Fungal contributions to carbon flow and nutrient cycling during decomposition of standing Typha domingensis leaves in a subtropical freshwater marsh

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Vegetation feedbacks of nutrient addition lead to a weaker carbon sink in an ombrotrophic bog

To study vegetation feedbacks of nutrient addition on carbon sequestration capacity, we investigated vegetation and ecosystem CO₂ exchange at Mer Bleue Bog, Canada in plots that had been fertilized with nitrogen (N) or with N plus phosphorus (P) and potassium (K) for 7–12 years. Gross photosynthesis, ecosystem respiration, and net CO₂ exchange were measured weekly during May–September 2011 using climate‐controlled chambers. A substrate‐induced respiration technique was used to determine the functional ability of the microbial community. The highest N and NPK additions were associated with 40% less net CO₂ uptake than the control. In the NPK additions, a diminished C sink potential was due to a 20–30% increase in ecosystem respiration, while gross photosynthesis rates did not change as greater vascular plant biomass compensated for the decrease in Sphagnum mosses. In the highest N‐only treatment, small reductions in gross photosynthesis and no change in ecosystem respiration led to the reduced C sink. Substrate‐induced microbial respiration was significantly higher in all levels of NPK additions compared with control. The temperature sensitivity of respiration in the plots was lower with increasing cumulative N load, suggesting more labile sources of respired CO₂. The weaker C sink potential could be explained by changes in nutrient availability, higher woody : foliar ratio, moss loss, and enhanced decomposition. Stronger responses to NPK fertilization than to N‐only fertilization for both shrub biomass production and decomposition suggest that the bog ecosystem is N‐P/K colimited rather than N‐limited. Negative effects of further N‐only deposition were indicated by delayed spring CO₂ uptake. In contrast to forests, increased wood formation and surface litter accumulation in bogs seem to reduce the C sink potential owing to the loss of peat‐forming Sphagnum.     

Effect of nitrogen addition on decomposition of Calamagrostis angustifolia litters from freshwater marshes of Northeast China

Wetland ecosystems store a large amount of organic carbon (C) in soils, due to the slow decomposition rates of plant litter and soil organic matter. Increased nitrogen (N) availability induced by human activities and global warming may accelerate litter decomposition and affect soil organic C dynamics in wetlands. In the present study, we investigated the effect of N addition on decomposition of Calamagrostis angustifolia litters from freshwater marshes in the Sanjiang Plain of Northeast China under field and laboratory conditions. First, we assessed the changes in initial litter chemical composition and subsequent decomposition following three years of N addition at the rate of 24 g N m⁻² year⁻¹ under field conditions. Our results showed that N addition increased litter N concentration and decreased C/N ratio, and thus stimulated litter decomposition. Secondly, we examined the effect of increased N availability (0, 25, 50 and 100 mg N g⁻¹ litter) on litter decomposition under laboratory conditions. Increased exterior N availability also enhanced microbial respiration and increased litter mass loss under both waterlogging and non-waterlogging conditions. In addition, waterlogging conditions inhibited microbial respiration and suppressed litter mass loss. These findings demonstrated that N addition increased litter decomposition rates through improved litter quality and enhanced microbial activity in freshwater marshes of Northeast China. This implies that increased N availability accelerates litter decomposition rates, and thus may cause substantial losses of soil C and diminish and even reverse the C sink function of wetlands in the Sanjiang Plain of Northeast China.     

