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Nitrogen (N) immobilization (Nim, including microbial N assimilation) and plant N uptake (PNU) are the two most important pathways of N retention in soils. The ratio of Nim to PNU (hereafter Nim:PNU ratio) generally reflects the degree of N limitation for plant growth in terrestrial ecosystems. However, the key factors driving the pattern of Nim:PNU ratio across global ecosystems remain unclear. Here, using a global data set of 1018 observations from 184 studies, we examined the relative importance of mycorrhizal associations, climate, plant, and soil properties on the Nim:PNU ratio across terrestrial ecosystems. Our results show that mycorrhizal fungi type (arbuscular mycorrhizal (AM) or ectomycorrhizal (EM) fungi) in combination with soil inorganic N mainly explain the global variation in the Nim:PNU ratio in terrestrial ecosystems. In AM fungi‐associated ecosystems, the relationship between Nim and PNU displays a weaker negative correlation (r = −.06, p <.001), whereas there is a stronger positive correlation (r =.25, p <.001) in EM fungi‐associated ecosystems. Our meta‐analysis thus suggests that the AM‐associated plants display a weak interaction with soil microorganisms for N absorption, while EM‐associated plants cooperate with soil microorganisms. Furthermore, we find that the Nim:PNU ratio for both AM‐ and EM‐associated ecosystems gradually converge around a stable value (13.8 ± 0.5 for AM‐ and 12.1 ± 1.2 for EM‐associated ecosystems) under high soil inorganic N conditions. Our findings highlight the dependence of plant–microbial interaction for N absorption on both plant mycorrhizal association and soil inorganic N, with the stable convergence of the Nim:PNU ratio under high soil N conditions. [ABSTRACT FROM AUTHOR]

Root exudates are an important pathway for plant–microbial interactions and are highly sensitive to climate change. However, how extreme drought affects root exudates and the main components, as well as species‐specific differences in response magnitude and direction, are poorly understood. In this study, root exudation rates of total carbon (C) and its components (e.g., sugar, organic acid, and amino acid) were measured under the control and extreme drought treatments (i.e., 70% throughfall reduction) by in situ collection of four tree species with different growth rates in a subtropical forest. We also quantified soil properties, root morphological traits, and mycorrhizal infection rates to examine the driving factors underlying variations in root exudation. Our results showed that extreme drought significantly decreased root exudation rates of total C, sugar, and amino acid by 17.8%, 30.8%, and 35.0%, respectively, but increased root exudation rate of organic acid by 38.6%, which were largely associated with drought‐induced changes in tree growth rates, root morphological traits, and mycorrhizal infection rates. Specifically, trees with relatively high growth rates were more responsive to drought for root exudation rates compared with those with relatively low growth rates, which were closely related to root morphological traits and mycorrhizal infection rates. These findings highlight the importance of plant growth strategy in mediating drought‐induced changes in root exudation rates. The coordinations among root exudation rates, root morphological traits, and mycorrhizal symbioses in response to drought could be incorporated into land surface models to improve the prediction of climate change impacts on rhizosphere C dynamics in forest ecosystems. [ABSTRACT FROM AUTHOR]

Tropical forests are among the most productive and vulnerable ecosystems in the planet. Several global forestation programs are aiming to plant millions of trees in tropical regions in the future decade. Mycorrhizal associations are known to largely influence forest soil carbon (C) stocks. However, to date, little is known on whether and how different tree mycorrhizal types affect soil respiration (Rs) and C stocks in tropical forests. In this study, we used a three-decade tropical common garden experiment, with three arbuscular mycorrhizal (AM) and three ectomycorrhizal (EM) monocultures, to investigate the impacts of tree mycorrhizal type on Rs and soil C stocks. Associating biotic (e.g. root biomass, litter dynamic, soil microbes) and abiotic factors (e.g. microclimate) were also measured. Our results showed that AM stands supported significantly higher Rs and soil C stock, litter turnover rate and fine root biomass than EM stands. Further statistical analysis displayed that tree mycorrhizal type was the most important factor in regulating Rs and soil C stock compared with other biotic or abiotic factors. Moreover, we found that mycorrhizal type directly and indirectly affected Rs and soil C stocks via fine root biomass and litter dynamic, i.e. litter production, litter standing crop and litter turnover rate. Our findings highlight important effects of tree mycorrhizal type on forest C cycle, suggesting that planting AM tree species could contribute to promotion of soil C stock in tropical ecosystems. [ABSTRACT FROM AUTHOR]

