Long-Term Alpine Plant Responses to Global Change Drivers Depend on Functional Traits.

Forecasting plant responses under global change is a critical but challenging endeavour. Despite seemingly idiosyncratic responses of species to global change, greater generalisation of 'winners' and 'losers' may emerge from considering how species functional traits influence responses and how these responses scale to the community level. Here, we synthesised six long-term global change experiments combined with locally measured functional traits. We quantified the change in abundance and probability of establishment through time for 70 alpine plant species and then assessed if leaf and stature traits were predictive of species and community responses across nitrogen addition, snow addition and warming treatments. Overall, we found that plants with more resource-acquisitive trait strategies increased in abundance but each global change factor was related to different functional strategies. Nitrogen addition favoured species with lower leaf nitrogen, snow addition favoured species with cheaply constructed leaves and warming showed few consistent trends. Community-weighted mean changes in trait values in response to nitrogen addition, snow addition and warming were often different from species-specific trait effects on abundance and establishment, reflecting in part the responses and traits of dominant species. Together, these results highlight that the effects of traits can differ by scale and response of interest.

Fast-slow traits predict competition network structure and its response to resources and enemies.

Plants interact in complex networks but how network structure depends on resources, natural enemies and species resource-use strategy remains poorly understood. Here, we quantified competition networks among 18 plants varying in fast-slow strategy, by testing how increased nutrient availability and reduced foliar pathogens affected intra- and inter-specific interactions. Our results show that nitrogen and pathogens altered several aspects of network structure, often in unexpected ways due to fast and slow growing species responding differently. Nitrogen addition increased competition asymmetry in slow growing networks, as expected, but decreased it in fast growing networks. Pathogen reduction made networks more even and less skewed because pathogens targeted weaker competitors. Surprisingly, pathogens and nitrogen dampened each other's effect. Our results show that plant growth strategy is key to understand how competition respond to resources and enemies, a prediction from classic theories which has rarely been tested by linking functional traits to competition networks.

Pseudomonas fluorescens Inoculation Enhances Salix matsudana Growth by Modifying Phyllosphere Microbiomes, Surpassing Nitrogen Fertilization.

The enhancement of plant growth by soil fertilization and microbial inoculation involves different mechanisms, particularly by altering the phyllosphere microbiome. This study investigated how nitrogen (N) fertilization, Pseudomonas fluorescens strain R124 inoculation and their combined effects influence the growth of different-aged Salix matsudana cuttings by modulating N dynamics within the phyllosphere microbiome. Results showed that P. fluorescens inoculation was significantly more effective than N fertilization alone, enhancing biomass, plant nutrient uptake, soil nutrient content and root development by 90.51%, 18.18%, 72.74% and 126.20%, respectively. Crucially, the inoculation notably shifted the beta-diversity of the phyllosphere microbial community, with K-strategy fungi enhancing plant N fixation and subsequent plant growth. Cuttings from middle-aged forests displayed more robust growth than those from young-aged, associated with a varied impact on phyllosphere fungi, notably increasing the relative abundance of Myriangiales in young (76.37%) and Capnodiales in middle-aged cuttings (42.37%), which improve phyllosphere stability and plant health. These findings highlight the effectiveness of microbial inoculation over N fertilization in promoting plant growth and provide valuable insights for the sustainable management of willow plantations at different stages of development.

Canopy nitrogen deposition enhances soil ecosystem multifunctionality in a temperate forest.

Nitrogen (N) deposition affects ecosystem functions crucial to human health and well-being. However, the consequences of this scenario for soil ecosystem multifunctionality (SMF) in forests are poorly understood. Here, we conducted a long-term field experiment in a temperate forest in China, where N deposition was simulated by adding N above and under the canopies. We discover that canopy N addition promotes SMF expression, whereas understory N addition suppresses it. SMF was regulated by fungal diversity in canopy N addition treatments, which is largely due to the strong resistance to soil acidification and efficient resource utilization characteristics of fungi. While in understory N addition treatments, SMF is regulated by bacterial diversity, which is mainly because of the strong resilience to disturbances and fast turnover of bacteria. Furthermore, rare microbial taxa may play a more important role in the maintenance of the SMF. This study provides the first evidence that N deposition enhanced SMF in temperate forests and enriches the knowledge on enhanced N deposition affecting forest ecosystems. Given the divergent results from two N addition approaches, an innovative perspective of canopy N addition on soil microbial diversity-multifunctionality relationships is crucial to policy-making for the conservation of soil microbial diversity and sustainable ecosystem management under enhanced N deposition. In future research, the consideration of canopy N processes is essential for more realistic assessments of the effects of atmospheric N deposition in forests.

