Microbial Decomposition Rates Research Articles

Vegetation Loss Following Vertical Drowning of Mississippi River Deltaic Wetlands Leads to Faster Microbial Decomposition and Decreases in Soil Carbon

AbstractWetland ecosystems hold nearly a third of the global soil carbon pool, but as wetlands rapidly disappear the fate of this stored soil carbon is unclear. The aim of this study was to quantify and then link potential rates of microbial decomposition after vertical drowning of vegetated tidal marshes in coastal Louisiana to known drivers of anaerobic decomposition altered by vegetation loss. Profiles of potential CH4 and CO2 production (surface to 60 cm deep) were measured during anaerobic incubations, organic matter chemistry was assessed with infrared spectroscopy, and soil porewater nutrients and redox potentials were measured in the field along a chronosequence of wetland loss. After vertical drowning, pond soils had lower redox potentials, higher pH values, lower soil carbon and nitrogen concentrations, lower lignin: polysaccharide ratios, more NH4+ and PO43−, and higher rates of potential CO2 release than vegetated marsh soils. Potential CH4 production was similar in vegetated marshes and open water ponds, with depth‐dependent decreases in CH4 production as soil carbon concentrations increased. In these anoxic soils, vegetation loss exerts a primary control on decomposition rates because flooding drives sustained increases in porewater nutrient availability (NH4+ and PO43, dissolved organic carbon) and decreases in redox potential (from −150 to −500 mV) that lead to higher potential CO2 fluxes within a few years. Without new carbon inputs following wetland loss, the sustained decomposition in open water ponds may lead to losses of stored soil carbon and could influence global carbon budgets.

Joint effects of warming and salinization on instream leaf litter decomposition assessed through a microcosm experiment

Stream ecosystem functioning is often impaired by warming and salinization, but the joint effect of both environmental stressors on key processes such as leaf litter decomposition is virtually unknown, particularly in the tropics. We experimentally explored how increased water temperature (26, 29 and 32°C) and salinity (no salt addition, 0.1, 1.0 and 10 g l−1 NaCl added) affected the rates of total, microbial and detritivore-mediated decomposition, in stream microcosms containing leaf litter of Ficus insipida and larvae of Chironomus sp. collected from tropical streams. Effects of temperature were strong and consistent with previous findings: it promoted microbial decomposition and reduced decomposition mediated by detritivores, which showed greater feeding activity at 26°C. Salinity was negatively correlated with microbial decomposition at 32°C; it also had a negative influence on detritivore-mediated decomposition, which was nevertheless non-significant due to the high detritivore mortality at higher salinities. Notably, total decomposition was reduced with the joint presence of both factors (32°C and salt addition treatments, compared to 26°C and no salt addition), indicating the existence of additive effects and highlighting the relevance of multiple-stressor contexts when assessing the consequences of global change on stream ecosystems.

Soil organic matter turnover: Global implications from δ13C and δ15N signatures

The turnover and residence time of carbon (C) and nitrogen (N) in soil is a fundamental parameter reflecting the rates of soil organic matter (SOM) transformation and the contribution of soils to greenhouse gases fluxes. Based on the global database of the stable isotope composition of C (δ13C) and N (δ15N) depending on soil depth (171 profiles), we assessed С and N turnover and related them to climate, biome types and soil properties. The 13C and 15N discrimination between the litter horizon and mineral soil was evaluated to explain the key processes of litter transformation. The 13C and 15N discrimination by microbial utilization of litter and SOM, as well as the continuous increase of δ13C and δ15N with depth, enabled to assess C and N turnover within SOM. N turnover was two times faster than that of C, which reflects i) repeated N recycling by microorganisms accelerating N turnover, ii) C loss as CO2 and input of new C atoms to cycling, which reduces the C turnover within soil, and iii) generally slower turnover of N free persistent organic compounds (e.g. lignin, suberin, cellulose) compared to the N containing compounds (e.g. amino acids, ribonucleic acids). An increase in temperature and precipitation accelerated C and N turnover because: i) higher microbial activity and SOM decomposition rate, ii) larger soil moisture and fast diffusion of dissolved organics towards exoenzymes, iii) downward transport of 13C-enriched organic matter (e.g. sugars, amino acids), and iii) leaching of 15N-depleted nitrates from the topsoil into subsoil and losses from the whole soil profile. Temperature accelerates SOM turnover stronger than precipitation. The temperature increase by 10 °C accelerates the C and N turnover for 40 %. SOM turnover is boosted by decreasing C/N ratio because: i) SOM with a high C/N ratio originated from litter is converted to microbially-derived SOM in mineral soil characterized by a low C/N ratio; ii) litter with a low C/N ratio is decomposed faster than litter with a high C/N; iii) microbial carbon-use efficiency increases with N availability. The biome type affects SOM decomposition by i) climate: slower turnover under wet and cold conditions, and ii) by litter quality: faster utilization of leaves than needles. Thus, the fastest C turnover is common under evergreen forests and the lowest under mixed and coniferous ones, whereas temperature and C/N ratio are the main factors controlling SOM turnover. Concluding, the assessment of SOM turnover by δ13C and δ15N approach showed two times faster N turnover compared to C, and specifics of SOM turnover depending on the biomes as well as climate conditions.