Stream acidification increases nitrogen uptake by leaf biofilms: implications at the ecosystem scale

1. While anthropogenic stream acidification is known to lower species diversity and impair decomposition, its effects on nutrient cycling remain unclear. The influence of acid‐stress on microbial physiology can have implications for carbon (C) and nitrogen (N) cycles, linking environmental conditions to ecosystem processes. 2. We collected leaf biofilms from streams spanning a gradient of pH (5.1–6.7), related to chronic acidification, to investigate the relationship between qCO₂ (biomass‐specific respiration; mg CO₂‐C g^?1 fungal C h^?1), a known indicator of stress, and biomass‐specific N uptake (μg NH₄‐N mg^?1 fungal biomass h^?1) at two levels of N availability (25 and 100 μg NH₄‐N L^?1) in experimental microcosms. 3. Strong patterns of increasing qCO₂ (i.e. increasing stress) and increasing microbial N uptake were observed with a decrease in ambient (i.e. chronic) stream pH at both levels of N availability. However, fungal biomass was lower on leaves from more acidic streams, resulting in lower overall respiration and N uptake when rates were standardized by leaf biomass. 4. Results suggest that chronic acidification decreases fungal metabolic efficiency because, under acid conditions, these organisms allocate more resources to maintenance and survival and increase their removal of N, possibly via increased exoenzyme production. At the same time, greater N availability enhanced N uptake without influencing CO₂ production, implying increased growth efficiency. 5. At the ecosystem level, reductions in growth because of chronic acidification reduce microbial biomass and may impair decomposition and N uptake; however, in systems where N is initially scarce, increased N availability may alleviate these effects. Ecosystem response to chronic stressors may be better understood by a greater focus on microbial physiology, coupled elemental cycling, and responses across several scales of investigation.     

有机碳质量对黄河三角洲芦苇凋落物分解及其温度敏感性的影响

凋落物分解速率及其温度敏感性Q_(10)能够影响凋落物对土壤的碳归还及其对全球变暖的响应。然而,凋落物有机碳质量对凋落物分解及其温度敏感性的影响研究仍不充分。以黄河三角洲芦苇(Phragmites australi)为例,通过凋落物袋法、室内模拟实验及固态~(13)C核磁共振技术,研究有机碳质量对凋落物分解及其温度敏感性的影响,探讨预测凋落物分解及其温度敏感性的指标。结果表明:(1)随着凋落物分解,易分解碳组分(烷氧碳、双烷氧碳)相对含量逐渐降低,而难分解碳组分(芳香碳)相对含量显著增加,疏水碳/亲水碳、芳香碳/烷氧碳比值逐渐增大,凋落物有机碳更加稳定,凋落物呼吸速率及失重率呈下降趋势。(2)凋落物失重主要受烷基碳、烷氧碳相对含量及C/N的影响,凋落物CO_2累积释放量主要受烷氧碳及双烷氧碳相对含量的影响。羰基碳相对含量可以用来解释Q_(10)的变异。因此,相对于生态化学计量比,烷基碳、烷氧碳、双烷氧碳、羰基碳相对含量是预测凋落物分解及其温度敏感性的敏感性指标。     

Positive climate feedbacks of soil microbial communities in a semi‐arid grassland

Soil microbial communities may be able to rapidly respond to changing environments in ways that change community structure and functioning, which could affect climate–carbon feedbacks. However, detecting microbial feedbacks to elevated CO₂ (eCO₂) or warming is hampered by concurrent changes in substrate availability and plant responses. Whether microbial communities can persistently feed back to climate change is still unknown. We overcame this problem by collecting microbial inocula at subfreezing conditions under eCO₂ and warming treatments in a semi‐arid grassland field experiment. The inoculant was incubated in a sterilised soil medium at constant conditions for 30 days. Microbes from eCO₂ exhibited an increased ability to decompose soil organic matter (SOM) compared with those from ambient CO₂ plots, and microbes from warmed plots exhibited increased thermal sensitivity for respiration. Microbes from the combined eCO₂ and warming plots had consistently enhanced microbial decomposition activity and thermal sensitivity. These persistent positive feedbacks of soil microbial communities to eCO₂ and warming may therefore stimulate soil C loss.     

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.     

