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.     

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.     

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.     

Effects of experimental warming of air,soil and permafrost on carbon balance in Alaskan tundra

The carbon (C) storage capacity of northern latitude ecosystems may diminish as warming air temperatures increase permafrost thaw and stimulate decomposition of previously frozen soil organic C. However, warming may also enhance plant growth so that photosynthetic carbon dioxide (CO₂) uptake may, in part, offset respiratory losses. To determine the effects of air and soil warming on CO₂ exchange in tundra, we established an ecosystem warming experiment – the Carbon in Permafrost Experimental Heating Research (CiPEHR) project – in the northern foothills of the Alaska Range in Interior Alaska. We used snow fences coupled with spring snow removal to increase deep soil temperatures and thaw depth (winter warming) and open‐top chambers to increase growing season air temperatures (summer warming). Winter warming increased soil temperature (integrated 5–40 cm depth) by 1.5 °C, which resulted in a 10% increase in growing season thaw depth. Surprisingly, the additional 2 kg of thawed soil C m^?2 in the winter warming plots did not result in significant changes in cumulative growing season respiration, which may have been inhibited by soil saturation at the base of the active layer. In contrast to the limited effects on growing‐season C dynamics, winter warming caused drastic changes in winter respiration and altered the annual C balance of this ecosystem by doubling the net loss of CO₂ to the atmosphere. While most changes to the abiotic environment at CiPEHR were driven by winter warming, summer warming effects on plant and soil processes resulted in 20% increases in both gross primary productivity and growing season ecosystem respiration and significantly altered the age and sources of CO₂ respired from this ecosystem. These results demonstrate the vulnerability of organic C stored in near surface permafrost to increasing temperatures and the strong potential for warming tundra to serve as a positive feedback to global climate change.     

Effect of warming and grazing on litter mass loss and temperature sensitivity of litter and dung mass loss on the Tibetan plateau

Knowledge about the role of litter and dung decomposition in nutrient cycling and response to climate change and grazing in alpine ecosystems is still rudimentary. We conducted two separate studies to assess the relative role of warming and grazing on litter mass loss and on the temperature sensitivity of litter and dung mass loss. Experiments were conducted for 1–2 years under a controlled warming–grazing system and along an elevation gradient from 3200 to 3800 m. A free‐air temperature enhancement system (FATE) using infrared heaters and grazing significantly increased soil temperatures (average 0.5–1.6 °C) from 0 to 40 cm depth, but neither warming nor grazing affected soil moisture except early in the growing seasons at 30 cm soil depth. Heaters caused greater soil warming at night‐time compared with daytime, but grazing resulted in greater soil warming during daytime compared with night‐time. Annual average values of the soil temperature at 5 cm were 3.2, 2.4 and 0.3 °C at 3200, 3600 and 3800 m, respectively. Neither warming nor grazing caused changes of litter quality for the first year of the controlled warming–grazing experiment. The effects of warming and grazing on litter mass losses were additive, increasing litter mass losses by about 19.3% and 8.3%, respectively, for the 2‐year decomposition periods. The temperature sensitivity of litter mass losses was approximately 11% °C^?1 based on the controlled warming–grazing experiment. The annual cumulative litter mass loss was approximately 2.5 times that of dung along the elevation gradient. However, the temperature sensitivity (about 18% °C^?1) of the dung mass loss was about three times that of the litter mass loss. These results suggest greater warming at night‐time compared with daytime may accelerate litter mass loss, and grazing will enhance carbon loss to atmosphere in the region through a decrease of litter biomass and an increase of dung production with an increase of stocking rate in future warmer conditions.     

