Reprint of “Soil extracellular enzyme dynamics in a changing climate”

Assays for extracellular enzyme activity (EEA) have become a common tool for studying soil microbial responses in climate change experiments. Nevertheless, measures of potential EEA, which are conducted under controlled conditions, often do not account for the direct effects of climate change on EEA that occur as a result of the temperature and moisture dependence of enzyme activity in situ. Likewise, the indirect effects of climate on EEA in the field, that occur via effects on microbial enzyme producers, must be assessed in the context of potential changes in plant and soil faunal communities. Here, EEA responses to warming and altered precipitation in field studies are reviewed, with the goal of evaluating the role of EEA in enhancing our understanding of soil and ecosystem responses to climate change. Seasonal and interannual variation in EEA responses to climate change treatments are examined, and potential interactions with elevated atmospheric CO₂, increased atmospheric N deposition and changes in disturbance regimes are also explored. It is demonstrated that in general, soil moisture manipulations in field studies have had a much greater influence on potential EEA than warming treatments. However, these results may simply reflect the low magnitude of soil warming achieved in many field experiments. In addition, changes in plant species composition over the longer term in response to warming could strongly affect EEA. Future challenges involve extending studies of potential EEA to address EEA responses to climate change in situ, and gaining further insights into the mechanisms, such as enzyme production, stabilization and turnover, that underlie EEA responses.     

Seasonal influence of climate manipulation on microbial community structure and function in mountain soils

Microbial communities drive soil organic matter (SOM) decomposition through the production of a variety of extracellular enzymes. Climate change impact on soil microbial communities and soil enzymatic activities can therefore strongly affect SOM turnover, and thereby determine the fate of ecosystems and their role as carbon sinks or sources.To simulate projected impacts of climate change on Swiss Jura subalpine grassland soils, an altitudinal soil transplantation experiment was set up in October 2009. On the fourth year of this experiment, we measured microbial biomass (MB), microbial community structure (MCS), and soil extracellular enzymatic activities (EEA) of nine hydrolytic and oxidative extracellular enzymes in the transplanted soils on a seasonal basis.We found a strong sampling date effect and a smaller but significant effect of the climate manipulation (soil transplantation) on EEA. Overall EEA was higher in winter and spring but enzymes linked to N and P cycles showed higher potential activities in autumn, suggesting that other factors than soil microclimate controlled their pool size, such as substrate availability. The climate warming manipulation decreased EEA in most cases, with oxidative enzymes more concerned than hydrolytic enzymes. In contrast to EEA, soil MB was more affected by the climate manipulation than by the seasons. Transplanting soils to lower altitudes caused a significant decrease in soil MB, but did not affect soil MCS. Conversely, a clear shift in soil MCS was observed between winter and summer. Mass-specific soil EEA (EEA normalized by MB) showed a systematic seasonal trend, with a higher ratio in winter than in summer, suggesting that the seasonal shift in MCS is accompanied by a change in their activities. Surprisingly, we observed a significant decrease in soil organic carbon (SOC) concentration after four years of soil transplantation, as compared to the control site, which could not be linked to any microbial data.We conclude that medium term (four years) warming and decreased precipitation strongly affected MB and EEA but not MCS in subalpine grassland soils, and that those shifts cannot be readily linked to the dynamics of soil carbon concentration under climate change.     

现代气候变化情景下土壤微生物活性和碳动态研究进展

Microbial activities are affected by a myriad of factors with end points involved in nutrient cycling and carbon sequestration issues. Because of their prominent role in the global carbon balance and their possible role in carbon sequestration, soil microbes are very important organisms in relation to global climate changes. This review focuses mainly on the responses of soil microbes to climate changes and subsequent effects on soil carbon dynamics. An overview table regarding extracellular enzyme activities (EAA) with all relevant literature data summarizes the effects of different ecosystems under various experimental treatments on EAA. Increasing temperature, altered soil moisture regimes, and elevated carbon dioxide significantly affect directly or indirectly soil microbial activities. High temperature regimes can increase the microbial activities which can provide positive feedback to climate change, whereas lower moisture condition in pedosystem can negate the increase, although the interactive effects still remain unanswered. Shifts in soil microbial community in response to climate change have been determined by gene probing, phospholipid fatty acid analysis (PLFA), terminal restriction length polymorphism (TRFLP), and denaturing gradient gel electrophoresis (DGGE), but in a recent investigations, omic technological interventions have enabled determination of the shift in soil microbe community at a taxa level, which can provide very important inputs for modeling C sequestration process. The intricacy and diversity of the soil microbial population and how it responds to climate change are big challenges, but new molecular and stable isotope probing tools are being developed for linking fluctuations in microbial diversity to ecosystem function.     