Subtropical evergreen broadleaf forests (SEBF) are experiencing and expected to suffer more frequent and severe drought events. However, how the hydraulic traits directly link to the mortality and recovery of SEBF trees remains unclear. In this study, we conducted a drought–rewatering experiment on tree seedlings of five dominant species to investigate how the hydraulic traits were related to tree mortality and the resistance and recovery of photosynthesis (A) and transpiration (E) under different drought severities. Species with greater embolism resistance (P50) survived longer than those with a weaker P50. However, there was no general hydraulic threshold associated with tree mortality, with the lethal hydraulic failure varying from 64% to 93% loss of conductance. The photosynthesis and transpiration of tree species with a greater P50 were more resistant to and recovered faster from drought than those with lower P50. Other plant traits could not explain the interspecific variation in tree mortality and drought resistance and recovery. These results highlight the unique importance of embolism resistance in driving carbon and water processes under persistent drought across different trees in SEBFs. The absence of multiple efficient drought strategies in SEBF seedlings implies the difficulty of natural seedling regeneration under future droughts, which often occurs after destructive disturbances (e.g., extreme drought events and typhoon), suggesting that this biome may be highly vulnerable to co‐occurring climate extremes. [ABSTRACT FROM AUTHOR]

Plants and microbes are the primary drivers in affecting the formation and accrual of soil organic carbon (SOC) for natural ecosystems. However, experimental evidence elucidating their underlying mechanisms for SOC accumulation remains elusive. Here, we quantified plant and microbial contributions to SOC accrual in successional subtropical forests by measuring leaf-, root-, and microbial biomarkers, root and leaf litter inputs, and microbial C decomposition. The long-term monitoring results showed that SOC accumulated rapidly at the early-successional stage, but changed little at the mid- and late-successional stages. SOC accrual rate was positively correlated with fine-root production and microbial C turnover, but negatively with annual litterfall. Biomarker data exhibited that the rapid SOC accumulation was jointly driven by root- and microbe-derived C inputs from the early- to mid-successional stages. In contrast, aboveground litterfall considerably contributed to soil C accrual from the mid- to late-successional stages compared to belowground processes, although SOC accumulation is low. Our study revealed the importance of root production and microbial anabolism in SOC accrual at the early stages of forest succession. Incorporating these effects of belowground C inputs on SOC formation and accumulation into earth system models might improve model performance and projection of long-term soil C dynamics. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) is one of the most important components in the global carbon cycle, which is largely influenced by climate and plant traits. Although previous studies have examined the impacts of climatic factors (e.g., mean annual temperature (MAT) and precipitation (MAP)) or plant traits (e.g., leaf area index, leaf nitrogen) on DOC, the relative importance of climate and plant traits on DOC flux remains unclear on a global scale. In this study, we compiled 153 pairs of DOC observational data from 84 forest sites to explore the relative importance of climate and plant traits on DOC flux with a linear mixed model, variance partitioning, and random forest approaches. Our results showed that DOC fluxes from throughfall and the litter layer were higher in broadleaved forests than those in coniferous forests. Throughfall-DOC flux increased significantly with MAT and MAP in coniferous forests, but that from the litter layer showed no significant correlations with climate factors. In broadleaved forests, throughfall-DOC flux increased with potential evapotranspiration (PET), while that from the litter layer was positively correlated with MAT. Meanwhile, throughfall-DOC flux had negative relationships with specific leaf area (SLA), leaf nitrogen content (LN), and leaf phosphorus content (LP) in broadleaved forests, but it showed a positive correlation with SLA in coniferous forests. Litter-layer-DOC flux increased with LN in broadleaved forests, but this correlation was the opposite in coniferous forests. Using the variance partitioning approach, plant traits contributed to 29.0% and 76.4% of the variation of DOC from throughfall and litter layer, respectively, whereas climate only explained 19.1% and 8.3%, respectively. These results indicate that there is a more important contribution by plant traits than by climate in driving the spatial variability of global forest DOC flux, which may help enhance forest management as a terrestrial carbon sink in the future. Our findings suggest the necessity of incorporating plant traits into land surface models for improving predictions regarding the forest carbon cycle. [ABSTRACT FROM AUTHOR]