Subsea permafrost organic carbon stocks are large and of dominantly low reactivity

Subsea permafrost carbon pools below the Arctic shelf seas are a major unknown in the global carbon cycle. We combine a numerical model of sedimentation and permafrost evolution with simplified carbon turnover to estimate accumulation and microbial decomposition of organic matter on the pan-Arctic shelf over the past four glacial cycles. We find that Arctic shelf permafrost is a globally important long-term carbon sink storing 2822 (1518–4982) Pg OC, double the amount stored in lowland permafrost. Although currently thawing, prior microbial decomposition and organic matter aging limit decomposition rates to less than 48 Tg OC/yr (25–85) constraining emissions due to thaw and suggesting that the large permafrost shelf carbon pool is largely insensitive to thaw. We identify an urgent need to reduce uncertainty in rates of microbial decomposition of organic matter in cold and saline subaquatic environments. Large emissions of methane more likely derive from older and deeper sources than from organic matter in thawing permafrost.

Decomposition rate as an emergent property of optimal microbial foraging

Decomposition kinetics are fundamental for quantifying carbon and nutrient cycling in terrestrial and aquatic ecosystems. Several theories have been proposed to construct process-based kinetics laws, but most of these theories do not consider that microbial decomposers can adapt to environmental conditions, thereby modulating decomposition. Starting from the assumption that a homogeneous microbial community maximizes its growth rate over the period of decomposition, we formalize decomposition as an optimal control problem where the decomposition rate is a control variable. When maintenance respiration is negligible, we find that the optimal decomposition kinetics scale as the square root of the substrate concentration, resulting in growth kinetics following a Hill function with exponent 1/2 (rather than the Monod growth function). When maintenance respiration is important, optimal decomposition is a more complex function of substrate concentration, which does not decrease to zero as the substrate is depleted. With this optimality-based formulation, a trade-off emerges between microbial carbon-use efficiency (ratio of growth rate over substrate uptake rate) and decomposition rate at the beginning of decomposition. In environments where carbon substrates are easily lost due to abiotic or biotic factors, microbes with higher uptake capacity and lower efficiency are selected, compared to environments where substrates remain available. The proposed optimization framework provides an alternative to purely empirical or process-based formulations for decomposition, allowing exploration of the effects of microbial adaptation on element cycling.

Temperature Sensitivity of Topsoil Organic Matter Decomposition Does Not Depend on Vegetation Types in Mountains.

Rising air temperatures caused by global warming affects microbial decomposition rate of soil organic matter (SOM). The temperature sensitivity of SOM decomposition (Q10) may depend on SOM quality determined by vegetation type. In this study, we selected a long transect (3.6 km) across the five ecosystems and short transects (0.1 km) from grazed and ungrazed meadows to forests in the Northwest Caucasus to consider different patterns in Q10 changes at shift of the vegetation belts. It is hypothesized that Q10 will increase along altitudinal gradient in line with recalcitrance of SOM according to kinetics-based theory. The indicators of SOM quality (BR:C, respiration per unit of soil C; MBC:C, ratio of microbial biomass carbon to soil carbon; soil C:N ratio) were used for checking the hypothesis. It was shown that Q10 did not differ across vegetation types within long and short transects, regardless differences in projective cover (14–99%) and vegetation species richness (6–12 units per plot). However, Q10 value differed between the long and short transects by almost two times (on average 2.4 vs. 1.4). Such a difference was explained by environmental characteristics linked with terrain position (slope steepness, microclimate, and land forms). The Q10 changes across studied slopes were driven by BR:C for meadows (R2 = 0.64; negative relationship) and pH value for forests (R2 = 0.80; positive relationship). Thus, proxy of SOM quality explained Q10 variability only across mountain meadows, whereas for forests, soil acidity was the main driver of microbial activity.

