in N uptake observed in CO2 -enriched plots at this site. Fertilization generally decreased tree reliance on mycorrhizae as tip production declined with the addition of nitrogen as has been shown in many other studies. Annual NPP of mycorrhizal tips was greatest during years with warm January temperatures and during years with cool spring temperatures. A 2 °C increase in average late spring temperatures (May and June) decreased annual production of mycorrhizal root tip length by 50%. This has important implications for ecosystem function in a warmer world in addition to potential for forest soils to sequester atmospheric C.

Synthesis and modeling perspectives of rhizosphere priming

The rhizosphere priming effect (RPE) is a mechanism by which plants interact with soil functions. The large impact of the RPE on soil organic matter decomposition rates (from 50% reduction to 380% increase) warrants similar attention to that being paid to climatic controls on ecosystem functions. Furthermore, global increases in atmospheric CO2 concentration and surface temperature can significantly alter the RPE. Our analysis using a game theoretic model suggests that the RPE may have resulted from an evolutionarily stable mutualistic association between plants and rhizosphere microbes. Through model simulations based on microbial physiology, we demonstrate that a shift in microbial metabolic response to different substrate inputs from plants is a plausible mechanism leading to positive or negative RPEs. In a case study of the Duke Free-Air CO2 Enrichment experiment, performance of the PhotoCent model was significantly improved by including an RPE-induced 40% increase in soil organic matter decomposition rate for the elevated CO2 treatment--demonstrating the value of incorporating the RPE into future ecosystem models. Overall, the RPE is emerging as a crucial mechanism in terrestrial ecosystems, which awaits substantial research and model development.

Three decades of research at Flakaliden advancing whole-tree physiology, forest ecosystem and global change research

Nutrient supply often limits growth in forest ecosystems and may limit the response of growth to an increase in other resources, or to more favorable environmental factors such as temperature and soil water. To explore the consequences and mechanisms of optimum nutrient supply for forest growth, the Flakaliden research site was established in 1986 on a young Norway spruce site with nutrient-poor soil. This special section on research at Flakaliden presents five papers that explore different facets of nutrition, atmospheric CO2 concentration, [CO2], and increased temperature treatments, using the original experiment as a base. Research at Flakaliden shows the dominant role of nutrition in controlling the response of growth to the increased photosynthesis promoted by elevated [CO2] and temperature. Experiments with whole-tree chambers showed that all treatments (air temperature warming, elevated [CO2] and optimum nutrition) increased shoot photosynthesis by 30-50%, but growth only increased with [CO2] when combined with the optimum nutrition treatment. Elevated [CO2] and temperature increased shoot photosynthesis by increasing the slope between light-saturated photosynthesis and foliar nitrogen by 122%, the initial slope of the light response curve by 52% and apparent quantum yield by 10%. Optimum nutrition also decreased photosynthetic capacity by 17%, but increased it by 62% in elevated [CO2], as estimated from wood δ(13)C. Elevated air temperature advanced spring recovery of photosynthesis by 37%, but spring frost events remained the controlling factor for photosynthetic recovery, and elevated [CO2] did not affect this. Increased nutrient availability increased wood growth primarily through a 50% increase in tracheid formation, mostly during the peak growth season. Other notable contributions of research at Flakaliden include exploring the role of optimal nutrition in large-scale field trials with foliar analysis, using an ecosystem approach for multifactor experiments, development of whole-tree chambers allowing inexpensive environmental manipulations, long-term deployment of shoot chambers for continuous measurements of gas exchange and exploring the ecosystem response to soil and aboveground tree warming. The enduring legacy of Flakaliden will be the rich data set of long-term, multifactor experiments that has been and will continue to be used in many modeling and cross-site comparison studies.