Carbon quality and soil microbial property control the latitudinal pattern in temperature sensitivity of soil microbial respiration across Chinese forest ecosystems

Understanding the temperature sensitivity (Q₁₀) of soil organic C (SOC) decomposition is critical to quantifying the climate–carbon cycle feedback and predicting the response of ecosystems to climate change. However, the driving factors of the spatial variation in Q₁₀ at a continental scale are fully unidentified. In this study, we conducted a novel incubation experiment with periodically varying temperature based on the mean annual temperature of the soil origin sites. A total of 140 soil samples were collected from 22 sites along a 3,800 km long north–south transect of forests in China, and the Q₁₀ of soil microbial respiration and corresponding environmental variables were measured. Results showed that changes in the Q₁₀ values were nonlinear with latitude, particularly showing low Q₁₀ values in subtropical forests and high Q₁₀ values in temperate forests. The soil C:N ratio was positively related to the Q₁₀ values, and coniferous forest soils with low SOC quality had higher Q₁₀ values than broadleaved forest soils with high SOC quality, which supported the “C quality temperature” hypothesis. Out of the spatial variations in Q₁₀ across all ecosystems, gram‐negative bacteria exhibited the most importance in regulating the variation in Q₁₀ and contributed 25.1%, followed by the C:N ratio (C quality), fungi, and the fungi:bacteria ratio. However, the dominant factors that regulate the regional variations in Q₁₀ differed among the tropical, subtropical, and temperate forest ecosystems. Overall, our findings highlight the importance of C quality and microbial controls over Q₁₀ value in China's forest ecosystems. Meanwhile, C dynamics in temperate forests under a global warming scenario can be robustly predicted through the incorporation of substrate quality and microbial property into models.     

The toughest animals of the Earth versus global warming: Effects of long‐term experimental warming on tardigrade community structure of a temperate deciduous forest

Understanding how different taxa respond to global warming is essential for predicting future changes and elaborating strategies to buffer them. Tardigrades are well known for their ability to survive environmental stressors, such as drying and freezing, by undergoing cryptobiosis and rapidly recovering their metabolic function after stressors cease. Determining the extent to which animals that undergo cryptobiosis are affected by environmental warming will help to understand the real magnitude climate change will have on these organisms. Here, we report on the responses of tardigrades within a five‐year‐long, field‐based artificial warming experiment, which consisted of 12 open‐top chambers heated to simulate the projected effects of global warming (ranging from 0 to 5.5°C above ambient temperature) in a temperate deciduous forest of North Carolina (USA). To elucidate the effects of warming on the tardigrade community inhabiting the soil litter, three community diversity indices (abundance, species richness, and Shannon diversity) and the abundance of the three most abundant species (Diphascon pingue, Adropion scoticum, and Mesobiotus sp.) were determined. Their relationships with air temperature, soil moisture, and the interaction between air temperature and soil moisture were tested using Bayesian generalized linear mixed models. Despite observed negative effects of warming on other ground invertebrates in previous studies at this site, long‐term warming did not affect the abundance, richness, or diversity of tardigrades in this experiment. These results are in line with previous experimental studies, indicating that tardigrades may not be directly affected by ongoing global warming, possibly due to their thermotolerance and cryptobiotic abilities to avoid negative effects of stressful temperatures, and the buffering effect on temperature of the soil litter substrate.     

Phenological responses of Icelandic subarctic grasslands to short‐term and long‐term natural soil warming

The phenology of vegetation, particularly the length of the growing season (LOS; i.e., the period from greenup to senescence), is highly sensitive to climate change, which could imply potent feedbacks to the climate system, for example, by altering the ecosystem carbon (C) balance. In recent decades, the largest extensions of LOS have been reported at high northern latitudes, but further warming‐induced LOS extensions may be constrained by too short photoperiod or unfulfilled chilling requirements. Here, we studied subarctic grasslands, which cover a vast area and contain large C stocks, but for which LOS changes under further warming are highly uncertain. We measured LOS extensions of Icelandic subarctic grasslands along natural geothermal soil warming gradients of different age (short term, where the measurements started after 5 years of warming and long term, i.e., warmed since ≥50 years) using ground‐level measurements of normalized difference vegetation index. We found that LOS linearly extended with on average 2.1 days per °C soil warming up to the highest soil warming levels (ca. +10°C) and that LOS had the potential to extend at least 1 month. This indicates that the warming impact on LOS in these subarctic grasslands will likely not saturate in the near future. A similar response to short‐ and long‐term warming indicated a strong physiological control of the phenological response of the subarctic grasslands to warming and suggested that genetic adaptations and community changes were likely of minor importance. We conclude that the warming‐driven extension of the LOSs of these subarctic grasslands did not saturate up to +10°C warming, and hence that growing seasons of high‐latitude grasslands are likely to continue lengthening with future warming (unless genetic adaptations or species shifts do occur). This persistence of the warming‐induced extension of LOS has important implications for the C‐sink potential of subarctic grasslands under climate change.     

