Effects of Substrate Addition on Soil Respiratory Carbon Release Under Long-Term Warming and Clipping in a Tallgrass Prairie

Regulatory mechanisms of soil respiratory carbon (C) release induced by substrates (i.e., plant derived substrates) are critical for predicting ecosystem responses to climate change, but the mechanisms are not well understood. In this study, we sampled soils from a long-term field manipulative experiment and conducted a laboratory incubation to explore the role of substrate supply in regulating the differences in soil C release among the experimental treatments, including control, warming, clipping, and warming plus clipping. Three types of substrates (glucose, C₃ and C₄ plant materials) were added with an amount equal to 1% of soil dry weight under the four treatments. We found that the addition of all three substrates significantly stimulated soil respiratory C release in all four warming and clipping treatments. In soils without substrate addition, warming significantly stimulated soil C release but clipping decreased it. However, additions of glucose and C₃ plant materials (C₃ addition) offset the warming effects, whereas C₄ addition still showed the warming-induced stimulation of soil C release. Our results suggest that long-term warming may inhibit microbial capacity for decomposition of C₃ litter but may enhance it for decomposition of C₄ litter. Such warming-induced adaptation of microbial communities may weaken the positive C-cycle feedback to warming due to increased proportion of C₄ species in plant community and decreased litter quality. In contrast, clipping may weaken microbial capacity for warming-induced decomposition of C₄ litter but may enhance it for C₃ litter. Warming- and clipping-induced shifts in microbial metabolic capacity may be strongly associated with changes in plant species composition and could substantially influence soil C dynamics in response to global change.     

Low soil moisture suppresses the thermal compensatory response of microbial respiration

The thermal compensatory response of microbial respiration contributes to a decrease in warming-induced enhancement of soil respiration over time, which could weaken the positive feedback between the carbon cycle and climate warming. Climate warming is also predicted to cause a worldwide decrease in soil moisture, which has an effect on the microbial metabolism of soil carbon. However, whether and how changes in moisture affect the thermal compensatory response of microbial respiration are unexplored. Here, using soils from an 8-year warming experiment in an alpine grassland, we assayed the thermal response of microbial respiration rates at different soil moisture levels. The results showed that relatively low soil moisture suppressed the thermal compensatory response of microbial respiration, leading to an enhanced response to warming. A subsequent moisture incubation experiment involving off-plot soils also showed that the response of microbial respiration to 100 d warming shifted from a slight compensatory response to an enhanced response with decreasing incubation moisture. Further analysis revealed that such respiration regulation by moisture was associated with shifts in enzymatic activities and carbon use efficiency. Our findings suggest that future drought induced by climate warming might weaken the thermal compensatory capacity of microbial respiration, with important consequences for carbon–climate feedback.     

Soil microbial responses to warming and increased precipitation and their implications for ecosystem C cycling

A better understanding of soil microbial ecology is critical to gaining an understanding of terrestrial carbon (C) cycle–climate change feedbacks. However, current knowledge limits our ability to predict microbial community dynamics in the face of multiple global change drivers and their implications for respiratory loss of soil carbon. Whether microorganisms will acclimate to climate warming and ameliorate predicted respiratory C losses is still debated. It also remains unclear how precipitation, another important climate change driver, will interact with warming to affect microorganisms and their regulation of respiratory C loss. We explore the dynamics of microorganisms and their contributions to respiratory C loss using a 4-year (2006–2009) field experiment in a semi-arid grassland with increased temperature and precipitation in a full factorial design. We found no response of mass-specific (per unit microbial biomass C) heterotrophic respiration to warming, suggesting that respiratory C loss is directly from microbial growth rather than total physiological respiratory responses to warming. Increased precipitation did stimulate both microbial biomass and mass-specific respiration, both of which make large contributions to respiratory loss of soil carbon. Taken together, these results suggest that, in semi-arid grasslands, soil moisture and related substrate availability may inhibit physiological respiratory responses to warming (where soil moisture was significantly lower), while they are not inhibited under elevated precipitation. Although we found no total physiological response to warming, warming increased bacterial C utilization (measured by BIOLOG EcoPlates) and increased bacterial oxidation of carbohydrates and phenols. Non-metric multidimensional scaling analysis as well as ANOVA testing showed that warming or increased precipitation did not change microbial community structure, which could suggest that microbial communities in semi-arid grasslands are already adapted to fluctuating climatic conditions. In summary, our results support the idea that microbial responses to climate change are multifaceted and, even with no large shifts in community structure, microbial mediation of soil carbon loss could still occur under future climate scenarios.     