Shifts in soil ammonia-oxidizing community maintain the nitrogen stimulation of nitrification across climatic conditions.

Anthropogenic nitrogen (N) loading alters soil ammonia-oxidizing archaea (AOA) and bacteria (AOB) abundances, likely leading to substantial changes in soil nitrification. However, the factors and mechanisms determining the responses of soil AOA:AOB and nitrification to N loading are still unclear, making it difficult to predict future changes in soil nitrification. Herein, we synthesize 68 field studies around the world to evaluate the impacts of N loading on soil ammonia oxidizers and nitrification. Across a wide range of biotic and abiotic factors, climate is the most important driver of the responses of AOA:AOB to N loading. Climate does not directly affect the N-stimulation of nitrification, but does so via climate-related shifts in AOA:AOB. Specifically, climate modulates the responses of AOA:AOB to N loading by affecting soil pH, N-availability and moisture. AOB play a dominant role in affecting nitrification in dry climates, while the impacts from AOA can exceed AOB in humid climates. Together, these results suggest that climate-related shifts in soil ammonia-oxidizing community maintain the N-stimulation of nitrification, highlighting the importance of microbial community composition in mediating the responses of the soil N cycle to N loading.

Depth-dependent responses of soil organic carbon under nitrogen deposition.

Emerging evidence points out that the responses of soil organic carbon (SOC) to nitrogen (N) addition differ along the soil profile, highlighting the importance of synthesizing results from different soil layers. Here, using a global meta-analysis, we found that N addition significantly enhanced topsoil (0-30 cm) SOC by 3.7% (±1.4%) in forests and grasslands. In contrast, SOC in the subsoil (30-100 cm) initially increased with N addition but decreased over time. The model selection analysis revealed that experimental duration and vegetation type are among the most important predictors across a wide range of climatic, environmental, and edaphic variables. The contrasting responses of SOC to N addition indicate the importance of considering deep soil layers, particularly for long-term continuous N deposition. Finally, the lack of depth-dependent SOC responses to N addition in experimental and modeling frameworks has likely resulted in the overestimation of changes in SOC storage under enhanced N deposition.

Multi-ecosystem services differently affected by over-canopy and understory nitrogen additions in a typical subtropical forest.

Obtaining a holistic understanding of the impacts of atmospheric nitrogen deposition on multiple ecosystem services of forest is essential for developing comprehensive and sustainable strategies, particularly in heavy N deposition regions such as subtropical China. However, such impacts remain incompletely understood, with most previous studies focus on individual ecosystem function or service via understory N addition experiments. To address this knowledge gap, we quantified the effects of over-canopy and understory N additions on multiple ecosystem services based on a 7-year large-scale field experiment in a typical subtropical forest. Our results showed continued over-canopy N addition with 50 kg ha-1 year-1 over a period of 4-7 years significantly increased plant nutrient retention, but did not affect the services of soil nutrient accumulation, water yield, C sequestration (in plants and soil), or oxygen release. There were trade-offs between the soil and plant on providing the services of nutrient accumulation/retention and C sequestration under over-canopy N addition. However, without uptake and retention of tree canopy, the trade-off between soil and plant were more weaken under the understory N addition with 50 kg ha-1 year-1 , and their relationships were even synergetic under the understory N addition with 25 kg ha-1 year-1 . The results suggest that understory N addition cannot accurately simulate the effects of atmospheric N deposition on multiple services, along with mutual relationships. Interestingly, the services of plant N, P retention, and C sequestration exhibited a synergetic increase under the over-canopy N addition but a decrease under the understory N addition. Our results also found tree layer plays a primary role in providing plant nutrient retention service and is sensitive to atmospheric N deposition. Further studies are needed to investigate the generalized effects of forest canopy processes on alleviating the threaten of global change factors in different forest ecosystems.