Design and performance of combined infrared canopy and belowground warming in the B4WarmED (Boreal Forest Warming at an Ecotone in Danger) experiment

Conducting manipulative climate change experiments in complex vegetation is challenging, given considerable temporal and spatial heterogeneity. One specific challenge involves warming of both plants and soils to depth. We describe the design and performance of an open‐air warming experiment called Boreal Forest Warming at an Ecotone in Danger (B4WarmED) that addresses the potential for projected climate warming to alter tree function, species composition, and ecosystem processes at the boreal‐temperate ecotone. The experiment includes two forested sites in northern Minnesota, USA, with plots in both open (recently clear‐cut) and closed canopy habitats, where seedlings of 11 tree species were planted into native ground vegetation. Treatments include three target levels of plant canopy and soil warming (ambient, +1.7 °C, +3.4 °C). Warming was achieved by independent feedback control of voltage input to aboveground infrared heaters and belowground buried resistance heating cables in each of 72‐7.0 m² plots. The treatments emulated patterns of observed diurnal, seasonal, and annual temperatures but with superimposed warming. For the 2009 to 2011 field seasons, we achieved temperature elevations near our targets with growing season overall mean differences (?T_below) of +1.84 °C and +3.66 °C at 10 cm soil depth and (?T^above) of +1.82 °C and +3.45 °C for the plant canopies. We also achieved measured soil warming to at least 1 m depth. Aboveground treatment stability and control were better during nighttime than daytime and in closed vs. open canopy sites in part due to calmer conditions. Heating efficacy in open canopy areas was reduced with increasing canopy complexity and size. Results of this study suggest the warming approach is scalable: it should work well in small‐statured vegetation such as grasslands, desert, agricultural crops, and tree saplings (<5 m tall).     

Moderate Warming in Microcosm Experiment Does Not Affect Microbial Communities in Temperate Vineyard Soils

Changes in the soil microbial community structure can lead to dramatic changes in the soil ecosystem. Temperature, which is projected to increase with climate change, is commonly assumed to affect microbial communities, but its effects on agricultural soils are not fully understood. We collected soil samples from six vineyards characterised by a difference of about 2 °C in daily soil temperature over the year and simulated in a microcosm experiment different temperature regimes over a period of 1 year: seasonal fluctuations in soil temperature based on the average daily soil temperature measured in the field; soil temperature warming (2 °C above the normal seasonal temperatures); and constant temperatures normally registered in these temperate soils in winter (3 °C) and in summer (20 °C). Changes in the soil bacterial and fungal community structures were analysed by automated ribosomal intergenic spacer analysis (ARISA). We did not find any effect of warming on soil bacterial and fungal communities, while stable temperatures affected the fungal more than the bacterial communities, although this effect was soil dependent. The soil bacterial community exhibited soil-dependent seasonal fluctuations, while the fungal community was mainly stable. Each soil harbours different microbial communities that respond differently to seasonal temperature fluctuations; therefore, any generalization regarding the effect of climate change on soil communities should be made carefully.     

Thermal adaptation of soil microbial respiration to elevated temperature

In the short‐term heterotrophic soil respiration is strongly and positively related to temperature. In the long‐term, its response to temperature is uncertain. One reason for this is because in field experiments increases in respiration due to warming are relatively short‐lived. The explanations proposed for this ephemeral response include depletion of fast‐cycling, soil carbon pools and thermal adaptation of microbial respiration. Using a > 15 year soil warming experiment in a mid‐latitude forest, we show that the apparent ‘acclimation’ of soil respiration at the ecosystem scale results from combined effects of reductions in soil carbon pools and microbial biomass, and thermal adaptation of microbial respiration. Mass‐specific respiration rates were lower when seasonal temperatures were higher, suggesting that rate reductions under experimental warming likely occurred through temperature‐induced changes in the microbial community. Our results imply that stimulatory effects of global temperature rise on soil respiration rates may be lower than currently predicted.     

