亚热带溪流中树叶凋落物多酚含量对树叶分解过程的影响

利用邻二氮菲-铁(Ⅲ)分光光度法测定了蒲桃与人面子树叶在二级溪流中分解时植物多酚含量的变化,并研究了多酚含量对树叶分解速率、底栖动物定殖以及微生物呼吸量的影响。结果表明,蒲桃叶片中初始多酚含量(19%)比人面子树叶的含量高(6%),其分解速率(分解系数k=0.01d^-1)比人面子树叶慢(k=0.04d^-1)。研究还发现,蒲桃叶片上的微生物呼吸量比人面子的低(每克叶片单位时间的耗氧量分别为0.2和0.4毫克),而且底栖动物中撕食者的种类和数量也比人面子树叶少。说明蒲桃叶片中较高的多酚含量抑制了微生物的活性和底栖动物的取食,从而减缓了蒲桃树叶的分解进程,表明叶片中多酚含量与树叶凋落物分解速率呈负相关关系。     

Response of Aquatic Leaf Associated Microbial Communities to Elevated Leachate DOC: A Microcosm Study

In Central Europe climate change will increase summer droughts, which cause both, premature leaf fall and fragmentation of small streams during summer and early autumn. As a consequence dissolved organic carbon (DOC) leached from leaves will be dispersed into pools with long water residence time. A microcosm experiment was performed to test the effect of high concentrations of leachate DOC and the relative importance of labile and refractory leachate compounds on leaf associated microbial parameters. In microcosms leaf discs colonized in a stream were exposed to high concentrations of either leaf leachate, glucose or tannic acid. Leaf associated respiration, fungal sporulation, leaf mass loss and fungal biomass (ergosterol) were measured during a 3 weeks experimental period and compared to control without DOC amendment. The results imply that depending on source and composition elevated leachate DOC may have variable effects on microbial mediated litter decomposition. Our findings suggest reduced microbial decomposition rates in pools of fragmented streams receiving premature leaf fall. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Effects of Exotic Grasses on Litter Decomposition in a Hawaiian Woodland: The Importance of Indirect Effects

Exotic grasses and grass-fueled fires have altered plant species composition in the seasonal submontane woodlands of Hawaii Volcanoes National Park. These changes have altered both structural and functional aspects of the plant community, which could, in turn, have consequences for litter decomposition and nitrogen (N) dynamics. In grass-invaded unburned woodland, grass removal plots within the woodland, and woodland converted to grassland by fire, we compared whole-system fluxes and the contributions of individual species to annual aboveground fine litterfall and litterfall N, and litter mass and net N loss. We assessed the direct contribution of grass biomass to decomposition and N dynamics, and we determined how grasses affected decomposition processes indirectly via effects on native species and alteration of the litter layer microenvironment. Grasses contributed 35% of the total annual aboveground fine litterfall in the invaded woodland. However, total litterfall mass and N were not different between the invaded woodland and the grass removal treatment because of compensation by the native tree Metrosideros polymorpha, which increased litter production by 37% ± 5% when grasses were removed. The 0.3 g N m^–2/y^–1 contained in this production increase was equal to the N contained in grass litter. Litter production and litterfall N was lowest in the grassland due to the loss of native litter inputs. Decomposition of litterfall on an area basis was highest in the grass-invaded woodland. We attributed this effect to increased inherent decomposability of native litter in the presence of grasses because (a) the microenvironment of the three vegetation treatments had little effect on decomposition of common litter types and (b) M. polymorpha litter produced in the invaded woodland decomposed faster than that produced in the grass removal plots due to higher lignin concentrations in the latter than in the former. Area-weighted decomposition was lowest in the grassland due to the absence of native litter inputs. Across all treatments, most litter types immobilized N throughout the incubation, and litter net N loss on an area basis was not different among treatments. Our results support the idea that the effects of a plant species or growth form on decomposition cannot be determined in isolation from the rest of the community or from the direct effects of litter quality and quantity alone. In this dry woodland, exotic grasses significantly altered decomposition processes through indirect effects on the quantity and quality of litter produced by native species.     