Experimental warming alters the relative survival and emigration of two dung beetle species from an Australian dung pat community

To preserve insect‐mediated ecosystem services under ongoing climate change, it is necessary to first understand the impact that warming will have on the insects that provide or mediate these services. Furthermore, because responses of a species may be modified by interactions with competitors, it is informative to examine warming effects on organisms and service provision under competition. Dung beetles provide numerous services to agriculture by burying the manure of other animals. To understand the potential impacts of climate warming on ecosystem service provision, we exposed two dung beetle species (Sisyphus rubrus and Euoniticellus fulvus (Coleoptera: Scarabaeidae)), occurring together in the same experimental pats, to warming and measured reproduction (dung ball production and burial, brood production, and egg laying), pat departure behaviour and survival of both species. These two species are likely competitors in pastures in northern New South Wales. To simulate climate warming, we used custom‐built chambers to add offsets (+0, +2 or +4°C) to field recorded, diurnally fluctuating baseline temperatures. There was no direct effect of increased temperature on any measured trait in either species. We did find however that the relative survival of the two species depended on temperature; S. rubrus had a higher probability (resulting in greater odds) of surviving than E. fulvus in the +0 and +4°C offset chambers, but not in the +2°C offset chambers. Likewise, the relative likelihood of the different species leaving a dung pat was temperature dependent; in the +2°C offset chambers, E. fulvus were more likely to leave than S. rubrus, but not in the +0 and +4°C offsets chambers. Our results highlight that it may be important for future studies to consider warming effects on relative survival and emigration because such effects could potentially lead to changes in dung beetle species composition.     

Soil–plant N processes in a High Arctic ecosystem,NW Greenland are altered by long‐term experimental warming and higher rainfall

Rapid temperature and precipitation changes in High Arctic tundra ecosystems are altering the biogeochemical cycles of carbon (C) and nitrogen (N), but in ways that are difficult to predict. The challenge grows from the uncertainty of N cycle responses and the extent to which shifts in soil N are coupled with the C cycle and productivity of tundra systems. We used a long‐term (since 2003) experiment of summer warming and supplemental summer water additions to a High Arctic ecosystem in NW Greenland, and applied a combination of discrete sampling and in situ soil core incubations to measure C and N pools and seasonal microbial processes that might control plant‐available N. We hypothesized that elevated temperature and increased precipitation would stimulate microbial activity and net inorganic N mineralization, thereby increasing plant N‐availability through the growing season. While we did find increased N mineralization rates under both global change scenarios, water addition also significantly increased net nitrification rates, loss of NO₃^?‐N via leaching, and lowered rates of labile organic N production. We also expected the chronic warming and watering would lead to long‐term changes in soil N‐cycling that would be reflected in soil δ¹⁵N values. We found that soil δ¹⁵N decreased under the different climate change scenarios. Our results suggest that temperature accelerates biological processes and existing C and N transformations, but moisture increases soil hydraulic connectivity and so alters the pathways, and changes the fate of the products of C and N transformations. In addition, our findings indicate that warmer, wetter High Arctic tundra will be cycling N and C in ways that may transform these landscapes in part leading to greater C sequestration, but simultaneously, N losses from the upper soil profile that may be transported to depth dissolved in water and or transported off site in lateral flow.     

