Detritus Quality Controls Macrophyte Decomposition under Different Nutrient Concentrations in a Eutrophic Shallow Lake, North China

Macrophyte decomposition is important for carbon and nutrient cycling in lake ecosystems. Currently, little is known about how this process responds to detritus quality and water nutrient conditions in eutrophic shallow lakes in which incomplete decomposition of detritus accelerates the lake terrestrialization process. In this study, we investigated the effects of detritus quality and water nutrient concentrations on macrophyte decomposition in Lake Baiyangdian, China, by analyzing the decomposition of three major aquatic plants at three sites with different pollution intensities (low, medium, and high pollution sites). Detritus quality refers to detritus nutrient contents as well as C∶N, C∶P, and N∶P mass ratios in this study. Effects of detritus mixtures were tested by combining pairs of representative macrophytes at ratios of 75∶25, 50∶50 and 25∶75 (mass basis). The results indicate that the influence of species types on decomposition was stronger than that of site conditions. Correlation analysis showed that mass losses at the end of the experimental period were significantly controlled by initial detritus chemistry, especially by the initial phosphorus (P) content, carbon to nitrogen (C∶N), and carbon to phosphorus (C∶P) mass ratios in the detritus. The decomposition processes were also influenced by water chemistry. The NO(3)-N and NH(4)-N concentrations in the lake water retarded detritus mass loss at the low and high pollution sites, respectively. Net P mineralization in detritus was observed at all sites and detritus P release at the high pollution site was slower than at the other two sites. Nonadditive effects of mixtures tended to be species specific due to the different nutrient contents in each species. Results suggest that the nonadditive effects varied significantly among different sites, indicating that interactions between the detritus quality in species mixtures and site water chemistry may be another driver controlling decomposition in eutrophic shallow lakes.     

Effect of temperature on submerged macrophyte litter decomposition within sediments from a large shallow and subtropical freshwater lake

In shallow aquatic systems, the majority of organic matter mineralization occurs in the sediments. Several factors including temperature control mineralization rates, however, the underlying causes of the effects are not well understood in subtropical lakes. In this study, we determined the influence of temperature on organic matter degradation by taking sediments from four sites in a subtropical large shallow freshwater lake, and monitoring organic matter composition and enzymes in microcosm experiments at five temperatures from 5 to 40°C. Following a three-month incubation, it was found that the mineralization of submerged plants in sediments was strongly influenced by temperature. Removal efficiency of total organic carbon in sediments ranged from 4.3 to 22.6% at 5°C, and reached 46.7–55.5% at 40°C. In addition, the removal efficiency of organic matter and the relative recalcitrant carbon decomposition depended on sediment type. For sediments in the site located in the lake center, recalcitrant and labile carbon decomposition had equivalent responses to the different temperatures. For sediments with dominance of submerged macrophytes, the humic acids were low even at high temperature. Thus, the annual deposition of plant litter in sediments favored organic carbon decomposition rather than humification.     

桂西北喀斯特区原生林与次生林凋落叶降解和养分释放

凋落叶降解及养分释放研究对喀斯特生态脆弱区森林生态系统的恢复与重建具有重要指导意义。本文选取桂西北喀斯特区3种原生林与3种次生林进行比较,研究其凋落叶降解与降解过程中的营养元素释放规律以及降解速率的影响因子。结果表明,原生林凋落叶的降解速率略大于次生林。C、N、K元素在前180天释放速率较快,随后趋于稳定。次生林凋落叶总P含量在降解初始阶段呈净积累,随后净释放,而原生林的凋落叶在降解360天后仍呈现P素净积累。相关分析表明,凋落叶降解速率与凋落叶初始总N、木质素含量及木质素:N比值呈负相关,与C:N比呈正相关。综合比较发现,次生林圆叶乌桕(Sapium rotundifolium Hemsl)凋落叶的降解速率与养分释放速率较快,是喀斯特退化土地及植被恢复过程中潜在的优势种和建群种。     

Nutrient fluxes via litterfall and leaf litter decomposition vary across a gradient of soil nutrient supply in a lowland tropical rain forest

