Accelerated terrestrial ecosystem carbon turnover and its drivers

Unprotected carbon dominates decadal soil carbon increase

Changes in the distribution of soil organic carbon (SOC) fractions - particulate organic carbon (POC; unprotected carbon) vs. mineral-associated organic carbon (MAOC; protected carbon) - affect SOC storage and stability. Here, we compile a SOC fraction dataset from 7219 soil samples across six continents. From 2000 to 2022, POC increases by 21.8% while MAOC decreases by 5.3%, leading to a net gain of 11.0% in SOC storage and a 29.1% increase in the POC/MAOC ratio. However, relative to undisturbed natural ecosystems, SOC declines in planted forests, grazed grasslands, and croplands with management practices such as heavy grazing and conventional tillage, primarily due to rapid declines in POC. Our results highlight that the global increase in SOC is mainly driven by POC, but warn of decreasing SOC stability associated with climate change and human activities. Thus, maintaining soil carbon sinks requires targeted strategies focusing on POC.

Enhancing drought tolerance in Pisum sativum and Vicia faba through interspecific interactions with a mixed inoculum of Rhizobium laguerreae and non-host beneficial rhizobacteria

Introduction

Harnessing plant growth-promoting rhizobia presents a sustainable and cost-effective method to enhance crop performance, particularly under drought stress. This study evaluates the variability of plant growth-promoting (PGP) traits among three strains of Rhizobium laguerreae LMR575, LMR571, and LMR655, and two native PGP strains Bacillus LMR698 and Enterobacter aerogenes LMR696. The primary objective was to assess the host range specificity of these strains and their effectiveness in improving drought tolerance in three legume species: Pisum sativum, Vicia faba, and Phaseolus vulgaris.

Methods

In-vitro experiments were conducted to assess the PGP traits of the selected strains, including phosphate solubilization, indole-3-acetic acid (IAA) production, and siderophore production. Greenhouse trials were also performed using a mixed inoculum of performing strains to evaluate their effects on plant physiological and biochemical traits under drought conditions.

Results

Significant variability in PGP traits was observed among the strains. R. laguerreae LMR655 exhibited the highest phosphate solubilization (113.85 mg mL-1 PO4 2-), while R. laguerreae LMR571 produced the highest IAA concentration (25.37 mg mL-1). E. aerogenes LMR696 demonstrated 82% siderophore production. Symbiotic interactions varied, with R. laguerreae LMR571 and LMR655 forming associations with P. sativum and V. faba, but none establishing compatibility with P. vulgaris. Greenhouse experiments showed that a mixed inoculum of R. laguerreae LMR571, LMR655, and E. aerogenes LMR696 significantly improved proline, total soluble sugars, proteins, and chlorophyll content under drought stress, with V. faba showing the strongest response.

Discussion

These findings highlight the importance of strain selection based on host specificity and PGP potential. The enhanced drought tolerance observed suggests that tailored microbial inoculants can improve legume resilience in water-limited environments. This study provides valuable insights for optimizing bioinoculant formulations to enhance crop performance under drought stress.

Biodiversity conservation in the context of climate change: Facing challenges and management strategies

Biodiversity conservation amidst the uncertainty of climate change presents unique challenges that necessitate precise management strategies. The study reported here was aimed at refining understanding of these challenges and to propose specific, actionable management strategies. Employing a quantitative literature analysis, we meticulously examined 1268 research articles from the Web of Science database between 2005 and 2023. Through Cite Spaces and VOS viewer software, we conducted a bibliometric analysis and thematic synthesis to pinpoint emerging trends, key themes, and the geographical distribution of research efforts. Our methodology involved identifying patterns within the data, such as frequency of keywords, co-authorship networks, and citation analysis, to discern the primary focus areas within the field. This approach allowed us to distinguish between research concentration areas, specifically highlighting a predominant interest in Environmental Sciences Ecology (67.59 %) and Biodiversity Conservation (22.63 %). The identification of adaptive management practices and ecosystem services maintenance are central themes in the research from 2005 to 2023. Moreover, challenges such as understanding phenological shifts, invasive species dynamics, and anthropogenic pressures critically impact biodiversity conservation efforts. Our findings underscore the urgent need for precise, data-driven decision-making processes in the face of these challenges. Addressing the gaps identified, our study proposes targeted solutions, including the establishment of germplasm banks for at-risk species, the development of advanced genomic and microclimate models, and scenario analysis to predict and mitigate future conservation challenges. These strategies are aimed at enhancing the resilience of biodiversity against the backdrop of climate change through integrated, evidence-based approaches. By leveraging the compiled and analyzed data, this study offers a foundational framework for future research and practical action in biodiversity conservation strategies, demonstrating a path forward through detailed analysis and specified solutions.