Shift in community structure in an early‐successional Mediterranean shrubland driven by long‐term experimental warming and drought and natural extreme droughts

Global warming and recurring drought are expected to accelerate water limitation for plant communities in semiarid Mediterranean ecosystems and produce directional shifts in structure and composition that are not easily detected, and supporting evidence is scarce. We conducted a long‐term (17 years) nocturnal‐warming (+0.6°C) and drought (?40% rainfall) experiments in an early‐successional Mediterranean shrubland to study the changes in community structure and composition, contrasting functional groups and dominant species, and the superimposed effects of natural extreme drought. Species richness decreased in both the warming and drought treatments. Responses to the moderate warming were associated with decreases in herb abundance, and responses to the drought were associated with decreases in both herb and shrub abundances. The drought also significantly decreased community diversity and evenness. Changes in abundance differed between herbs (decreases) and shrubs (increases or no changes). Both warming and drought, especially drought, increased the relative species richness and abundance of shrubs, favoring the establishment of shrubs. Both warming and drought produced significant shifts in plant community composition. Experimental warming shifted the community composition from Erica multiflora toward Rosmarinus officinalis, and drought consistently shifted the composition toward Globularia alypum. The responses in biodiversity (e.g., community biodiversity, changes of functional groups and compositional shifts) were also strongly correlated with atmospheric drought (SPEI) in winter–spring and/or summer, indicating sensitivity to water limitation in this early‐successional Mediterranean ecosystem, especially to natural extreme droughts. Our results suggest that the shifts in species assembles and community diversity and composition are accelerated by the long‐term nocturnal‐warming and drought, combined with natural severe droughts, and that the magnitude of the impacts of climate change is also correlated with the successional status of ecosystem. The results thus highlight the necessity for assessing the impacts on ecosystemic functioning and services and developing effective measures for conserving biodiversity.     

Carbon dynamics in the future forest: the importance of long‐term successional legacy and climate–fire interactions

Understanding how climate change may influence forest carbon (C) budgets requires knowledge of forest growth relationships with regional climate, long‐term forest succession, and past and future disturbances, such as wildfires and timber harvesting events. We used a landscape‐scale model of forest succession, wildfire, and C dynamics (LANDIS‐II) to evaluate the effects of a changing climate (A2 and B1 IPCC emissions; Geophysical Fluid Dynamics Laboratory General Circulation Models) on total forest C, tree species composition, and wildfire dynamics in the Lake Tahoe Basin, California, and Nevada. The independent effects of temperature and precipitation were assessed within and among climate models. Results highlight the importance of modeling forest succession and stand development processes at the landscape scale for understanding the C cycle. Due primarily to landscape legacy effects of historic logging of the Comstock Era in the late 1880s, C sequestration may continue throughout the current century, and the forest will remain a C sink (Net Ecosystem Carbon Balance > 0), regardless of climate regime. Climate change caused increases in temperatures limited simulated C sequestration potential because of augmented fire activity and reduced establishment ability of subalpine and upper montane trees. Higher temperatures influenced forest response more than reduced precipitation. As the forest reached its potential steady state, the forest could become C neutral or a C source, and climate change could accelerate this transition. The future of forest ecosystem C cycling in many forested systems worldwide may depend more on major disturbances and landscape legacies related to land use than on projected climate change alone.     

Drought and its legacy modulate the post‐fire recovery of soil functionality and microbial community structure in a Mediterranean shrubland

The effects of drought on soil dynamics after fire are poorly known, particularly its long‐term (i.e., years) legacy effects once rainfall returns to normal. Understanding this is particularly important for nutrient‐poor soils in semi‐arid regions affected by fire, in which rainfall is projected to decrease with climate change. Here, we studied the effects of post‐fire drought and its legacy on soil microbial community structure and functionality in a Cistus‐Erica shrubland (Spain). Rainfall total and patterns were experimentally modified to produce an unburned control (natural rainfall) and four burned treatments: control (natural rainfall), historical control (long‐term average rainfall), moderate drought (percentile 8 historical rainfall, 5 months of drought per year), and severe drought (percentile 2, 7 months of drought). Soil nutrients and microbial community composition (ester‐linked fatty acid approach) and functionality (enzyme activities and C mineralization rate) were monitored during the first 4 years after fire under rainfall treatments, plus two additional ones without them (six post‐fire years). We found that the recovery of burned soils was lower under drought. Post‐fire drought increased nitrate in the short term and reduced available phosphorus, exchangeable potassium, soil organic matter, enzyme activities, and carbon mineralization rate. Moreover, drought decreased soil total microbial biomass and fungi, with bacteria becoming relatively more abundant. Two years after discontinuing the drought treatments, the drought legacy was significant for available phosphorus and enzyme activities. Although microbial biomass did not show any drought legacy effect, the proportion of fungi and bacteria (mainly gram‐positive) did, being lower and higher, respectively, in former drought‐treated plots. We show that drought has an important impact on soil processes, and that some of its effects persist for at least 2 years after the drought ended. Therefore, drought and its legacy effects can be important for modeling biogeochemical processes in burned soils under future climate change.     

