Warming-induced Increases Research Articles

Microbial Diversity Losses Constrain the Capacity of Soils to Mitigate Climate Change.

Soil microbes may adapt to climate warming, potentially reducing the warming-induced increase in microbial carbon emissions such as carbon dioxide, and thereby helping to mitigate climate change. Yet, soil microbes are subjected to various global change stresses (e.g., warming, drought, flooding, and land-use changes), altering their biodiversity, which challenges microbial adaptation to climate change. Here, we created microbial diversity gradients in microcosms at two different temperatures using soils from a 2000-km field survey. We found that reduced microbial diversity weakens the thermal adaptation of soil microbial respiration and can further enhance the microbial respiratory temperature sensitivity over time. Our analyses further revealed that the negative impact of microbial diversity losses is linked to the decline of keystone microbial taxa, which can adapt to temperature changes and are crucial for the community's ability to compensate for the temperature-driven effects on soil respiration in the long term. Taken together, our study provides new insights into the key role of microbial diversity in driving the thermal response of soil heterotrophic respiration, suggesting that any global change-driven shifts in microbial diversity can have critical consequences for the future of carbon stocks.

Anticipating how rain-on-snow events will change through the 21st century: lessons from the 1997 new year’s flood event

The California-Nevada 1997 New Year’s flood was an atmospheric river (AR)-driven rain-on-snow (RoS) event and remains the costliest in their history. The joint occurrence of saturated soils, rainfall, and snowmelt generated inundation throughout northern California-Nevada. Although AR RoS events are projected to occur more frequently with climate change, the warming sensitivity of their flood drivers across scales remains understudied. We leverage the regionally refined mesh capabilities of the Energy Exascale Earth System Model (RRM-E3SM) to recreate the 1997 New Year’s flood with horizontal grid spacings of 3.5 km across California, with forecast lead times of up to 4 days, and across six warming levels ranging from pre-industrial conditions to +3.5∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$+3.5\\,^\\circ$$\\end{document}C. We describe the sensitivity of the flood drivers to warming including AR duration and intensity, precipitation phase, intensity and efficiency, snowpack mass and energy changes, and runoff efficiency. Our findings indicate current levels of climate change negligibly influence the flood drivers. At warming levels ≥1.7∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\ge 1.7\\,^\\circ$$\\end{document}C, AR hazard potential increases, snowpack nonlinearly decreases, antecedent soil moisture decreases (except where the snowline retreats), and runoff decreases (except in the southern Sierra Nevada where antecedent snowpack persists). Storm total precipitation increases, but at rates below warming-induced increases in saturation-specific humidity. Warming intensifies short-duration, high-intensity rainfall, particularly where snowfall-to-rainfall transitions occur. This study highlights the nonlinear tradeoffs in 21st-century RoS flood hazards with warming and provides water management and infrastructure investment adaptation considerations.

Enhanced response of soil respiration to experimental warming upon thermokarst formation

As global temperatures continue to rise, a key uncertainty of terrestrial carbon (C)–climate feedback is the rate of C loss upon abrupt permafrost thaw. This type of thawing—termed thermokarst—may in turn accelerate or dampen the response of microbial degradation of soil organic matter and carbon dioxide (CO2) release to climate warming. However, such impacts have not yet been explored in experimental studies. Here, by experimentally warming three thermo-erosion gullies in an upland thermokarst site combined with incubating soils from five additional thermokarst-impacted sites on the Tibetan Plateau, we investigate how warming responses of soil CO2 release would change upon upland thermokarst formation. Our results show that warming-induced increase in soil CO2 release is ~5.5 times higher in thermokarst features than the adjacent non-thermokarst landforms. This larger warming response is associated with the lower substrate quality and higher abundance of microbial functional genes for recalcitrant C degradation in thermokarst-affected soils. Taken together, our study provides experimental evidence that warming-associated soil CO2 loss becomes stronger upon abrupt permafrost thaw, which could exacerbate the positive soil C–climate feedback in permafrost-affected regions.