A decrease in the age of respired carbon from the terrestrial biosphere and increase in the asymmetry of its distribution

We provide here a model-based estimate of the transit time of carbon through the terrestrial biosphere, since the time of carbon uptake through photosynthesis until its release through respiration. We explored the consequences of increasing productivity versus increasing respiration rates on the transit time distribution and found that while higher respiration rates induced by higher temperature increase the transit time because older carbon is respired, increases in productivity cause a decline in transit times because more young carbon is available to supply increased metabolism. The combined effect of increases in temperature and productivity results in a decrease in transit times, with the productivity effect dominating over the respiration effect. By using an ensemble of simulation trajectories from the Carbon Data Model Framework (CARDAMOM), we obtained time-dependent transit time distributions incorporating the twentieth century global change. In these simulations, transit time declined over the twentieth century, suggesting an increased productivity effect that augmented the amount of respired young carbon, but also increasing the release of old carbon from high latitudes. The transit time distribution of carbon becomes more asymmetric over time, with more carbon transiting faster through tropical and temperate regions, and older carbon being respired from high latitude regions. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Study on the spatial spillover effect of land use type change on carbon emissions

Land use change affects the terrestrial carbon cycle, a crucial factor in attaining energy conservation and emission reduction under climate change. This study constructs panel data for thirteen Hangzhou districts and municipalities from 2000 to 2020. Using the spatial Durbin model, it analyzes the spatial spillover effect of land use change on carbon emissions. The results show that the spatial distribution of carbon emissions in Hangzhou continues to increase with positive spatial autocorrelation, and the spatial distribution shows "high-high" and "low-low" clustering. The expansion of construction land is the main reason for the increase in carbon emissions, and the inhibitory effect of water area on carbon emissions is more potent than woodland. The area of cultivated land and construction land has a positive spillover effect on carbon emissions, while the woodland area has a negative spillover effect on carbon emissions. To promote urban low-carbon development, maximizing the spatial spillover effect of land use and establishing a collaborative governance system between districts and counties is crucial.

Long-term temporal patterns in ecosystem carbon flux components and overall balance in a heathland ecosystem

Terrestrial ecosystems have strong feedback to atmospheric CO₂ concentration and climate change. However, the long-term whole life cycle dynamics of ecosystem carbon (C) fluxes and overall balance in some ecosystem types, such as heathland ecosystems, have not been thoroughly explored. We studied the changes in ecosystem CO₂ flux components and overall C balance over a full ecosystem lifecycle in stands of Calluna vulgaris (L.) Hull by using a chronosequence of 0, 12, 19 and 28 years after vegetation cutting. Overall, the ecosystem C balance was highly nonlinear over time and exhibited a sinusoidal-like curvature of C sink/source change over the three-decade timescale. After cutting, plant-related C flux components of gross photosynthesis (P_G)_, aboveground autotrophic respiration (R_aa) and belowground autotrophic respiration (R_ba) were higher at the young age (12 years) than at middle (19 years) and old (28 years) ages. The young ecosystem was a C sink (12 years: -0.374 kg C m^-2 year^-1) while it became a C source with aging (19 years: 0.218 kg C m^-2 year^-1) and when dying (28 years: 0.089 kg C m^-2 year^-1). The post-cutting C compensation point was observed after four years, while the cumulative C loss in the period after cutting had been compensated by an equal amount of C uptake after seven years. Annual ecosystem C payback from the ecosystem to the atmosphere started after 16 years. This information may be used directly for optimizing vegetation management practices for maximal ecosystem C uptake capacity. Our study highlights that whole life cycle observational data of changes in C fluxes and balance in ecosystems are important and the ecosystem model needs to take the successional stage and vegetation age into account when projecting component C fluxes, ecosystem C balance, and overall feedback to climate change.

