Does mixing litter of different qualities alter stream microbial diversity and functioning on individual litter species?

We examined effects of leaf litter quality and species mixing on microbial community diversity and litter processing in a forested headwater stream. Single- and mixed-species litter from dominant tree species ( Liriodendron tulipifera , Acer rubrum , Quercus prinus , Rhododendron maximum ) were incubated in a southern Appalachian headwater stream. Litter carbon-to-nitrogen ratios (C:N), mass loss, microbial respiration, and microbial community diversity were analyzed on individual litter species after incubation. Initial C:N varied widely among individual litter species, and these differences persisted throughout the 50-day incubation period. Litter C:N of the recalcitrant species R. maximum remained higher than that of all other litter species, and C:N of R. maximum and L. tulipifera increased when both species were present together in a mixture. Although mass loss of individual species was generally unaffected by mixing, microbial respiration was greater on A. rubrum and Q. prinus litter incubated with R. maximum compared to either species alone. Enhanced resource heterogeneity, which was experimentally achieved by litter mixing low- and higher-quality litter species, resulted in apparent shifts in microbial community diversity on individual litter species. Responses of bacterial and fungal community diversity to litter mixing varied among individual litter species. Our results suggest that changes in tree species composition in riparian forests and subsequent changes in litter resource heterogeneity could alter stream microbial community diversity and function. As bacteria and fungi are important decomposers of plant litter in aquatic ecosystems, resource-dependent changes in microbial communities could alter detrital processing dynamics in streams.     

Spatial separation of litter decomposition and mycorrhizal nitrogen uptake in a boreal forest

Our understanding of how saprotrophic and mycorrhizal fungi interact to re-circulate carbon and nutrients from plant litter and soil organic matter is limited by poor understanding of their spatiotemporal dynamics. In order to investigate how different functional groups of fungi contribute to carbon and nitrogen cycling at different stages of decomposition, we studied changes in fungal community composition along vertical profiles through a Pinus sylvestris forest soil. We combined molecular identification methods with 14C dating of the organic matter, analyses of carbon:nitrogen (C:N) ratios and 15N natural abundance measurements. Saprotrophic fungi were primarily confined to relatively recently (< 4 yr) shed litter components on the surface of the forest floor, where organic carbon was mineralized while nitrogen was retained. Mycorrhizal fungi dominated in the underlying, more decomposed litter and humus, where they apparently mobilized N and made it available to their host plants. Our observations show that the degrading and nutrient-mobilizing components of the fungal community are spatially separated. This has important implications for biogeochemical studies of boreal forest ecosystems.     

Interactions between an arbuscular mycorrhizal fungus and a soil microbial community mediating litter decomposition

We investigated arbuscular mycorrhizal fungi (AMF) alteration of microbial mediation of litter decomposition. AMF (Glomus hoi) were either allowed access to or excluded from Plantago lanceolata L. root litter embedded in soil; litter was labeled with either (13) C only or (13) C and (15) N. Plant N uptake was significantly increased if AMF accessed the litter, and (15) N analysis of the plant material indicated that 2-3% of plant N originated from litter. Succession of the soil community mediating decomposition was assessed by phospholipid fatty acids (PLFA) combined with (13) C-PLFA. During the first 21?days of decomposition, saprotrophic fungi and Gram-negative bacteria were the dominant consumers of litter C. As decomposition progressed however, (13) C content of the fungal biomarkers declined substantially, and Gram-negative and Gram-positive bacteria became the primary reservoirs of labeled litter C. The putative PLFA marker for AMF (16:1ω5c) originated primarily from non-AMF sources. In AMF-invaded root litter, Gram-negative, Gram-positive, and 16:1ω5c markers became less (13) C-enriched relative to their counterparts in non-AMF-invaded microcosms during active decomposition. These patterns of (13) C: (12) C enrichment may result from AMF supply of (12) C from the plant to the decomposing soil microbial community; such C inputs could alter the microbial mediation of litter decomposition.     

