Publications

The CO₂ fertilization effect in tropical forests is a key factor for the global land carbon sink. We show that the normalized CO₂ effect on tropical vegetation carbon was c. 70% lower in seedling CO₂ experiments without nutrient fertilizers and c. 50% and 70% lower in models that consider nitrogen and phosphorus cycles, based on two model ensembles. The inadequate representation or lack of nutrient cycles in Earth System models likely leads to overestimating future tropical carbon gains.

Background: Research on the changes of organic carbon (OC) and total nitrogen (TN) contents in soil aggregates is crucial for better evaluating C and N dynamics in paddy soil.

Aims: We investigated the effects of wheat straw incorporation on soil aggregate distributions and stability, the contents of OC and TN in bulk soil, and aggregates as well as their relationships.

Methods: A rice paddy field experiment in China with four treatments: (1) unfertilized control (CK), (2) mineral NPK fertilizer (NPK), (3) NPK plus medium-rate wheat straw incorporation (MSNPK), and (4) NPK plus high-rate wheat straw incorporation (HSNPK). Soil samples were collected from four soil layers (0–5, 5–10, 10–20, and 20–30 cm, respectively) after the rice harvest, and the samples were separated into four classes of aggregates with different sizes (>5, 5–2, 2–0.25, and <0.25 mm).

Results: The >5 mm fraction dominated aggregate size distribution, but the 2–0.25 mm fraction had the highest OC and TN contents throughout soil profile among treatments. The proportions of >5 and 5–2 mm aggregates, mean weight diameter (MWD), geometric mean diameter (GMD), the proportion of aggregates greater than 0.25 mm (R_0.25), bulk soil organic carbon (SOC) and TN contents, and OC and TN contents in aggregates were highest in HSNPK and lowest in CK at either depth. Both bulk SOC and TN contents positively correlated with R_0.25, MWD, GMD, and OC and TN contents in aggregates, and positively and negatively correlated with all aggregates (p < 0.05), except between bulk TN and the proportion of 5–2 mm aggregates. The OC content in the 5–2 mm aggregates (increase in MSE = 23.87%) and the TN content in the 2–0.25 mm aggregates (increase in MSE = 13.89%) were more important than other driving factors for bulk SOC and TN, respectively.

Conclusion: In conclusion, these results confirmed that the application of HSNPK increased bulk SOC and TN contents, soil aggregates distributions (i.e. >5 mm and 5–2 mm fractions) and stability, and the OC and TN contents in all aggregates at both depths in rice paddy fields of China, which might improve soil C and N sinks in agricultural ecosystems.

Effects of experimental eCO2, warming, nutrient addition and/or water addition/removal on carbon and nutrient cycle related variables New in v1.0.2: new table mesi_methods_comments.csv, describing methodologies of response variable measurements, plus further relevant comments new table mesi_template.csv, providing a template that can be used for contributing new experimental data soil pHw (water) and pHKCl (KCl solution) distinguished in main and soil tables leaf_n/p/k: distinction made between leaf_n_mass and leaf_n_area missing root/shoot ratio

A poor understanding of the fraction of global plant biomass occurring belowground as roots limits our understanding of present and future ecosystem function and carbon pools. Here we create a database of root-mass fractions (RMFs), an index of plant below- versus aboveground biomass distributions, and generate quantitative, spatially explicit global maps of RMFs in trees, shrubs and grasses. Our analyses reveal large gradients in RMFs both across and within vegetation types that can be attributed to resource availability. High RMFs occur in cold and dry ecosystems, while low RMFs dominate in warm and wet regions. Across all vegetation types, the directional effect of temperature on RMFs depends on water availability, suggesting feedbacks between heat, water and nutrient supply. By integrating our RMF maps with existing aboveground plant biomass information, we estimate that in forests, shrublands and grasslands, respectively, 22%, 47% and 67% of plant biomass exists belowground, with a total global belowground fraction of 24% (20–28%), that is, 113 (90–135) Gt carbon. By documenting the environmental correlates of root biomass allocation, our results can inform model projections of global vegetation dynamics under current and future climate scenarios.