Canopy and understory nitrogen additions differently affect soil microbial residual carbon in a temperate forest.

Atmospheric nitrogen (N) deposition in forests can affect soil microbial growth and turnover directly through increasing N availability and indirectly through altering plant-derived carbon (C) availability for microbes. This impacts microbial residues (i.e., amino sugars), a major component of soil organic carbon (SOC). Previous studies in forests have so far focused on the impact of understory N addition on microbes and microbial residues, but the effect of N deposition through plant canopy, the major pathway of N deposition in nature, has not been explicitly explored. In this study, we investigated whether and how the quantities (25 and 50 kg N ha-1 year-1) and modes (canopy and understory) of N addition affect soil microbial residues in a temperate broadleaf forest under 10-year N additions. Our results showed that N addition enhanced the concentrations of soil amino sugars and microbial residual C (MRC) but not their relative contributions to SOC, and this effect on amino sugars and MRC was closely related to the quantities and modes of N addition. In the topsoil, high-N addition significantly increased the concentrations of amino sugars and MRC, regardless of the N addition mode. In the subsoil, only canopy N addition positively affected amino sugars and MRC, implying that the indirect pathway via plants plays a more important role. Neither canopy nor understory N addition significantly affected soil microbial biomass (as represented by phospholipid fatty acids), community composition and activity, suggesting that enhanced microbial residues under N deposition likely stem from increased microbial turnover. These findings indicate that understory N addition may underestimate the impact of N deposition on microbial residues and SOC, highlighting that the processes of canopy N uptake and plant-derived C availability to microbes should be taken into consideration when predicting the impact of N deposition on the C sequestration in temperate forests.

Microbial life-history strategies mediate microbial carbon pump efficacy in response to N management depending on stoichiometry of microbial demand.

The soil microbial carbon pump (MCP) is increasingly acknowledged as being directly linked to soil organic carbon (SOC) accumulation and stability. Given the close coupling of carbon (C) and nitrogen (N) cycles and the constraints imposed by their stoichiometry on microbial growth, N addition might affect microbial growth strategies with potential consequences for necromass formation and carbon stability. However, this topic remains largely unexplored. Based on two multi-level N fertilizer experiments over 10 years in two soils with contrasting soil fertility located in the North (Cambisol, carbon-poor) and Southwest (Luvisol, carbon-rich), we hypothesized that different resource demands of microorganism elicit a trade-off in microbial growth potential (Y-strategy) and resource-acquisition (A-strategy) in response to N addition, and consequently on necromass formation and soil carbon stability. We combined measurements of necromass metrics (MCP efficacy) and soil carbon stability (chemical composition and mineral associated organic carbon) with potential changes in microbial life history strategies (assessed via soil metagenomes and enzymatic activity analyses). The contribution of microbial necromass to SOC decreased with N addition in the Cambisol, but increased in the Luvisol. Soil microbial life strategies displayed two distinct responses in two soils after N amendment: shift toward A-strategy (Cambisol) or Y-strategy (Luvisol). These divergent responses are owing to the stoichiometric imbalance between microbial demands and resource availability for C and N, which presented very distinct patterns in the two soils. The partial correlation analysis further confirmed that high N addition aggravated stoichiometric carbon demand, shifting the microbial community strategy toward resource-acquisition which reduced carbon stability in Cambisol. In contrast, the microbial Y-strategy had the positive direct effect on MCP efficacy in Luvisol, which greatly enhanced carbon stability. Such findings provide mechanistic insights into the stoichiometric regulation of MCP efficacy, and how this is mediated by site-specific trade-offs in microbial life strategies, which contribute to improving our comprehension of soil microbial C sequestration and potential optimization of agricultural N management.

Eastern Canadian boreal forest soil and foliar chemistry show evidence of resilience to long-term nitrogen addition.