Changes in variability of soil moisture alter microbial community C and N resource use

Grassland ecosystems contain ∼12% of global soil organic carbon (C) stocks and are located in regions where global climate change will likely alter the timing and size of precipitation events, increasing soil moisture variability. In response to increased soil moisture variability and other forms of stress, microorganisms can induce ecosystem-scale alterations in C and N cycling processes through alterations in their function. We explored the influence of physiological stress on microbial communities by manipulating moisture variability in soils from four grassland sites in the Great Plains, representing a precipitation gradient of 485-1003 mm y⁻¹. Keeping water totals constant, we manipulated the frequency and size of water additions and dry down periods in these soils by applying water in two different, two-week long wetting-drying cycles in a 72-day laboratory incubation. To assess the effects of the treatments on microbial community function, we measured C mineralization, N dynamics, extracellular enzyme activities (EEA) and a proxy for substrate use efficiency. In soils from all four sites undergoing a long interval (LI) treatment for which added water was applied once at the beginning of each two-week cycle, 1.4-2.0 times more C was mineralized compared to soils undergoing a short interval (SI) treatment, for which four wetting events were evenly distributed over each two-week cycle. A proxy for carbon use efficiency (CUE) suggests declines in this parameter with the greater soil moisture stress imposed in LI soils from all four different native soil moisture regimes. A decline in CUE in LI soils may have been related to an increased effort by microbes to obtain N-rich organic substrates for use as protection against osmotic shock, consistent with EEA data. These results contrast with similar in situ studies of response to increased soil moisture variability and may indicate divergent autotrophic vs. heterotrophic responses to increased moisture variability. Increases in microbial N demand and decreases in microbial CUE with increased moisture variability observed in this study, regardless of the soils’ site of origin, imply that these systems may experience enhanced heterotrophic CO₂ release and declines in plant-available N with climate change. This has particularly important implications for C budgets in these grasslands when coupled with the declines in net primary productivity reported in other studies as a result of increases in precipitation variability across the region.     

Spatial patterns of top soil carbon sensitivity to climate variables in northern Chinese grasslands

Abstract

Estimation of the sensitivity for soil organic carbon to climate change is critical for evaluating the potential response of the terrestrial biosphere to global change. In this study, we integrated CENTURY 4.5 model with GIS to assess the soil organic carbon sensitivity to climate variable shifting and atmospheric carbon dioxide enrichment in northern Chinese grasslands. The response of top soil (0–20 cm) organic carbon to climate change depended on the relative sensitivity of net primary productivity and soil respiration. A 4°C increase in soil temperature led to a loss of 4.7% of soil organic carbon in the Alpine Meadow region, but the same temperature increase led to a maximum loss of only 2.3% of soil organic carbon in the Temperate Steppe region. The effects of precipitation changes on soil organic carbon were varied depending on the moisture level of the local grassland system. The direct effect of carbon dioxide enrichment was to reduce carbon loss throughout northern Chinese grasslands, especially in droughty regions. Alpine Meadow was the most sensitive region under climate change, and it will become the biggest potential carbon source in Chinese grasslands as climate warming continues to occur. Increased atmospheric carbon dioxide concentrations led to net carbon sequestration in all grasslands and tended to diminish the carbon loss driven by precipitation and temperature changes.     