The extent to which plant communities are determined by resource availability is a central theme in ecosystem science, but patterns of small-scale variation in resource availability are poorly known. Studies of carbon (C) and nutrient cycling provide insights into factors limiting tree growth and forest productivity. To investigate rates of tropical forest litter production and decomposition in relation to nutrient availability and topography in the absence of confounding large-scale variation in climate and altitude we quantified nutrient fluxes via litterfall and leaf litter decomposition within three distinct floristic associations of tropical rain forest growing along a soil fertility gradient at the Sepilok Forest Reserve (SFR), Sabah, Malaysia. The quantity and nutrient content of small litter decreased along a gradient of soil nutrient availability from alluvial forest (most fertile) through sandstone forest to heath forest (least fertile). Temporal variation in litterfall was greatest in the sandstone forest, where the amount of litter was correlated negatively with rainfall in the previous month. Mass loss and N and P release were fastest from alluvial forest litter, and slowest from heath forest litter. All litter types decomposed most rapidly in the alluvial forest. Stand-level N and P use efficiencies (ratios of litter dry mass to nutrient content) were greatest for the heath forest followed by the sandstone ridge, sandstone valley and alluvial forests, respectively. We conclude that nutrient supply limits productivity most in the heath forest and least in the alluvial forest. Nutrient supply limited productivity in sandstone forest, especially on ridge and hill top sites where nutrient limitation may be exacerbated by reduced rates of litter decomposition during dry periods. The fluxes of N and P varied significantly between the different floristic communities at SFR and these differences may contribute to small-scale variation in species composition.     

不同土壤厚度、水分和种植方式对喀斯特两种草本凋落物分解质量损失和化学计量特征的影响

为了探究生长期间不同土壤厚度、水分及种植方式处理对草本植物凋落物分解质量损失和化学计量特征的的影响,采用分解袋法,在露天分解床上分解经过生长期间2种土壤厚度(对照土壤厚度和浅土处理)、2种水分(正常灌水处理和干旱处理)和2种种植方式(单种和混种)处理的苇状羊茅(Festuca arundinacea Schreb.)和黑麦草(Lolium perenne L.)凋落物,研究生长期间土壤厚度和水分减少及不同的种植方式是否通过改变两物种初始凋落物质量、产量和组分来影响自身凋落物分解。结果发现:(1)与对照组(CK)相比,在干旱组(D)和浅土+干旱组(SD),两物种地上、根系和总的凋落物质量损失率、初始N和P含量均显著增加,凋落物产量、C/N和C/P显著降低,凋落物地上组分比大体上无显著变化;而不同的种植方式处理对各组分质量损失率、元素含量、计量比、凋落物产量和组分比大体上无显著影响;(2)两物种地上、根系和总凋落物的质量损失率分别与地上、根系和总N含量呈显著正相关,与C/N呈显著负相关,与凋落量呈显著负相关,而总凋落物质量损失率与地上凋落物组分比呈显著正相关。结果表明,生长期间干旱和浅土+干旱处理能够通过影响苇状羊茅和黑麦草的初始凋落物质量、产量和组分比来加快地上、根系和总凋落物分解,其中凋落物N含量和C/N是影响两物种凋落物分解快慢的主要原因。     

Decomposition dynamics in mixed-species leaf litter

Literature on plant leaf litter decomposition is substantial, but only in recent years have potential interactions among leaves of different species during decomposition been examined. We review emerging research on patterns of mass loss, changes in nutrient concentration, and decomposer abundance and activity when leaves of different species are decaying in mixtures. Approximately 30 papers have been published that directly examine decomposition in leaf mixtures as well as in all component species decaying alone. From these litter‐mix experiments, it is clear that decomposition patterns are not always predictable from single‐species dynamics. (Characteristics of decomposition in litter‐mixes that deviate from responses predicted from decomposition of single‐species litters alone are designated "non‐additive"; "additive" responses in mixes are predictable from component species decaying alone.) Non‐additive patterns of mass loss were observed in 67% of tested mixtures; mass loss is often (though not always) increased when litters of different species are mixed. Observed mass loss in some mixtures is as much as 65% more extensive than expected from decomposition of single‐species litter, but more often mass loss in mixtures exceeds expected decay by 20% or less. Nutrient transfer among leaves of different species is striking, with 76% of the mixtures showing non‐additive dynamics of nutrient concentrations. Non‐additive patterns in the abundance and activity of decomposers were observed in 55% and 65% of leaf mixes, respectively. We discuss some methodological details that likely contribute to conflicting results among mixed‐litter studies to date. Enough information is available to begin formulating mechanistic hypotheses to explain patterns in litter‐mix experiments. Emerging patterns in the mixed‐litter decomposition literature have implications for relationships between biodiversity and ecosystem function (in this case, the function being decomposition), and for potential mechanisms through which invasive plant species could alter carbon and nutrient dynamics in ecosystems.     