Predominant role of water in regulating soil and microbial respiration and their responses to climate change in a semiarid grassland

Climate change can profoundly impact carbon (C) cycling of terrestrial ecosystems. A field experiment was conducted to examine responses of total soil and microbial respiration, and microbial biomass to experimental warming and increased precipitation in a semiarid temperate steppe in northern China since April 2005. We measured soil respiration twice a month over the growing seasons, soil microbial biomass C (MBC) and N (MBN), microbial respiration (MR) once a year in the middle growing season from 2005 to 2007. The results showed that interannual variations in soil respiration, MR, and microbial biomass were positively related to interannual fluctuations in precipitation. Laboratory incubation with a soil moisture gradient revealed a constraint of the temperature responses of MR by low soil moisture contents. Across the 3 years, experimental warming decreased soil moisture, and consequently caused significant reductions in total and microbial respiration, and microbial biomass, suggesting stronger negatively indirect effects through warming‐induced water stress than the positively direct effects of elevated temperature. Increased evapotranspiration under experimental warming could have reduced soil water availability below a stress threshold, thus leading to suppression of plant growth, root and microbial activities. Increased precipitation significantly stimulated total soil and microbial respiration and all other microbial parameters and the positive precipitation effects increased over time. Our results suggest that soil water availability is more important than temperature in regulating soil and microbial respiratory processes, microbial biomass and their responses to climate change in the semiarid temperate steppe. Experimental warming caused greater reductions in soil respiration than in gross ecosystem productivity (GEP). In contrast, increased precipitation stimulated GEP more than soil respiration. Our observations suggest that climate warming may cause net C losses, whereas increased precipitation may lead to net C gains in the semiarid temperate steppe. Our findings highlight that unless there is concurrent increase in precipitation, the temperate steppe in the arid and semiarid regions of northern China may act as a net C source under climate warming.     

Ecosystem Carbon Fluxes in Response to Warming and Clipping in a Tallgrass Prairie

Global warming and land-use change could have profound impacts on ecosystem carbon (C) fluxes, with consequent changes in C sequestration and its feedback to climate change. However, it is not well understood how net ecosystem C exchange (NEE) and its components respond to warming and mowing in tallgrass prairie. We conducted two warming experiments, one long term with a 1.7°C increase in a C₄-dominated grassland (Experiment 1), and one short term with a 2.8°C increase in a C₃-dominated grassland (Experiment 2), to investigate main and interactive effects of warming and clipping on ecosystem C fluxes in the Great Plains of North America during 2009–2011. An infrared radiator was used to simulate climate warming and clipping once a year mimicked mowing in both experiments. The results showed that warming significantly increased ecosystem respiration (ER), slightly increased GPP, with the net outcome (NEE) being little changed in Experiment 1. In contrast, warming significantly suppressed GPP and ER in both years, with the net outcome being enhanced in NEE (more C sequestration) in 2009–2010 in Experiment 2. The C₄-dominated grassland showed a much higher optimum temperature for C fluxes than the C₃-dominated grassland, which may partly contribute to the different warming effects in the two experiments. Clipping significantly enhanced GPP, ER, and NEE in both experiments but did not significantly interact with warming in impacting C fluxes in either experiment. The warming-induced changes in ecosystem C fluxes correlated significantly with C₄ biomass proportion but not with warming-induced changes in either soil temperature or soil moisture across the plots in the experiments. Our results demonstrate that carbon fluxes in the tallgrass prairie are highly sensitive to climate warming and clipping, and C₃/C₄ plant functional types may be important factor in determining ecosystem response to climate change.     