Nitrogen availability affects saprotrophic basidiomycetes decomposing pine needles in a long term laboratory study

Fungi, especially basidiomycetous litter decomposers, are pivotal to the turnover of soil organic matter in forest soils. Many litter decomposing fungi have a well-developed capacity to translocate resources in their mycelia, a feature that may significantly affect carbon (C) and nitrogen (N) dynamics in decomposing litter. In an eight-month long laboratory study we investigated how the external availability of N affected the decomposition of Scots pine needles, fungal biomass production, N retention and N-mineralization by two litter decomposing fungi – Marasmius androsaceus and Mycena epipterygia. Glycine additions had a general, positive effect on fungal biomass production and increased accumulated needle mass loss after 8 months, suggesting that low N availability may limit fungal growth and activity in decomposing pine litter. Changes in the needle N pool reflected the dynamics of the fungal mycelium. During late decomposition stages, redistribution of mycelium and N out from the decomposed needles was observed for M. epipterygia, suggesting autophagous self degradation.     

Effects of grassland species on decomposition of litter and soil microbial communities

Decomposition of litter is greatly influenced not only by its chemical composition but also by activities of soil decomposers. By using leaf litter from 15 plant species collected from semi-natural and improved grasslands, we examined (1) how interspecific differences in the chemical composition of litter influence the abundance and composition of soil bacterial and fungal communities and (2) how such changes in microbial communities are related to the processes of decomposition. The litter from each species was incubated in soil of a standard composition for 60 days under controlled conditions. After incubation, the structure of bacterial and fungal communities in the soil was examined using phospholipid fatty-acid analysis and denaturing gradient gel electrophoresis. Species from improved grasslands had significantly higher rates of nitrogen mineralization and decomposition than those from semi-natural grasslands because the former were richer in nitrogen. Litter from improved grasslands was also richer in Gram-positive bacteria, whereas that from semi-natural grasslands was richer in actinomycetes and fungi. Nitrogen content of litter also influenced the composition of the fungal community. Changes in the composition of both bacterial and fungal communities were closely related to the rate of litter decomposition. These results suggest that plant species greatly influence litter decomposition not only through influencing the quality of substrate but also through changing the composition of soil microbial communities.     

Effect of Inorganic Nutrients on Relative Contributions of Fungi and Bacteria to Carbon Flow from Submerged Decomposing Leaf Litter

The relative contributions of fungi and bacteria to carbon flow from submerged decaying plant litter at different levels of inorganic nutrients (N and P) were studied. We estimated leaf mass loss, fungal and bacterial biomass and production, and microbial respiration and constructed partial carbon budgets for red maple leaf disks precolonized in a stream and then incubated in laboratory microcosms at two levels of nutrients. Patterns of carbon flow for leaf disks colonized with the full microbial assemblage were compared with those colonized by bacteria but in which fungi were greatly reduced by placing leaf disks in colonization chambers sealed with membrane filters to exclude aquatic hyphomycete conidia but not bacterial cells. On leaves colonized by the full microbial assemblage, elevated nutrient concentrations stimulated fungi and bacteria to a similar degree. Peak fungal and bacterial biomass increased by factors of 3.9 and 4.0; cumulative production was 3.9 and 5.1 times higher in the high nutrient in comparison with the low nutrient treatment, respectively. Fungi dominated the total microbial biomass (98.4 to 99.8%) and cumulative production (97.3 and 96.5%), and the fungal yield coefficient exceeded that of bacteria by a factor of 36 and 27 in low- and high-nutrient treatments, respectively. Consequently, the dominant role of fungi in leaf decomposition did not change as a result of nutrient manipulation. Carbon budgets indicated that 8% of leaf carbon loss in the low-nutrient treatment and 17% in the high-nutrient treatment were channeled to microbial (essentially fungal) production. Nutrient enrichment had a positive effect on rate of leaf decomposition only in microcosms with full microbial assemblages. In treatments where fungal colonization was reduced, cumulative bacterial production did not change significantly at either nutrient level and leaf decomposition rate was negatively affected (high nutrients), suggesting that bacterial participation in carbon flow from decaying leaf litter is low regardless of the presence of fungi and nutrient availability. Moreover, 1.5 and 2.3 times higher yield coefficients of bacteria in the reduced fungal treatments at low and high nutrients, respectively (percentage of leaf carbon loss channeled to bacterial production), suggest that bacteria are subjected to strong competition with fungi for resources available in leaf litter.     