Fine scale variability in soil extracellular enzyme activity is insensitive to rain events and temperature in a mesic system

While soil extracellular enzyme assays (EEAs) are frequently used to infer soil microbial function, the data typically reflect a small number of sampling points across a season, and it is unclear to what extent soil EEA may vary on the time scale of days to weeks. Rain events, in particular, may cause rapid shifts in EEA, and fine scale temporal data are needed to properly assess the generality of EEA data collected at coarser time scales. We examined soil EEA 2-3 times per week in the field from June to November in the context of natural rain events and temperature fluctuations, and explored how long-term water addition altered EEA responses. We also tested the short-term effects of water addition on the distribution of EEA in intact soil mesocoms and leachate. There was little temporal variation in EEA for the hydrolases phosphatase, N-acetyl-glucosaminidase and β-glucosidase, despite the occurrence of multiple large rain events and large soil temperature fluctuations. Phenol oxidase activity correlated significantly with seasonal trends in temperature and soil moisture, but was highly variable at short time scales, and the latter did not correlate significantly with short-term changes in soil microclimate. EEA generally increased in response to long-term water addition, and in soil mesocosms water addition did not significantly redistribute EEA among the upper and lower soil layers, and leachate EEA was three orders of magnitude lower than soil EEA. Overall, our results reveal relatively minor short-term variation in EEA for hydrolase enzymes, and no discernable response to temperature fluctuations or precipitation over the short term. However, high short-term variation in phenol oxidase activity suggests that it may be difficult to infer temporal trends in EEA for this enzyme from a limited number of sampling points.     

中国北方温带草原土壤微生物对气温变暖与添加氮素的响应特征

The responses of soil microbes to global warming and nitrogen enrichment can profoundly affect terrestrial ecosystem functions and the ecosystem feedbacks to climate change. However, the interactive effect of warming and nitrogen enrichment on soil microbial community is unclear. In this study, individual and interactive effects of experimental warming and nitrogen addition on the soil microbial community were investigated in a long-term field experiment in a temperate steppe of northern China. The field experiment started in 2006 and soils were sampled in 2010 and analyzed for phospholipid fatty acids to characterize the soil microbial communities. Some soil chemical properties were also determined. Five-year experimental warming significantly increased soil total microbial biomass and the proportion of Gram-negative bacteria in the soils. Long-term nitrogen addition decreased soil microbial biomass at the 0-10 cm soil depth and the relative abundance of arbuscular mycorrhizal fungi in the soils. Little interactive effect on soil microbes was detected when experimental warming and nitrogen addition were combined. Soil microbial biomass positively correlated with soil total C and N, but basically did not relate to the soil C/N ratio and pH. Our results suggest that future global warming or nitrogen enrichment may significantly change the soil microbial communities in the temperate steppes in northern China.     

增温对寒潮期间亚热带常绿阔叶天然林土壤呼吸的影响

全球变暖增加寒潮天气发生的频率和强度,影响土壤呼吸及其各组分,但有关增温和寒潮对亚热带森林土壤呼吸及其各组分的影响研究仍十分缺乏。通过壕沟法分离土壤呼吸,并利用土壤呼吸高频自动监测系统研究增温对寒潮期间亚热带常绿阔叶天然林土壤总呼吸、根呼吸与微生物呼吸的影响。结果表明：(1)寒潮发生时,对照和增温处理中土壤总呼吸速率分别显著下降45.93%和25.68%,土壤微生物呼吸速率分别显著下降51.25%和35.54%。但寒潮并没有影响增温处理中根呼吸速率,而对照处理中根呼吸速率在寒潮时显著下降39.72%。(2)观测期间,增温对总呼吸和根呼吸的日动态模式的影响在寒潮不同阶段具有明显差异,增温导致寒潮发生前后土壤总呼吸和根呼吸日峰值出现时间分别提前1,2 h,而寒潮发生时,对照和增温处理中土壤总呼吸和根呼吸的日峰值出现时间同步。(3)观测期间,增温后土壤总呼吸、根呼吸和微生物呼吸的温度敏感性(Q₁₀值)均下降,而根呼吸的Q₁₀值均高于微生物呼吸。因此,准确了解寒潮等极端天气下的土壤总呼吸、根呼吸和微生物呼吸的变化及其对增温的响应,对于提高气候变暖后土...     