The long‐term contribution of global forest restoration to support multiple dimensions of biodiversity and ecosystem function remains largely illusive across contrasting climates and forest types. This hampers the capacity to predict the future of forest rewilding under changing global climates. Here, 120 studies are synthesized across five continents, and it is found that forest restoration promotes multiple dimensions of biodiversity and ecosystem function such as soil fertility, plant biomass, microbial habitat, and carbon sequestration across contrasting climates and forest types. Based on global relationship between stand age and soil organic carbon stock, planting 350 million hectares of forest under the UN Bonn Challenge can sequester >30 Gt soil C in the surface 20 cm over the next century. However, these findings also indicate that predicted increases in temperature and reductions in precipitation can constrain the positive effects of forest rewilding on biodiversity and ecosystem function. Further, important tradeoffs are found in very old forests, with considerable disconnection between biodiversity and ecosystem function. Together, these findings provide evidence of the importance of the multidimensional rewilding of forests, suggesting that on‐going climatic changes may dampen the expectations of the positive effects of forest restoration on biodiversity and ecosystem function. [ABSTRACT FROM AUTHOR]

Both biochar and nitrogen (N) addition have been proposed for enhancing plant productivity and increasing carbon (C) sequestration. Although numerous studies have been conducted to examine responses of soil greenhouse gas (GHG) fluxes to biochar or N addition, biochar is often co‐applied with N fertilizer and the interactive effects of the two factors still remain unclear. In this study, we performed a meta‐analysis of manipulative experiments with 267 two‐factor observations to quantify the main and interactive effects of biochar and N addition on soil GHG fluxes at a global scale. Our results showed that biochar addition significantly increased soil CO2 emission by 10.1%, but decreased N2O emission by 14.7%. Meanwhile, N addition increased both soil CO2 and N2O emissions by 11.6% and 288%, respectively. The combination of biochar and N addition also exhibited significant positive effect on CO2 (+18.0%) and N2O (+148%) emissions, but there were non‐significant changes in CH4 fluxes. Consequently, antagonistic interaction between biochar and N addition was observed in soil GHG fluxes and their global warming potential (GWP), except for CH4 uptake showing an additive interaction. This synthesis highlights the importance of the interactive effects between biochar and N addition, providing a quantitative basis to develop sustainable strategies toward widespread application of biochar to preserve cropping system and mitigate climate change. [ABSTRACT FROM AUTHOR]

Coastal wetlands are increasingly recognized as important ecosystems for long-term carbon (C) storage. However, how soil aggregation mediates C accumulation and sequestration in these ecosystems remains unclear. Using the 13C isotope tracer from the invasion of a C4 plant, Spartina alterniflora, into the native ecosystem originally covered by C3 plants across Eastern Chinese coastal wetlands, we investigated a potential C stabilization process via soil structural protection. We quantified changes in soil aggregates, soil organic carbon (SOC), soil total nitrogen (STN), and natural 13C isotope abundance within aggregate fractions across a chronosequence of 0-, 4-, 8-, and 12-year S. alterniflora invasion. Our results showed that soil aggregate stability increased significantly along the chronosequence. Meanwhile, SOC and STN concentrations increased with invasion time in the whole soil and aggregate fractions, which were linked to increasing soil aggregate stability. The contribution of S. alterniflora-derived SOC increased from 18.96 to 40.24% in the 0–20 cm layer and from 4.66 to 32.04% in the 20–40 cm layer across the chronosequence from 4 to 12 years with the highest proportion observed in macro-aggregates. Our results indicate that invasion of S. alterniflora to coastal wetlands can sequester more C largely due to formation and stabilization of soil aggregates by soil structural protection. [ABSTRACT FROM AUTHOR]