Distinct patterns of soil bacterial and fungal community assemblages in subtropical forest ecosystems under warming

Climate change globally affects soil microbial community assembly across ecosystems. However, little is known about the impact of warming on the structure of soil microbial communities or underlying mechanisms that shape microbial community composition in subtropical forest ecosystems. To address this gap, we utilized natural variation in temperature via an altitudinal gradient to simulate ecosystem warming. After 6 years, microbial co-occurrence network complexity increased with warming, and changes in their taxonomic composition were asynchronous, likely due to contrasting community assembly processes. We found that while stochastic processes were drivers of bacterial community composition, warming led to a shift from stochastic to deterministic drivers in dry season. Structural equation modelling highlighted that soil temperature and water content positively influenced soil microbial communities during dry season and negatively during wet season. These results facilitate our understanding of the response of soil microbial communities to climate warming and may improve predictions of ecosystem function of soil microbes in subtropical forests.     

Enhanced decomposition of stable soil organic carbon and microbial catabolic potentials by long‐term field warming

Quantifying soil organic carbon (SOC) decomposition under warming is critical to predict carbon–climate feedbacks. According to the substrate regulating principle, SOC decomposition would decrease as labile SOC declines under field warming, but observations of SOC decomposition under warming do not always support this prediction. This discrepancy could result from varying changes in SOC components and soil microbial communities under warming. This study aimed to determine the decomposition of SOC components with different turnover times after subjected to long‐term field warming and/or root exclusion to limit C input, and to test whether SOC decomposition is driven by substrate lability under warming. Taking advantage of a 12‐year field warming experiment in a prairie, we assessed the decomposition of SOC components by incubating soils from control and warmed plots, with and without root exclusion for 3 years. We assayed SOC decomposition from these incubations by combining inverse modeling and microbial functional genes during decomposition with a metagenomic technique (GeoChip). The decomposition of SOC components with turnover times of years and decades, which contributed to 95% of total cumulative CO₂ respiration, was greater in soils from warmed plots. But the decomposition of labile SOC was similar in warmed plots compared to the control. The diversity of C‐degradation microbial genes generally declined with time during the incubation in all treatments, suggesting shifts of microbial functional groups as substrate composition was changing. Compared to the control, soils from warmed plots showed significant increase in the signal intensities of microbial genes involved in degrading complex organic compounds, implying enhanced potential abilities of microbial catabolism. These are likely responsible for accelerated decomposition of SOC components with slow turnover rates. Overall, the shifted microbial community induced by long‐term warming accelerates the decomposition of SOC components with slow turnover rates and thus amplify the positive feedback to climate change.     

全球变暖背景下土壤微生物呼吸的热适应性:证据、机理和争议

土壤微生物呼吸的热适应性被认为是决定陆地生态系统对全球变暖反馈作用的潜在重要机制,可能显著改变未来的气候变化趋势,然而学术界对于这一机制是否真实存在尚有分歧。阐述了土壤微生物呼吸的热适应性概念,从证据、机理和争议3方面对已有研究进展进行了综述和分析。土壤微生物呼吸的热适应性是微生物在群落尺度上对温度变化的适应性,具有坚实的生物学与生态学理论基础,研究者们运用各类指标已在许多实验中证实土壤微生物物种及群落的呼吸过程能够在高温环境产生适应性变化。土壤微生物呼吸的热适应性机理涉及生物膜结构变化、酶活性变化、微生物碳分配比例变化和微生物群落结构变化等方面。关于土壤微生物呼吸热适应性的争议可能是由研究方法、微生物物种及环境条件的差异引起的。根据对已有研究的分析,认为土壤微生物呼吸的热适应性是真实存在的,未来的研究可进一步探索土壤微生物呼吸的热适应性机理,深入研究环境和全球变化对土壤微生物呼吸的热适应性影响,定量评估土壤微生物呼吸的热适应性对陆地生态系统反馈过程的影响。     

Identifying the microbial taxa that consistently respond to soil warming across time and space