Predation structures food webs, influences energy flow, and alters rates and pathways of nutrient cycling through ecosystems, effects that are well documented for macroscopic predators. In the microbial world, predatory bacteria are common, yet little is known about their rates of growth and roles in energy flows through microbial food webs, in part because these are difficult to quantify. Here, we show that growth and carbon uptake were higher in predatory bacteria compared to nonpredatory bacteria, a finding across 15 sites, synthesizing 82 experiments and over 100,000 taxon-specific measurements of element flow into newly synthesized bacterial DNA. Obligate predatory bacteria grew 36% faster and assimilated carbon at rates 211% higher than nonpredatory bacteria. These differences were less pronounced for facultative predators (6% higher growth rates, 17% higher carbon assimilation rates), though high growth and carbon assimilation rates were observed for some facultative predators, such as members of the genera Lysobacter and Cytophaga, both capable of gliding motility and wolf-pack hunting behavior. Added carbon substrates disproportionately stimulated growth of obligate predators, with responses 63% higher than those of nonpredators for the Bdellovibrionales and 81% higher for the Vampirovibrionales, whereas responses of facultative predators to substrate addition were no different from those of nonpredators. This finding supports the ecological theory that higher productivity increases predator control of lower trophic levels. These findings also indicate that the functional significance of bacterial predators increases with energy flow and that predatory bacteria influence element flow through microbial food webs.

Global change has resulted in chronic shifts in fire regimes. Variability in the sensitivity of tree communities to multi-decadal changes in fire regimes is critical to anticipating shifts in ecosystem structure and function, yet remains poorly understood. Here, we address the overall effects of fire on tree communities and the factors controlling their sensitivity in 29 sites that experienced multi-decadal alterations in fire frequencies in savanna and forest ecosystems across tropical and temperate regions. Fire had a strong overall effect on tree communities, with an average fire frequency (one fire every three years) reducing stem density by 48% and basal area by 53% after 50 years, relative to unburned plots. The largest changes occurred in savanna ecosystems and in sites with strong wet seasons or strong dry seasons, pointing to fire characteristics and species composition as important. Analyses of functional traits highlighted the impact of fire-driven changes in soil nutrients because frequent burning favoured trees with low biomass nitrogen and phosphorus content, and with more efficient nitrogen acquisition through ectomycorrhizal symbioses. Taken together, the response of trees to altered fire frequencies depends both on climatic and vegetation determinants of fire behaviour and tree growth, and the coupling between fire-driven nutrient losses and plant traits.

Terrestrial ecosystems remove about 30 per cent of the carbon dioxide (CO₂) emitted by human activities each year¹, yet the persistence of this carbon sink depends partly on how plant biomass and soil organic carbon (SOC) stocks respond to future increases in atmospheric CO₂ (refs. ^2,3). Although plant biomass often increases in elevated CO₂ (eCO₂) experiments^4,5,6, SOC has been observed to increase, remain unchanged or even decline⁷. The mechanisms that drive this variation across experiments remain poorly understood, creating uncertainty in climate projections^8,9. Here we synthesized data from 108 eCO₂ experiments and found that the effect of eCO₂ on SOC stocks is best explained by a negative relationship with plant biomass: when plant biomass is strongly stimulated by eCO₂, SOC storage declines; conversely, when biomass is weakly stimulated, SOC storage increases. This trade-off appears to be related to plant nutrient acquisition, in which plants increase their biomass by mining the soil for nutrients, which decreases SOC storage. We found that, overall, SOC stocks increase with eCO₂ in grasslands (8 ± 2 per cent) but not in forests (0 ± 2 per cent), even though plant biomass in grasslands increase less (9 ± 3 per cent) than in forests (23 ± 2 per cent). Ecosystem models do not reproduce this trade-off, which implies that projections of SOC may need to be revised.

Secondary succession on abandoned agricultural lands can produce climate change mitigation co-benefits, such as soil carbon sequestration. However, the accumulation of soil organic carbon (SOC) in Mediterranean regions has been difficult to predict and is subject to multiple environmental and land management factors. Gains, losses, and no significant changes have all been reported. Here we compile chronosequence data (n = 113) from published studies and new field sites to assess the response of SOC to agricultural land abandonment in peninsular Spain. We found an overall SOC accumulation rate of +2.3% yr⁻¹ post-abandonment. SOC dynamics are highly variable and context-dependent. Minimal change occurs on abandoned cereal croplands compared to abandoned woody croplands (+4% yr⁻¹). Accumulation is most prevalent within a Goldilocks climatic window of ~13–17 °C and ~450–900 mm precipitation, promoting >100% gains after three decades. Our secondary forest field sites accrued 40.8 Mg C ha⁻¹ (+172%) following abandonment and displayed greater SOC and N depth heterogeneity than natural forests demonstrating the long-lasting impact of agriculture. Although changes in regional climate and crop types abandoned will impact future carbon sequestration, abandonment remains a low-cost, long-term natural climate solution best incorporated in tandem with other multipurpose sustainable land management strategies.