The boreal forest is one of the world's largest terrestrial biome and plays crucial roles in global biogeochemical cycles, such as carbon (C) sequestration in vegetation and soil. However, the impacts of decades of N deposition on N-limited ecosystems, like the eastern Canadian boreal forest, remain unclear. For 13 years, N deposition was simulated by periodically adding ammonium nitrate on soils of two boreal coniferous forests (i.e., balsam fir and black spruce) of eastern Canada, at low (LN) and high (HN) rates, corresponding to 3 and 10 times the ambient N deposition, respectively. We show that more than a decade of N addition had no strong effects on mineral soil C, N, P, and cation concentrations and on foliar total Ca, K, Mg, and Mn concentrations. In organic soil, C stock was not affected by N addition while N stock increased, and exchangeable Ca2+ and Mg2+ decreased at the balsam fir site under HN treatment. At both sites, LN treatment had nearly no impact on foliage and soil chemistry but foliar N and N:P significantly increased under HN treatment, potentially leading to foliar nutrient imbalance. Overall, our work indicates that, in the eastern Canadian boreal forest, soil and foliar nutrient concentrations and stocks are resilient to increasing N deposition potentially because, in the context of N limitation, extra N would be rapidly immobilized by soil micro-organisms and vegetation. These findings could improve modeling future boreal forest soil C stocks and biomass growth and could help in planning forest management strategies in eastern Canada.

Cooling has stimulated soil carbon storage in forest ecosystems.

The interactive effect of soil cooling and nitrogen (N) addition can accurately simulate climatic and anthropogenic effects on terrestrial and other land-based ecosystems, but direct empirical measurements on the effects of cooling and N addition on soil carbon (C) and N are lacking. Hence, transplanting soils into colder regions was used to evaluate the effects of cooling and N addition on soil C and N. We used PVCs of 30 cm in height and 8 cm in diameter to extract soil samples. Soil C and N were significantly (P < 0.05) increased by transplanting soils into colder regions. In contrast, cooling has insignificantly (P > 0.05) increased the soil dissolved organic C (DOC) and dissolved organic (DON), but the effect was negatively significant on soil pH compared to the C/N ratio. Similarly, N addition significantly increased the measured soil N stock. However, the effect was negatively significant on soil pH (P < 0.05) compared to the C/N ratio (P > 0.05). Nevertheless, the interaction of cooling and N addition did not affect the soil C and N storage. A similar effect was observed on the soil DOC and DON. The results presented here illustrate that transplanting soils into colder regions and N deposition has perfectly simulated the effects of climate-forcing factors on soil C and N storage in terrestrial and other land-based ecosystems. Accordingly, this study suggests that low temperatures have stimulated the accumulation of the measured soil organic and dissolved properties, but the effect is less consequential when low temperature interacts with N addition in high-elevation areas where ecosystem structures and functions are limited by temperature and may serve as a baseline for future research on land feedbacks to the climate system.

Contribution of mycorrhizal symbiosis and root strategy to red clover aboveground biomass under nitrogen addition and phosphorus distribution.

Soil nutrients exhibit heterogeneity in their spatial distribution, presenting challenges to plant acquisition. Notably, phosphorus (P) heterogeneity is a characteristic feature of soil, necessitating the development of adaptive strategies by plants to cope with this phenomenon. To address this, fully crossed three-factor experiments were conducted using red clover within rhizoboxes. Positions of P in three conditions, included P even distribution (even P), P close distribution (close P), and P far distribution (far P). Concurrently, N addition was two amounts(0 and 20 mg kg- 1), both with and without AMF inoculation. The findings indicated a decrease in aboveground biomass attributable to uneven P distribution, whereas N and AMF demonstrated the potential to affect aboveground biomass. In a structural equation model, AMF primarily increased aboveground biomass by enhancing nodule number and specific leaf area (SLA). In contrast, N addition improved aboveground biomass through increased nodule number or direct effects. Subsequently, a random forest model indicated that under the far P treatment, fine root length emerged as the primary factor affecting aboveground biomass, followed by thickest root length. Conversely, in the even P treatment, the thickest root length was of paramount importance. In summary, when confronted with uneven P distribution, clover plants adopted various root foraging strategies. AMF played a pivotal role in elevating nodule number, and SLA.