Nitrogen limitation and microbial diversity at the treeline

Tree growth limitation at treeline has mainly been studied in terms of carbon limitation while effects and mechanisms of potential nitrogen (N) limitation are barely known, especially in the southern hemisphere. We investigated how soil abiotic properties and microbial community structure and composition change from lower to upper sites within three vegetation belts (Nothofagus betuloides and N. pumilio forests, and alpine vegetation) across an elevation gradient (from 0 to 650 m a.s.l.) in Cordillera Darwin, southern Patagonia. Increasing elevation was associated with a decrease in soil N‐NH₄⁺ availability within the N. pumilio and the alpine vegetation belt. Within the alpine vegetation belt, a concurrent increase in the soil C:N ratio was associated with a shift from bacterial‐dominated in lower alpine sites to fungal‐dominated microbial communities in upper alpine sites. Lower forested belts (N. betuloides, N. pumilio) exhibited more complex patterns both in terms of soil properties and microbial communities. Overall, our results concur with recent findings from high‐latitude and altitude ecosystems showing decreased nutrient availability with elevation, leading to fungal‐dominated microbial communities. We suggest that growth limitation at treeline may result, in addition to proximal climatic parameters, from a competition between trees and soil microbial communities for limited soil inorganic N. At higher elevation, soil microbial communities could have comparably greater capacities to uptake soil N than trees, and the shift towards a fungal‐dominated community would favour N immobilization over N mineralization. Though evidences of altered nutrient dynamics in tree and alpine plant tissue with increasing altitude remain needed, we contend that the measured residual low amount of inorganic N available for trees in the soil could participate to the establishment limitation. Finally, our results suggest that responses of soil microbial communities to elevation could be influenced by functional properties of forest communities for instance through variations in litter quality.     

Nitrogen input quality changes the biochemical composition of soil organic matter stabilized in the fine fraction: a long-term study

The chemical composition of soil organic matter (SOM) is a key determinant of its biological stability. Our objective in this study was to evaluate the effects of various sources of supplemental N on the chemical composition of SOM in the fine (<5 μm) mineral fraction. Treatments were fallow, maize/soybean in rotation, and continuous maize receiving no fertilizer (maize0N), synthetic fertilizer N (maize + N), or composted manure (maize + manure). The chemical structures in SOM associated with the fine fraction were determined using XANES spectroscopy at the C and N K-edges, which was assessed using multidimensional scaling. Analysis of amino sugar biomarkers were used to evaluate the fungal:bacterial contributions to the SOM. The addition of N to soils (i.e., maize + N, maize + manure, and maize/soybean treatments) resulted in the enrichment of proteinaceous compounds. Soils which did not receive supplemental N (i.e., fallow and maize0N treatments) were enriched in plant-derived compounds (e.g., aromatics, phenolics, carboxylic acids and aliphatic compounds), suggesting that decomposition of plant residues was constrained by N-limitation. Microbial populations assessed by amino sugar biomarker ratios showed that the highest contributions to SOM by bacteria occurred in the maize + manure treatment (high N input), and by fungi in the fallow treatment (low N input). The SOM in the maize + N and maize/soybean treatments was enriched in N-bonded aromatics; we attribute this enrichment to the abiotic reaction of inorganic N with organic C structures. The SOM in the maize + manure treatment was enriched in pyridinic-N, likely as a result of intense microbial processing and high SOM turnover. The presences of signals for ketone and pyrrole compounds in XANES spectra suggest their use as biomarkers for microbially transformed and stabilized SOM. The SOM in the maize + manure treatment was enriched in ketones which are likely microbial by-products of fatty acid catabolism. Pyrrole compounds, which may accumulate over the long term as by-products of protein transformations by an N-limited microbial community, were dominant in the fallow soil. A combination of molecular spectroscopy and biomarker analysis showed that the source of supplemental N to soil influences the stable C- and N-containing compounds of SOM in a long-term field study. Indeed, any increase in N availability allowed the microbial community to transform plant material into microbial by-products which occur as stable SOM compounds in the fine soil fraction.     