Carbon dynamics at frost-patterned tundra driven by long-term vegetation change rather than by short-term non-growing season warming

Frost-patterned grounds, such as mostly barren frost boils surrounded by denser vegetation, are typical habitat mosaics in tundra. Plant and microbial processes in these habitats may be susceptible to short-term warming outside the growing season, while the areal cover of barren frost boils has decreased during the past decades due to climate warming-induced shrub expansion. The relative importance of such short-term and long-term climate impacts on carbon (C) dynamics remains unknown. We measured ecosystem CO₂ uptake and release (in the field), microbial respiration (in the laboratory), as well as microbial biomass N and soil extractable N in frost boils and the directly adjacent heath in late spring and late summer. These habitats had been experimentally warmed with insulating fleeces from late September until late May for three consecutive years, which allowed us to investigate the direct short-term effects of warming and longer-term, indirect climate effects via vegetation establishment into frost boils. Non-growing season warming increased C uptake at the frost boils in late spring and decreased it in late summer, while the timing and direction of responses was opposite for the heath. Experimental warming had no effects on microbial or ecosystem C release or soil N at either of the habitats. However, C cycling was manifold higher at the heath compared to the frost boils, likely because of a higher SOM stock in the soil. Short-term climate change can thus directly alter ecosystem C uptake at frost-patterned grounds but will most likely not affect microbial C release. We conclude that the C dynamics at frost-patterned grounds under a changing climate depend most strongly on the potential of vegetation to encroach into frost boils in the long-term.     

全球气候变暖对凋落物分解的影响

凋落物分解作为生态系统核心过程,参与生态系统碳的周转与循环,影响生态系统碳的收支平衡,调控生态系统对全球气候变暖的反馈结果。全球气候变暖通过环境因素、凋落物数量和质量以及分解者3个方面,直接或间接地作用于凋落物分解过程,并进一步影响土壤养分周转和碳库动态。气候变暖可通过升高温度和改变实际蒸散量等环境因素直接作用于凋落物分解。气候变暖可引起植物物种短期内碳、氮和木质素等化学性质的改变以及群落中物种组成的长期变化从而改变凋落物质量。在凋落物分解过程中,土壤分解者亚系统作为主要生命组分(土壤动物和微生物)彼此相互作用、相互协调共同参与调节凋落物的分解过程。凋落物分解可以通过改变土壤微生物量、微生物活动和群落结构来加快微生物养分的固定或矿化,以形成新的养分利用模式来改变土壤有机质从而对气候变化做出响应。未来凋落物分解的研究方向应基于大尺度跨区域分解实验和长期实验,关注多个因子交互影响下,分解过程中碳、氮养分释放、地上/地下凋落物分解生物学过程与联系、分解者亚系统营养级联效应等方面。     

Warmer winters increase the rhizosphere carbon flow to mycorrhizal fungi more than to other microorganisms in a temperate grassland

A decisive set of steps in the terrestrial carbon (C) cycle is the fixation of atmospheric C by plants and the subsequent C‐transfer to rhizosphere microorganisms. With climate change winters are expected to become milder in temperate ecosystems. Although the rate and pathways of rhizosphere C input to soil could be impacted by milder winters, the responses remain unknown. To address this knowledge‐gap, a winter‐warming experiment was established in a seminatural temperate grassland to follow the C flow from atmosphere, via the plants, to different groups of soil microorganisms. In situ ¹³CO₂ pulse labelling was used to track C into signature fatty acids of microorganisms. The winter warming did not result in any changes in biomass of any of the groups of microorganisms. However, the C flow from plants to arbuscular mycorrhizal (AM) fungi, increased substantially by winter warming. Saprotrophic fungi also received large amounts of plant‐derived C—indicating a higher importance for the turnover of rhizosphere C than biomass estimates would suggest—still, this C flow was unaffected by winter warming. AM fungi was the only microbial group positively affected by winter warming—the group with the closest connection to plants. Winter warming resulted in higher plant productivity earlier in the season, and this aboveground change likely induced plant nutrient limitation in warmed plots, thus stimulating the plant dependence on, and C allocation to, belowground nutrient acquisition. The preferential C allocation to AM fungi was at the expense of C flow to other microbial groups, which were unaffected by warming. Our findings imply that warmer winters may shift rhizosphere C‐fluxes to become more AM fungal‐dominated. Surprisingly, the stimulated rhizosphere C flow was matched by increased microbial turnover, leading to no accumulation of soil microbial biomass.     