Offsetting global warming‐induced elevated greenhouse gas emissions from an arable soil by biochar application

Global warming will likely enhance greenhouse gas (GHG) emissions from soils. Due to its slow decomposability, biochar is widely recognized as effective in long‐term soil carbon (C) sequestration and in mitigation of soil GHG emissions. In a long‐term soil warming experiment (+2.5 °C, since July 2008) we studied the effect of applying high‐temperature Miscanthus biochar (0, 30 t/ha, since August 2013) on GHG emissions and their global warming potential (GWP) during 2 years in a temperate agroecosystem. Crop growth, physical and chemical soil properties, temperature sensitivity of soil respiration (R_s), and metabolic quotient (qCO₂) were investigated to yield further information about single effects of soil warming and biochar as well as on their interactions. Soil warming increased total CO₂ emissions by 28% over 2 years. The effect of warming on soil respiration did not level off as has often been observed in less intensively managed ecosystems. However, the temperature sensitivity of soil respiration was not affected by warming. Overall, biochar had no effect on most of the measured parameters, suggesting its high degradation stability and its low influence on microbial C cycling even under elevated soil temperatures. In contrast, biochar × warming interactions led to higher total N₂O emissions, possibly due to accelerated N‐cycling at elevated soil temperature and to biochar‐induced changes in soil properties and environmental conditions. Methane uptake was not affected by soil warming or biochar. The incorporation of biochar‐C into soil was estimated to offset warming‐induced elevated GHG emissions for 25 years. Our results highlight the suitability of biochar for C sequestration in cultivated temperate agricultural soil under a future elevated temperature. However, the increased N₂O emissions under warming limit the GHG mitigation potential of biochar.     

模拟增温对西藏高原高寒草甸土壤供氮潜力的影响

过去几十年青藏高原呈现显著的增温趋势,冬季增温幅度显著高于生长季的季节非对称特征。气候变暖会对生态系统氮素循环产生重要影响,但关于全年增温与冬季增温对高寒生态系统氮循环的不同影响仍缺乏研究。在青藏高原高寒草甸区开展模拟增温试验,研究季节非对称增温对高寒草甸生态系统氮循环的影响。该试验布设于2010年7月,设置3种处理(不增温、冬季增温与全年增温)。研究结果发现,开顶箱增温装置造成了小环境的暖干化:显著提高了地表空气温度和表层土壤温度,降低了表层土壤含水量。冬季增温会加剧土壤中氮素的流失,所以在经历了冬季增温后土壤氮含量显著降低;在生长季节,土壤氮素周转速率受土壤水分的调控,在降雨较少的季节,增温引起的土壤含水量降低会抑制土壤氮周转速率。对于土壤微生物量而言,高寒草甸土壤微生物量碳表现出明显的季节动态,在生长季旺盛期较低,在生长季末期和初冬季节反而较高,这说明为了降低对土壤养分的竞争,高寒草甸植物氮吸收与土壤微生物氮固持在时间上存在分离。研究结果表明,冬季增温导致的土壤养分含量变化会影响随后生长季植物群落的生产力、结构组成与碳氮循环等过程,对生态系统过程产生深远的影响。     

A method for experimental heating of intact soil profiles for application to climate change experiments

A new system for simulating future belowground temperature increases was conceived, simulated, constructed and tested in a temperate deciduous forest in Oak Ridge, TN, USA. The new system uses low‐wattage, 3 m deep heaters installed around the circumference of a defined soil volume. The heaters add the necessary energy to achieve a set soil temperature differential within the treatment area and add exterior energy inputs equal to those, which might be lost from lateral heat conduction. The method, which was designed to work in conjunction with aboveground heated chambers, requires only two control sensor positions one for aboveground air temperatures at 1 m and another for belowground temperatures at 0.8 m. The method is capable of achieving temperature differentials of at least +4.0±0.5 °C for soils to a measured depth of ?2 m. These +4 °C differential soil temperatures were sustained in situ throughout 2009, and both diurnal and seasonal cycles at all soil depths were retained using this simple heating approach. Measured mean energy inputs required to sustain the target heating level of +4 °C over the 7.1 m² target area were substantial for aboveground heating (21.1 kW h day^?1 m^?2), but 16 times lower for belowground heaters (1.3 kW h day^?1 m^?2). Observations of soil CO₂ efflux from the surface of the target soil volumes showed CO₂ losses throughout 2009 that were elevated above the temperature response curve that have been reported in previous near‐surface soil warming studies. Stimulation of biological activity within previously undisturbed deep‐soil carbon stocks is the hypothesized source. Long‐term research programs may be able to apply this new heating method that captures expected future warming and temperature dynamics throughout the soil profile to address uncertainties in process‐level responses of microbial, plant and animal communities in whole, intact ecosystems.     