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

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

Lignin decomposition is sustained under fluctuating redox conditions in humid tropical forest soils

Lignin mineralization represents a critical flux in the terrestrial carbon (C) cycle, yet little is known about mechanisms and environmental factors controlling lignin breakdown in mineral soils. Hypoxia is thought to suppress lignin decomposition, yet potential effects of oxygen (O₂) variability in surface soils have not been explored. Here, we tested the impact of redox fluctuations on lignin breakdown in humid tropical forest soils during ten‐week laboratory incubations. We used synthetic lignins labeled with ¹³C in either of two positions (aromatic methoxyl or propyl side chain C_β) to provide highly sensitive and specific measures of lignin mineralization seldom employed in soils. Four‐day redox fluctuations increased the percent contribution of methoxyl C to soil respiration relative to static aerobic conditions, and cumulative methoxyl‐C mineralization was statistically equivalent under static aerobic and fluctuating redox conditions despite lower soil respiration in the latter treatment. Contributions of the less labile lignin C_β to soil respiration were equivalent in the static aerobic and fluctuating redox treatments during periods of O₂ exposure, and tended to decline during periods of O₂ limitation, resulting in lower cumulative C_β mineralization in the fluctuating treatment relative to the static aerobic treatment. However, cumulative mineralization of both the C_β‐ and methoxyl‐labeled lignins nearly doubled in the fluctuating treatment relative to the static aerobic treatment when total lignin mineralization was normalized to total O₂ exposure. Oxygen fluctuations are thought to be suboptimal for canonical lignin‐degrading microorganisms. However, O₂ fluctuations drove substantial Fe reduction and oxidation, and reactive oxygen species generated during abiotic Fe oxidation might explain the elevated contribution of lignin to C mineralization. Iron redox cycling provides a potential mechanism for lignin depletion in soil organic matter. Couplings between soil moisture, redox fluctuations, and lignin breakdown provide a potential link between climate variability and the biochemical composition of soil organic matter.     

Microbial Biomass,Respiration, and Decomposition of Hura crepitans L. (Euphorbiaceae) Leaves in a Tropical Stream1

The processing of leaves in temperate streams has been the subject of numerous studies but equivalent tropical ecosystems have received little attention. We investigated leaf breakdown of a tropical tree species (Hura crepitans, Euphorbiaceae), in a tropical stream using leaf bags (0.5 mm mesh) over a period of 24 days. We followed the loss of mass and the changes in adenosine triphosphate (ATP) concentrations and respiration rates associated with the decomposing leaves. The breakdown rate was fast (k=?0.0672/d, k_d=?0.0031/degree‐day), with 81 percent loss of the initial mass within 24 days. This high rate was probably related to the stable and high water temperature (22°C) favoring strong biological activity. Respiration rates increased until day 16 (1.1 mg O₂/h/g AFDM), but maximum ATP concentrations were attained at day 9 (725 nmol ATP/g AFDM) when leaf mass remaining was 52 percent. To determine the relative importance of fungi and bacteria during leaf decomposition, ATP concentrations, and respiration rates were determined in samples treated with antibiotics, after incubation in the stream. The results of the samples treated with the antifungal or the bacterial antibiotic suggest a higher contribution of the fungal community for total microbial biomass and a higher contribution of the bacterial community for microbial respiration rates, especially during the later stages of leaf decomposition. However, these results should be analyzed with caution since both antibacterial and antifungal agents did not totally eliminate microbial activity and biomass.     

ATP and Ergosterol as Indicators of Fungal Biomass during Leaf Decomposition in Streams: a Comparative Study