长期模拟升温对崇明东滩湿地土壤微生物生物量的影响

以崇明东滩芦苇湿地为对象,采用开顶室生长箱(Open top chambers OTCs)原位模拟大气升温试验,研究了连续升温8a对崇明东滩湿地0—40cm土层土壤微生物生物量碳氮含量的影响。结果表明:连续升温显著提高了崇明东滩湿地土壤微生物生物量碳氮含量,从土壤表层到深层(0—10,10—20,20—30,30—40cm),微生物生物量碳分别增加了39.32%、70.79%、65.20%、74.09%,微生物生物量氮分别增加了66.46%、178.27%、47.24%、64.11%。但升温对土壤微生物生物量的影响因不同土层和不同季节并未表现出统一的规律,长期模拟升温显著提高4月0—20cm土层和7月0—40cm土层微生物生物量碳氮含量,对10月0—40cm土层微生物生物量碳含量没有影响,但是显著提高了10月0—40cm土层微生物生物量氮含量,同时,微生物生物量碳氮比在7月也显著提高。相关分析表明:无论在升温条件还是在对照条件下,土壤温度、含水量、总氮与土壤微生物生物量碳氮及微生物生物量碳氮比均无相关关系,升温条件下,有机碳与微生物生物量碳氮含量以及微生物生物量碳氮比呈显著正相关,但是在对照条件下有机碳与微生物生物量碳氮含量以及微生物生物量碳氮比呈显著负相关。因此,土壤有机碳是影响土壤微生物生物量碳氮含量对长期模拟升温响应的重要生态因子。     

Climate Change Affects Carbon Allocation to the Soil in Shrublands

Climate change may affect ecosystem functioning through increased temperatures or changes in precipitation patterns. Temperature and water availability are important drivers for ecosystem processes such as photosynthesis, carbon translocation, and organic matter decomposition. These climate changes may affect the supply of carbon and energy to the soil microbial population and subsequently alter decomposition and mineralization, important ecosystem processes in carbon and nutrient cycling. In this study, carried out within the cross-European research project CLIMOOR, the effect of climate change, resulting from imposed manipulations, on carbon dynamics in shrubland ecosystems was examined. We performed a ¹⁴C-labeling experiment to probe changes in net carbon uptake and allocation to the roots and soil compartments as affected by a higher temperature during the year and a drought period in the growing season. Differences in climate, soil, and plant characteristics resulted in a gradient in the severity of the drought effects on net carbon uptake by plants with the impact being most severe in Spain, followed by Denmark, with the UK showing few negative effects at significance levels of p 0.10. Drought clearly reduced carbon flow from the roots to the soil compartments. The fraction of the ¹⁴C fixed by the plants and allocated into the soluble carbon fraction in the soil and to soil microbial biomass in Denmark and the UK decreased by more than 60%. The effects of warming were not significant, but, as with the drought treatment, a negative effect on carbon allocation to soil microbial biomass was found. The changes in carbon allocation to soil microbial biomass at the northern sites in this study indicate that soil microbial biomass is a sensitive, early indicator of drought- or temperature-initiated changes in these shrubland ecosystems. The reduced supply of substrate to the soil and the response of the soil microbial biomass may help to explain the observed acclimation of CO₂ exchange in other ecosystems.     