Atmospheric carbon dioxide concentration ([CO₂]) is increasing, which increases leaf-scale photosynthesis and intrinsic water-use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO₂] increase and thus climate change. However, ecosystem CO₂ responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO₂]-driven terrestrial carbon sink can appear contradictory. Here we synthesize theory and broad, multidisciplinary evidence for the effects of increasing [CO₂] (iCO₂) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre-industrial times. Established theory, supported by experiments, indicates that iCO₂ is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO₂ responses are high in comparison to experiments and predictions from theory. Plant mortality and soil carbon iCO₂ responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO₂, albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.

A global synthesis of experiments reveals that increases in plant biomass under conditions of elevated CO₂ mean that plants need to mine the soil for nutrients, which decreases soil’s ability to store carbon. In forests, elevated CO₂ generally seems to greatly increase plant biomass, but not soil carbon. In grasslands, by contrast, it causes small changes in biomass and large increases in soil carbon.

Background and aims: Through agriculture and industry, humans are increasing the deposition and availability of nitrogen (N) in ecosystems worldwide. Carbon (C) isotope tracers provide useful insights into soil C dynamics, as they allow to study soil C pools of different ages. We evaluated to what extent N enrichment affects soil C dynamics in experiments that applied C isotope tracers.
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Methods: Using meta-analysis, we synthesized data from 35 published papers. We made a distinction between “new C” and “old C” stocks, i.e., soil C derived from plant C input since the start of the isotopic enrichment, or unlabeled, pre-existing soil C.

Results: Averaged across studies, N addition increased new soil C stocks (+30.3%), total soil C stocks (+6.1%) and soil C input proxies (+30.7%). Although N addition had no overall, average, effect on old soil C stocks and old soil C respiration, old soil C stocks increased with the amount of N added and respiration of old soil C declined. Nitrogen-induced effects on new soil C and soil C input both decreased with the amount of extraneous N added in control treatments.

Conclusion: Although our findings require additional confirmation from long-term field experiments, our analysis provides isotopic evidence that N addition stimulates soil C storage both by increasing soil C input and (at high N rates) by decreasing decomposition of old soil C. Furthermore, we demonstrate that the widely reported saturating response of plant growth to N enrichment also applies to new soil C storage.

Increased human-derived nitrogen (N) deposition to terrestrial ecosystems has resulted in widespread phosphorus (P) limitation of net primary productivity. However, it remains unclear if and how N-induced P limitation varies over time. Soil extracellular phosphatases catalyze the hydrolysis of P from soil organic matter, an important adaptive mechanism for ecosystems to cope with N-induced P limitation. Here we show, using a meta-analysis of 140 studies and 668 observations worldwide, that N stimulation of soil phosphatase activity diminishes over time. Whereas short-term N loading (≤5 years) significantly increased soil phosphatase activity by 28%, long-term N loading had no significant effect. Nitrogen loading did not affect soil available P and total P content in either short- or long-term studies. Together, these results suggest that N-induced P limitation in ecosystems is alleviated in the long-term through the initial stimulation of soil phosphatase activity, thereby securing P supply to support plant growth. Our results suggest that increases in terrestrial carbon uptake due to ongoing anthropogenic N loading may be greater than previously thought.

The widespread historical and ongoing abandonment of agricultural lands worldwide presents important opportunities for promoting climate change mitigation through carbon sequestration. The default management outcome of abandonment is natural regeneration through ecological succession. However, several different management strategies and new land uses for abandoned agricultural lands have been recommended by the scientific community in recent years. This paper reviews the foremost proposed strategies and compares their soil carbon sequestration potentials. Six major categories have been proposed globally. Each proposal has positive and negative outcomes depending on site-specific factors and management objectives. Accordingly, no single strategy is ideal in all scenarios and a combination of strategies addresses multiple rural development goals concurrently. A combination of passive and active management techniques is the most effective approach for maximizing soil carbon sequestration over large geographic scales, while other strategies can be designed to also promote low-carbon land use practices and fossil fuel substitution. The implications of each proposal highlighted here demonstrates the positive role that abandoned agricultural lands can serve in climate change mitigation efforts, supporting policymakers tasked with planning the future of regions undergoing abandonment.