Differential responses of soil phosphorus fractions to varied nitrogen compound additions in a meadow steppe.

Nitrogen (N) addition can greatly influence soil inorganic phosphorus (Pi) and organic phosphorus (Po) transformations. However, whether and how the N compound forms may differentially affect the soil P fractions remain unclear. Here, we investigated the responses of soil Pi (labile Pi, moderately-occluded Pi, and recalcitrant Pi) and Po fractions (labile Po and stable Po) to varying addition rates of three N compounds ((NH4)2SO4, NH4NO3, and urea) in a meadow steppe in northern China. Our studies revealed that with increasing N addition rate, soil labile and moderately-occluded Pi increased, accompanied by decreases in soil recalcitrant Pi. This shift was attributed to N-induced soil acidification, which accelerated the conversion of recalcitrant Pi into labile and moderately-occluded Pi. Soil labile Po decreased with increasing rate of N addition, whilst soil stable Po was not affected. Regardless of the compound forms, N addition increased soil Olsen-P, suggesting a potential alleviation of P limitation in this grassland ecosystem. The effect of N addition on soil labile Pi was significantly greater with addition of urea than with addition of either (NH4)2SO4 or NH4NO3, indicating that urea was more efficient in enhancing soil P availability. Addition of (NH4)2SO4 imposed a more pronounced positive effect on soil moderately-occluded Pi than the addition of either NH4NO3 or urea, mainly due to the greater mobilization of recalcitrant Pi as a result of higher soil acidification strength of (NH4)2SO4. These findings underscore the importance of considering the distinct effects of different N compounds when studying grassland soil P dynamics and availability in response to N addition.

Canopy nitrogen addition and understory removal destabilize the microbial community in a subtropical Chinese fir plantation.

Subtropical Chinese fir plantations have been experiencing increased nitrogen deposition and understory management because of human activities. Nevertheless, effect of increased nitrogen deposition and understory removal in the plantations on microbial community stability and the resulting consequences for ecosystem functioning is still unclear. We carried out a 5-year experiment of canopy nitrogen addition (2.5 g N m-2 year-1), understory removal, and their combination to assess their influences on microbial community stability and functional potentials in a subtropical Chinese fir plantation. Nitrogen addition, understory removal, and their combination reduced soil bacterial diversity (OUT richness, Inverse Simpson index, Shannon index, and phylogenetic diversity) by 11-18%, 15-24%, and 19-31%; reduced fungal diversity indexes by 3-5%, 5-6%, and 5-7%, respectively. We found that environmental filtering and interspecific interactions together determined changes in bacterial community stability, while changes in fungal community stability were mainly caused by environmental filtering. Fungi were more stable than bacteria under disturbances, possibly from having a more stable network structure. Furthermore, we found that microbial community stability was linked to changes in microbial community functional potentials. Importantly, we observed synergistic interactions between understory removal and nitrogen addition on bacterial diversity, network structure, and community stability. These findings suggest that understory plants play a significant role in promoting soil microbial community stability in subtropical Chinese fir plantations and help to mitigate the negative impacts of nitrogen addition. Hence, it is crucial to retain understory vegetation as important components of subtropical plantations.

Climate warming interacts with other global change drivers to influence plant phenology: A meta-analysis of experimental studies.

Shifts in plant phenology influence ecosystem structures and functions, yet how multiple global change drivers interact to affect phenology remains elusive. We conducted a meta-analysis of 242 published articles to assess interactions between warming (W) and other global change drivers including nitrogen addition (N), increased precipitation (IP), decreased precipitation (DP) and elevated CO2 (eCO2 ) on multiple phenophases in experimental studies. We show that leaf out and first flowering were most strongly affected by warming, while warming and decreased precipitation were the most pronounced drivers for leaf colouring. Moreover, interactions between warming and other global change drivers were common and both synergistic and antagonistic interactions were observed: interactions W + IP and W + eCO2 were frequently synergistic, whereas interactions W + N and W + DP were mostly antagonistic. These findings demonstrate that global change drivers often affect plant phenology interactively. Incorporating the multitude of interactions into models is crucial for accurately predicting plant responses to global changes.