Ergosterol and ATP concentrations, microbial respiration and sporulation rates of aquatic hyphomycetes associated with leaves of Castanea sativa decomposing in a 5^th order stream were determined periodically over a period of 102 days in order to compare ergosterol and ATP as indicators of fungal biomass. ATP and ergosterol concentrations exhibited a significant positive correlation (F = 4.459, DF = 28, P < 0.001) during the first stages of leaf breakdown (until day 39), i.e., during periods of increasing fungal biomass. No correlation was found between ATP and ergosterol concentrations during later stages of decomposition (days 39 to 102). Respiration rates increased rapidly up to 0.525 mg O₂ h^–1 g^–1 AFDM during the first month and remained high until the end of the experiment. Sporulation rates peaked at day 9 (1069 conidia day^–1 mg^–1 AFDM) and decreased during later stages of decomposition. ATP‐to‐biomass conversion factors were determined for both fungi (0.59 μmol ATP g^–1 dry mass) and bacteria (1.30 μmol ATP g^–1 dry mass) collected from the stream and grown in the laboratory. Estimates of fungal biomass based on ATP concentrations were similar to those calculated from ergosterol concentrations during the first 39 days of breakdown. The results here presented suggest that ATP is a reliable method to quantify microbial biomass in streams and that the relative importance of bacteria increases at later stages of decomposition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Correlation of foliage and litter chemistry of sugar maple, Acer saccharum, as affected by elevated CO2 and varying N availability, and effects on decomposition

Rising atmospheric carbon dioxide has the potential to alter leaf litter chemistry, potentially affecting decomposition and rates of carbon and nitrogen cycling in forest ecosystems. This study was conducted to determine whether growth under elevated atmospheric CO₂ altered the quality and microbial decomposition of leaf litter of a widely distributed northern hardwood species at sites of low and high soil nitrogen availability. In addition, we assessed whether the carbon–nutrient balance (CNB) and growth differentiation balance (GDB) hypotheses could be extended to predict changes in litter quality in response to resource availability. Sugar maple (Acer saccharum) was grown in the field in open‐top chambers at 36 and 55 Pa partial pressure CO₂, and initial soil mineralization rates of 45 and 348 μg N g^?1 d^?1. Naturally senesced leaf litter was assessed for chemical composition and incubated in the laboratory for 111 d. Microbial respiration and the production of dissolved organic carbon (DOC) were quantified as estimates of decomposition. Elevated CO₂ and low soil nitrogen resulted in higher litter concentrations of nonstructural carbohydrates and condensed tannins, higher C/N ratios and lower N concentrations. Soil N availability appears to have had a greater effect on litter quality than did atmospheric CO₂, although the treatments were additive, with highest concentrations of nonstructural carbohydrates and condensed tannins occurring under elevated CO₂–low soil N. Rates of microbial respiration and the production of DOC were insensitive to differences in litter quality. In general, concentrations of litter constituents, except for starch, were highly correlated to those in live foliage, and the CNB/GDB hypotheses proved useful in predicting changes in litter quality. We conclude the chemical composition of sugar maple litter will change in the future in response to rising atmospheric CO₂, and that soil N availability will exert a major control. It appears that microbial metabolism will not be directly affected by changes in litter quality, although conclusions regarding decomposition as a whole must consider the entire soil food web.     

Biotic degradation at night,abiotic degradation at day: positive feedbacks on litter decomposition in drylands

The arid and semi‐arid drylands of the world are increasingly recognized for their role in the terrestrial net carbon dioxide (CO₂) uptake, which depends largely on plant litter decomposition and the subsequent release of CO₂ back to the atmosphere. Observed decomposition rates in drylands are higher than predictions by biogeochemical models, which are traditionally based on microbial (biotic) degradation enabled by precipitation as the main mechanism of litter decomposition. Consequently, recent research in drylands has focused on abiotic mechanisms, mainly photochemical and thermal degradation, but they only partly explain litter decomposition under dry conditions, suggesting the operation of an additional mechanism. Here we show that in the absence of precipitation, absorption of dew and water vapor by litter in the field enables microbial degradation at night. By experimentally manipulating solar irradiance and nighttime air humidity, we estimated that most of the litter CO₂ efflux and decay occurring in the dry season was due to nighttime microbial degradation, with considerable additional contributions from photochemical and thermal degradation during the daytime. In a complementary study, at three sites across the Mediterranean Basin, litter CO₂ efflux was largely explained by litter moisture driving microbial degradation and ultraviolet radiation driving photodegradation. We further observed mutual enhancement of microbial activity and photodegradation at a daily scale. Identifying the interplay of decay mechanisms enhances our understanding of carbon turnover in drylands, which should improve the predictions of the long‐term trend of global carbon sequestration.     