Towards a functional assessment of stream integrity: A first large-scale application using leaf litter decomposition

• Litter decomposition exhibits large variability at the regional scale (83 streams). • Litter decomposition does not correlate with the European WFD structural indicators. • Functional and structural indicators are complementary. • A grid is given to translate decomposition rates into functional quality classes. There is increasing recognition that indicators of ecosystem functioning are needed to improve current stream monitoring schemes. However, to date, no attempt has been made to include functional metrics in large-scale routine monitoring programs under the European Water Framework Directive (WFD). One reason is the uncertainty if functional indicators really carry new and independent information about stream status or, to the contrary, remain broadly redundant with existing indicators of biological stream quality (based on stream communities). A second reason is that, despite increasing scientific knowledge on how anthropogenic pressures influence ecosystem process rates, no ‘ready for use’ tool is available to translate them into stream ecosystem status. Litter decomposition probably ranges among the most documented stream ecosystem processes, and its potential for bioindication has been repeatedly demonstrated during the past decade. Here we report an extensive comparison of routine French structural indicators (I 2 M 2 , IBGN, IBD) with alder litter (microbial and total) decomposition rates in 83 streams located in south-western France. Expectedly, microbial decomposition rates were positively correlated with fungal biomass and activity (conidial production rate), while total and invertebrate-driven decomposition rates increased together with detritivore density and diversity in litter bags. By contrast, correlations between litter decomposition rates and routine structural indicators were clearly weak (Spearman’s ρ < 0.20), providing evidence that functional indicators bring new information. We devised a new interpretation grid to translate litter decomposition rates into stream ecological quality in a WFD-compatible design. This system considers the litter decomposition rates measured in both coarse (total decomposition) and fine (microbial decomposition) mesh bags as uncorrelated metrics of ecosystem functioning ( P = 0.898). Streams are considered functionally impaired when total and/or microbial decomposition rates are either higher or lower than in a reference condition. Applying our classification system to the 83 study sites allowed to assign them to 5 functional quality classes: bad (4 streams), poor (26), fair (29), good (21) and high (3). This classification did not match the classes obtained by conventional WFD structural indicators, reflecting that both functional and structural indicators are needed to obtain a comprehensive assessment of stream ecosystem integrity.

Climatic controls on stable carbon and nitrogen isotope compositions of temperate grasslands in northern China

The natural abundances of stable carbon (C) and nitrogen (N) isotopes (δ13C and δ15N) are extensively used to indicate the C and N biogeochemical cycles at large spatial scales. However, the spatial patterns of δ13C and δ15N in plant-soil systems of grasslands in northern China and their main driving factors across regional climatic gradient are still not well understood. We measured plant and soil δ13C and δ15N compositions as well as their associated environmental factors across 2000 km climatic gradient (-0.2 to 9 °C; 152 to 502 mm) in grasslands of northern China. The soil δ13C and δ15N values in surface were lower than those in bottom for temperate typical steppe but had no significant differences for temperate meadow steppe and temperate desert steppe. Soil δ13C values declined with increasing soil organic carbon (SOC) but increased as mean annual temperature (MAT). These changes were attributed to the microbial decomposition rate. The δ15N values in soil and plant were negatively correlated with MAT and mean annual precipitation (MAP), which were mainly related to the low soil organic matter mineralization rate and the shift of dominant species from C4 to C3. Our results indicate the spatial patterns and different influencing factors on δ13C and δ15N values along the climatic gradient in grasslands of northern China. The findings will provide scientific references for future research on the C and N biogeochemical cycles of temperate grasslands.