Invasion by a Perennial Herb Increases Decomposition Rate and Alters Nutrient Availability in Warm Temperate Lowland Forest Remnants

We determined the impact of the invasive herb, Tradescantia fluminensis Vell., on litter decomposition and nutrient availability in a remnant of New Zealand lowland podocarp–broadleaf forest. Using litter bags, we found that litter beneath mats of Tradescantia decomposed at almost twice the rate of litter placed outside the mat. Values of k (decomposition quotient) were 9.44±0.42 yrs for litter placed beneath Tradescantia and 5.42±0.42 yrs for litter placed in native, non-Tradescantia plots. The impact of Tradescantia on decomposition was evident through the smaller forest floor mass in Tradescantia plots (2.65±1.05 t ha⁻¹) compared with non-Tradescantia plots (5.05±1.05 t ha⁻¹), despite similar quantities of annual leaf litterfall into Tradescantia plots (6.85±0.85 t ha⁻¹ yr⁻¹) and non-Tradescantia plots (7.45±1.05 t ha⁻¹ yr⁻¹). Moreover, there was increased plant nitrate available, as captured on resin bags, in Tradescantia plots (25.77 ± 8.32 cmol(−)/kg resin) compared with non-Tradescantia plots (9.55±3.72 cmol(−)/kg resin). Finally, the annual nutrient uptake by Tradescantia represented a large proportion of nutrients in litterfall (41% N, 61% P, 23% Ca, 46% Mg and 83% K), exceeded the nutrient content of the forest floor (except Ca), but was a small proportion of the topsoil nutrient pools. Taken together, our results show that Tradescantia increases litter decomposition and alters nutrient availability, effects that could influence the long-term viability of the majority of podocarp–broadleaf forest remnants affected with Tradescantia in New Zealand. These impacts are likely mostly due to Tradescantia's vegetation structure (i.e., tall, dense mats) and associated microclimate, compared with native ground covers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Invasion of temperate deciduous broadleaf forests by N-fixing tree species – consequences for stream ecosystems

Biological invasions are a major threat to biodiversity and ecosystem functioning. Forest invasion by alien woody species can have cross-ecosystem effects. This is especially relevant in the case of stream–riparian forest meta-ecosystems as forest streams depend strongly on riparian vegetation for carbon, nutrients and energy. Forest invasion by woody species with dissimilar characteristics from native species may be particularly troublesome. The invasion of temperate deciduous broadleaf forests with low representation of nitrogen (N)-fixing species by N-fixers has the potential to induce ecosystem changes at the stream level. Although effects of tree invasion on stream ecosystems have been under assessed, knowledge of native and invasive tree characteristics allows prediction of invasion effects on streams. Here we present a conceptual model to predict the effects of forest invasion by alien N-fixing species on streams, using as a background the invasion of temperate deciduous broadleaf forests by leguminous Acacia species, which are among the most aggressive invaders worldwide. Effects are discussed using a trait-based approach to allow the model to be applied to other pairs of invaded ecosystem–invasive species, taking into account differences in species traits and environmental conditions. Anticipated effects of N-fixing species invasions include changes in water quality (increase in N concentration) and quantity (decrease in flow) and changes in litter input characteristics (altered diversity, seasonality, typology, quantity and quality). The magnitude of these changes will depend on the magnitude of differences in species traits, the extent and duration of the invasion and stream characteristics (e.g. basal nutrient concentration). The extensive literature on effects of nutrient enrichment of stream water, water scarcity and changes in litter input characteristics on aquatic communities and processes allows prediction of invasion effects on stream structure and function. The magnitude of invasion effects on aquatic communities and processes may, however, depend on interactions among different pathways (e.g. effects mediated by increases in stream nutrient concentration may contrast with those mediated by decreases in water availability or by decreases in litter nutritional quality). A review of the literature addressing effects of increasing cover of N-fixing species on streams suggests a wide application of the model, while it highlights the need to consider differences in the type of system and species when making generalizations. Changes induced by N-fixing species invasion on streams can jeopardize multiple ecosystem services (e.g. good quality water, hydroelectricity, leisure activities), with relevant social and economic consequences.     

Dynamics of yeast populations recovered from decaying leaves in a nonpolluted stream: a 2-year study on the effects of leaf litter type and decomposition time

Here we report on the results of a survey of the yeast populations occurring on submerged leaves (alder, eucalyptus and oak) in a natural mountain stream, during different phases of their decomposition and through two consecutive years. Leaf litter mass loss, total yeast counts, Shannon-Weiner index (H'), yeast community structure and physiologic abilities were analyzed to evaluate the dynamics of yeast communities during decay. Seventy-two yeast taxa were recorded, and in all litter types, species of basidiomycetous affinity predominated over ascomycetous ones. Discriminant analysis of presence/absence data (yeast species) showed significant differences both among substrate types (P<0.0026) and with decomposition time (P<0.0001). Carbon and nitrogen source utilization by yeast strains also varied with the substrate (P<0.0001) and decomposition time (P<0.0001). Further conclusions were that: (1) all litter types have in common ubiquitous yeast species, such as Cryptococcus albidus, Debaryomyces hansenii and Rhodotorula glutinis, among the common 20 yeast species; (2) only a few species were dominant, and most species were rare, being recorded once or twice throughout decomposition; and (3) the order of yeast appearance, and their substrate assimilation patterns, strongly suggest a succession phenomenon. Finally, explanations for the distribution patterns and variations in yeast communities are discussed.     