Climate warming shifts riverine macroinvertebrate communities to be more sensitive to chemical pollutants.

Freshwaters are highly threatened ecosystems that are vulnerable to chemical pollution and climate change. Freshwater taxa vary in their sensitivity to chemicals and changes in species composition can potentially affect the sensitivity of assemblages to chemical exposure. Here we explore the potential consequences of future climate change on the composition and sensitivity of freshwater macroinvertebrate assemblages to chemical stressors using the UK as a case study. Macroinvertebrate assemblages under end of century (2080-2100) and baseline (1980-2000) climate conditions were predicted for 608 UK sites for four climate scenarios corresponding to mean temperature changes of 1.28 to 3.78°C. Freshwater macroinvertebrate toxicity data were collated for 19 chemicals and the hierarchical species sensitivity distribution model was used to predict the sensitivity of untested taxa using relatedness within a Bayesian approach. All four future climate scenarios shifted assemblage compositions, increasing the prevalence of Mollusca, Crustacea and Oligochaeta species, and the insect taxa of Odonata, Chironomidae, and Baetidae species. Contrastingly, decreases were projected for Plecoptera, Ephemeroptera (except for Baetidae) and Coleoptera species. Shifts in taxonomic composition were associated with changes in the percentage of species at risk from chemical exposure. For the 3.78°C climate scenario, 76% of all assemblages became more sensitive to chemicals and for 18 of the 19 chemicals, the percentage of species at risk increased. Climate warming-induced increases in sensitivity were greatest for assemblages exposed to metals and were dependent on baseline assemblage composition, which varied spatially. Climate warming is predicted to result in changes in the use, environmental exposure and toxicity of chemicals. Here we show that, even in the absence of these climate-chemical interactions, shifts in species composition due to climate warming will increase chemical risk and that the impact of chemical pollution on freshwater macroinvertebrate biodiversity may double or quadruple by the end of the 21st century.

Nighttime warming and nitrogen addition effects on the microclimate of a freshwater wetland dominated by Phragmites australis

The critical impacts of microclimate on carbon (C) cycling have been widely reported. However, the potential effects of global change on wetland microclimate remain unclear, primarily because of the absence of field manipulative experiment in inundated wetland. This study was designed to examine the effects of nighttime warming and nitrogen (N) addition on air, water, and sediment temperature and also reveal the controlling factors in a Phragmites australis dominated freshwater wetland on the North China Plain. Nighttime warming increased daily air, water, and sediment temperature by 0.24 °C, 0.27 °C, and 0.36 °C, respectively. The diurnal temperature range of water was decreased by 0.44 °C under nighttime warming, whereas warming had no effect on diurnal temperature range of air and sediment. In addition, N addition caused a reduction of 0.20 °C and 0.14 °C in daily water and sediment temperature by increasing vegetation coverage. There was a significant interaction between nighttime warming and N addition on water temperature. Furthermore, the vapor pressure deficit is the main factor affecting the extent of the warming-induced increases in air temperature. The changes of height and leaf area index of Phragmites australis are responsible for the cooling effects in the N addition plots. This study provides empirical evidence for the positive climate warming – microclimate feedback in freshwater wetland. However, N deposition leads to decreased water and sediment temperature. Our findings highlight the importance of incorporating the differential impacts of nighttime warming and N addition on air, water, and sediment temperature into the predictions of wetland C cycling responses to climate change.

Acidification Offset Warming-Induced Increase in N2O Production in Estuarine and Coastal Sediments.