Cross-Ecosystem Comparisons of In Situ Plant Uptake of Amino Acid-N and NH4 +

Plant and microbial use of nitrogen (N) can be simultaneously mutualistic and competitive, particularly in ecosystems dominated by mycorrhizal fungi. Our goal was to quantify plant uptake of organic and inorganic N across a broad latitudinal gradient of forest ecosystems that varied with respect to overstory taxon, edaphic characteristics, and dominant mycorrhizal association. Using ¹³C and ¹⁵N, we observed in situ the cycling dynamics of NH₄ ⁺ and glycine through various soil pools and fine roots over 14 days. Recovery of ¹⁵N as soil N varied with respect to N form, forest type, and sampling period; however, there were similarities in the cycling dynamics of glycine and NH₄ ⁺ among all forest types. Microbial immobilization of ¹⁵N was immediately apparent for both treatments and represented the largest sink (~25%) for ¹⁵N among extractable soil N pools during the first 24 h. In contrast, fine roots were a relatively small sink (<10%) for both N forms, but fine root ¹³C enrichment indicated that plants in all forest types absorbed glycine intact, suggesting that plants and microbes effectively target the same labile soil N pools. Relative uptake of amino acid-N versus NH₄ ⁺ varied significantly among sites and approximately half of this variation was explained by mycorrhizal association. Estimates of plant uptake of amino acid-N relative to NH₄ ⁺ were 3× higher in ectomycorrhizal-dominated stands (1.6 ± 0.2) than arbuscular mycorrhizae-dominated stands (0.5 ± 0.1). We conclude that free amino acids are an important component of the N economy in all stands studied; however, in these natural environments plant uptake of organic N relative to inorganic N is explained as much by mycorrhizal association as by the availability of N forms per se.     

Anthropogenic nitrogen enrichment enhances soil carbon accumulation by impacting saprotrophs rather than ectomycorrhizal fungal activity

There is evidence that anthropogenic nitrogen (N) deposition enhances carbon (C) sequestration in boreal forest soils. However, it is unclear how free‐living saprotrophs (bacteria and fungi, SAP) and ectomycorrhizal (EM) fungi responses to N addition impact soil C dynamics. Our aim was to investigate how SAP and EM communities are impacted by N enrichment and to estimate whether these changes influence decay of litter and humus. We conducted a long‐term experiment in northern Sweden, maintained since 2004, consisting of ambient, low N additions (0, 3, 6, and 12 kg N ha^?1 year^?1) simulating current N deposition rates in the boreal region, as well as a high N addition (50 kg N ha^?1 year^?1). Our data showed that long‐term N enrichment impeded mass loss of litter, but not of humus, and only in response to the highest N addition treatment. Furthermore, our data showed that EM fungi reduced the mass of N and P in both substrates during the incubation period compared to when only SAP organisms were present. Low N additions had no effect on microbial community structure, while the high N addition decreased fungal and bacterial biomasses and altered EM fungi and SAP community composition. Actinomycetes were the only bacterial SAP to show increased biomass in response to the highest N addition. These results provide a mechanistic understanding of how anthropogenic N enrichment can influence soil C accumulation rates and suggest that current N deposition rates in the boreal region (≤12 kg N ha^?1 year^?1) are likely to have a minor impact on the soil microbial community and the decomposition of humus and litter.     