Responses of Soil Enzyme Activities and Microbial Community Composition to Moisture Regimes in Paddy Soils Under Long-Term Fertilization Practices

The effects of fertilization on activity and composition of soil microbial community depend on nutrient and water availability;however,the combination of these factors on the response of microorganisms was seldom studied.This study investigated the responses of soil microbial community and enzyme activities to changes in moisture along a gradient of soil fertility formed within a long-term(24 years)field experiment.Soils(0–20 cm)were sampled from the plots under four fertilizer treatments:i)unfertilized control(CK),ii)organic manure(M),iii)nitrogen,phosphorus,and potassium fertilizers(NPK),and iv)NPK plus M(NPK+M).The soils were incubated at three moisture levels:constant submergence,five submerging-draining cycles(S-D cycles),and constant moisture content at 40%water-holding capacity(low moisture).Compared with CK,fertilization increased soil organic carbon(SOC) by 30.1%–36.3%,total N by 27.3%–38.4%,available N by 35.9%–56.4%,available P by 61.4%–440.9%,and total P by 28.6%–102.9%.Soil fertility buffered the negative effects of moisture on enzyme activities and microbial community composition.Enzyme activities decreased in response to submergence and S-D cycles versus low moisture.Compared with low moisture,S-D cycles increased total phospholipid fatty acids(PLFAs)and actinomycete,fungal,and bacterial PLFAs.The increased level of PLFAs in the unfertilized soil after five S-D cycles was greater than that in the fertilized soil.Variations in soil microbial properties responding to moisture separated CK from the long-term fertilization treatments.The coefficients of variation of microbial properties were negatively correlated with SOC,total P,and available N.Soils with higher fertility maintained the original microbial properties more stable in response to changes in moisture compared to low-fertility soil.     

Enchytraeids in a changing climate: A mini-review

The climate is undergoing rapid changes with rising atmospheric CO₂ concentration, increasing temperatures and changes in the hydrological regimes resulting in more frequent and intense drought periods. These three climate change factors will, separately and in combination, affect the biotic and abiotic components of soil communities. This paper reviews the impact of climate change on field communities of enchytraeids with special emphasis on Cognettia sphagnetorum because most relevant studies have involved this species. C. sphagnetorum prefers cold and wet environments and several studies suggest that reductions of soil moisture may have dramatic consequences for C. sphagnetorum and other enchytraeid species. Effects of changing temperatures are less clear partly because thermal conditions influence soil moisture, which complicate the predictions of the outcome from such changes. The predicted increasing annual mean temperature may be stimulating and expand the season for growth and reproduction of enchytraeids; on the other hand, an increased frequency of extreme weather events, with heat waves during summer and bare soil freezes during autumn and spring, may occasionally cause severe mortality. Stimulating effects of increased atmospheric CO₂ have been observed, perhaps due to increased food availability via root and litter production. However, effects of CO₂ are also influenced by moisture and temperature. Generally, there is a lack of research looking into the complicated interactions between various climate change factors, and little is known about the potential of enchytraeids to adapt to a changing climate. Existing data suggest that C. sphagnetorum is not capable of adapting to a drier climate, thus, a decline in abundance and distribution of this species is possible. Since enchytraeids are of ecological significance in some types of habitats, a reduction may result in serious disruption in the functioning of these decomposer communities.     

Detecting microbial N-limitation in tussock tundra soil: Implications for Arctic soil organic carbon cycling

More than a third of the global soil organic carbon (SOC) pool is estimated to be stored in northern latitudes. While the primary regulators of microbially-mediated decomposition in physically unprotected organic soils are typically attributed to abiotic factors ( e.g. temperature and moisture), in extremely nutrient-poor environments such as the Alaskan Arctic tussock tundra, evidence from field studies suggests that low N-availability may also strongly limit microbial growth, and thus the rate of SOC decomposition. However, there have been few direct tests of microbial nutrient-limitation, particularly in Arctic systems. We predicted that during the Arctic summer growing season, when both plants and microbes are competing for mineralized nutrients, N-availability in tussock tundra soil is so low that it will limit microbial biomass production, and thus decomposition potential. We tested this prediction by adding N and C to tussock tundra organic soil and tracking microbial responses to these additions. We used a combination of approaches to identify microbial N-limitation, including changes in microbial biomass, C-mineralization, substrate use efficiency, and extracellular enzyme activity. The Arctic soil's microbial community demonstrated strong signals of N-limitation, with N-addition increasing all aspects of decomposition tested, including extracellular enzyme activity, the rate-limiting step in decomposition. The corresponding C-addition experiment did not similarly influence the microbial activity of the tundra soil. These results suggest that tundra SOC decomposition is at least seasonally constrained by N-availability through microbial N-limitation. Therefore, explicitly including N as a regulator of microbial growth in this N-poor system is critical to accurately modeling the effects of climatic warming on Arctic SOC decomposition rates.     