Soil microbial communities are the key drivers of many terrestrial biogeochemical processes. However, we currently lack a generalizable understanding of how these soil communities will change in response to predicted increases in global temperatures and which microbial lineages will be most impacted. Here, using high‐throughput marker gene sequencing of soils collected from 18 sites throughout North America included in a 100‐day laboratory incubation experiment, we identified a core group of abundant and nearly ubiquitous soil microbes that shift in relative abundance with elevated soil temperatures. We then validated and narrowed our list of temperature‐sensitive microbes by comparing the results from this laboratory experiment with data compiled from 210 soils representing multiple, independent global field studies sampled across spatial gradients with a wide range in mean annual temperatures. Our results reveal predictable and consistent responses to temperature for a core group of 189 ubiquitous soil bacterial and archaeal taxa, with these taxa exhibiting similar temperature responses across a broad range of soil types. These microbial ‘bioindicators’ are useful for understanding how soil microbial communities respond to warming and to discriminate between the direct and indirect effects of soil warming on microbial communities. Those taxa that were found to be sensitive to temperature represented a wide range of lineages and the direction of the temperature responses were not predictable from phylogeny alone, indicating that temperature responses are difficult to predict from simply describing soil microbial communities at broad taxonomic or phylogenetic levels of resolution. Together, these results lay the foundation for a more predictive understanding of how soil microbial communities respond to soil warming and how warming may ultimately lead to changes in soil biogeochemical processes.     

Convergent ecosystem responses to 23‐year ambient and manipulated warming link advancing snowmelt and shrub encroachment to transient and long‐term climate–soil carbon feedback

Ecosystem responses to climate change can exert positive or negative feedbacks on climate, mediated in part by slow‐moving factors such as shifts in vegetation community composition. Long‐term experimental manipulations can be used to examine such ecosystem responses, but they also present another opportunity: inferring the extent to which contemporary climate change is responsible for slow changes in ecosystems under ambient conditions. Here, using 23 years of data, we document a shift from nonwoody to woody vegetation and a loss of soil carbon in ambient plots and show that these changes track previously shown similar but faster changes under experimental warming. This allows us to infer that climate change is the cause of the observed shifts in ambient vegetation and soil carbon and that the vegetation responses mediate the observed changes in soil carbon. Our findings demonstrate the realism of an experimental manipulation, allow attribution of a climate cause to observed ambient ecosystem changes, and demonstrate how a combination of long‐term study of ambient and experimental responses to warming can identify mechanistic drivers needed for realistic predictions of the conditions under which ecosystems are likely to become carbon sources or sinks over varying timescales.     

Allometric constraints on,and trade‐offs in,belowground carbon allocation and their control of soil respiration across global forest ecosystems

To fully understand how soil respiration is partitioned among its component fluxes and responds to climate, it is essential to relate it to belowground carbon allocation, the ultimate carbon source for soil respiration. This remains one of the largest gaps in knowledge of terrestrial carbon cycling. Here, we synthesize data on gross and net primary production and their components, and soil respiration and its components, from a global forest database, to determine mechanisms governing belowground carbon allocation and their relationship with soil respiration partitioning and soil respiration responses to climatic factors across global forest ecosystems. Our results revealed that there are three independent mechanisms controlling belowground carbon allocation and which influence soil respiration and its partitioning: an allometric constraint; a fine‐root production vs. root respiration trade‐off; and an above‐ vs. belowground trade‐off in plant carbon. Global patterns in soil respiration and its partitioning are constrained primarily by the allometric allocation, which explains some of the previously ambiguous results reported in the literature. Responses of soil respiration and its components to mean annual temperature, precipitation, and nitrogen deposition can be mediated by changes in belowground carbon allocation. Soil respiration responds to mean annual temperature overwhelmingly through an increasing belowground carbon input as a result of extending total day length of growing season, but not by temperature‐driven acceleration of soil carbon decomposition, which argues against the possibility of a strong positive feedback between global warming and soil carbon loss. Different nitrogen loads can trigger distinct belowground carbon allocation mechanisms, which are responsible for different responses of soil respiration to nitrogen addition that have been observed. These results provide new insights into belowground carbon allocation, partitioning of soil respiration, and its responses to climate in forest ecosystems and are, therefore, valuable for terrestrial carbon simulations and projections.     