全球森林土壤微生物生物量碳氮磷化学计量的季节动态

土壤微生物生物量在森林生态系统中充当具有生物活性的养分积累和储存库。土壤微生物转化有机质为植物提供可利用养分, 与植物的相互作用维系着陆地生态系统的生态功能。同时, 土壤微生物也与植物争夺营养元素, 在季节交替过程和植物的生长周期中呈现出复杂的互利-竞争关系。综合全球数据对温带、亚热带和热带森林土壤微生物生物量碳(C)、氮(N)、磷(P)含量及其化学计量比值的季节动态进行分析, 发现温带和亚热带森林的土壤微生物生物量C、N、P含量均呈现夏季低、冬季高的格局。热带森林四季的土壤微生物生物量C、N、P含量都低于温带和亚热带森林, 且热带森林土壤微生物生物量C含量、N含量在秋季相对最低, 土壤微生物生物量P含量四季都相对恒定。温带森林的土壤微生物生物量C:N在春季显著高于其他两个森林类型; 热带森林的土壤微生物生物量C:N在秋季显著高于其他2个森林类型。温带森林土壤微生物生物量N:P和C:P在四季都保持相对恒定, 而热带森林土壤微生物生物量N:P和C:P在夏季高于其他3个季节。阔叶树的土壤微生物生物量C含量、N含量、N:P、C:P在四季都显著高于针叶树; 而针叶树的土壤微生物生物量P含量在四季都显著高于阔叶树。在春季和冬季时, 土壤微生物生物量C:N在阔叶树和针叶树之间都没有显著差异; 但是在夏季和秋季, 针叶树的土壤微生物生物量C:N显著高于阔叶树。对于土壤微生物生物量的变化来说, 森林类型是主要的显著影响因子, 季节不是显著影响因子, 暗示土壤微生物生物量的季节波动是随着植物其内在固有的周期变化而变化。植物和土壤微生物密切作用表现出来的对养分的不同步吸收是保留养分和维持生态功能的一种权衡机制。     

Responses of soil respiration to elevated CO2, air warming, and changing soil water availability in a model old-field grassland

Responses of soil respiration to atmospheric and climatic change will have profound impacts on ecosystem and global carbon (C) cycling in the future. This study was conducted to examine effects on soil respiration of the concurrent driving factors of elevated atmospheric CO₂ concentration, air warming, and changing precipitation in a constructed old‐field grassland in eastern Tennessee, USA. Model ecosystems of seven old‐field species were established in open‐top chambers and treated with factorial combinations of ambient or elevated (+300 ppm) CO₂ concentration, ambient or elevated (+3 °C) air temperature, and high or low soil moisture content. During the 19‐month experimental period from June 2003 to December 2004, higher CO₂ concentration and soil water availability significantly increased mean soil respiration by 35.8% and 15.7%, respectively. The effects of air warming on soil respiration varied seasonally from small reductions to significant increases to no response, and there was no significant main effect. In the wet side of elevated CO₂ chambers, air warming consistently caused increases in soil respiration, whereas in the other three combinations of CO₂ and water treatments, warming tended to decrease soil respiration over the growing season but increase it over the winter. There were no interactive effects on soil respiration among any two or three treatment factors irrespective of time period. Treatment‐induced changes in soil temperature and moisture together explained 49%, 44%, and 56% of the seasonal variations of soil respiration responses to elevated CO₂, air warming, and changing precipitation, respectively. Additional indirect effects of seasonal dynamics and responses of plant growth on C substrate supply were indicated. Given the importance of indirect effects of the forcing factors and plant community dynamics on soil temperature, moisture, and C substrate, soil respiration response to climatic warming should not be represented in models as a simple temperature response function, and a more mechanistic representation including vegetation dynamics and substrate supply is needed.     