Plants and vegetation play a critical—but largely unpredictable—role in global environmental changes due to the multitude of contributing processes at widely different spatial and temporal scales. In this Perspective, we explore approaches to master this complexity and improve our ability to predict vegetation dynamics by explicitly taking account of principles that constrain plant and ecosystem behaviour: natural selection, self-organization and entropy maximization. These ideas are increasingly being used in vegetation models, but we argue that their full potential has yet to be realized. We demonstrate the power of natural selection-based optimality principles to predict photosynthetic and carbon allocation responses to multiple environmental drivers, as well as how individual plasticity leads to the predictable self-organization of forest canopies. We show how models of natural selection acting on a few key traits can generate realistic plant communities and how entropy maximization can identify the most probable outcomes of community dynamics in space- and time-varying environments. Finally, we present a roadmap indicating how these principles could be combined in a new generation of models with stronger theoretical foundations and an improved capacity to predict complex vegetation responses to environmental change.

Nitrogen (N) and phosphorus (P) limitation constrains the magnitude of terrestrial carbon uptake in response to elevated carbon dioxide and climate change. However, global maps of nutrient limitation are still lacking. Here we examined global N and P limitation using the ratio of site-averaged leaf N and P resorption efficiencies of the dominant species across 171 sites. We evaluated our predictions using a global database of N- and P-limitation experiments based on nutrient additions at 106 and 53 sites, respectively. Globally, we found a shift from relative P to N limitation for both higher latitudes and precipitation seasonality and lower mean annual temperature, temperature seasonality, mean annual precipitation and soil clay fraction. Excluding cropland, urban and glacial areas, we estimate that 18% of the natural terrestrial land area is significantly limited by N, whereas 43% is relatively P limited. The remaining 39% of the natural terrestrial land area could be co-limited by N and P or weakly limited by either nutrient alone. This work provides both a new framework for testing nutrient limitation and a benchmark of N and P limitation for models to constrain predictions of the terrestrial carbon sink.

Nutrient availability influences virtually every aspect of an ecosystem, and is a critical modifier of ecosystem responses to global change. Although this crucial role of nutrient availability in regulating ecosystem structure and functioning has been widely acknowledged, nutrients are still often neglected in observational and experimental synthesis studies due to difficulties in comparing the nutrient status across sites. In the current study, we explain different nutrient-related concepts and discuss the potential of soil-, plant- and remote sensing-based metrics to compare the nutrient status across space. Based on our review and additional analyses on a dataset of European, managed temperate and boreal forests (ICP [International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests] Forests dataset), we conclude that the use of plant- and remote sensing-based metrics that rely on tissue stoichiometry is limited due to their strong dependence on species identity. The potential use of other plant-based metrics such as Ellenberg indicator values and plant-functional traits is also discussed. We conclude from our analyses and review that soil-based metrics have the highest potential for successful intersite comparison of the nutrient status. As an example, we used and adjusted a soil-based metric, previously developed for conifer forests across Sweden, against the same ICP Forests data. We suggest that this adjusted and further adaptable metric, which included the organic carbon concentration in the upper 20 cm of the soil (including the organic fermentation-humus [FH] layer), the C:N ratio and pH_CaCl2 of the FH layer, can be used as a complementary tool along with other indicators of nutrient availability, to compare the background nutrient status across temperate and boreal forests dominated by spruce, pine or beech. Future collection and provision of harmonized soil data from observational and experimental sites is crucial for further testing and adjusting the metric.

Vegetation impacts on ecosystem functioning are mediated by mycorrhizas, plant–fungal associations formed by most plant species. Ecosystems dominated by distinct mycorrhizal types differ strongly in their biogeochemistry. Quantitative analyses of mycorrhizal impacts on ecosystem functioning are hindered by the scarcity of information on mycorrhizal distributions. Here we present global, high-resolution maps of vegetation biomass distribution by dominant mycorrhizal associations. Arbuscular, ectomycorrhizal, and ericoid mycorrhizal vegetation store, respectively, 241 ± 15, 100 ± 17, and 7 ± 1.8 GT carbon in aboveground biomass, whereas non-mycorrhizal vegetation stores 29 ± 5.5 GT carbon. Soil carbon stocks in both topsoil and subsoil are positively related to the community-level biomass fraction of ectomycorrhizal plants, though the strength of this relationship varies across biomes. We show that human-induced transformations of Earth’s ecosystems have reduced ectomycorrhizal vegetation, with potential ramifications to terrestrial carbon stocks. Our work provides a benchmark for spatially explicit and globally quantitative assessments of mycorrhizal impacts on ecosystem functioning and biogeochemical cycling.