Circumpolar assessment of permafrost C quality and its vulnerability over time using long‐term incubation data

High‐latitude ecosystems store approximately 1700 Pg of soil carbon (C), which is twice as much C as is currently contained in the atmosphere. Permafrost thaw and subsequent microbial decomposition of permafrost organic matter could add large amounts of C to the atmosphere, thereby influencing the global C cycle. The rates at which C is being released from the permafrost zone at different soil depths and across different physiographic regions are poorly understood but crucial in understanding future changes in permafrost C storage with climate change. We assessed the inherent decomposability of C from the permafrost zone by assembling a database of long‐term (>1 year) aerobic soil incubations from 121 individual samples from 23 high‐latitude ecosystems located across the northern circumpolar permafrost zone. Using a three‐pool (i.e., fast, slow and passive) decomposition model, we estimated pool sizes for C fractions with different turnover times and their inherent decomposition rates using a reference temperature of 5 °C. Fast cycling C accounted for less than 5% of all C in both organic and mineral soils whereas the pool size of slow cycling C increased with C : N. Turnover time at 5 °C of fast cycling C typically was below 1 year, between 5 and 15 years for slow turning over C, and more than 500 years for passive C. We project that between 20 and 90% of the organic C could potentially be mineralized to CO₂ within 50 incubation years at a constant temperature of 5 °C, with vulnerability to loss increasing in soils with higher C : N. These results demonstrate the variation in the vulnerability of C stored in permafrost soils based on inherent differences in organic matter decomposability, and point toward C : N as an index of decomposability that has the potential to be used to scale permafrost C loss across landscapes.     

Plant phenological responses to a long‐term experimental extension of growing season and soil warming in the tussock tundra of Alaska

Climate warming is strongly altering the timing of season initiation and season length in the Arctic. Phenological activities are among the most sensitive plant responses to climate change and have important effects at all levels within the ecosystem. We tested the effects of two experimental treatments, extended growing season via snow removal and extended growing season combined with soil warming, on plant phenology in tussock tundra in Alaska from 1995 through 2003. We specifically monitored the responses of eight species, representing four growth forms: (i) graminoids (Carex bigellowii and Eriophorum vaginatum); (ii) evergreen shrubs (Ledum palustre, Cassiope tetragona, and Vaccinium vitis‐idaea); (iii) deciduous shrubs (Betula nana and Salix pulchra); and (iv) forbs (Polygonum bistorta). Our study answered three questions: (i) Do experimental treatments affect the timing of leaf bud break, flowering, and leaf senescence? (ii) Are responses to treatments species‐specific and growth form‐specific? and (iii) Which environmental factors best predict timing of phenophases? Treatment significantly affected the timing of all three phenophases, although the two experimental treatments did not differ from each other. While phenological events began earlier in the experimental plots relative to the controls, duration of phenophases did not increase. The evergreen shrub, Cassiope tetragona, did not respond to either experimental treatment. While the other species did respond to experimental treatments, the total active period for these species did not increase relative to the control. Air temperature was consistently the best predictor of phenology. Our results imply that some evergreen shrubs (i.e., C. tetragona) will not capitalize on earlier favorable growing conditions, putting them at a competitive disadvantage relative to phenotypically plastic deciduous shrubs. Our findings also suggest that an early onset of the growing season as a result of decreased snow cover will not necessarily result in greater tundra productivity.     

Response of soil surface CO2 flux in a boreal forest to ecosystem warming

Soil surface carbon dioxide (CO₂) flux (R_S) was measured for 2 years at the Boreal Soil and Air Warming Experiment site near Thompson, MB, Canada. The experimental design was a complete random block design that consisted of four replicate blocks, with each block containing a 15 m × 15 m control and heated plot. Black spruce [Picea mariana (Mill.) BSP] was the overstory species and Epilobium angustifolium was the dominant understory. Soil temperature was maintained (～5 °C) above the control soil temperature using electric cables inside water filled polyethylene tubing for each heated plot. Air inside a 7.3‐m‐diameter chamber, centered in the soil warming plot, contained approximately nine black spruce trees was heated ～5 °C above control ambient air temperature allowing for the testing of soil‐only warming and soil+air warming. Soil surface CO₂ flux (R_S) was positively correlated (P < 0.0001) to soil temperature at 10 cm depth. Soil surface CO₂ flux (R_S) was 24% greater in the soil‐only warming than the control in 2004, but was only 11% greater in 2005, while R_S in the soil+air warming treatments was 31% less than the control in 2004 and 23% less in 2005. Live fine root mass (< 2 mm diameter) was less in the heated than control treatments in 2004 and statistically less (P < 0.01) in 2005. Similar root mass between the two heated treatments suggests that different heating methods (soil‐only vs. soil+air warming) can affect the rate of decomposition.     