Annual variation in soil respiration and its components in a coppice oak forest in Central Italy

In order to investigate the annual variation of soil respiration and its components in relation to seasonal changes in soil temperature and soil moisture in a Mediterranean mixed oak forest ecosystem, we set up a series of experimental treatments in May 1999 where litter (no litter), roots (no roots, by trenching) or both were excluded from plots of 4 m². Subsequently, we measured soil respiration, soil temperature and soil moisture in each plot over a year after the forest was coppiced. The treatments did not significantly affect soil temperature or soil moisture measured over 0–10 cm depth. Soil respiration varied markedly during the year with high rates in spring and autumn and low rates in summer, coinciding with summer drought, and in winter, with the lowest temperatures. Very high respiration rates, however, were observed during the summer immediately after rainfall events. The mean annual rate of soil respiration was 2.9 µ mol m^?2 s^?1, ranging from 1.35 to 7.03 µmol m^?2 s^?1. Soil respiration was highly correlated with temperature during winter and during spring and autumn whenever volumetric soil water content was above 20%. Below this threshold value, there was no correlation between soil respiration and soil temperature, but soil moisture was a good predictor of soil respiration. A simple empirical model that predicted soil respiration during the year, using both soil temperature and soil moisture accounted for more than 91% of the observed annual variation in soil respiration. All the components of soil respiration followed a similar seasonal trend and were affected by summer drought. The Q₁₀ value for soil respiration was 2.32, which is in agreement with other studies in forest ecosystems. However, we found a Q₁₀ value for root respiration of 2.20, which is lower than recent values reported for forest sites. The fact that the seasonal variation in root growth with temperature in Mediterranean ecosystems differs from that in temperate regions may explain this difference. In temperate regions, increases in size of root populations during the growing season, coinciding with high temperatures, may yield higher apparent Q₁₀ values than in Mediterranean regions where root growth is suppressed by summer drought. The decomposition of organic matter and belowground litter were the major components of soil respiration, accounting for almost 55% of the total soil respiration flux. This proportion is higher than has been reported for mature boreal and temperate forest and is probably the result of a short‐term C loss following recent logging at the site. The relationship proposed for soil respiration with soil temperature and soil moisture is useful for understanding and predicting potential changes in Mediterranean forest ecosystems in response to forest management and climate change.     

Warming and drying suppress microbial activity and carbon cycling in boreal forest soils

Climate warming is expected to have particularly strong effects on tundra and boreal ecosystems, yet relatively few studies have examined soil responses to temperature change in these systems. We used closed‐top greenhouses to examine the response of soil respiration, nutrient availability, microbial abundance, and active fungal communities to soil warming in an Alaskan boreal forest dominated by mature black spruce. This treatment raised soil temperature by 0.5 °C and also resulted in a 22% decline in soil water content. We hypothesized that microbial abundance and activity would increase with the greenhouse treatment. Instead, we found that bacterial and fungal abundance declined by over 50%, and there was a trend toward lower activity of the chitin‐degrading enzyme N‐acetyl‐glucosaminidase. Soil respiration also declined by up to 50%, but only late in the growing season. These changes were accompanied by significant shifts in the community structure of active fungi, with decreased relative abundance of a dominant Thelephoroid fungus and increased relative abundance of Ascomycetes and Zygomycetes in response to warming. In line with our hypothesis, we found that warming marginally increased soil ammonium and nitrate availability as well as the overall diversity of active fungi. Our results indicate that rising temperatures in northern‐latitude ecosystems may not always cause a positive feedback to the soil carbon cycle, particularly in boreal forests with drier soils. Models of carbon cycle‐climate feedbacks could increase their predictive power by incorporating heterogeneity in soil properties and microbial communities across the boreal zone.     