Global warming and acidification, induced by a substantial increase in anthropogenic CO2 emissions, are expected to have profound impacts on biogeochemical cycles. However, underlying mechanisms of nitrous oxide (N2O) production in estuarine and coastal sediments remain rarely constrained under warming and acidification. Here, the responses of sediment N2O production pathways to warming and acidification were examined using a series of anoxic incubation experiments. Denitrification and N2O production were largely stimulated by the warming, while N2O production decreased under the acidification as well as the denitrification rate and electron transfer efficiency. Compared to warming alone, the combination of warming and acidification decreased N2O production by 26 ± 4%, which was mainly attributed to the decline of the N2O yield by fungal denitrification. Fungal denitrification was mainly responsible for N2O production under the warming condition, while bacterial denitrification predominated N2O production under the acidification condition. The reduced site preference of N2O under acidification reflects that the dominant pathways of N2O production were likely shifted from fungal to bacterial denitrification. In addition, acidification decreased the diversity and abundance of nirS-type denitrifiers, which were the keystone taxa mediating the low N2O production. Collectively, acidification can decrease sediment N2O yield through shifting the responsible production pathways, partly counteracting the warming-induced increase in N2O emissions, further reducing the positive climate warming feedback loop.

Radial growth responses of Larix gmelinii to drought events in dry and wet areas of northern temperate forests

Drought stress caused by global climate warming affects tree growth in both dry and wet areas. However, the differences in tree growth responses to drought in dry and wet areas are poorly understood. Here, we collected 93 tree cores to analyze the differences in the radial growth responses of larch (Larix gmelinii) under climate change and tree growth resilience under drought events in the Altai Mountains (dry area) and Changbai Mountains (wet area). The results showed that larch growth in the Altai Mountains was significantly positively correlated with the self-calibrating Palmer Drought Severity Index (sc-PDSI) in all months and precipitation in the previous growth season and May, whereas it was significantly (p < 0.05) negatively correlated with temperature in May and the previous June to August. In the Changbai Mountains, larch growth was significantly positively correlated with May maximum and mean temperature, and significantly positively and negatively correlated with precipitation in April and May，respectively (p < 0.05). The mean resistance (recovery) of larch growth to drought in wet areas were significantly stronger (weaker) than that in dry areas (p < 0.05). Moreover, strengthening the drought frequency led to a significant (p < 0.05) decline in larch resistance in dry areas. Therefore, warming-induced increases in drought stress will aggravate negative impacts on the radial growth of larch forests in temperate dry areas but not in wet areas in the future.

Precipitation increment reinforced warming-induced increases in soil mineral-associated and particulate organic matter under agricultural ecosystem

Exploring the dynamics of soil carbon (C) response to multi-faceted global climate change plays a vital role in facilitating the estimation of carbon-climate feedback. However, the direction and extent of the warming effect on soil carbon pool within different soil fractions considered simultaneously in increased precipitation and decreased precipitation system are not well understood. Here we established a 2-year field manipulation experiment based on open top chamber (OTC) to assess the effects of warming and altering precipitation regimes (40 % reduction, normal, and 40 % increment) on soil physical fractions formation and distribution. Our results showed that in the precipitation increment, warming significantly increased particulate organic carbon (POC) and mineral-associated carbon (MAOC) by an average of 48.10 % and 21.28 % in the 0–5 cm soil layer. And the formation mechanism of POC and MAOC were different. POC was directly affected by the coupling between plant input and C-degradation fungal functional genes adjusted by dissolved organic carbon (DOC) and total nitrogen (TN); and MAOC was affected by increasing labile carbon inputs (such as DOC) of maize residue and belowground biomass. However, precipitation reduction neutralized the beneficial effects of warming, mainly through reducing plant inputs and residue decomposition, thereby suppressing C-degradation fungal functional genes and labile carbon supply. Overall, our results provide new insights into the potential mechanisms through which interaction of warming and altered precipitation controls soil C dynamics.