Grass species litter have varied trait response to the photodegradation and microbial decomposition in tropical savanna grasslands, South Africa

Purpose of the paper: This paper evaluated the effects of microbial and photodegradation on the ten grass species in a tropical grassland ecosystem in South Africa. Despite continuous necromass accumulation in tropical grasslands, the process that governs how the grass litter gets to the soil has often been ignored. In the absence of fire, abiotic factors and biotic factors are drivers of this process of nutrient cycling. Methods and approaches: A classical litterbag experiment was set up to simulate photodegradation (standing litter) versus microbial (soil surface) decomposition using 10 savanna types of grass from the start of the dry season until the beginning of spring. The rates of photodegradation versus microbial decomposition were compared for tropical savanna grasses and plant traits or predictors of decomposition were also investigated. Main results: The results showed that the grass litter decomposed more rapidly when exposed to the activity of soil microbes at the soil surface (k = 0.0028 ) than when held above ground (k = 0.0018) and subjected to photodegradation only. However, examination of the individual litter species revealed up to 4 fold variation between the slowest (0.0001) and fastest (0.006) decomposing species. Differences in decomposition rates were more pronounced for litter decomposed at the soil surface than held above the soil surface. Multiple linear regression analyses did not resolve clear differences between measures of litter quality as drivers of decomposition in the two treatments. Implications: Photodegradation appeared to be a rate-controlling step during the process of decomposition in tropical savanna grasslands. Therefore grasslands dominated by photodegradation-resistant grasses were likely to need fire to get rid of moribund litter, which may be the reason why mesic grasslands need to burn to recycle nutrients.

Soil VOC emissions of a Mediterranean woodland are sensitive to shrub invasion.

Many belowground processes, such as soil respiration and soil-atmosphere VOC (volatile organic compounds) exchange, are closely linked to soil microbiological processes. However, little is known about how changes in plant species cover, i.e. after plant invasion, alter these soil processes. In particular, the response of soil VOC emissions to plant invasion is not well understood. We analysed soil VOC emissions and soil respiration of a Mediterranean cork oak (Quercus suber) ecosystem, comparing soil VOC emissions from a non-invaded Q. suber woodland to one invaded by the shrub Cistus ladanifer. Soil VOC emissions were determined under controlled conditions using online proton-transfer time-of-flight mass spectrometry. Net soil VOC emissions were measured by exposing soils with or without litter to different temperature and soil moisture conditions. Soil VOC emissions were sensitive to C. ladanifer invasion. Highest net emission rates were determined for oxygenated VOC (acetaldehyde, acetone, methanol, acetic acid), and high temperatures enhanced total VOC emissions. Invasion affected the relative contribution of various VOC. Methanol and acetaldehyde were emitted exclusively from litter and were associated with the non-invaded sites. In contrast, acetone emissions increased in response to shrub presence. Interestingly, low soil moisture enhanced the effect of shrub invasion on VOC emissions. Our results indicate that shrub invasion substantially influences important belowground processes in cork oak ecosystems, in particular soil VOC emissions. High soil moisture is suggested to diminish the invasion effect through a moisture-induced increase in microbial decomposition rates of soil VOC.

Effect of riparian soil moisture on bacterial, fungal and plant communities and microbial decomposition rates in boreal stream-side forests