Responses of soil microarthropods to warming and increased precipitation in a semiarid temperate steppe

Soil microarthropods are an important component in soil food webs and their responses to climate change could have profound impacts on ecosystem functions. As part of a long-term manipulative experiment, with increased temperature and precipitation in a semiarid temperate steppe in the Mongolian Plateau which started in 2005, this study was conducted to examine effects of climate change on the abundance of soil microarthropods. Experimental warming had slightly negative but insignificant effects on the abundance of mites (−14.6%) and Collembola (−11.7%). Increased precipitation greatly enhanced the abundance of mites and Collembola by 117 and 45.3%, respectively. The response direction and magnitude of mites to warming and increased precipitation varied with suborder, leading to shifts in community structure. The positive relationships of mite abundance with plant cover, plant species richness, and soil microbial biomass nitrogen suggest that the responses of soil microarthropods to climate change are largely regulated by food resource availability. The findings of positive dependence of soil respiration upon mite abundance indicate that the potential contribution of soil fauna to soil CO₂ efflux should be considered when assessing carbon cycling of semiarid grassland ecosystems under climate change scenarios.     

自然培养条件下全球变暖对山毛榉林土壤微生物活性的影响

Microbial activity in soil is known to be controlled by various factors. However, the operating mechanisms have not yet been clearly identified, particularly under climate change conditions, although they are crucial for understanding carbon dynamics in terrestrial ecosystems. In this study, a natural incubation experiment was carried out using intact soil cores transferred from high altitude(1 500 m) to low(900 m) altitude to mimic climate change scenarios in a typical cold-temperate mountainous area in Japan. Soil microbial activities, indicated by substrate-induced respiration(SIR) and metabolic quotient(q CO2), together with soil physicalchemical properties(abiotic factors) and soil functional enzyme and microbial properties(biotic factors), were investigated throughout the growing season in 2013. Results of principal component analysis(PCA) indicated that soil microbial biomass carbon(MBC) andβ-glucosidase activity were the most important factors characterizing the responses of soil microbes to global warming. Although there was a statistical difference of 2.82 ℃ between the two altitudes, such variations in soil physical-chemical properties did not show any remarkable effect on soil microbial activities, suggesting that they might indirectly impact carbon dynamics through biotic factors such as soil functional enzymes. It was also found that the biotic factors mainly controlled soil microbial activities at elevated temperature,which might trigger the inner soil dynamics to respond to the changing environment. Future studies should hence take more biotic variables into account for accurately projecting the responses of soil metabolic activities to climate change.     

Climate change goes underground: effects of elevated atmospheric CO<Subscript>2</Subscript> on microbial community structure and activities in the rhizosphere

General concern about climate change has led to growing interest in the responses of terrestrial ecosystems to elevated concentrations of CO₂ in the atmosphere. Experimentation during the last two to three decades using a large variety of approaches has provided sufficient information to conclude that enrichment of atmospheric CO₂ may have severe impact on terrestrial ecosystems. This impact is mainly due to the changes in the organic C dynamics as a result of the effects of elevated CO₂ on the primary source of organic C in soil, i.e., plant photosynthesis. As the majority of life in soil is heterotrophic and dependent on the input of plant-derived organic C, the activity and functioning of soil organisms will greatly be influenced by changes in the atmospheric CO₂ concentration. In this review, we examine the current state of the art with respect to effects of elevated atmospheric CO₂ on soil microbial communities, with a focus on microbial community structure. On the basis of the existing information, we conclude that the main effects of elevated atmospheric CO₂ on soil microbiota occur via plant metabolism and root secretion, especially in C3 plants, thereby directly affecting the mycorrhizal, bacterial, and fungal communities in the close vicinity of the root. There is little or no direct effect on the microbial community of the bulk soil. In particular, we have explored the impact of these changes on rhizosphere interactions and ecosystem processes, including food web interactions.     