Amazon Basin climate under global warming: the role of the sea surface temperature

The Hadley Centre coupled climate-carbon cycle model (HadCM3LC) predicts loss of the Amazon rainforest in response to future anthropogenic greenhouse gas emissions. In this study, the atmospheric component of HadCM3LC is used to assess the role of simulated changes in mid-twenty-first century sea surface temperature (SST) in Amazon Basin climate change. When the full HadCM3LC SST anomalies (SSTAs) are used, the atmosphere model reproduces the Amazon Basin climate change exhibited by HadCM3LC, including much of the reduction in Amazon Basin rainfall. This rainfall change is shown to be the combined effect of SSTAs in both the tropical Atlantic and the Pacific, with roughly equal contributions from each basin. The greatest rainfall reduction occurs from May to October, outside of the mature South American monsoon (SAM) season. This dry season response is the combined effect of a more rapid warming of the tropical North Atlantic relative to the south, and warm SSTAs in the tropical east Pacific. Conversely, a weak enhancement of mature SAM season rainfall in response to Atlantic SST change is suppressed by the atmospheric response to Pacific SST. This net wet season response is sufficient to prevent dry season soil moisture deficits from being recharged through the SAM season, leading to a perennial soil moisture reduction and an associated 30% reduction in annual Amazon Basin net primary productivity (NPP). A further 23% NPP reduction occurs in response to a 3.5 degrees C warmer air temperature associated with a global mean SST warming.     

Accumulation of soil carbon under elevated CO2 unaffected by warming and drought

Elevated atmospheric CO₂ concentration and climate change may substantially alter soil carbon (C) dynamics, which in turn may impact future climate through feedback cycles. However, only very few field experiments worldwide have combined elevated CO₂ (eCO₂) with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of C cycling in ecosystems. We exposed a temperate heath/grassland to eCO₂, warming, and drought, in all combinations for 8 years. At the end of the study, soil C stocks were on average 0.927 kg C/m² higher across all treatment combinations with eCO₂ compared to ambient CO₂ treatments (equal to an increase of 0.120 ± 0.043 kg C m^?2 year^?1), and showed no sign of slowed accumulation over time. However, if observed pretreatment differences in soil C are taken into account, the annual rate of increase caused by eCO₂ may be as high as 0.177 ± 0.070 kg C m^?2 year^?1. Furthermore, the response to eCO₂ was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO₂ observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO₂ concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long‐term measurements of changes in soil C in response to the three major climate change‐related global changes, eCO₂, warming, and changes in precipitation patterns, are, therefore, urgently needed.     

Effects of seasonal and decadal warming on soil enzymatic activity in a P‐deficient Mediterranean shrubland

Soil enzymes are central in the response of terrestrial ecosystems to climate change, and their study can be crucial for the models’ implementation. We investigated for 1 year the effects of warming and seasonality on the potential activities of five soil extracellular enzymes and their relationships with soil moisture, phosphorus (P) concentration, and other soil parameters in a P‐limited Mediterranean semiarid shrubland. The site was continuously subjected to warming since 1999, and we compared data from this study to analogous data from 2004. Warming uniformly increased all enzymes activities, but only when a sufficient amount of soil water was available. Seasonality unevenly altered enzyme activities, thus affecting enzymatic stoichiometry. P deficiency affected enzymatic stoichiometry, favoring the activities of the phosphatases. The effect of warming was stronger in 2014 than 2004, excluding the hypothesis of acclimation of rhizospheric responses to higher temperatures and suggesting that further increases in extracellular enzymatic activities are to be expected if sufficient water is available. Climatic warming will likely generally stimulate soil enzymatic activities and accelerate nutrient mineralization and similar ecological processes such as the production and degradation of biomass and changes in community composition, but which will be limited by water availability, especially in Mediterranean soils in summer. Winters in such ecosystems will benefit from a general increase in activity and production, but biological activity could even decrease in summer, potentially leading to a negative overall balance of nutrient mineralization. This study suggests that a general increase in activity due to warming could lead to faster mineralization of soil organic matter and water consumption in colder climates, until one of these factors in turn becomes limiting. Such trade‐offs between water and temperature in relation with enzyme activity should be considered in biogeochemical models.