Additive Effects of Warming and Increased Nitrogen Deposition in a Temperate Old Field: Plant Productivity and the Importance of Winter

Both climate warming and atmospheric nitrogen (N) deposition are predicted to alter plant productivity and species composition over the next century. However, the extent to which their effects may interact is unclear. For example, over winter, the effects of warming on soil freezing dynamics may promote ecosystem N losses, thereby limiting increases in productivity in response to warming, yet these losses may be compensated for by increased N deposition. We measured plant production and species composition in response to warming (winter-only or year-round) and N addition in a temperate old field. We used shoot allometric relationships to estimate aboveground production non-destructively and sampled root biomass destructively throughout two growing seasons. We also used spectral data (normalized difference vegetation index—NDVI) to examine the treatment effects on the timing of plant green-up and senescence. In 2007, which featured an exceptionally dry summer, there were no treatment effects on plant growth. However, in 2008, warming (both winter-only and year-round) and N addition combined approximately doubled aboveground productivity, and these effects were additive. Warming increased root biomass, but no N effect was evident. Conversely, N addition increased NDVI, but NDVI was unresponsive to warming. Overall, our results do not support the hypothesis that warming-induced changes to soil freezing dynamics limit plant productivity in our system. On the contrary, they demonstrate that winter warming alone can increase primary productivity to the same extent as year-round warming, and that this effect may interact very strongly with interannual variation in precipitation.     

Microbial responses to inorganic nutrient amendment overridden by warming: Consequences on soil carbon stability

Eutrophication and climate warming, induced by anthropogenic activities, are simultaneously occurring worldwide and jointly affecting soil carbon stability. Therefore, it is of great interest to examine whether and how they interactively affect soil microbial community, a major soil carbon driver. Here, we showed that climate warming, simulated by southward transferring Mollisol soil in agricultural ecosystems from the cold temperate climate zone (N) to warm temperate climate (C) and subtropical climate zone (S), decreased soil organic matter (SOM) by 6%–12%. In contrast, amendment with nitrogen, phosphorus and potassium enhanced plant biomass by 97% and SOM by 6% at the N site, thus stimulating copiotrophic taxa but reducing oligotrophic taxa in relative abundance. However, microbial responses to nutrient amendment were overridden by soil transfer in that nutrient amendment had little effect at the C site but increased recalcitrant carbon‐degrading fungal Agaricomycetes and Microbotryomycetes taxa derived from Basidiomycota by 4‐17 folds and recalcitrant carbon‐degrading genes by 23%–40% at the S site, implying a possible priming effect. Consequently, SOM at the S site was not increased by nutrient amendment despite increased plant biomass by 108%. Collectively, we demonstrate that soil transfer to warmer regions overrides microbial responses to nutrient amendment and weakens soil carbon sequestration.     

Interactive effects of wildfire and permafrost on microbial communities and soil processes in an Alaskan black spruce forest

Boreal forests contain significant quantities of soil carbon that may be oxidized to CO₂ given future increases in climate warming and wildfire behavior. At the ecosystem scale, decomposition and heterotrophic respiration are strongly controlled by temperature and moisture, but we questioned whether changes in microbial biomass, activity, or community structure induced by fire might also affect these processes. We particularly wanted to understand whether postfire reductions in microbial biomass could affect rates of decomposition. Additionally, we compared the short‐term effects of wildfire to the long‐term effects of climate warming and permafrost decline. We compared soil microbial communities between control and recently burned soils that were located in areas with and without permafrost near Delta Junction, AK. In addition to soil physical variables, we quantified changes in microbial biomass, fungal biomass, fungal community composition, and C cycling processes (phenol oxidase enzyme activity, lignin decomposition, and microbial respiration). Five years following fire, organic surface horizons had lower microbial biomass, fungal biomass, and dissolved organic carbon (DOC) concentrations compared with control soils. Reductions in soil fungi were associated with reductions in phenol oxidase activity and lignin decomposition. Effects of wildfire on microbial biomass and activity in the mineral soil were minor. Microbial community composition was affected by wildfire, but the effect was greater in nonpermafrost soils. Although the presence of permafrost increased soil moisture contents, effects on microbial biomass and activity were limited to mineral soils that showed lower fungal biomass but higher activity compared with soils without permafrost. Fungal abundance and moisture were strong predictors of phenol oxidase enzyme activity in soil. Phenol oxidase enzyme activity, in turn, was linearly related to both ¹³C lignin decomposition and microbial respiration in incubation studies. Taken together, these results indicate that reductions in fungal biomass in postfire soils and lower soil moisture in nonpermafrost soils reduced the potential of soil heterotrophs to decompose soil carbon. Although in the field increased rates of microbial respiration can be observed in postfire soils due to warmer soil conditions, reductions in fungal biomass and activity may limit rates of decomposition.     