Riparian habitats of boreal forests are considered as hotspots for biochemical processes and biodiversity, and varying width riparian buffers have been proposed to protect species diversity of the riparian forests. However, evidence of the role of soil moisture variation in shaping riparian biodiversity and ecosystem functioning remain scarce particularly regarding belowground diversity. We studied how distance from the stream and soil moisture of the riparian zone affected species richness and community composition of plants, bacteria, and fungi as well as microbial decomposition rates. Using a split-plot design with a plant survey and amplicon sequencing for microorganisms we identified taxa associated with different categories of moisture and distance from the stream along six headwater stream-sides in middle boreal forests in Northern Finland. Tea-bag Index was used to assess the decomposition rates. PERMANOVA and linear mixed-effect models were used to analyze the data. Variation in riparian soil moisture influenced species composition and richness of plants and bacteria. Plant communities also changed from herbaceous dominated to shrub dominated with increasing distance from the stream. Fungal communities, however, did not respond to soil moisture or distance from the stream, and there were only slight differences in fungal trophic guilds among moisture and distance categories. Decomposition of organic material by microorganisms was faster adjacent to the stream than further away, and moist riparian areas had higher decomposition rates than drier ones. Decomposition rates were positively related to pH, Ca, Mg and NH4 and soil temperature.Synthesis and applications We show that above- and belowground diversity and microbial decomposition are associated to soil moisture at riparian sites supporting the idea of leaving wider unmanaged buffers in moist habitats to safeguard the overall forest diversity. Our findings further emphasize the need to consider soil moisture when planning the measures for riparian protection as changes in riparian soil moisture could lead to deterioration of organic matter decomposition. Different responses of the examined plant and microbial communities to riparian soil conditions clearly imply that overall riparian diversity cannot be explained based on a single community type, and that different organisms may respond differently to human-induced changes in stream riparian zone.

Colonization and decomposition of litter produced by invasive Acacia dealbata and native tree species by stream microbial decomposers

Changes in forest composition and litter inputs to streams due to invasion by exotic tree species can affect the functioning of freshwater ecosystems. Acacia dealbata is an important invasive tree species in Mediterranean areas, and often replaces the native riparian vegetation. In this study, we assessed the chemical characteristics of three litter types produced by the invasive Ac. dealbata (leaflets, flowers and pods) and leaf litter produced by two native tree species with contrasting litter characteristics (Quercus robur and Alnus glutinosa). We then assessed litter decomposition and associated microbial activity (i.e., overall microbial metabolism as respiration, fungal growth as biomass accumulation, and reproduction by aquatic hyphomycetes as conidial production), and the aquatic hyphomycetes community structure, in laboratory microcosms. In general, Ac. dealbata pods supported lower microbial activity and decomposed slower than all other litter types, due to their low nutrient concentrations and high carbon:nutrients molar ratio. Alnus glutinosa leaf litter supported high microbial activity and decomposed fast, due to its relatively high nutrient concentrations, low carbon:nutrients molar ratios and low lignin concentration. Acacia dealbata leaflets and flowers and Q. robur leaf litter generally had similar microbial activity and decomposition rates, intermediate between those of Ac. dealbata pods and Al. glutinosa leaf litter, likely due to trade-offs between nutrient concentrations and concentrations of structural and secondary compound among litter types. Aquatic hyphomycetes community structure also differed among litter types. For instance, Ac. dealbata pods had the lowest species richness per sampling date, but due to high dissimilarity among replicates, total species richness over the incubation period was comparable to that of other litter types. The invasion of native riparian forests by Ac. dealbata can affect the quality of litter inputs into streams, potentially affecting the community structure and activity of microbial decomposers, thus altering the functioning of stream ecosystems.

Effect of Riparian Soil Moisture on Bacterial, Fungal and Plant Communities and Microbial Decomposition Rates in Boreal Stream-Side Forests

Effect of Riparian Soil Moisture on Bacterial, Fungal and Plant Communities and Microbial Decomposition Rates in Boreal Stream-Side Forests