Impact assessment of urbanization on vegetation net primary productivity: A case study of the core development area in central plains urban agglomeration, China

Rapid urbanization process has a negative or positive impact on vegetation growth. Net primary productivity (NPP) is an effective indicator to characterize vegetation growth status. Taking the core development area of the Central Plains urban agglomeration as the study area, we estimated the NPP and its change trend in the past four decades using the Carnegie-Ames-Stanford Approach (CASA) model and statistical analysis based on meteorological and multi-source remote sensing data. Meanwhile, combined with the urbanization impact framework, we further analyzed urbanization's direct and indirect impact on NPP. The results showed that the urban area increased by 2688 km² during a high-speed urbanization process from 1983 to 2019. As a result of the intense urbanization process, a continuous NPP decrease (direct impact) can be seen, which aggravated along with the acceleration of the urban expansion, and the mean value of direct impact was 130.84 g C·m^-2·a^-1. Meanwhile, urbanization also had a positive impact on NPP (indirect impact). The indirect impact showed an increasing trend during urbanization with a mean value of 10.91 g C·m^-2·a^-1. The indirect impact was mainly related to temperature in climatic factors. The indirect impact has a seasonal heterogeneity, and high-temperature environments of urban areas are more effective in promoting vegetation growth in autumn and winter than in summer. Among different cities, high-speed development cities have higher indirect impact values than medium's and low's because of better ecological construction. This study is of great significance for understanding the impact of urbanization on vegetation growth in the Central Plains urban agglomeration area, supporting urban greening plans, and building sustainable and resilient urban agglomerations.

Pool dynamics of time-dependent compartmental systems with application to the terrestrial carbon cycle

Compartmental models play an important role to describe the dynamics of systems that involve mass movements between different types of pools. We develop a theory to analyse the average ages of mass in different pools in a linear compartmental system with time-dependent (i.e. non-autonomous) transfer rates, which involves transit times that characterize the average time a particle has spent in a particular pool. We apply our theoretical results to investigate a nine-dimensional compartmental system with time-dependent fluxes between pools modelling the carbon cycle which is a modification of the Carnegie-Ames-Stanford approach model. Knowledge of transit time and mean age allows calculation of carbon storage in a pool as a function of time. The general result that has important implications for understanding and managing carbon storage is that the change in storage in different pools does not change monotonically through time: as rates change monotonically a pool which initially shows a decrease may then show an increase in storage or vice versa. Thus caution is needed in extrapolating even the direction of future changes in storage in carbon storage in different pools with global change.

Statistical traps lurk in the research of soil carbon dynamics

Soil organic carbon (SOC) plays an essential role in sequestrating anthropogenic CO₂ emissions and mitigating climate change. Hence, accurate knowledge of the SOC dynamics and its controlling factors is a key part of the soil carbon cycling. Slessarev et al. (2022) question the major role of initial SOC in mediating changes in SOC, and illustrate the possible statistical artifacts that might be erroneously interpreted as causal. Slessarev and colleagues' insights make an important advance in the statistical evaluation of the regression problems in SOC, yielding more accurate inference.

More future synergies and less trade-offs between forest ecosystem services with natural climate solutions instead of bioeconomy solutions

To reach the Paris Agreement, societies need to increase the global terrestrial carbon sink. There are many climate change mitigation solutions (CCMS) for forests, including increasing bioenergy, bioeconomy, and protection. Bioenergy and bioeconomy solutions use climate-smart, intensive management to generate high quantities of bioenergy and bioproducts. Protection of (semi-)natural forests is a major component of "natural climate solution" (NCS) since forests store carbon in standing biomass and soil. Furthermore, protected forests provide more habitat for biodiversity and non-wood ecosystem services (ES). We investigated the impacts of different CCMS and climate scenarios, jointly or in isolation, on future wood ES, non-wood ES, and regulating ES for a major wood provider for the international market. Specifically, we projected future ES given by three CCMS scenarios for Sweden 2020-2100. In the long term, fulfilling the increasing wood demand through bioenergy and bioeconomy solutions will decrease ES multifunctionality, but the increased stand age and wood stocks induced by rising greenhouse gas (GHG) concentrations will partially offset these negative effects. Adopting bioenergy and bioeconomy solutions will have a greater negative impact on ES supply than adopting NCS. Bioenergy or bioeconomy solutions, as well as increasing GHG emissions, will reduce synergies and increase trade-offs in ES. NCS, by contrast, increases the supply of multiple ES in synergy, even transforming current ES trade-offs into future synergies. Moreover, NCS can be considered an adaptation measure to offset negative climate change effects on the future supplies of non-wood ES. In boreal countries around the world, forestry strategies that integrate NCS more deeply are crucial to ensure a synergistic supply of multiple ES.