Plasticity and constraint in growth and protein mineralization of ectomycorrhizal fungi under simulated nitrogen deposition

Ectomycorrhizal fungi allow their host plants access to organic forms of N through enzymatic mineralization of the substrate and enhanced absorption of amino acids and mineral N. The cost to the plant is carbohydrates that support fungal growth and metabolism. Enrichment of soils with mineral N, as through atmospheric deposition, may affect the growth and function of these fungi by direct effects of increased N availability on fungi and indirect effects through reduced plant C allocation to roots. We tested the potential of N enrichment and altered carbohydrate supply to affect the growth and protein mineralization activity of 10 ectomycorrhizal fungi in sterile liquid media. Nitrogen treatments consisted of organic N only vs organic plus mineral N. Carbon treatments consisted of 5 g per liter glucose vs. no glucose added. Fungi differed widely in their growth and mineralization responses to these variables. Seven of 10 fungi had at least 20% reduced growth with reduced carbohydrates. Only 2 of 10 increased growth by 20% or more with increased mineral N. Carbohydrates affected growth more in a purely organic N environment suggesting an energy limitation to mineralization. Protein mineralization activity tended to be depressed by reductions in carbohydrates and increased by increased mineral N. The high sensitivity of fungal growth to carbohydrates suggests important indirect effects of N enrichment via altered C allocation in host trees. Principal Components analysis separated most fungal species along an axis representing a gradient from high protein mineralization efficiency to high intrinsic growth rate. Those fungi with slow growth and efficient mineralization activity corresponded closely to fungi often cited as late successional species, while fungi with high growth rates and low mineralization efficiency are often categorized as early successional. One fungus, Cenococcum geophillum, separated from others on an axis representing strong N dependence in growth. Nitrogen enrichment has the potential to alter the composition and function of the ectomycorrhizal fungus community. Physiological differences among species provide a starting point for predicting community responses and anticipating ecosystem consequences.     

Fungal guilds are evenly distributed along a vertical spruce forest soil profile while individual fungi show pronounced niche partitioning

Saprotrophic and ectomycorrhizal (EcM) forest fungi decompose organic matter and mobilize nutrients for host plants, respectively. Competition between the two guilds may cause the so-called Gadgil effect, i.e., decreased litter decomposition rates resulting in increased carbon storage in soil. The Gadgil effect was supposed to even affect global climate, highlighting the necessity to understand fungal distribution and interactions in soil. Searching for evidence of competition between saprotrophic and mycorrhizal fungi, we analyzed the distribution of fungi along a well-stratified vertical spruce forest soil profile in two seasons, i.e., autumn and the following spring. The different soil strata (i.e., two mineral horizons and two organic layers) underneath the litter layer were colonized by distinct fungal communities, which included roughly consistent proportions of all fungal guilds and phyla at each time. However, the community composition changed quantitatively between the sampling dates. Along the vertical soil profile, it differed mostly between the organic layers and the mineral soil, which is supposed to be due to differences in the predominant energy sources (i.e., aboveground litter and rhizodeposition, respectively). Network analyses revealed co-occurrences (i.e., positive correlations of individual abundances) to outweigh mutual exclusions (i.e., negative correlations) between individual fungi in each soil stratum and season. This also applied for interactions between saprotrophic and EcM fungi. Network analyses therefore provided no indications for a possible Gadgil effect. However, considering individual nutrient use efficiencies might refine insights from network analyses in future studies and facilitate linking community dynamics to ecosystem processes.     