岷江上游华山松林冬季土壤呼吸对模拟增温的短期响应

冬季的土壤呼吸是生态系统呼吸的重要组成部分, 对气候变化的响应可能更为敏感。该文采用红外辐射加热器模拟土壤增温, 研究了岷江上游华山松(Pinus armandii)人工林冬季的土壤呼吸、微生物生物量及无机氮库对模拟增温的响应。结果表明: 在冬季(2009年11月-翌年3月), 模拟增温往往能显著提高土壤呼吸速率, 平均增幅达31.4%; 同样模拟增温使土壤微生物生物量碳、氮分别增加23.2%和22.7%, 而对微生物生物量碳氮比没有影响, 温度升高显著促进了微生物的生长, 但没有改变微生物的群落结构; 增温样地土壤的NO₃ ^--N和NH₄ ⁺-N浓度较对照分别增加了38.5%和12.3%, 增温显著提高了土壤的可溶性无机氮含量。综上所述, 该区针叶林冬季土壤呼吸、微生物生长和养分矿化对未来气候变暖非常敏感。     

Temperature and substrate controls on intra-annual variation in ecosystem respiration in two subarctic vegetation types

Arctic ecosystems are important in the context of climate change because they are expected to undergo the most rapid temperature increases, and could provide a globally significant release of CO₂ to the atmosphere from their extensive bulk soil organic carbon reserves. Understanding the relative contributions of bulk soil organic matter and plant‐associated carbon pools to ecosystem respiration is critical to predicting the response of arctic ecosystem net carbon balance to climate change. In this study, we determined the variation in ecosystem respiration rates from birch forest understory and heath tundra vegetation types in northern Sweden through a full annual cycle. We used a plant biomass removal treatment to differentiate bulk soil organic matter respiration from total ecosystem respiration in each vegetation type. Plant‐associated and bulk soil organic matter carbon pools each contributed significantly to ecosystem respiration during most phases of winter and summer in the two vegetation types. Ecosystem respiration rates through the year did not differ significantly between vegetation types despite substantial differences in biomass pools, soil depth and temperature regime. Most (76–92%) of the intra‐annual variation in ecosystem respiration rates from these two common mesic subarctic ecosystems was explained using a first‐order exponential equation relating respiration to substrate chemical quality and soil temperature. Removal of plants and their current year's litter significantly reduced the sensitivity of ecosystem respiration to intra‐annual variations in soil temperature for both vegetation types, indicating that respiration derived from recent plant carbon fixation was more temperature sensitive than respiration from bulk soil organic matter carbon stores. Accurate assessment of the potential for positive feedbacks from high‐latitude ecosystems to CO₂‐induced climate change will require the development of ecosystem‐level physiological models of net carbon exchange that differentiate the responses of major C pools, that account for effects of vegetation type, and that integrate over summer and winter seasons.     

Soil microbial responses to experimental warming and clipping in a tallgrass prairie

Global surface temperature is predicted to increase by 1.4–5.8°C by the end of this century. However, the impacts of this projected warming on soil C balance and the C budget of terrestrial ecosystems are not clear. One major source of uncertainty stems from warming effects on soil microbes, which exert a dominant influence on the net C balance of terrestrial ecosystems by controlling organic matter decomposition and plant nutrient availability. We, therefore, conducted an experiment in a tallgrass prairie ecosystem at the Great Plain Apiaries (near Norman, OK) to study soil microbial responses to temperature elevation of about 2°C through artificial heating in clipped and unclipped field plots. While warming did not induce significant changes in net N mineralization, soil microbial biomass and respiration rate, it tended to reduce extractable inorganic N during the second and third warming years, likely through increasing plant uptake. In addition, microbial substrate utilization patterns and the profiles of microbial phospholipid fatty acids (PLFAs) showed that warming caused a shift in the soil microbial community structure in unclipped subplots, leading to the relative dominance of fungi as evidenced by the increased ratio of fungal to bacterial PLFAs. However, no warming effect on soil microbial community structure was found in clipped subplots where a similar scale of temperature increase occurred. Clipping also significantly reduced soil microbial biomass and respiration rate in both warmed and unwarmed plots. These results indicated that warming‐led enhancement of plant growth rather than the temperature increase itself may primarily regulate soil microbial response. Our observations show that warming may increase the relative contribution of fungi to the soil microbial community, suggesting that shifts in the microbial community structure may constitute a major mechanism underlying warming acclimatization of soil respiration.     

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.