酸雨对中国陆地生态系统土壤呼吸影响的整合分析

PDF HTML阅读 XML下载 导出引用 引用提醒 酸雨对中国陆地生态系统土壤呼吸影响的整合分析 DOI: 10.5846/stxb202110202959 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（31901296）；广东省基础与应用基础研究基金项目（2021A1515010652）；广州市基础研究计划项目（202102020289） Effects of acid rain on soil respiration in Chinese terrestrial ecosystems: A meta-analysis Author: Affiliation: Fund Project: This work was supported by the National Natural Science Foundation of China (31901296), the Guangdong Basic and Applied Basic Research Foundation (2021A1515010652), and the Guangzhou Basic Research Program (2021000031). 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:通过整合分析（Meta-analysis）国内外公开发表的81篇模拟酸雨实验论文的2683条有效观测数据，量化了酸雨对中国3个主要陆地生态系统（森林、草地和农田）土壤呼吸（Rs）及其组分（自养呼吸（Ra）、异养呼吸（Rh））的影响。结果表明，酸雨显著降低了Rs （-9.6%）、Rh （-7.7%）和Ra （-11.7%）；酸雨pH越低，Rs及其组分的降幅越大；野外实验对Rh和Ra的负效应大于温室实验。酸雨对Rs的负效应在农田最大（-14.7%），草地次之（-10.8%），森林最小（-8.0%）；森林Rh、Ra对酸雨的响应与Rs一致，不同林型间差异不显著；草地Rh和Ra在酸雨处理下分别显著降低和增加。Rs、Rh与土壤pH显著正相关，与土壤有机碳（SOC）显著负相关；Rh和Ra分别与地上和地下生物量显著正相关。酸雨对Rs和Ra的负效应随纬度的增加而减弱，随年平均温的升高而增强，对Rs的正效应随年平均降水的降低而增强。研究表明，酸雨不仅降低了土壤pH，抑制了植物生长，减少了植物向土壤的碳输入，还降低了微生物活性，减少了Rh，导致SOC分解降低，因而未显著改变土壤碳库。研究结果将为全球变化背景下我国陆地生态系统的碳预算提供科学借鉴。 Abstract:Anthropogenic activities over the last four decades have increased depositions of sulfur dioxide (SO2) and nitrogen oxides (NOx) by three to five fold in China, which resulted in a serious problem of acid rain with adverse effects on terrestrial carbon cycle. Soil respiration (Rs) as the second largest flux for the most terrestrial carbon cycle, and its two components (i.e., autotrophic respiration (Ra) and heterotrophic respiration (Rh)) are extremely sensitive to anthropogenic acid rain due to its detrimental effects on plant growth and soil microbes. Although numerous ecosystem-level manipulative experiments were conducted to explore the underlying mechanisms how acid rain affected Rs, a quantitative synthesis of previous studies on the effects of acid rain on Rs and its two components across different terrestrial ecosystems was scarce. By conducting a meta-analysis of 2683 observations from 81 independent experimental studies, we quantified the directions and magnitudes of the responses of Rs, Ra, and Rh to simulated acid rain (SAR) in three major terrestrial ecosystems (i.e., forest, grassland, farmland) in China. Across all the experimental studies, our results showed that SAR significantly reduced Rs (-9.6%), Rh (-11.7%) and Ra (-11.7%), and the degree of reduction was proportional to the pH of SAR. The negative effects of SAR on Rh and Ra were greater in field experiment than that in pot experiment due to the differences in the intensity of SAR and experimental duration. Among different terrestrial ecosystem types, the negative effect of SAR on Rs was the most negative in farmland ecosystems (-14.7%), followed by grassland (-10.8%) and forest (-8.0%) ecosystems. The responses of Rh and Ra to SAR in forest ecosystems were consistent with that of Rs, and there were no significant differences among different forest types (i.e., coniferous forest, coniferous and broadleaved mixed forest and broadleaved forest). In grassland ecosystems, the response to SAR was negative for Rh, but positive for Ra. Linear-regression analysis showed that Rs and Rh were positively correlated with soil pH, but negatively correlated with soil organic carbon (SOC). Rh and Ra were positively correlated with aboveground biomass (AGB) and belowground biomass (BGB), respectively. Meta-regression analysis indicated that the negative effects of SAR on Rs and Ra were decreased with the increase of latitude, and increased with the increase of mean annual temperature (MAT), whereas the positive effect of SAR on Rs increased with a decrease in mean annual precipitation (MAP). Based on this meta-analysis, we found that SAR not only reduced soil pH, inhibited plant growth, and thus reduced plant carbon input to soil, but also inhibited microbial activities, reduced Rh, and SOC decomposition rate. Therefore, anthropogenic acid rain significantly altered the input and output of SOC, but did not significantly change the content of SOC in terrestrial ecosystems. These results will provide scientific basis for assessing the carbon budget of the Chinese terrestrial ecosystems in response to global change. 参考文献 相似文献 引证文献

Baja California Sur mangrove deep peat microbial communities cycle nitrogen but do not affect old carbon pool