Elevated CO₂ (eCO₂) experiments provide critical information to quantify the effects of rising CO₂ on vegetation^1,2,3,4,5,6. Many eCO₂ experiments suggest that nutrient limitations modulate the local magnitude of the eCO₂ effect on plant biomass^1,3,5, but the global extent of these limitations has not been empirically quantified, complicating projections of the capacity of plants to take up CO₂^7,8. Here, we present a data-driven global quantification of the eCO₂ effect on biomass based on 138 eCO₂ experiments. The strength of CO₂ fertilization is primarily driven by nitrogen (N) in ~65% of global vegetation and by phosphorus (P) in ~25% of global vegetation, with N- or P-limitation modulated by mycorrhizal association. Our approach suggests that CO₂ levels expected by 2100 can potentially enhance plant biomass by 12 ± 3% above current values, equivalent to 59 ± 13 PgC. The future effect of eCO₂ we derive from experiments is geographically consistent with past changes in greenness⁹, but is considerably lower than the past effect derived from models¹⁰. If borne out, our results suggest that the stimulatory effect of CO₂ on carbon storage could slow considerably this century. Our research provides an empirical estimate of the biomass sensitivity to eCO₂ that may help to constrain climate projections.

Land ecosystems sequester on average about a quarter of anthropogenic CO₂ emissions. It has been proposed that nitrogen (N) availability will exert an increasingly limiting effect on plants’ ability to store additional carbon (C) under rising CO₂, but these mechanisms are not well understood. Here, we review findings from elevated CO₂ experiments using a plant economics framework, highlighting how ecosystem responses to elevated CO₂ may depend on the costs and benefits of plant interactions with mycorrhizal fungi and symbiotic N-fixing microbes. We found that N-acquisition efficiency is positively correlated with leaf-level photosynthetic capacity and plant growth, and negatively with soil C storage. Plants that associate with ectomycorrhizal fungi and N-fixers may acquire N at a lower cost than plants associated with arbuscular mycorrhizal fungi. However, the additional growth in ectomycorrhizal plants is partly offset by decreases in soil C pools via priming. Collectively, our results indicate that predictive models aimed at quantifying C cycle feedbacks to global change may be improved by treating N as a resource that can be acquired by plants in exchange for energy, with different costs depending on plant interactions with microbial symbionts.

The recent study by Smith et al. concludes that Earth system models (ESMs) overestimate the effect of CO₂ fertilization on net primary productivity (NPP). Whilst this finding is possible, here we highlight that the satellite derived NPP estimates used are likely to underestimate the CO₂ fertilization effect because they do not account for the primary effect of CO₂ on photosynthesis. In addition, the calculation of NPP sensitivity to atmospheric CO2 is misleading, invalidating the comparison with free air CO₂ enrichment (FACE) data.

Plants buffer increasing atmospheric carbon dioxide (CO₂) concentrations through enhanced growth, but the question whether nitrogen availability constrains the magnitude of this ecosystem service remains unresolved. Synthesizing experiments from around the world, we show that CO₂ fertilization is best explained by a simple interaction between nitrogen availability and mycorrhizal association. Plant species that associate with ectomycorrhizal fungi show a strong biomass increase (30 ± 3%, P< 0.001) in response to elevated CO₂ regardless of nitrogen availability, whereas low nitrogen availability limits CO₂ fertilization (0 ± 5%, P = 0.946) in plants that associate with arbuscular mycorrhizal fungi. The incorporation of mycorrhizae in global carbon cycle models is feasible, and crucial if we are to accurately project ecosystem responses and feedbacks to climate change.

In this work, we apply a dynamic modelling approach to analyse the habitat loss of the Canarian houbara bustard (Chlamydotis undulata fuerteventurae). This tool allows us to assess the effects of the socio-economic and environmental interactions on the factors threatening the habitat and to carry out a prospective analysis. The results show a potential habitat loss of around 13 % during the period 1996–2011, the land uptake and increase in new roads and tracks being the factors contributing most. After model testing, a set of scenarios was explored. Under the business as usual (BAU) scenario, around 20 % of the habitat would be lost by the end of the period considered (2012–2025). The impact of the economic growth scenario on the habitat would mean an additional loss of around 21 % with respect to BAU, whereas under the recession scenario, the loss might be around 6.5 % lower than BAU. The policy of restoration of gavias—traditional farming systems—would suppose an additional loss of almost 6 %, relative to BAU. If this policy took place under economic growth conditions, it might mean an additional loss of almost 28 % relative to BAU. These results point to the existence of a potential trade-off between the recuperation of ecosystem services offered by restored gavias and the conservation of the houbara habitat, which must be addressed within the management processes, as well as to the need for compensatory measures to guarantee the conservation goals.