增温对高寒灌丛根际和非根际土壤微生物生物量碳氮的影响

本文对青藏高原东缘窄叶鲜卑花高寒灌丛生长季根际和非根际土壤微生物生物量碳和氮对增温的响应进行研究.结果表明: 窄叶鲜卑花灌丛生长季初期根际和非根际土壤微生物生物量碳和氮均显著高于生长季中期和末期.在多数时期,增温对根际土壤微生物生物量碳和氮的影响不显著.在非根际土壤中,增温对土壤微生物生物量碳和氮的影响因不同生长季节而不同: 增温使生长季初期土壤微生物生物量碳显著降低,而使土壤微生物生物量氮显著提高;生长季中期增温使土壤微生物生物量碳和氮显著提高;而在生长季末期增温对土壤微生物生物量碳和氮的影响不显著.土壤微生物生物量碳和氮的根际效应也因不同生长季节而不同: 土壤微生物生物量碳和氮在生长季初期表现为负根际效应,而在生长季中期表现为正根际效应;在生长季末期,土壤微生物生物量碳表现为负根际效应,土壤微生物生物量氮则表现为正根际效应.增温在生长季初期使土壤微生物生物量碳和氮的根际效应显著提高,而在生长季中期和末期使土壤微生物生物量碳和氮的根际效应降低.本研究初步阐明了气候变暖背景下高寒灌丛根际和非根际土壤生物学过程变化机理.     

川西高山林线土壤活性碳、氮对短期增温的响应

随着温室效应的加剧,受低温限制的高山林线生态系统对全球气候变暖较为敏感,可能直接影响到植物的生长和土壤碳氮过程.本研究假设气候变暖会改变高山生态系统土壤活性碳氮含量,在四川省理县米亚罗高山生态系统定位站,采用开顶式模拟增温装置(OTC)模拟增温对土壤活性碳、氮的短期影响.分别于2017年4、7和10月,采集OTC以及对照样地(CK)内土壤有机层和矿质土壤层的原状土壤,测定土壤可溶性有机碳(DOC)、土壤微生物生物量碳(MBC)、土壤可溶性有机氮(DON)和土壤微生物生物量氮(MBN)含量.结果表明: 模拟增温使年均气温升高0.88 ℃,土壤有机层和矿质土壤层的年均温度分别提高0.48和0.23 ℃.模拟增温没有显著改变土壤有机质和含水量,但显著提高了矿质土壤层的pH值,同时显著降低了非生长季矿质土壤层的DOC、DON含量;季节变化对两个层次的DOC、DON和MBN含量有极显著影响,而MBC没有明显的季节动态;增温和季节交互作用对矿质土壤层的DOC和DON有显著影响.土壤有机层的MBC、MBN含量显著高于矿质土壤层.土壤活性碳、氮与土壤有机质和含水量呈极显著正相关,MBC、MBN与土壤pH呈极显著正相关,MBN与土壤温度呈显著负相关.     