Mangroves provide important ecosystem services, including storing carbon belowground for millennia. Mangrove carbon storage relies in part on high primary productivity, but essential to the long-lived nature of this storage is the slow rate of microbial decomposition of peat. In this study, we (1) examined how carbon and nitrogen densities and microbial community composition vary with peat age and (2) describe the formation of peat deposits over time. At 4 mangrove sites near La Paz, Baja California Sur, Mexico, we cored the sediments until rejection and obtained 5 cm samples at 20 cm intervals. In these samples, we measured organic carbon (Corg), total nitrogen, δ13C, δ15N, and radiocarbon (14C) age. We observed peat carbon densities of 3.4 × 10-2± 0.2 × 10-2 g cm-3, Corg:N ratios of 42 ± 3, and inter-site variation in Corg:N that reflects differing preservation conditions. Recalcitrant organic matter sources and anaerobic conditions leave a strong imprint on peat microbial communities. Microbial community composition and diversity were driven by depth and sediment characteristics, including Corg:N ratio and 14C age. Carbon dating allowed us to reconstruct the accumulation of organic matter over the last 5029 ± 85 yr. Even over this long time scale, though microbes have evidently continuously cycled the peat nitrogen pool, peat carbon density remains effectively unchanged.

Temperature sensitivity of Antarctic soil-humic substance degradation by cold-adapted bacteria.

Heteropolymer humic substances (HS) are the largest constituents of soil organic matter and are key components that affect plant and microbial growth in maritime Antarctic tundra. We investigated HS decomposition in Antarctic tundra soils from distinct sites by incubating samples at 5°C or 8°C (within a natural soil thawing temperature range of -3.8°C to 9.6°C) for 90 days (average Antarctic summer period). This continuous 3-month artificial incubation maintained a higher total soil temperature than that in natural conditions. The long-term warming effects rapidly decreased HS content during the initial incubation, with no significant difference between 5°C and 8°C. In the presence of Antarctic tundra soil heterogeneity, the relative abundance of Proteobacteria (one of the major bacterial phyla in cold soil environments) increased during HS decomposition, which was more significant at 8°C than at 5°C. Contrasting this, the relative abundance of Actinobacteria (another major group) did not exhibit any significant variation. This microcosm study indicates that higher temperatures or prolonged thawing periods affect the relative abundance of cold-adapted bacterial communities, thereby promoting the rate of microbial HS decomposition. The resulting increase in HS-derived small metabolites will possibly accelerate warming-induced changes in the Antarctic tundra ecosystem.

“Sinking dead”—How zooplankton carcasses contribute to particulate organic carbon flux in the subantarctic Southern Ocean

AbstractZooplankton carcasses are an important, yet understudied, pathway of the biological gravitational pump. To understand their contribution to the downward carbon flux in the subantarctic, carcasses of the copepod Neocalanus tonsus were analyzed for carbon content, microbial remineralization rates, and sinking velocities. In addition, the sensitivity of carcass flux to varying mortality, microbial turnover, and sinking velocity rates was analyzed and compared to carbon flux measurements from sediment traps. Microbial decomposition rates (between 0.02 and 0.16 d−1) were comparable to those of marine snow, highlighting the importance of carcasses as microbial hotspots. High sinking velocities (730 ± 182 m d−1) suggest that particulate organic carbon flux to the deep ocean is substantial. Carcass flux is sensitive to a change in sinking velocity but appears less sensitive to fluctuations in microbial decomposition rate. More research on zooplankton mortality and the factors that influence carcass sinking through the water column is needed to quantify the carcass‐mediated carbon export and enable their inclusion in marine ecosystem and biogeochemical models.

Functional fungal communities dominate wood decomposition and are modified by wood traits in a subtropical forest