Younger carbon dominates global soil carbon efflux

Soil carbon (C) is comprised of a continuum of organic compounds with distinct ages (i.e., the time a C atom has experienced in soil since the C atom entered soil). The contribution of different age groups to soil C efflux is critical for understanding soil C stability and persistence, but is poorly understood due to the complexity of soil C pool age structure and potential distinct turnover behaviors of age groups. Here, we build upon the quantification of soil C transit times to infer the age of C atoms in soil C efflux (a_efflux ) from seven sequential soil layer depths down to 2 m at a global scale, and compare this age with radiocarbon-inferred ages of C retained in corresponding soil layers (a_soil ). In the whole 0-2 m soil profile, the mean a_efflux is 194 21 1021 (mean with 5%-95% quantiles) year and is just about one-eighth of a_soil ( 1476 717 2547 year), demonstrating that younger C dominates soil C efflux. With increasing soil depth, both a_efflux and a_soil are increased, but their disparities are markedly narrowed. That is, the proportional contribution of relatively younger soil C to efflux is decreased in deeper layers, demonstrating that C inputs (new and young) stay longer in deeper layers. Across the globe, we find large spatial variability of the contribution of soil C age groups to C efflux. Especially, in deep soil layers of cold regions (e.g., boreal forests and tundra), a_efflux may be older than a_soil , suggesting that older C dominates C efflux only under a limited range of conditions. These results imply that most C inputs may not contribute to long-term soil C storage, particularly in upper layers that hold the majority of new C inputs.

Effects of land use and climate on carbon and nitrogen pool partitioning in European mountain grasslands

European mountain grasslands are increasingly affected by land-use changes and climate, which have been suggested to exert important controls on grassland carbon (C) and nitrogen (N) pools. However, so far there has been no synthetic study on whether and how land-use changes and climate interactively affect the partitioning of these pools amongst the different grassland compartments. We analyzed the partitioning of C and N pools of 36 European mountain grasslands differing in land-use and climate with respect to above- and belowground phytomass, litter and topsoil (top 23 cm). We found that a reduction of management intensity and the abandonment of hay meadows and pastures increased above-ground phytomass, root mass and litter as well as their respective C and N pools, concurrently decreasing the fractional contribution of the topsoil to the total organic carbon pool. These changes were strongly driven by the cessation of cutting and grazing, a shift in plant functional groups and a related reduction in litter quality. Across all grasslands studied, variation in the impact of land management on the topsoil N pool and C/N-ratio were mainly explained by soil clay content combined with pH. Across the grasslands, below-ground phytomass as well as phytomass- and litter C concentrations were inversely related to the mean annual temperature; furthermore, C/N-ratios of phytomass and litter increased with decreasing mean annual precipitation. Within the topsoil compartment, C concentrations decreased from colder to warmer sites, and increased with increasing precipitation. Climate generally influenced effects of land use on C and N pools mainly through mean annual temperature and less through mean annual precipitation. We conclude that site-specific conditions need to be considered for understanding the effects of land use and of current and future climate changes on grassland C and N pools.