The importance of bacterivorous protists in the decomposition of stream leaf litter

1. Allochthonous organic matter, in the form of senesced leaves, is a major source of carbon supporting detrital food webs. While studies have documented the role of bacteria and fungi in the decomposition of leaf litter, little information is available regarding the role of protists in the decomposition process. 2. We tested the hypothesis that the presence of stream‐dwelling bacterivorous protists leads to an increased rate of leaf decomposition through grazing pressure on bacteria. We isolated live protists from decomposing leaves collected in a stream in Northern Virginia, U.S.A. (Goose Creek) and established laboratory cultures of common bacterivorous protists. 3. Recently senesced leaves from the field were used in laboratory microcosm experiments to determine if the rate of litter decomposition differed between four treatments: bacteria only, bacteria + flagellates, bacteria + flagellates + ciliates, autoclaved stream water (control). We determined the dry weight of leaf remaining, bacterial abundance, flagellate abundance and ciliate abundance for each replicate on days 0, 7, 14, 30, 60 and 120. 4. The rate of leaf decomposition was significantly higher in treatments with protists than without and bacterial abundance declined in protist treatments compared with bacteria only treatment. Weight loss in the presence of flagellates was three to four times higher when protists were present compared with treatments with bacteria alone. These results provide experimental evidence that protists could play a significant role in the detrital processes of streams.     

高寒森林不同生境凋落叶分解过程中水溶性组分动态特征

凋落叶分解所产生的水溶性组分(water soluble matter)是森林水陆不同生境碳和养分迁移的重要载体。本研究通过布设高寒森林4种代表物种凋落叶分解袋,即康定柳(Salix paraplesia)、高山杜鹃(Rhododendron lapponicum)、方枝柏(Sabina saltuaria)和四川红杉(Larix mastersiana),探讨其在林下地表、溪流和河岸带3种生境下不同分解时期(冻结初期、冻结期、融化期、生长季节、生长季节后期)的水溶性组分及水溶性碳含量动态及其影响因素。结果表明:经两年的分解,发现溪流显著促进了凋落叶中水溶性组分和水溶性碳的释放;同一物种凋落叶在不同生境下水溶性组分和水溶性碳损失差异显著(P<0.05),整体表现为溪流>河岸带>林下;在分解初期水溶性组分含量有明显的降低;在整个分解过程中,水溶性组分(-70.43%)和水溶性碳(-84.31%)含量变化基本一致且呈明显降低趋势。此外,凋落叶中水溶性组分和水溶性碳的释放速率受时间、物种以及区域环境因子(温度、p H值、营养成分)的调控。这些结果表明,高寒森林凋落叶中水溶性...     

亚热带森林树种多样性对凋落叶分解胞外酶活性的影响

森林生物多样性与生态系统功能关系是当前群落生态学的热点研究领域。然而, 以往研究更多聚焦在森林植物多样性丧失对群落生产力的影响, 而对森林凋落物分解的相关研究稍显不足。森林凋落叶分解的快慢直接受控于凋落物分解者分泌的胞外酶的活性, 后者更是指示森林生态系统养分循环的重要指标之一。本研究依托我国江西亚热带森林生物多样性与生态系统功能控制实验, 通过对不同植物多样性梯度样方内目标树种凋落叶胞外酶活性、理化性质以及腐生真菌的分析, 探索树种多样性丧失对胞外酶活性的影响及其调控机制, 以探讨森林树种多样性对地表、地下生态过程和功能的影响。结果表明, 样方水平树种多样性丧失显著影响胞外酶的活性, 除单种样方外, 随着样方水平树种丰富度的增加, 胞外酶活性呈现出增长趋势; 与碳周转相关的α-葡萄糖苷酶(AG)、β-葡萄糖苷酶(BG)、纤维二糖水解酶(CB)在树种多样性最大时活性达到最高; 而木糖苷酶(XS)以及与氮、磷和顽拗有机养分分解相关的N-乙酰-β-氨基葡萄糖苷酶(NAG)、酸性磷酸酶(AP)和多酚氧化酶(PPO)在树种多样性较低时活性较高。针对目标树种周围的邻居树种多样性进一步分析发现, 各胞外酶活性随着邻居树种多样性的变化呈“单峰”响应趋势, 酶活性大多在邻居树种丰富度为6时呈现峰值。研究发现真菌分解者在胞外酶活性对植物多样性的响应上可能存在重要的调控作用, 可以推测树种多样性通过改变腐生真菌分解者的群落结构和多度, 从而影响胞外酶活性。     