Light and dissolved phosphorus interactively affect microbial metabolism,stoichiometry and decomposition of leaf litter

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Decomposition of soil and plant carbon from pasture systems after 9 years of exposure to elevated CO2: impact on C cycling and modeling

Elevated atmospheric CO₂ may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO₂ decomposes differently from plant material produced under ambient CO₂. Moreover, a long‐term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO₂ made in coupled climate–soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO₂ at two levels of N fertilizer (140 vs. 560 kg ha^?1 yr^?1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO₂ under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO₂ and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO₂ did not affect decomposition rates of the plant material. If under prolonged elevated CO₂ changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low‐N fertilization treatment was enhanced after exposure to elevated CO₂. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the ¹³C‐CO₂ signature, reflected the turnover times of SOM–C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO₂, as used in coupled climate–SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.     

Linking temperature sensitivity of soil organic matter decomposition to its molecular structure,accessibility, and microbial physiology

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme‐kinetic hypothesis suggests that decomposition of low‐quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high‐quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme‐substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low‐density fraction (LF) which represents readily accessible, mineral‐free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30‐days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6–1.8 g cm^?3) and bulk soil was measured by solid‐state ¹³C‐NMR. Decomposition Q₁₀ was significantly correlated with the abundance of aromatic plus alkyl‐C relative to O‐alkyl‐C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two‐ to three‐fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C‐use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme‐kinetic interpretation of widely observed C quality‐temperature relationship for short‐term decomposition. Factors controlling long‐term decomposition Q₁₀ are more complex due to protective effect of mineral matrix and thus remain as a central question.     

Seasonal fluctuations in microbial activity in Antarctic moss peat

The Signy Island terrestrial reference sites epitomize unpolluted maritime Antarctic tundra. The extreme transition from the harsh Antarctic winter to the milder summer facilitates studies of the effects of freeze-thaw cycles on microbial activity in moss peat. Seasonal monitoring of peat oxygen uptake showed a transient spring peak at c. 0^oC, attributed to microbial utilization of dissolved organic carbon (DOC). After a more gradual temperature-linked summer increase, autumnal freeze-thaw cycles stimulated a final pre-winter peak. The transient climaxes were associated with blooms of saccharolytic yeasts and microfungi. The bacterial population stabilized after a spring increase but then diversified as DOC became rate-limiting. Effects of pre-monitored spring freeze-thaw cycles on late-winter peat cores were simulated in a Gilson respirometer. In vitro perturbations demonstrated the regulatory effects of DOC availability, water content and temperature on peat respiration and microflora! composition. Comparative respirometry and loss in tensile strength of interred cotton strips showed a difference in decomposer activity beneath a relatively dry Polytrichum-Chorisodontium turf and a wet Cattiergon-Cephalozielta carpet. This was associated with water content and anaerobiosis. Cellulolysis accelerated during the growing season and increased with depth, despite anaerobic conditions. Estimates of annual bryophyte decomposition are presented for use in an Antarctic ecosystem model.     