Predicting future carbon (C) dynamics in grassland ecosystems requires knowledge of how grazing and global climate change (e.g., warming, elevated CO2, increased precipitation, drought, and N fertilization) interact to influence C storage and release. Here, we synthesized data from 223 grassland studies to quantify the individual and interactive effects of herbivores and climate change on ecosystem C pools and soil respiration (Rs). Our results showed that grazing overrode global climate change factors in regulating grassland C storage and release (i.e., Rs). Specifically, grazing significantly decreased aboveground plant C pool (APCP), belowground plant C pool (BPCP), soil C pool (SCP), and Rs by 19.1%, 6.4%, 3.1%, and 4.6%, respectively, while overall effects of all global climate change factors increased APCP, BPCP, and Rs by 6.5%, 15.3%, and 3.4% but had no significant effect on SCP. However, the combined effects of grazing with global climate change factors also significantly decreased APCP, SCP, and Rs by 4.0%, 4.7%, and 2.7%, respectively but had no effect on BPCP. Most of the interactions between grazing and global climate change factors on APCP, BPCP, SCP, and Rs were additive instead of synergistic or antagonistic. Our findings highlight the dominant effects of grazing on C storage and Rs when compared with the suite of global climate change factors. Therefore, incorporating the dominant effect of herbivore grazing into Earth System Models is necessary to accurately predict climate–grassland feedbacks in the Anthropocene. Dominant effects of grazing on C storage and Rs when compared with the suite of global climate change factors. Most of the interactions between grazing and global climate change factors on C storage and release were additive instead of synergistic or antagonistic. [ABSTRACT FROM AUTHOR]

As a climate change mitigation strategy, biochar application to soil has been demonstrated to increase soil carbon (C) sequestration and reduce greenhouse gas (GHG) emission. Although numerous manipulative studies have been conducted, it is still not fully understood how biochar application affects soil respiration ( Rs ) and its components (i.e., autotrophic [ Ra ] and heterotrophic respiration [ Rh ]) in forest ecosystems, especially in subtropical forests. In this study, we performed a meta-analysis of forest ecosystems and a field experiment with biochar amendments of 0, 10, and 30 t ha −1 in a subtropical forest in Zhejiang, China to examine the effects of biochar application on Rs and its components. Our results showed that biochar application significantly increased Rs by 20.92% at the global scale with an increase of 20.25% in temperate forests and a nonsignificant effect in subtropical forests. Responses of Rs to biochar application varied with experimental methods and soil textures. Similarly, our field experiment showed that biochar amendment did not significantly affect Ra, Rh , and Rs in a subtropical forest in Eastern China. Specifically, the average Rs under biochar amendments of 0, 10, and 30 t ha −1 were 2.37, 2.06 and 2.15 μmol m −2 s −1 , respectively ( P > 0.05). Both Rs and Rh were positively correlated with microbial biomass C (MBC) and negatively with dissolved organic C (DOC). Both apparent temperature sensitivity ( Q 10 ) of Rh and Rs were significantly higher under biochar treatments than in the control. Our findings indicate the importance of the differential effects of biochar application on Rs in different forest types for C sequestration, which may inform ecosystem and regional models to improve prediction of biochar effects on forest C dynamics and climate-biosphere feedbacks. [ABSTRACT FROM AUTHOR]

Biochar application to soils may increase carbon (C) sequestration due to the inputs of recalcitrant organic C. However, the effects of biochar application on the soil greenhouse gas ( GHG) fluxes appear variable among many case studies; therefore, the efficacy of biochar as a carbon sequestration agent for climate change mitigation remains uncertain. We performed a meta-analysis of 91 published papers with 552 paired comparisons to obtain a central tendency of three main GHG fluxes (i.e., CO2, CH4, and N2O) in response to biochar application. Our results showed that biochar application significantly increased soil CO2 fluxes by 22.14%, but decreased N2O fluxes by 30.92% and did not affect CH4 fluxes. As a consequence, biochar application may significantly contribute to an increased global warming potential ( GWP) of total soil GHG fluxes due to the large stimulation of CO2 fluxes. However, soil CO2 fluxes were suppressed when biochar was added to fertilized soils, indicating that biochar application is unlikely to stimulate CO2 fluxes in the agriculture sector, in which N fertilizer inputs are common. Responses of soil GHG fluxes mainly varied with biochar feedstock source and soil texture and the pyrolysis temperature of biochar. Soil and biochar pH, biochar applied rate, and latitude also influence soil GHG fluxes, but to a more limited extent. Our findings provide a scientific basis for developing more rational strategies toward widespread adoption of biochar as a soil amendment for climate change mitigation. [ABSTRACT FROM AUTHOR]