Plant diversity does not buffer drought effects on early‐stage litter mass loss rates and microbial properties

Human activities are decreasing biodiversity and changing the climate worldwide. Both global change drivers have been shown to affect ecosystem functioning, but they may also act in concert in a non‐additive way. We studied early‐stage litter mass loss rates and soil microbial properties (basal respiration and microbial biomass) during the summer season in response to plant species richness and summer drought in a large grassland biodiversity experiment, the Jena Experiment, Germany. In line with our expectations, decreasing plant diversity and summer drought decreased litter mass loss rates and soil microbial properties. In contrast to our hypotheses, however, this was only true for mass loss of standard litter (wheat straw) used in all plots, and not for plant community‐specific litter mass loss. We found no interactive effects between global change drivers, that is, drought reduced litter mass loss rates and soil microbial properties irrespective of plant diversity. High mass loss rates of plant community‐specific litter and low responsiveness to drought relative to the standard litter indicate that soil microbial communities were adapted to decomposing community‐specific plant litter material including lower susceptibility to dry conditions during summer months. Moreover, higher microbial enzymatic diversity at high plant diversity may have caused elevated mass loss of standard litter. Our results indicate that plant diversity loss and summer drought independently impede soil processes. However, soil decomposer communities may be highly adapted to decomposing plant community‐specific litter material, even in situations of environmental stress. Results of standard litter mass loss moreover suggest that decomposer communities under diverse plant communities are able to cope with a greater variety of plant inputs possibly making them less responsive to biotic changes.     

Nitrogen availability mediates soil carbon cycling response to climate warming: A meta-analysis

Global climate warming may induce a positive feedback through increasing soil carbon (C) release to the atmosphere. Although warming can affect both C input to and output from soil, direct and convincing evidence illustrating that warming induces a net change in soil C is still lacking. We synthesized the results from field warming experiments at 165 sites across the globe and found that climate warming had no significant effect on soil C stock. On average, warming significantly increased root biomass and soil respiration, but warming effects on root biomass and soil respiration strongly depended on soil nitrogen (N) availability. Under high N availability (soil C:N ratio < 15), warming had no significant effect on root biomass, but promoted the coupling between effect sizes of root biomass and soil C stock. Under relative N limitation (soil C:N ratio > 15), warming significantly enhanced root biomass. However, the enhancement of root biomass did not induce a corresponding C accumulation in soil, possibly because warming promoted microbial CO₂ release that offset the increased root C input. Also, reactive N input alleviated warming-induced C loss from soil, but elevated atmospheric CO₂ or precipitation increase/reduction did not. Together, our findings indicate that the relative availability of soil C to N (i.e., soil C:N ratio) critically mediates warming effects on soil C dynamics, suggesting that its incorporation into C-climate models may improve the prediction of soil C cycling under future global warming scenarios.     

Light and heavy fractions of soil organic matter in response to climate warming and increased precipitation in a temperate steppe

Soil is one of the most important carbon (C) and nitrogen (N) pools and plays a crucial role in ecosystem C and N cycling. Climate change profoundly affects soil C and N storage via changing C and N inputs and outputs. However, the influences of climate warming and changing precipitation regime on labile and recalcitrant fractions of soil organic C and N remain unclear. Here, we investigated soil labile and recalcitrant C and N under 6 years' treatments of experimental warming and increased precipitation in a temperate steppe in Northern China. We measured soil light fraction C (LFC) and N (LFN), microbial biomass C (MBC) and N (MBN), dissolved organic C (DOC) and heavy fraction C (HFC) and N (HFN). The results showed that increased precipitation significantly stimulated soil LFC and LFN by 16.1% and 18.5%, respectively, and increased LFC:HFC ratio and LFN:HFN ratio, suggesting that increased precipitation transferred more soil organic carbon into the quick-decayed carbon pool. Experimental warming reduced soil labile C (LFC, MBC, and DOC). In contrast, soil heavy fraction C and N, and total C and N were not significantly impacted by increased precipitation or warming. Soil labile C significantly correlated with gross ecosystem productivity, ecosystem respiration and soil respiration, but not with soil moisture and temperature, suggesting that biotic processes rather than abiotic factors determine variations in soil labile C. Our results indicate that certain soil carbon fraction is sensitive to climate change in the temperate steppe, which may in turn impact ecosystem carbon fluxes in response and feedback to climate change.     