Arbuscular mycorrhizal fungi benefit plants in response to major global change factors.

Land plants play a key role in global carbon cycling, but the potential role of arbuscular mycorrhizal fungi (AMF) in the responses of a wide range of plant species to global change factors (GCFs) remains limited. Based on 1100 paired observations from 181 plant species, we conducted a meta-analysis to test the role of AMF in plant responses to four GCFs: drought, warming, nitrogen (N) addition and elevated CO2 . We show that AMF significantly ameliorate the negative effects of drought on plant performance. The GCFs N addition and elevated CO2 significantly enhance the performance of AM plants but not of non-inoculated plants. AM plants show better performance than their non-inoculated counterparts under warming, although neither of them showed a significant response to this GCF. These results suggest that AMF benefit plants in response to GCFs. Our study highlights the importance of AMF in enhancing plant performance under ongoing global change.

Effects of nitrogen deposition on carbon and nutrient cycling along a natural soil acidity gradient as revealed by metagenomics.

Nitrogen (N) deposition and soil acidification are environmental challenges affecting ecosystem functioning, health, and biodiversity, but their effects on functional genes are poorly understood. Here, we utilized metabarcoding and metagenomics to investigate the responses of soil functional genes to N deposition along a natural soil pH gradient. Soil N content was uncorrelated with pH, enabling us to investigate their effects separately. Soil acidity strongly and negatively affected the relative abundances of most cluster of orthologous gene categories of the metabolism supercategory. Similarly, soil acidity negatively affected the diversity of functional genes related to carbon and N but not phosphorus cycling. Multivariate analyses showed that soil pH was the most important factor affecting microbial and functional gene composition, while the effects of N deposition were less important. Relative abundance of KEGG functional modules related to different parts of the studied cycles showed variable responses to soil acidity and N deposition. Furthermore, our results suggested that the diversity-function relationship reported for other organisms also applies to soil microbiomes. Since N deposition and soil pH affected microbial taxonomic and functional composition to a different extent, we conclude that N deposition effects might be primarily mediated through soil acidification in forest ecosystems.

Apparent thermal acclimation of soil heterotrophic respiration mainly mediated by substrate availability.

Multiple lines of existing evidence suggest that increasing CO2 emission from soils in response to rising temperature could accelerate global warming. However, in experimental studies, the initial positive response of soil heterotrophic respiration (RH ) to warming often weakens over time (referred to apparent thermal acclimation). If the decreased RH 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 RH to long-term warming. Furthermore, N addition appeared to alleviate the apparent acclimation of RH 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.

The potential bias of nitrogen deposition effects on primary productivity and biodiversity.

Atmospheric nitrogen (N) deposition is composed of both inorganic nitrogen (IN) and organic nitrogen (ON), and these sources of N may exhibit different impacts on ecosystems. However, our understanding of the impacts of N deposition is largely based on experimental gradients of INs or more rarely ONs. Thus, the effects of N deposition on ecosystem productivity and biodiversity may be biased. We explored the differential impacts of N addition with different IN:ON ratios (0:10, 3:7, 5:5, 7:3, and 10:0) on aboveground net primary productivity (ANPP) of plant community and plant diversity in a typical temperate grassland with a long-term N addition experiment. Soil pH, litter biomass, soil IN concentration, and light penetration were measured to examine the potential mechanisms underlying species loss with N addition. Our results showed that N addition significantly increased plant community ANPP by 68.33%-105.50% and reduced species richness by 16.20%-37.99%. The IN:ON ratios showed no significant effects on plant community ANPP. However, IN-induced species richness loss was about 2.34 times of ON-induced richness loss. Soil pH was positively related to species richness, and they exhibited very similar response patterns to IN:ON ratios. It implies that soil acidification accounts for the different magnitudes of species loss with IN and ON additions. Overall, our study suggests that it might be reasonable to evaluate the effects of N deposition on plant community ANPP with either IN or ON addition. However, the evaluation of N deposition on biodiversity might be overestimated if only IN is added or underestimated if only ON is added.

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

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