A data-driven estimate of litterfall and forest carbon turnover and the drivers of their inter-annual variabilities in forest ecosystems across China

Strong influences of climate and land-cover changes on terrestrial ecosystems urgently need to re-estimate forest carbon turnover time (τ_forest), i.e., the residence time of carbon (C) in the living forest carbon reservoir in China, to reduce uncertainties in ecosystem carbon sinks under ongoing climate change. However, in absence of accurate carbon loss (e.g., forest litterfall), τ_forest estimate based on the non-steady-state assumption (NSSA) in forest ecosystems across China is still unclear. In this study, thus, we first compiled a litterfall dataset with 1025 field observations, and applied a Random Forest (RF) algorithm with the linkage of gridded environmental variables to predict litterfall from 2000 to 2019 with a fine spatial resolution of 1 km and a temporal resolution of one year. Finally, τ_forest was also estimated with the data-driven litterfall product. Results showed that RF algorithm could well predict the spatial and temporal patterns of forest litterfall with a model efficiency of 0.58 and root mean square error of 78.7 g C m^-2 year^-1. Mean litterfall was 205.4 ± 1.1 Tg C year^-1 (mean ± standard error) with a significant increasing trend of 0.65 ± 0.14 Tg C year^-2 from 2000 to 2019 (p < 0.01), indicating an increasing carbon loss from litterfall. Mean τ_forest was 26.2 ± 0.1 years with a significant decreasing trend of -0.11 ± 0.02 years (p < 0.01) from 2000 to 2019. Climate change dominated the inter-annual variability of τ_forest in high latitude areas, and land-cover change dominated the regions with intensive human activities. These findings suggested that carbon loss from vegetation to the atmosphere becomes more quickly in recent decades, with significant implication for vegetation carbon cycling-climate feedbacks. Meanwhile, the developed litterfall and τ_forest datasets can serve as a benchmark for biogeochemical models to accurately estimate global carbon cycling.

Is There Spatial and Temporal Variability in the Response of Plant Canopy and Trunk Growth to Climate Change in a Typical River Basin of Arid Areas

The response of plants to climate change has become a topical issue. However, there is no consensus on the synergistic processes of the canopy and trunk growth within different vegetation types, or on the consistency of the response of the canopy and trunk to climate change. This paper is based on Normalized Difference Vegetation Index (NDVI), tree-ring width index (TRW) and climate data from the Irtysh River basin, a sensitive area for climate change in Central Asia. Spatial statistical methods and correlation analysis were used to analyze the spatial and temporal trends of plants and climate, and to reveal the differences in the canopy and trunk response mechanisms to climate within different vegetation types. The results show a warming and humidifying trend between 1982 and 2015 in the study area, and NDVI and TRW increases in different vegetation type zones. On an interannual scale, temperature is the main driver of the canopy growth in alpine areas and precipitation is the main limiting factor for the canopy growth in lower altitude valley and desert areas. The degree of response of the trunk to climatic factors decreases with increasing altitude, and TRW is significantly correlated with mean annual temperature, precipitation and SPEI in desert areas. On a monthly scale, the earlier and longer growing season due to the accumulation of temperature and precipitation in the early spring and late autumn periods contributes to two highly significant trends of increase in the canopy from March to May and August to October. Climatic conditions during the growing season are the main limiting factor for the growth of the trunk, but there is considerable variation in the driving of the trunk in different vegetation type zones. The canopy growth is mainly influenced by climatic factors in the current month, while there is a 1–2-month lag effect in the response of the trunk to climatic factors. In addition, the synergy between the canopy and the trunk is gradually weakened with increasing altitude (correlation coefficient is 0.371 in alpine areas, 0.413 in valley areas and 0.583 in desert areas). These findings help to enrich the understanding of the response mechanisms to climate change in different vegetation type zones and provide a scientific basis for the development of climate change response measures in Central Asia.

Light and Water Conditions Co-Regulated Stomata and Leaf Relative Uptake Rate (LRU) during Photosynthesis and COS Assimilation: A Meta-Analysis