Microbial activity and soil respiration under nitrogen addition in Alaskan boreal forest

Climate warming could increase rates of soil organic matter turnover and nutrient mineralization, particularly in northern high‐latitude ecosystems. However, the effects of increasing nutrient availability on microbial processes in these ecosystems are poorly understood. To determine how soil microbes respond to nutrient enrichment, we measured microbial biomass, extracellular enzyme activities, soil respiration, and the community composition of active fungi in nitrogen (N) fertilized soils of a boreal forest in central Alaska. We predicted that N addition would suppress fungal activity relative to bacteria, but stimulate carbon (C)‐degrading enzyme activities and soil respiration. Instead, we found no evidence for a suppression of fungal activity, although fungal sporocarp production declined significantly, and the relative abundance of two fungal taxa changed dramatically with N fertilization. Microbial biomass as measured by chloroform fumigation did not respond to fertilization, nor did the ratio of fungi : bacteria as measured by quantitative polymerase chain reaction. However, microbial biomass C : N ratios narrowed significantly from 16.0 ± 1.4 to 5.2 ± 0.3 with fertilization. N fertilization significantly increased the activity of a cellulose‐degrading enzyme and suppressed the activities of protein‐ and chitin‐degrading enzymes but had no effect on soil respiration rates or ¹⁴C signatures. These results indicate that N fertilization alters microbial community composition and allocation to extracellular enzyme production without affecting soil respiration. Thus, our results do not provide evidence for strong microbial feedbacks to the boreal C cycle under climate warming or N addition. However, organic N cycling may decline due to a reduction in the activity of enzymes that target nitrogenous compounds.     

Nitrogen increases soil organic carbon accrual and alters its functionality

Nitrogen (N) availability has been considered as a critical factor for the cycling and storage of soil organic carbon (SOC), but effects of N enrichment on the SOC pool appear highly variable. Given the complex nature of the SOC pool, recent frameworks suggest that separating this pool into different functional components, for example, particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), is of great importance for understanding and predicting SOC dynamics. Importantly, little is known about how these N-induced changes in SOC components (e.g., changes in the ratios among these fractions) would affect the functionality of the SOC pool, given the differences in nutrient density, resistance to disturbance, and turnover time between POC and MAOC pool. Here, we conducted a global meta-analysis of 803 paired observations from 98 published studies to assess the effect of N addition on these SOC components, and the ratios among these fractions. We found that N addition, on average, significantly increased POC and MAOC pools by 16.4% and 3.7%, respectively. In contrast, both the ratios of MAOC to SOC and MAOC to POC were remarkably decreased by N enrichment (4.1% and 10.1%, respectively). Increases in the POC pool were positively correlated with changes in aboveground plant biomass and with hydrolytic enzymes. However, the positive responses of MAOC to N enrichment were correlated with increases in microbial biomass. Our results suggest that although reactive N deposition could facilitate soil C sequestration to some extent, it might decrease the nutrient density, turnover time, and resistance to disturbance of the SOC pool. Our study provides mechanistic insights into the effects of N enrichment on the SOC pool and its functionality at global scale, which is pivotal for understanding soil C dynamics especially in future scenarios with more frequent and severe perturbations.     

Nitrogen and Phosphorus Limitation over Long-Term Ecosystem Development in Terrestrial Ecosystems

Nutrient limitation to net primary production (NPP) displays a diversity of patterns as ecosystems develop over a range of timescales. For example, some ecosystems transition from N limitation on young soils to P limitation on geologically old soils, whereas others appear to remain N limited. Under what conditions should N limitation and P limitation prevail? When do transitions between N and P limitation occur? We analyzed transient dynamics of multiple timescales in an ecosystem model to investigate these questions. Post-disturbance dynamics in our model are controlled by a cascade of rates, from plant uptake (very fast) to litter turnover (fast) to plant mortality (intermediate) to plant-unavailable nutrient loss (slow) to weathering (very slow). Young ecosystems are N limited when symbiotic N fixation (SNF) is constrained and P weathering inputs are high relative to atmospheric N deposition and plant N:P demand, but P limited under opposite conditions. In the absence of SNF, N limitation is likely to worsen through succession (decades to centuries) because P is mineralized faster than N. Over long timescales (centuries and longer) this preferential P mineralization increases the N:P ratio of soil organic matter, leading to greater losses of plant-unavailable N versus P relative to plant N:P demand. These loss dynamics favor N limitation on older soils despite the rising organic matter N:P ratio. However, weathering depletion favors P limitation on older soils when continual P inputs (e.g., dust deposition) are low, so nutrient limitation at the terminal equilibrium depends on the balance of these input and loss effects. If NPP switches from N to P limitation over long time periods, the transition time depends most strongly on the P weathering rate. At all timescales SNF has the capacity to overcome N limitation, so nutrient limitation depends critically on limits to SNF.     