施氮和增雨对内蒙古半干旱地区草原土壤微生物群落碳源利用潜力的影响

已有许多研究证明,中国北方草地生态系统的植物群落结构和组成对气候变化和氮沉降较为敏感,但是关于草原土壤微生物群落响应多重环境因子变化方面的研究较薄弱。水和氮是陆地生态系统生产力的两大限制性因子。本研究在内蒙古多伦半干旱草原地区进行增雨和施氮的野外控制试验,以模拟未来该地区的降水变化和氮沉降,使用微生物群落水平生理图谱法,监测样地土壤理化指标和土壤微生物群落碳源利用潜力的变化。3年的跟踪监测结果显示:增雨显著提高了半干旱草原地区土壤含水量和有机质含量;施氮和增雨同时施氮则显著提高了土壤可溶性氮含量,降低了土壤pH;施氮和增雨都没有单独引起土壤微生物群落碳源利用潜力的显著变化,而在同时增雨和施氮试验处理下,微生物群落碳源利用潜力得到提高,说明在水和氮都充足的条件下,土壤微生物碳源利用潜力才会显著提高。以上研究结果预示着在未来降雨增加和氮沉降的全球变化背景下,中国北方半干旱草地生态系统的碳循环速率可能会加快。     

The impact of long-term CO2 enrichment and moisture levels on soil microbial community structure and enzyme activities

Soil organic matter is the most important reservoir of terrestrial organic C and minor changes in the balance may have a significant impact on the climate. However, the response of microbial decomposers of soil C to global changes is not fully apprehended. This is particularly the case with regard to the interactive effects of the various climatic changes. Here, we present data from the Giessen Free Air CO₂ Experiment (Gi-FACE, University of Giessen, Germany) in which the CO₂ concentration at a grassland site was increased by 20% relative to atmospheric levels during a period of 10 years. The site included a slope that resulted in differences in average soil moisture. The effects of CO₂ and soil moisture on soil microbial community structure, measured by Denaturing Gradient Gel Electrophoresis (DGGE), PhosphoLipid Fatty Acids (PLFA) and enzyme activity profiles were determined. Total carbon, nitrogen and phosphorous contents were also determined. Soil moisture explained a large part of the variance in the microbial community structure data, by affecting fungi and bacteria. Furthermore, the CO₂ treatment had no significant effect on either overall PLFA or DGGE profiles, despite the fact that the fungal:bacterial PLFA ratio was altered. Overall enzyme activity profiles were also only affected by soil moisture levels, although the CO₂ treatment induced a significant increase of the acid phosphatase activity. Finally, neither soil moisture nor elevated CO₂ induced changes in the soil C stock.     

Long-term experimental warming reduces soil nematode populations in the McMurdo Dry Valleys, Antarctica

Climate models predict significant future warming in polar regions. In the McMurdo Dry Valleys, Antarctica, projected summer climate warming is expected to increase snow and glacial melt, resulting in higher stream discharge, rising lake levels, and an increase in areas of moist soil, but the potential influence of warming and associated changes in hydrology on the soil ecosystem is poorly understood. To examine the effects of soil warming and changes in the availability of liquid water on populations of soil invertebrates and their habitat, we established a full-factorial warming and water addition experiment at one experimental site in each of the three hydrologic basins of Taylor Valley, Antarctica, and measured responses over 8 years. We hypothesized that an increase in temperature and moisture together would enhance habitat suitability for soil invertebrates thereby increasing abundance, biomass and diversity of the soil animal communities. Instead, warming treatments had an overall negative effect on density and body size of the microbial-feeding nematode Scottnema lindsayae, the dominant animal in the dry valleys, which decreased by 42% in warmed plots. While experimental moisture additions as a single annual pulse had no effect on nematodes, the surface flooding of one site from rapid melting of upslope subsurface ice (the result of an unusual natural warming event) drastically altered soil moisture, salinity, and animal communities; mortality of S. lindsayae increased and densities decreased. This extreme soil wetting event also resulted in an increase in chlorophyll a and populations of Eudorylaimus spp, a nematode species that prefers moist to wet habitats and feeds on soil micro-algae. Our results suggest that warming in the dry valleys could significantly affect soil nematode populations and species composition both directly and indirectly by altering species-specific habitat suitability for soil biota.     