The direct and indirect effects of the environmental factors on global terrestrial gross primary productivity over the past four decades

Gross primary productivity (GPP) is jointly controlled by the structural and physiological properties of the vegetation canopy and the changing environment. Recent studies showed notable changes in global GPP during recent decades and attributed it to dramatic environmental changes. Environmental changes can affect GPP by altering not only the biogeochemical characteristics of the photosynthesis system (direct effects) but also the structure of the vegetation canopy (indirect effects). However, comprehensively quantifying the multi-pathway effects of environmental change on GPP is currently challenging. We proposed a framework to analyse the changes in global GPP by combining a nested machine-learning model and a theoretical photosynthesis model. We quantified the direct and indirect effects of changes in key environmental factors (atmospheric CO2 concentration, temperature, solar radiation, vapour pressure deficit (VPD), and soil moisture (SM)) on global GPP from 1982 to 2020. The results showed that direct and indirect absolute contributions of environmental changes on global GPP were 0.2819 Pg C yr−2 and 0.1078 Pg C yr−2. Direct and indirect effects for single environmental factors accounted for 1.36%–51.96% and 0.56%–18.37% of the total environmental effect. Among the direct effects, the positive contribution of elevated CO2 concentration on GPP was the highest; and warming-induced GPP increase counteracted the negative effects. There was also a notable indirect effect, mainly through the influence of the leaf area index. In particular, the rising VPD and declining SM negatively impacted GPP more through the indirect pathway rather than the direct pathway, but not sufficient to offset the boost of warming over the past four decades. We provide new insights for understanding the effects of environmental changes on vegetation photosynthesis, which could help modelling and projection of the global carbon cycle in the context of dramatic global environmental change.

Effects of experimental warming on soil enzyme activities in an alpine swamp meadow on the Qinghai-Tibetan Plateau

Information about the response of soil enzymes to field warming in permafrost regions is scarce, and the potential mechanisms by which warming combined with biotic and abiotic factors affect soil enzyme activities in alpine grasslands remain unclear. A 3-year in situ experiment with two warming levels (2.7 °C and 5.3 °C) was conducted in an alpine swamp meadow to investigate the effects of experimental warming on 5 soil enzyme activities involved in soil carbon and nitrogen cycling, and the relationships between soil enzyme activities and soil variables related to physicochemical properties and nutrient levels were examined. Results showed that warming had obvious positive effects on soil moisture (SM), soil organic carbon, total nitrogen, and NH4+-N contents, especially in the surface soil layer. Soil NO3--N tended to decrease under moderate warming, while it significantly increased under high warming. Meanwhile, during the entire growing season, warming strongly enhanced invertase and amylase activities by 39.2–49.0% and 109.9–191.0%, respectively. In contrast, urease activities were significantly decreased by 36.8–55.7% in warming plots and there were no significant differences in catalase or cellulase activities among treatments during the whole growing season, while catalase activities in warming plots were significantly decreased by 2.6–4.0% in June. These inconsistent responses of soil enzyme activities to experimental warming could be partially explained by warming-induced changes in soil variables. Redundancy analysis showed that soil NO3--N and SM were the most important factors, which explained 29.8% and 19.7% of the variations in soil enzyme activities, respectively, suggesting that the warming-induced increase in SM could weaken the soil aeration status and enhance enzyme and substrate diffusion, strongly affecting soil nitrification and microbial enzyme production.

Short-term warming-induced increase in non-microbial carbon emissions from semiarid abandoned farmland soils

Soil warming is expected to accelerate microbial carbon (C) degradation, increasing carbon dioxide (CO2) emissions. This process greatly contributes to global warming. To validate this positive feedback loop in semiarid restored ecosystems, we examined the effects of 2 years of open-top chamber warming on soil CO2 efflux (soil respiration) and microbial metabolic characteristics in abandoned farmland on the Chinese Loess Plateau. Soil warming of 1.3 °C above ambient at 10 cm depth significantly stimulated CO2 emissions by 40.5 %. Despite short-term warming alleviating the C (energy) limitation on microbial activity, no substantial differences in microbial C use efficiency or biomass turnover rate were noted compared to those in the control. Importantly, we found little evidence that warming altered the expression levels of carbohydrate metabolism genes annotated using the eggNOG, KEGG, and CAZy databases. All expressed genes targeting the degradation of diverse carbonaceous components were assigned to bacterial taxa unaffected by warming. Therefore, short-term warming stimulated soil CO2 emissions from non-microbial sources but did not drive the abandoned farmland in the reverse direction to a C source. Our findings highlight the importance of conducting long-term comparisons of microbial metabolic profiles and soil respiration components to elucidate the mechanisms underlying changes in C-cycling in ecosystems that have undergone restoration and experienced warming.