外来种湿地松凋落物对土壤微生物群落结构和功能的影响

外来种湿地松是我国亚热带地区大面积造林树种,研究其凋落物影响下土壤微生物群落结构和功能的变化,对于全面科学的评价湿地松的生态影响具有重要的理论与实践意义。通过凋落物袋 小盆模拟试验研究,分两个时期(分解5个月和18个月)比较了外来种湿地松与本地种马尾松的凋落物对土壤微生物群落结构(磷脂脂肪酸)和功能(碳代谢)的影响,结果表明：(1)外来种湿地松凋落物的C:N高于本地种马尾松;(2)两个时期,湿地松凋落物处理土壤细菌和放线菌的磷脂脂肪酸含量均低于马尾松,18个月时湿地松凋落物处理土壤真菌含量和群落真菌/细菌显著高于马尾松处理;(3)湿地松凋落物影响下土壤微生物群落功能多样性显著低于马尾松;(4)土壤微生物群落的结构显著影响微生物的活性和功能多样性：土壤微生物群落碳源代谢的强度、多样性及丰富度与细菌磷脂脂肪酸含量呈极显著正相关,细菌特征脂肪酸14:0、15:0、a15:0、i16:0、16:1ω7c、a17:0和cy19:0的含量显著影响土壤微生物群落碳代谢功能。上述结果表明：与本地种马尾松相比,引进种湿地松的凋落物显著改变了土壤微生物群落结构,降低了微生物群落的功能。     

Species richness–decomposition relationships depend on species dominance

Human disturbances both decrease the number of species in ecosystems and change their relative abundances. Here we present field evidence demonstrating that shifts in species abundances can have effects on ecosystem functioning that are as great as those from shifts in species richness. We investigated spatial and temporal variability of leaf decomposition rates and community metrics of leaf‐eating invertebrates (shredders) in streams. The shredder community composition dramatically influenced the diversity–function relationship; decomposition was much higher for a given species richness at sites with high species dominance than at sites where dominance was low. Decomposition rates also markedly depended on the identity of the dominant species. Further, dominance effects on decomposition varied seasonally and the number of species required for maintaining decomposition increased with increasing evenness. These findings reveal important but less obvious aspects of the biodiversity–ecosystem functioning relationship.     

Higher temperature reduces the effects of litter quality on decomposition by aquatic fungi

1. We investigated the effects of riparian plant diversity (species number and identity) and temperature on microbially mediated leaf decomposition by assessing fungal biodiversity, fungal reproduction and leaf mass loss. 2. Leaves of five riparian plant species were first immersed in a stream to allow microbial colonisation and were then exposed, alone or in all possible combinations, at 16 or 24 °C in laboratory microcosms. 3. Fungal biodiversity was reduced by temperature but was not affected by litter diversity. Temperature altered fungal community composition with species of warmer climate, such as Lunulospora curvula, becoming dominant. 4. Fungal reproduction was affected by litter diversity, but not by temperature. Fungal reproduction in leaf mixtures did not differ or was lower than that expected from the weighted sum of fungal sporulation on individual leaf species. At the higher temperature, the negative effect of litter diversity on fungal reproduction decreased with the number of leaf species. 5. Leaf mass loss was affected by the identity of leaf mixtures (i.e. litter quality), but not by leaf species number. This was mainly explained by the negative correlation between leaf decomposition and initial lignin concentration of leaves. 6. At 24 °C, the negative effects of lignin on microbially mediated leaf decomposition diminished, suggesting that higher temperatures may weaken the effects of litter quality on plant litter decomposition in streams. 7. The reduction in the negative effects of lignin at the higher temperature resulted in an increased microbially mediated litter decomposition, which may favour invertebrate‐mediated litter decomposition leading to a depletion of litter stocks in streams.