Livestock grazing activities potentially alter ecosystem carbon (C) and nitrogen (N) cycles in grassland ecosystems. Despite the fact that numerous individual studies and a few meta-analyses had been conducted, how grazing, especially its intensity, affects belowground C and N cycling in grasslands remains unclear. In this study, we performed a comprehensive meta-analysis of 115 published studies to examine the responses of 19 variables associated with belowground C and N cycling to livestock grazing in global grasslands. Our results showed that, on average, grazing significantly decreased belowground C and N pools in grassland ecosystems, with the largest decreases in microbial biomass C and N (21.62% and 24.40%, respectively). In contrast, belowground fluxes, including soil respiration, soil net N mineralization and soil N nitrification increased by 4.25%, 34.67% and 25.87%, respectively, in grazed grasslands compared to ungrazed ones. More importantly, grazing intensity significantly affected the magnitude (even direction) of changes in the majority of the assessed belowground C and N pools and fluxes, and C : N ratio as well as soil moisture. Specifically,light grazing contributed to soil C and N sequestration whereas moderate and heavy grazing significantly increased C and N losses. In addition, soil depth, livestock type and climatic conditions influenced the responses of selected variables to livestock grazing to some degree. Our findings highlight the importance of the effects of grazing intensity on belowground C and N cycling, which may need to be incorporated into regional and global models for predicting effects of human disturbance on global grasslands and assessing the climate-biosphere feedbacks. [ABSTRACT FROM AUTHOR]

As the second largest carbon (C) flux between the atmosphere and terrestrial ecosystems, soil respiration (Rs) plays vital roles in regulating atmospheric CO2 concentration ([CO2]) and climatic dynamics in the earth system. Although numerous manipulative studies and a few meta-analyses have been conducted to determine the responses of Rs and its two components [i.e., autotrophic (Ra) and heterotrophic (Rh) respiration] to single global change factors, the interactive effects of the multiple factors are still unclear. In this study, we performed a meta-analysis of 150 multiple-factor (≥2) studies to examine the main and interactive effects of global change factors on Rs and its two components. Our results showed that elevated [ CO2] (E), nitrogen addition (N), irrigation (I), and warming (W) induced significant increases in Rs by 28.6%, 8.8%, 9.7%, and 7.1%, respectively. The combined effects of the multiple factors, EN, EW, DE, IE, IN, IW, IEW, and DEW, were also significantly positive on Rs to a greater extent than those of the single-factor ones. For all the individual studies, the additive interactions were predominant on Rs (90.6%) and its components (≈70.0%) relative to synergistic and antagonistic ones. However, the different combinations of global change factors (e.g., EN, NW, EW, IW) indicated that the three types of interactions were all important, with two combinations for synergistic effects, two for antagonistic, and five for additive when at least eight independent experiments were considered. In addition, the interactions of elevated [ CO2] and warming had opposite effects on Ra and Rh, suggesting that different processes may influence their responses to the multifactor interactions. Our study highlights the crucial importance of the interactive effects among the multiple factors on Rs and its components, which could inform regional and global models to assess the climate-biosphere feedbacks and improve predictions of the future states of the ecological and climate systems. [ABSTRACT FROM AUTHOR]