High Concentration of Sulphate Coupled with Climate Warming Generates Ecosystem Feedback Under Sub-Oxic Conditions at Sediment-Water Interface in the Ganga River.

Here, we quantified sediment phosphorus (P) release in relation to concentrations of dissolved oxygen (DO) and sulphate, and increase in temperature in a major river of India subjected to long-term human perturbations. We found a substantial increase in sediment P release, an ecosystem feedback, at higher concentrations of sulphate, more towards the lower end of DO concentrations. A 2°C warming increased sediment P release upto 25.21% and caused a drop in DO level by 16%. Our findings reconcile the observed sulphate-driven changes in sediment P release across systems, and provide first experimental evidence of warming-induced increases. Our results imply that aquatic ecosystems will undergo self-fertilizing effect as the planet warming interacts with other human perturbations. This has implications for eutrophication linkages and ecosystem functioning.

Five-year warming does not change soil organic carbon stock but alters its chemical composition in an alpine peatland

Climate warming may promote soil organic carbon (SOC) decomposition and alter SOC stocks in terrestrial ecosystems, which would in turn affect climate warming. We manipulated a warming experiment using open-top chambers to investigate the effect of warming on SOC stock and chemical composition in an alpine peatland in Zoigê on the eastern Tibetan Plateau, China. Results showed that 5 years of warming soil temperatures enhanced ecosystem respiration during the growing season, promoted above- and belowground plant biomass, but did not alter the SOC stock. However, labile O-alkyl C and relatively recalcitrant aromatic C contents decreased, and alkyl C content increased. Warming also increased the amount of SOC stored in the silt-clay fraction (< 0.053 mm), but this was offset by warming-induced decreases in the SOC stored within micro- and macroaggregates (0.053–0.25 and > 0.25 mm, respectively). These changes in labile and recalcitrant C were largely associated with warming-induced increases in soil microbial biomass C, fungal diversity, enzyme activity, and functional gene abundance related to the decomposition of labile and recalcitrant C compounds. The warming-induced accumulation of SOC stored in the silt-clay fraction could increase SOC persistence in alpine peatland ecosystems. Our findings suggest that mechanisms mediated by soil microbes account for the changes in SOC chemical composition and SOC in different aggregate size fractions, which is of great significance when evaluating SOC stability under climate warming conditions.

Soil microbial respiration adapts to higher and longer warming experiments at the global scale

Warming can affect soil microbial respiration by changing microbial biomass and community composition. The responses of soil microbial respiration to warming under experimental conditions are also related to background conditions and the experimental setup, such as warming magnitude, duration, and methods. However, the global pattern of soil microbial respiration in response to warming and underlying mechanisms remain unclear. Here, we conducted a global meta-analysis of the response of soil microbial respiration to warming by synthesizing data from 187 field experiments. We found that experimental warming significantly increased soil microbial respiration and microbial biomass carbon by 11.8% and 6.4%, respectively. The warming-induced increase in microbial carbon decomposition was positively correlated with increased microbial biomass carbon, but not community composition. Moreover, the positive response of soil microbial respiration marginally increased with warming magnitude, particularly in short-term experiments, but soil microbial respiration adapted to higher warming at longer timescales. Warming method did not significantly affect the response of microbial respiration, except for a significant effect with open top chamber warming. In addition, the impact of warming on soil microbial respiration was more pronounced in wetter sites and in sites with lower soil pH and higher soil organic carbon. Our findings suggest that warming stimulates microbial respiration mainly by increasing microbial biomass carbon. We also highlight the importance of the combination of warming magnitude and duration in regulating soil microbial respiration responses, and the dependence of warming effects upon background precipitation and soil conditions. These findings can advance our understanding of soil carbon losses and carbon-climate feedbacks in a warm world.