Wood decomposition is a fundamental process of the carbon cycle in forest ecosystems and differs under varying environmental conditions. However, it remains unclear whether exposure situation and litter removal affect wood decomposition, especially in subtropical forests. Therefore, we chose wood from four dominant species and carried out an experiment with treatments consisting of placing wood in ground contact with and without litter input and above ground exposure. The experiment was performed for 2.5 consecutive years in the subtropical forest of Southwest China to reveal the potential effects of microenvironmental changes due to above ground exposure and nutrient input changes due to litter removal. In this study, neither above ground exposure nor litter removal significantly changed the fungal communities, microbial respiration rates or decomposition rates of the wood, but significant differences among tree species were observed. The abundance of Ascomycota (70.2%) was higher than that of Basidiomycota (24.3%), and there was a significant negative relationship between their abundances, suggesting competition. Moreover, negative (Ascomycota) and positive (Basidiomycota) relationships with microbial respiration and explained 21.5 and 25.5% of the variation in microbial respiration, respectively. The wood density was directly controlled by the sugar, cellulose, and lignin contents and influenced the water content in the wood. The abundances of saprotrophic and pathotrophic fungi were significantly and directly regulated by the water content of the wood. The abundance of pathotrophic fungi was unaffected by wood traits, but these fungi may limit saprotrophic fungal colonization, thereby affecting microbial respiration and decomposition processes. We confirmed that the saprotrophic fungal abundance, rather than fungal diversity, determined wood microbial respiration. These results are of great significance for the comprehensive assessment of wood decomposition and the carbon cycle in subtropical forests, although long-term fungal community dynamics and decomposition rates under different conditions require further study.

Aerobic environments in combination with substrate additions to soil significantly reshape depth-dependent microbial distribution patterns in Zoige peatlands, China

Climate warming is leading to the water table drawdown in peatlands, and a shift in the structure and productivity of vegetation communities. These events may alter the availability of oxygen and substrate utilized by soil microbes, which may impact the microbial decomposition rate in the different peat layers. We investigated the bacterial and fungal communities in surface (0–25 cm), subsurface (25–75 cm) and deep (> 76 cm) layers, and assessed the response of these communities to aerobic conditions and substrate of varying complexity additions to soils in Zoige peatlands, China. Bacterial and fungal communities varied in different peat layers, with the largest community sizes in the surface layer. The aerobic incubation significantly altered bacterial and fungal community compositions in the subsurface and deep layers. Both bacterial and fungal community size increased by 155% and 53% in the deep layer, respectively. Under aerobic conditions, substrate additions to soils significantly shifted fungal community compositions across all soil layers but not bacteria. Soil substrate increased the fungal community size by 130%–1000% in the subsurface layer without substantially altering bacterial abundance in any of the layers. Our results highlight the importance of vertical stratification in bacterial and fungal communities and the depth-dependent changes that occur under the influence of an aerobic environment and changes in the quality of the substrate. We suggest that fungi may be more sensitive to climate change than bacteria, especially in subsurface layers. Thus, this is an essential factor that needs to be considered when analyzing the role of microbes in peatland carbon dynamics.

Green and brown stream trophic food chains show specific responses to constant or hump-shaped inputs of copper

The brown food chain (based on decomposers) co-exists in streams with the green food chain (based on primary producers). The two trophic chains perform specific ecosystem functions which may be altered by the effect of contaminants. Copper is a common contaminant with recognized effects on several compartments of the two trophic chains. We applied it in two separate mesocosm experiments, in which we tested the effects of copper after contrasting patterns of contaminant exposure (constant vs hump-shaped). The constant input simulated a chronic contamination (average of 20 μg/L Cu), while the hump-shaped simulated the steady arrival of copper, the occurrence of a peak (reaching ca. 60 μg/L Cu), and its progressive decrease (down to 10–15 μg/L Cu). In the green trophic food chain, copper exposure decreased the total chlorophyll-a as well as the basal fluorescence and the photosynthetic yield. The treatment receiving hump-shaped inputs caused the highest mortality of the green food chain consumer, the snail Radix balthica. In the chronic copper exposure, mortality achieved a maximum of 80% by the end of the experiment but occurred later than that in the hump-shaped treatment. Effects on the brown food chain were not so pronounced; the microbial decomposition rate of leaflitter decreased nearly ca. 50% after 14 days of copper exposure. Effects on decomposition translated into the ingestion performance of detritivores, which decreased in the two copper treatments. Our results provide evidence that copper affected the two trophic food chains. The hump-shaped arrival included a peak of high concentration, which caused lethal effects on the consumers, but also a decreasing limb, which allowed a partial recovery of the algal photosynthetic variables. Our results suggest the need to consider the different compartments and functions performed within the stream trophic web when evaluating the effects of a contaminant in a river ecosystem.