贺兰山不同林分凋落物微生物群落特征与影响因素

贺兰山是我国重要生态屏障,贺兰山生态森林生态系统保护受到极大关注,森林凋落物及土壤微生物对全球气候变化研究具有重要意义。目前,贺兰山不同林分的凋落物分解过程中微生物群落结构特征差异、不同凋落物化学组成对微生物群落结构的影响尚不清楚。以贺兰山具有代表性的3种林分（油松林、青海云杉林以及油松-山杨混交林）凋落物为研究对象,开展凋落物化学组成、微生物群落组成及多样性特征研究,揭示不同林分凋落物的优势微生物群落特征和影响因子。结果表明,3种林分凋落物的细菌和真菌在多个多样性指数之间差异性均不显著,但是在多样性指数中真菌PD whole tree指数显著大于细菌,真菌Shannon指数与Ghao1指数却显著小于细菌。在门水平上不同林分凋落物的微生物优势菌类无显著差异,但在属水平上差异显著,而且细菌差异小于真菌,在各个分类水平上,凋落物细菌和真菌群落组成均表现为油松-山杨混交林＜青海云杉林＜青海云杉林,凋落物微生物多样性在青海云杉林中最为丰富。细菌不同节点间连接线负相关数量略大于正相关,真菌则相反。油松林凋落物与其它林分凋落物相比,微生物群落之间联系更加紧密。油松林凋落物OC含量最大、青海云杉林凋落物的TK含量最大、油松-山杨混交林的TN含量最大,且在3种林分中显著差异。相关性分析表明OC、TN、TP、TK是影响凋落物细菌和真菌群落组成及多样性的主要因素,冗余分析表明不同林分凋落物的微生物多样性指数受养分影响,凋落物OC、TN、TP、TK是影响微生物群落组成和多样性的重要因素,其中OC与微生物群落多样性相关性最显著,是影响凋落物细菌和真菌群落组成和多样性最主要的因子。     

Antagonism between Bacteria and Fungi on Decomposing Aquatic Plant Litter

Bacterial and fungal decomposers of aquatic plant litter may exhibit either synergistic or antagonistic interactions, which are likely to influence microbial growth as well as the decomposition of litter and, eventually, the carbon metabolism of aquatic systems. To elucidate such interactions, we inoculated decomposing Phragmites culms in microcosms with fungal isolates and with natural communities of bacteria and fungi in different combinations. The development of fungal and bacterial biomass and the carbon dynamics were studied during several months of degradation. The results show a bilateral antagonistic relationship between bacteria and fungi. After 3 months, fungal biomass accumulation was approximately 12 times higher in the absence than in the presence of bacteria. Bacterial biomass accumulation was about double in the absence of fungi compared to when fungi were present. Similar interactions developed between a natural assemblage of bacteria and five different fungal strains isolated from Phragmites litter (three identified hyphomycetes and two unidentified strains). Despite the great difference in biomass development between the treatments, the carbon metabolism was similar regardless of whether fungi and/or bacteria were present alone or in coexistence. We suggest that the antagonism between bacteria and fungi is an important controlling factor for microbial colonization and growth on aquatic plant litter.     