East Antarctica ice sheet in Schirmacher Oasis, Central Dronning Maud Land, during the past 158 ka

Varied geomorphic landforms along the coast of eastern Antarctica suggest that the most recent phase of ice retreat was spatially heterogeneous. Ice retreat here comprised; a thinning of the East Antarctic Ice Sheet (EAIS) by up to 500 m and the recession of the ice wall in kilometre. This retreat deposited moraines over the Schirmacher Oasis (SO) in central Dronning Maud Land with minimal reworking. The present study used optical dating of the recessional moraines to determine the timing of their final emplacement. Three phases of moraine deposition, during 158–125 ka; 76–50 ka and 22 ka to present, were inferred. It is suggested that decreased sea surface temperatures and increased sea ice cover of the surrounding oceans limited the moisture supply and led to the retreat of ice. By ~ 35 ka the SO became ice-free and has remained so, ever since. The inference that the ice sheet in this region was moisture limited, implies that in a global warming context, this region may not contribute to an increase in sea-level. Instead, a warming-induced increase in moisture supply may even add to ice cover in the region.

Biochar amendments and climate warming affected nitrification associated N2O and NO production in a vegetable field

Soil nitrification driven by ammonia-oxidizing microorganisms is the most important source of nitrous oxide (N2O) and nitric oxide (NO). Biochar amendment has been proposed as the most promising measure for combating climate warming; both have the potential to regulate the soil nitrification process. However, the comprehensive impacts of different aged biochars and warming combinations on soil nitrification-related N2O and NO production are not well understood. Here, 1-octyne and acetylene were used to investigate the relative contributions of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential nitrification-mediated N2O and NO production from the fertilized vegetable soil with different aged biochar amendments and soil temperatures in microcosm incubations. Results demonstrated that AOB dominated nitrification-related N2O and NO production across biochar additions and climate warming. Biochar amendment did not significantly influence the relative contribution of AOB and AOA to N2O and NO production. Field-aged biochar markedly reduced N2O and NO production via inhibiting AOB-amoA gene abundance and AOB-dependent N2O yield while fresh- and lab-aged biochar produced negligible effects on AOB-dependent N2O yield. Climate warming significantly increased N2O production and AOB-dependent N2O yield but less so on NO production. Notably, the relative contribution of AOB to N2O production was enhanced by climate warming, whereas AOB-derived NO showed the opposite tendency. Overall, the results revealed that field-aged biochar contributed to mitigating warming-induced increases in N2O and NO production via inhibiting AOB-amoA gene abundance and AOB-dependent N2O yield. Our findings provided guidance for mitigating nitrogen oxide emissions in intensively managed vegetable production under the context of biochar amendments and climate warming.

Warming-induced tree growth may help offset increasing disturbance across the Canadian boreal forest.

Large projected increases in forest disturbance pose a major threat to future wood fiber supply and carbon sequestration in the cold-limited, Canadian boreal forest ecosystem. Given the large sensitivity of tree growth to temperature, warming-induced increases in forest productivity have the potential to reduce these threats, but research efforts to date have yielded contradictory results attributed to limited data availability, methodological biases, and regional variability in forest dynamics. Here, we apply a machine learning algorithm to an unprecedented network of over 1 million tree growth records (1958 to 2018) from 20,089 permanent sample plots distributed across both Canada and the United States, spanning a 16.5 °C climatic gradient. Fitted models were then used to project the near-term (2050 s time period) growth of the six most abundant tree species in the Canadian boreal forest. Our results reveal a large, positive effect of increasing thermal energy on tree growth for most of the target species, leading to 20.5 to 22.7% projected gains in growth with climate change under RCP 4.5 and 8.5. The magnitude of these gains, which peak in the colder and wetter regions of the boreal forest, suggests that warming-induced growth increases should no longer be considered marginal but may in fact significantly offset some of the negative impacts of projected increases in drought and wildfire on wood supply and carbon sequestration and have major implications on ecological forecasts and the global economy.