Resistance of microbial and soil properties to warming treatment seven years after boreal fire

Boreal forests store a large fraction of global terrestrial carbon and are susceptible to environmental change, particularly rising temperatures and increased fire frequency. These changes have the potential to drive positive feedbacks between climate warming and the boreal carbon cycle. Because few studies have examined the warming response of boreal ecosystems recovering from fire, we established a greenhouse warming experiment near Delta Junction, Alaska, seven years after a 1999 wildfire. We hypothesized that experimental warming would increase soil CO₂ efflux, stimulate nutrient mineralization, and alter the composition and function of soil fungal communities. Although our treatment resulted in 1.20 °C soil warming, we found little support for our hypothesis. Only the activities of cellulose- and chitin-degrading enzymes increased significantly by 15% and 35%, respectively, and there were no changes in soil fungal communities. Warming resulted in drier soils, but the corresponding change in soil water potential was probably not sufficient to limit microbial activity. Rather, the warming response of this soil may be constrained by depletion of labile carbon substrates resulting from combustion and elevated soil temperatures in the years after the 1999 fire. We conclude that positive feedbacks between warming and the microbial release of soil carbon are weak in boreal ecosystems lacking permafrost. Since permafrost-free soils underlie 45-60% of the boreal zone, our results should be useful for modeling the warming response during recovery from fire in a large fraction of the boreal forest.     

Mineralization and carbon turnover in subarctic heath soil as affected by warming and additional litter

Arctic soil carbon (C) stocks are threatened by the rapidly advancing global warming. In addition to temperature, increasing amounts of leaf litter fall following from the expansion of deciduous shrubs and trees in northern ecosystems may alter biogeochemical cycling of C and nutrients. Our aim was to assess how factorial warming and litter addition in a long-term field experiment on a subarctic heath affect resource limitation of soil microbial communities (measured by thymidine and leucine incorporation techniques), net growing-season mineralization of nitrogen (N) and phosphorus (P), and carbon turnover (measured as changes in the pools during a growing-season-long field incubation of soil cores in situ). The mainly N limited bacterial communities had shifted slightly towards limitation by C and P in response to seven growing seasons of warming. This and the significantly increased bacterial growth rate under warming may partly explain the observed higher C loss from the warmed soil. This is furthermore consistent with the less dramatic increase in the contents of dissolved organic carbon (DOC) and dissolved organic N (DON) in the warmed soil than in the soil from ambient temperature during the field incubation. The added litter did not affect the carbon content, but it was a source of nutrients to the soil, and it also tended to increase bacterial growth rate and net mineralization of P. The inorganic N pool decreased during the field incubation of soil cores, especially in the separate warming and litter addition treatments, while gross mineralized N was immobilized in the biomass of microbes and plants transplanted into the incubates soil cores, but without any significant effect of the treatments. The effects of warming plus litter addition on bacterial growth rates and of warming on C and N transformations during field incubation suggest that microbial activity is an important control on the carbon balance of arctic soils under climate change.     

Effects of litter addition and warming on soil carbon, nutrient pools and microbial communities in a subarctic heath ecosystem

Climatic warming leads to the expansion of deciduous shrubs and trees in the Arctic. This leads to higher leaf litter inputs, which together with warming may alter the rate of carbon and nutrient cycling in the arctic ecosystems. We assessed effects of factorial warming and additional litter on the soil ecosystem of a subarctic heath in a 7-year-long field experiment. Fine root biomass, dissolved organic carbon (DOC) and total C concentration increased in response to warming, which probably was a result of the increased vegetation cover. Litter addition increased the concentration of inorganic P in the uppermost 5 cm soil, while decreasing the pool of total P per unit area of the organic profile and having no significant effects on N concentrations or pools. Microbial biomass C and N were unaffected by the treatments, while the microbial biomass P increased significantly with litter addition. Soil ergosterol concentration was also slightly increased by the added litter in the uppermost soil, although not statistically significantly. According to a principal component analysis of the phospholipid fatty acid profiles, litter addition differed from the other treatments by increasing the relative proportion of biomarkers for Gram-positive bacteria. The combined warming plus litter addition treatment decreased the soil water content in the uppermost 5 cm soil, which was a likely reason for many interactions between the effects of warming and litter addition. The soil organic matter quality of the combined treatment was also clearly different from the control based on a near-infrared reflectance (NIR) spectroscopic analysis, implying that the treatment altered the composition of soil organic matter. However, it appears that the biological processes and the microbial community composition responded more to the soil and litter moisture conditions than to the change in the quality of the organic matter.