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

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Nitrogen deposition,vegetation burning and climate warming act independently on microbial community structure and enzyme activity associated with decomposing litter in low‐alpine heath

Low‐alpine heathlands are thought to be particularly sensitive to nitrogen (N) deposition, climate and land management change, yet little is known about how these factors regulate key belowground processes, like litter turnover, under field conditions. Here we use an in situ factorial field experiment to test the effects of increased atmospheric N deposition, climate manipulation and past vegetation burning, and their interactions, on litter decomposition and the activity and diversity of associated microorganisms. The use of litter from within (native) and outwith (standard) the experimental plots also enabled us to test whether decomposition and microbial functional diversity is driven primarily by soil conditions or litter chemistry. In general, extracellular enzyme activities of litter were driven by additions of simulated N deposition with phosphatase being the most responsive. We found that standard litter incubated in plots that had been burnt 8 years previously decomposed slower and lost less N and phosphorus than in unburnt plots. This material also had associated with it the greatest activity of glucosidase and the least diverse microbial community, as assessed by culture‐independent methods. Although all treatments significantly affected microbial diversity, burning explained most of the variability, indicating a close coupling between plant and microbial communities in these treatments. A striking feature of all the data relating to both standard and native litter was an almost complete lack of interactive effects between the treatments. The lack of interactions between the treatments indicates that each perturbation might affect different mechanisms in the decomposition process (including the composition of associated microbial communities) and nutrient cycling.     

凋落物多样性及组成对凋落物分解和土壤微生物群落的影响

凋落物分解是生态系统营养物质循环的核心过程,而土壤微生物群落在凋落物分解过程中扮演着极其重要且不可替代的角色。随着生物多样性的丧失日益严峻,探讨凋落物多样性及组成对凋落物分解和土壤微生物群落的影响,不仅有助于了解凋落物分解的内在机制,而且可为退化草原生态系统的恢复提供参考。以内蒙古呼伦贝尔草原退化恢复群落中的草本植物为研究对象,依据植物多度、盖度、频度和物种的重要值及其在群落中的恢复程度筛选出排序前4的羊草（Leymus chinensis）、茵陈蒿（Artemisia capillaris）、麻花头（Serratula centauroides）、二裂委陵菜（Potentilla bifurca）的凋落物为实验材料,通过设置3种凋落物多样性水平（1,2,4）,包括11种凋落物组合（单物种凋落物共4种,两物种凋落物混合共6种,四物种凋落物混合共1种）,利用磷脂脂肪酸（PLFA）方法来研究分解60 d后凋落物多样性及组成对凋落物分解和土壤微生物群落的影响。结果表明：（1）凋落物物种多样性仅对C残余率具有显著影响,表现在两物种混合凋落物C残余率显著低于单物种凋落物,而凋落物组成对所观测的4个凋落物分解参数（质量、C、N残余率以及C/N）均具有显著影响;（2）凋落物物种多样性对细菌（B）含量具有显著影响,而凋落物组成对真菌（F）含量具有显著影响,两者对F/B以及微生物总量均无显著影响;（3）冗余分析结果表明凋落物组成与凋落物分解相关指标（凋落物质量、C、N残余率及C/N）和土壤微生物（真菌、细菌含量）的相关关系高于凋落物多样性。（4）进一步建立结构方程模型（Structural Equation Model,SEM）发现,凋落物初始C含量对凋落物质量、C、N残余率及C/N有显著正的直接影响;凋落物木质素含量对凋落物质量、C、N残余率有显著正的直接影响;凋落物初始N含量对N残余率有显著正的直接影响,而对C残余率及C/N有显著负的直接影响;凋落物初始C/N对凋落物质量、N残余率有显著正的直接影响,而对C/N有显著负的直接影响。此外,凋落物初始C、N、木质素含量及C/N均对真菌含量具有显著正影响,并可通过真菌对凋落物质量分解产生显著负的间接影响。以上结果表明该退化恢复区域优势种凋落物分解以初始C、木质素为主导,主要通过土壤真菌影响凋落物的分解进程,这将减缓凋落物的分解速率进而减慢草原生态系统的进程。这些结果为凋落物多样性及组成对自身分解和土壤微生物群落的影响提供了实验依据,也为进一步分析凋落物分解内在机制以及草原生态系统的恢复提供了数据参考。