Warming-induced increase in power demand and CO2 emissions in Qatar and the Middle East

Rising global temperatures in the Arabian Peninsula region caused by climate change have increased the demand for air conditioning, resulting in more electricity consumption and CO2 emissions. This paper treats Qatar as a representative country for understanding the effect of future regional warming on the electricity demand and CO2 emissions We first develop a model that relates daily electricity demand with temperature. By combining this model with temperature projections from the CMIP6 database (bias adjusted and statistically downscaled) and population and GDP projections from four shared socioeconomic pathways (SSPs), we can calculate Qatar's demand for electricity until the end of the century. The model identifies an average sensitivity of +4.2%/°C for the electricity demand and projects an increase in electricity demand by 5–35% due to warming alone at the end of this century. The model suggests that under SSP1-2.6, warming-induced CO2 emissions could be offset by carbon intensity improvements. Furthermore, under SSP5-8.5, assuming no carbon intensity improvement, future warming could add 20–35% of CO2 emissions per year by the end of the century, with half of the electricity demand related to more frequent hot days. We further found that the temperature effect on power demand and CO2 emissions is small compared to the effects from socioeconomic factors such as population, GDP, and carbon intensity.

Nitrogen availability determines ecosystem productivity in response to climate warming.

One of the major uncertainties for carbon-climate feedback predictions is an inadequate understanding of the mechanisms governing variations in ecosystem productivity response to warming. Temperature and water availability are regarded as the primary controls over the direction and magnitude of warming effects, but some unexplained results signal that our understanding is incomplete. Using two complementary meta-analyses, we present evidence that soil nitrogen (N) availability drives the warming effects on ecosystem productivity more strongly than thermal and hydrological factors over a broad geographical scale. First, by synthesizing temperature manipulation experiments, a meta-regression model analysis showed that the warming effect on productivity is mainly driven by its effect on soil N availability. Sites with a higher warming-induced increase in N availability were characterized by stronger productivity enhancement and vice versa, suggesting that N is a limiting factor across sites. Second, a synthesis of full-factorial warming × N addition experiments demonstrated that N addition significantly weakened the positive warming effect, because the additional N induced by warming may not further benefit plant growth when N limitation is relieved, providing experimental evidence that N regulates the warming effect. Furthermore, we demonstrated that warming effects on soil N availability were modulated by changes in dissolved organic N and soil microbes. Overall, our findings enrich a new mechanistic understanding of the varying magnitudes of observed productivity response to warming, and the N scaling of warming effects may help to constrain climate projections.

Sustained increases in soil respiration accompany increased carbon input under long-term warming across global grasslands

Respiratory effluxes of carbon (C) from the soil to the atmosphere are expected to rise with temperature, potentially intensifying future climate warming. However, whether and how this increase would be sustained under long-term warming is not well understood. Here, we combined a manipulation experiment in an alpine meadow with a global meta-analysis to explore the mechanisms underlying the long-term responses of soil respiration to climate warming. The results from the experiment in the alpine meadow showed that the warming-induced increase in net primary productivity (NPP, 23.6 %) explained 52 % of the increase in soil respiration across 6 years. In contrast, the warming-induced changes in soil moisture, soil temperature, microbial biomass C or nitrogen were not significantly correlated with soil respiration responses. Consistently, in the global meta-analysis, both soil respiration and NPP continually increased over the years by an average of 9.5 % and 15.9 %, respectively. The increases in soil respiration were also primarily correlated with the continued increases in NPP over this period. Notably, the sustained increase in soil respiration was mainly contributed by the response of autotrophic respiration, which was closely correlated with the sustained increase in belowground NPP under warming. The results from both our field experiment and meta-analysis suggest that the increased soil respiration under climate warming was at least partly from the stimulation of C input in grasslands. The simultaneous increases in soil respiration and NPP may counteract the expected positive terrestrial C-climate feedback and should be considered in land models to more accurately predict future climate change.