Soil biochemical/microbial indices as ecological indicators of land use change in mangrove forests

The objective of the study was to examine the long-term changes in biochemical/microbial indicators of soil quality due to clear felling of mangroves for establishment of plantations. The biochemical/microbial parameters included dissolved organic-C (DOC) and -N (DON), soil microbial biomass-C (SMBC), -N (SMBN) and -P (SMBP), soil respiration (SR), metabolic quotient (qCO₂), adenylates (ATP, AMP and ADP), adenylate energy charge (AEC), ergosterol and their ratios. Results revealed that the undisturbed mangroves possessed considerably greater amounts of soil organic C, DOC and DON. Consequently, SMBC, SMBN and SMBP showed marked reductions in the plantations suggesting an average loss of 66%, 49% and 75%, respectively due to changed land use. Likewise, SR decreased by 46.4% in the plantations. Enhanced qCO₂ levels in the plantations indicated a microbial community under stress with a high maintenance carbon demand, while lower qCO₂ levels in the mangroves indicated an efficient microbial community and a better use of available organic substrates. The levels of ATP, AMP and ADP followed a trend identical to that of SMB and SR. Greater ergosterol concentration led to greater ergosterol/SMBC ratio suggesting a shift in the microbial community structure from a primarily fungi dominated SMB in the mangroves to a fungi recessive SMB in the plantations.     

Does declining carbon‐use efficiency explain thermal acclimation of soil respiration with warming?

Enhanced soil respiration in response to global warming may substantially increase atmospheric CO₂ concentrations above the anthropogenic contribution, depending on the mechanisms underlying the temperature sensitivity of soil respiration. Here, we compared short‐term and seasonal responses of soil respiration to a shifting thermal environment and variable substrate availability via laboratory incubations. To analyze the data from incubations, we implemented a novel process‐based model of soil respiration in a hierarchical Bayesian framework. Our process model combined a Michaelis–Menten‐type equation of substrate availability and microbial biomass with an Arrhenius‐type nonlinear temperature response function. We tested the competing hypotheses that apparent thermal acclimation of soil respiration can be explained by depletion of labile substrates in warmed soils, or that physiological acclimation reduces respiration rates. We demonstrated that short‐term apparent acclimation can be induced by substrate depletion, but that decreasing microbial biomass carbon (MBC) is also important, and lower MBC at warmer temperatures is likely due to decreased carbon‐use efficiency (CUE). Observed seasonal acclimation of soil respiration was associated with higher CUE and lower basal respiration for summer‐ vs. winter‐collected soils. Whether the observed short‐term decrease in CUE or the seasonal acclimation of CUE with increased temperatures dominates the response to long‐term warming will have important consequences for soil organic carbon storage.     

Isopod effects on decomposition of litter produced under elevated CO2, N deposition and different soil types

The performance of Oniscus asellus (Isopoda) and its influence on litter mass loss and mineralization was assessed in a microcosm experiment, using beech (Fagus sylvatica) leaf litter that was produced on different soil types, contrasting atmospheric CO₂ concentrations, and different nitrogen deposition rates. Litter quality was significantly altered by these treatments, and many of the CO₂ and N effects differed between soil types. Litter quality affected subsequent litter mass loss rates, microbial respiration, and leaching of dissolved organic carbon (DOC) and nitrate. These effects were largely independent of the presence of isopods, even though isopods highly accelerated litter mass loss, stimulated microbial respiration by 37%, and increased nitrate leaching by 50%. Isopods did not change their relative rates of litter consumption and growth in response to litter quality. Isopod mortality, however, increased with increasing litter lignin/N ratios, and was significantly different between soil types, which may indicate long‐term effects on litter decomposition through altered isopod densities. Having the choice among the litter of three different species [maple (Acer pseudoplatanus), beech (Fagus sylvatica) and oak (Quercus robur)] grown at either ambient or elevated CO₂, isopods preferred maple to beech when all the litter was produced under elevated CO₂. This suggests that beyond changes in consumption, an altered food selection by isopods in a CO₂‐enriched atmosphere may affect the temporal and spatial composition of the litter layer in temperate forests. In contrast to previous findings, we conclude that isopods do not always increase their consumption rates, and hence do not differentially affect microbial decomposition in litter of poorer quality. Nevertheless changes in animal densities and/or shifts in their food preferences, could result in altered decomposition and carbon and nutrient turnover rates.