Roots and the associated rhizospheric activities regulate the mineralization of native soil organic matter (SOM), which is referred to as the rhizosphere priming effect (RPE). Although the importance of RPE for carbon cycle has increasingly been recognized, experimental evidence for how soil structural changes modulate the RPE is still unavailable. We addressed this issue by growing soybean plants (C 3) in a C 4 -derived soil in a continuous 13C- labeling greenhouse. We hypothesized that root-induced soil structural change regulated the RPE by destabilizing soil matrix-protected organic carbon. Our results showed that the RPE was tightly coupled with plant photosynthetic activity, the disruption of coarse macro-aggregates, and the increased release of dissolved organic carbon (DOC) from the soil matrix. These findings indicate that living roots together with rhizodeposits not only can directly stimulate rhizospheric microbial activities, but also can make soil matrix-protected organic carbon available to microbial attacks and further enhance the RPE. This study suggests that the RPE on SOM mineralization is intimately linked with the dynamics of soil structures and DOC, which should be considered in future studies on mechanistic understanding and modeling of the RPE. • Belowground respiration was tightly coupled with aboveground photosynthesis. • Soil structural changes modulated the rhizosphere priming effect. • Rhizosphere processes stimulated DOC release originated from soil organic matter. [ABSTRACT FROM AUTHOR]

To improve models for accurate projections, data assimilation, an emerging statistical approach to combine models with data, have recently been developed to probe initial conditions, parameters, data content, response functions and model uncertainties. Quantifying how many information contents are contained in different data streams is essential to predict future states of ecosystems and the climate. This study uses a data assimilation approach to examine the information contents contained in flux- and biometric-based data to constrain parameters in a terrestrial carbon (C) model, which includes canopy photosynthesis and vegetation–soil C transfer submodels. Three assimilation experiments were constructed with either net ecosystem exchange (NEE) data only or biometric data only [including foliage and woody biomass, litterfall, soil organic C (SOC) and soil respiration], or both NEE and biometric data to constrain model parameters by a probabilistic inversion application. The results showed that NEE data mainly constrained parameters associated with gross primary production (GPP) and ecosystem respiration (RE) but were almost invalid for C transfer coefficients, while biometric data were more effective in constraining C transfer coefficients than other parameters. NEE and biometric data constrained about 26% (6) and 30% (7) of a total of 23 parameters, respectively, but their combined application constrained about 61% (14) of all parameters. The complementarity of NEE and biometric data was obvious in constraining most of parameters. The poor constraint by only NEE or biometric data was probably attributable to either the lack of long-term C dynamic data or errors from measurements. Overall, our results suggest that flux- and biometric-based data, containing different processes in ecosystem C dynamics, have different capacities to constrain parameters related to photosynthesis and C transfer coefficients, respectively. Multiple data sources could also reduce uncertainties in parameter estimation if these data sources contain complementary information. [ABSTRACT FROM PUBLISHER]

Global climate change has greatly accelerated hydrological processes, causing significant increases in frequency and intensity of drought events, which may have a great impact on soil carbon (C) dynamics. Microbial residues are frequently represented as the important constituents involved in soil C formation and stability. However, how persistent drought influences microbial residues to regulate soil C cycling remains elusive, especially in forest ecosystems. In this study, we investigated drought effects on microbial (fungal and bacterial) residues and soil C dynamics by a 7-year filed experiment (2013−2020) with a 70 % rainfall reduction in a subtropical evergreen forest of Eastern China. Soil samples (0–10 cm) were collected in 2014, 2015, 2016, and 2020 with drought duration of 1, 2, 3, and 7 years, and soil moisture, soil temperature, soil organic carbon (SOC) and soil nitrogen (N) were measured. The contents of microbial (fungal and bacterial) residues were quantified by amino sugar biomarkers. Our results showed that fungal residues significantly decreased with increasing drought duration, which were mainly induced by the decline of soil N and soil moisture, causing a decrease in soil C under the 7-year persistent drought. In contrast, bacterial residues increased with drought duration due to fast adaption of bacteria to drought and aggregate protection. The retention of bacterial residues might be preserved as a "nutrient reservoir" to cope with the long-term drought in the near future. These results suggest that persistent drought-induced tradeoffs between fungal and bacterial residues mediate soil C dynamics. Fungal residues were the essential prerequisite for soil C sequestration in response to drought, while bacterial residues may mainly regulate soil C cycling under the prolonged drought. Therefore, microbial residues are imperative to understand the responses of microbial-derived C to drought, which could be integrated into terrestrial C models to accurately predict soil C dynamics in unpredictable climate regimes in forest ecosystems. • Drought reduced soil nutrients and moisture, resulting in decreased fungal residues. • Fungal residues significantly decreased with drought duration. • Bacterial residues increased with drought duration due to fast adaption to drought. • Bacterial residues preserved as a "nutrient reservoir" under persistent drought. [ABSTRACT FROM AUTHOR]