Influence of decomposer food web structure and nitrogen availability on plant growth

We studied the sensitivity of soil microbial communities and ecosystem processes to variation in the vertical and horizontal structure of decomposer food web under nitrogen poor and N-enriched conditions. Microcosms with humus and litter layer of boreal forest floor, birch seedlings infected with mycorrhizal fungi, and decomposer food webs with differing trophic group and species composition of soil fauna were constructed. During the second growing period for the birch, we irrigated half of the microcosms with urea solution, and the other half with de-ionised water to create two levels of N concentration in the substrate. During the experiment night time respirations of the microcosms were measured, and the water leached through the microcosms was analysed for concentration of mineral N, and nematode numbers. The microcosms were destructively sampled after 37 weeks for plant biomass and N uptake, structure of soil animal and microbial community (indicated by PLFA profiles), and physical and chemical properties of the humus and litter materials. Predatory mites and nematodes had a negative influence on the biomass of their microbivorous and microbi-detritivorous prey, and microbi-detritivores affected the biomass and community structure of microbes (indicated by PLFA-analysis). Moreover, predatory mites and nematodes increased microbial biomass and changed the microbial community structure. The decomposer food web structure affected also N uptake and growth of plants. Microbi-detritivorous fauna had a positive effect, whereas predators of microbial and detritus feeding fauna exerted a negative influence on plant N uptake and biomass production. The impact of a trophic group on the microbes and plant was also strongly dependent on species composition within the group. Nitrogen addition magnified the influence of food web structure on microbial biomass and plant N uptake. We suggest that addition of urea-N to the soil modified the animal-microbe interaction by increasing microbial growth and altering community structure of microbes. The presence of microbi-detritivores and predators reduced loss of carbon from the microcosms, and the food web structure influenced also water holding capacity of the materials. The changes in plant growth, nutrient cycling, size of N and C pools, and in the physical properties of the soil emphasize the importance and diversity of indirect consequences of decomposer food web structure. This revised version was published online in June 2006 with corrections to the Cover Date.     

Geothermally warmed soils reveal persistent increases in the respiratory costs of soil microbes contributing to substantial C losses

Increasing temperatures can accelerate soil organic matter decomposition and release large amounts of CO₂ to the atmosphere, potentially inducing positive warming feedbacks. Alterations to the temperature sensitivity and physiological functioning of soil microorganisms may play a key role in these carbon (C) losses. Geothermally active areas in Iceland provide stable and continuous soil temperature gradients to test this hypothesis, encompassing the full range of warming scenarios projected by the Intergovernmental Panel on Climate Change for the northern region. We took soils from these geothermal sites 7 years after the onset of warming and incubated them at varying temperatures and substrate availability conditions to detect persistent alterations of microbial physiology to long-term warming. Seven years of continuous warming ranging from 1.8 to 15.9 °C triggered a 8.6–58.0% decrease on the C concentrations in the topsoil (0–10 cm) of these sub-arctic silt-loam Andosols. The sensitivity of microbial respiration to temperature (Q₁₀) was not altered. However, soil microbes showed a persistent increase in their microbial metabolic quotients (microbial respiration per unit of microbial biomass) and a subsequent diminished C retention in biomass. After an initial depletion of labile soil C upon soil warming, increasing energy costs of metabolic maintenance and resource acquisition led to a weaker capacity of C stabilization in the microbial biomass of warmer soils. This mechanism contributes to our understanding of the acclimated response of soil respiration to in situ soil warming at the ecosystem level, despite a lack of acclimation at the physiological level. Persistent increases in the respiratory costs of soil microbes in response to warming constitute a fundamental process that should be incorporated into climate change-C cycling models.