As a trace gas involved in hydration during plant photosynthesis, carbonyl sulfide (COS) and its leaf relative uptake rate (LRU) is used to reduce the uncertainties in simulations of gross primary productivity (GPP). In this study, 101 independent observations were collected from 22 studies. We extracted the LRU, stomatal conductance (gs), canopy COS and carbon dioxide (CO2) fluxes, and relevant environmental conditions (i.e., light, temperature, and humidity), as well as the atmospheric COS and CO2 concentrations (Ca,COS and Ca,CO2). Although no evidence was found showing that gs regulates LRU, they responded in opposite ways to diurnal variations of environmental conditions in both mixed forests (LRU: Hedges’d = −0.901, LnRR = −0.189; gs: Hedges’d = 0.785, LnRR = 0.739) and croplands dominated by C3 plants (Hedges’d = −0.491, LnRR = −0.371; gs: Hedges’d = 1.066, LnRR = 0.322). In this process, the stomata play an important role in COS assimilation (R2 = 0.340, p = 0.020) and further influence the interrelationship of COS and CO2 fluxes (R2 = 0.650, p = 0.000). Slight increases in light intensity (R2 = 1, p = 0.002) and atmospheric drought (R2 = 0.885, p = 0.005) also decreased the LRU. The LRU saturation points of Ca,COS and Ca,CO2 were observed when ΔCa,COS ≈ 13 ppt (R2 = 0.580, p = 0.050) or ΔCa,CO2 ≈ −18 ppm (R2 = 0.970, p = 0.003). This study concluded that during plant photosynthesis and COS assimilation, light and water conditions co-regulated the stomata and LRU.

Country-level land carbon sink and its causing components by the middle of the twenty-first century

BACKGROUND

Countries have long been making efforts by reducing greenhouse-gas emissions to mitigate climate change. In the agreements of the United Nations Framework Convention on Climate Change, involved countries have committed to reduction targets. However, carbon (C) sink and its involving processes by natural ecosystems remain difficult to quantify.

METHODS

Using a transient traceability framework, we estimated country-level land C sink and its causing components by 2050 simulated by 12 Earth System Models involved in the Coupled Model Intercomparison Project Phase 5 (CMIP5) under RCP8.5.

RESULTS

The top 20 countries with highest C sink have the potential to sequester 62 Pg C in total, among which, Russia, Canada, USA, China, and Brazil sequester the most. This C sink consists of four components: production-driven change, turnover-driven change, change in instantaneous C storage potential, and interaction between production-driven change and turnover-driven change. The four components account for 49.5%, 28.1%, 14.5%, and 7.9% of the land C sink, respectively.

CONCLUSION

The model-based estimates highlight that land C sink potentially offsets a substantial proportion of greenhouse-gas emissions, especially for countries where net primary production (NPP) likely increases substantially and inherent residence time elongates.

Plant input does not exert stronger control on topsoil carbon persistence than climate in alpine grasslands

Chen et al. (Ecology Letters, 2021, 24, 1018) concluded that plant input governs topsoil carbon persistence in alpine grasslands. We demonstrated that the excluded direct effect of precipitation on topsoil Δ¹⁴ C in their analysis was significant and strong. Our results provide an alternative viewpoint on the drivers of soil carbon turnover.

Quantifying the contributions of human activities and climate change to vegetation net primary productivity dynamics in China from 2001 to 2016

Vegetation is an important component of the terrestrial ecosystem, driven by climate change and human activities. Quantifying the relative contributions of climate change and anthropogenic activities to vegetation dynamics are essential to cope with global climate change. In this paper, the relative contributions of anthropogenic activities and climate change to net primary productivity (NPP) in China were analyzed by a two-step methodology based on the residual trend analysis (RESTREND). Firstly, the unaltered natural vegetation only affected by climate change (V_climate) and the vegetation affected by climate change and human activities (V_{climate+human}) were separated by the multi-temporal land use land cover (LULC) data. Secondly, RESTREND was applied to NPP of V_climate and V_{climate+human}, respectively, to calculate contributions of climatic factors and human activities to vegetation growth. Results revealed that NPP exhibited a significant increase with 3.13 g C m^-2 yr^-1 from 2001 to 2016 in China. Climate change and human activities both made favorable impacts on vegetation growth during the study period. Besides, with the separation of V_climate and V_{climate+human}, contributions of climatic factors to vegetation changes increased from 1.57 to 1.88 g C m^-2 yr^-1, with the proportion of 60.06%. While contributions of human activities to NPP decreased from 1.56 to 1.25 g C m^-2 yr^-1, with the proportion of 39.94%. Moreover, the average contributions of precipitation, temperature, solar radiation, and other climatic factors to NPP over the entire country were 0.72, 0.24, 0.61, and 0.31 g C m^-2 yr^-1. Precipitation played a decisive role in vegetation changes in arid and semi-arid regions, temperature was the dominant factor for alpine vegetation dynamics, and solar radiation was beneficial to vegetation growth in most areas of China.