Root exudates determine plant's ability to acquire nutrients through influencing plant's interactions with soil microorganisms. Recent studies suggest that plant's associations with beneficial soil microorganisms explain variation in root exudation as plants opt to minimize the exudation cost through such symbiosis. Yet, we have a poor understanding of whether plants change their exudation rates through mycorrhizal symbiosis in soil environments with varying resource availability. Here, we report the effects of plant-mycorrhizal symbiosis on root exudation rates across a gradient of soil phosphorous (P) availability from a field experiment in subtropical forests. Root exudation rates were higher in plants partnering with arbuscular mycorrhizal fungi than those with ectomycorrhizal fungi, but this difference disappeared in soils with high P. Specific root surface area, specific root length and fine root vitality explained high root exudation in P-limited soils. These findings demonstrate that mycorrhizal symbiosis and root functional traits collectively determine the variation in root exudation in P-limited environments. • Root exudation rates were greater in AM trees than in ECM trees. • Root exudation rate decreased with soil P availability in sub-tropical forests. • Root morphology and vitality were the main drivers of root exudation rates. [ABSTRACT FROM AUTHOR]

• Treatments with nitrogen addition and/or irrigation markedly decreased root/shoot. • The synergistic interactions were more on AGB than BGB and root/shoot. • Variation of root/shoot was mainly caused by AGB for trees, but BGB for herbs. Knowledge of plant aboveground and belowground biomass (AGB and BGB) allocation is fundamental for our understanding of terrestrial carbon sequestration in a changing climate. However, how multiple global change factors interactively affect biomass allocation in terrestrial ecosystems remains unclear. We used meta-analysis to synthesize main and interactive effects of global change factors on AGB, BGB, and root/shoot based on 129 multiple-factor studies. Elevated CO 2 (E), nitrogen addition (N), warming (W), irrigation (I) and their combinations (EN, EW, NW, ENW, IE, IN, IW, IEN, INW and IENW) significantly increased AGB. However only half of the treatments (i.e., E, N, W, EN, EW, NW, IE and IW) stimulated BGB, leading to significant declines of root/shoot in treatments with I and/or N. Drought (D) significantly decreased both total biomass (14%) and AGB (47%), but increased root/shoot by 21% as well as DE and DW. Additive interactions between global change factors exhibited a predominance on both plant biomass (69.0%) and biomass allocation (64.8%). The proportion of synergistic interaction in AGB's responses to multiple global change factors was greater relative to that in BGB. Response correlation between AGB and root/shoot was observed in woody plants, while, in herbaceous ones, we found the correlation between BGB and root/shoot. Our findings highlight the importance of the interactive effects among global change factors on biomass allocation. Incorporating these interactions into global vegetation models may improve predictions of future global carbon storage and could inform sustainable strategies for grassland and plantation management in a future climate. [ABSTRACT FROM AUTHOR]

Multiple lines of existing evidence suggest that increasing CO 2 emission from soils in response to rising temperature could accelerate global warming. However, in experimental studies, the initial positive response of soil heterotrophic respiration (R H ) to warming often weakens over time (referred to apparent thermal acclimation). If the decreased R H is driven by thermal adaptation of soil microbial community, the potential for soil carbon (C) losses would be reduced substantially. In the meanwhile, the response could equally be caused by substrate depletion, and would then reflect the gradual loss of soil C. To address uncertainties regarding the causes of apparent thermal acclimation, we carried out sterilization and inoculation experiments using the soil samples from an alpine meadow with 6 years of warming and nitrogen (N) addition. We demonstrate that substrate depletion, rather than microbial adaptation, determined the response of R H to long-term warming. Furthermore, N addition appeared to alleviate the apparent acclimation of R H to warming. Our study provides strong empirical support for substrate availability being the cause of the apparent acclimation of soil microbial respiration to temperature. Thus, these mechanistic insights could facilitate efforts of biogeochemical modeling to accurately project soil C stocks in the future climate., (© 2022 John Wiley & Sons Ltd.)