Enhanced summer warming reduces fungal decomposer diversity and litter mass loss more strongly in dry than in wet tundra

Many Arctic regions are currently experiencing substantial summer and winter climate changes. Litter decomposition is a fundamental component of ecosystem carbon and nutrient cycles, with fungi being among the primary decomposers. To assess the impacts of seasonal climatic changes on litter fungal communities and their functioning, Betula glandulosa leaf litter was surface‐incubated in two adjacent low Arctic sites with contrasting soil moisture regimes: dry shrub heath and wet sedge tundra at Disko Island, Greenland. At both sites, we investigated the impacts of factorial combinations of enhanced summer warming (using open‐top chambers; OTCs) and deepened snow (using snow fences) on surface litter mass loss, chemistry and fungal decomposer communities after approximately 1 year. Enhanced summer warming significantly restricted litter mass loss by 32% in the dry and 17% in the wet site. Litter moisture content was significantly reduced by summer warming in the dry, but not in the wet site. Likewise, fungal total abundance and diversity were reduced by OTC warming at the dry site, while comparatively modest warming effects were observed in the wet site. These results suggest that increased evapotranspiration in the OTC plots lowered litter moisture content to the point where fungal decomposition activities became inhibited. In contrast, snow addition enhanced fungal abundance in both sites but did not significantly affect litter mass loss rates. Across sites, control plots only shared 15% of their fungal phylotypes, suggesting strong local controls on fungal decomposer community composition. Nevertheless, fungal community functioning (litter decomposition) was negatively affected by warming in both sites. We conclude that although buried soil organic matter decomposition is widely expected to increase with future summer warming, surface litter decay and nutrient turnover rates in both xeric and relatively moist tundra are likely to be significantly restricted by the evaporative drying associated with warmer air temperatures.     

Climatic effects on litter decomposition from arctic tundra to tropical rainforest

Climatic effects on the decomposition rates of various litter types in different environments must be known to predict how climatic changes would affect key functions of terrestrial ecosystems, such as nutrient and carbon cycling and plant growth. We developed regression models of the climatic effects on the first‐year mass loss of Scots pine needle litter in boreal and temperate forests across Europe (34 sites), and tested the applicability of these models for other litter types in different ecosystems from arctic tundra to tropical rainforest in Canada (average three year mass loss of 11 litter types at 18 sites), the USA and Central America (four litter types at 26 sites). A temperature variable (annual mean temperature, effective temperature sum or its logarithm) combined with a summer drought indicator (precipitation minus potential evapotranspiration between May and September) explained the first‐year mass loss of the Scots pine needle litter across Europe with a higher R² value than actual evapotranspiration (0.68–0.74 vs. 0.51) and with less systematic error for any sub‐region. The model with temperature sum and the summer drought indicator appeared best suited to the other litter types and environments. It predicted the climatic effects on the decomposition rates in North and Central America with least systematic error and highest R² values (0.72–0.80). Compared with Europe, the decomposition rate was significantly less sensitive to annual mean temperature in Canada, and to changes in actual evapotranspiration in the USA and Central America. A simple model distinguishing temperature and drought effects was able to explain the majority of climatic effects on the decomposition rates of the various litter types tested in the varying environments over the large geographical areas. Actual evapotranspiration summarizing the temperature and drought effects was not as general climatic predictor of the decomposition rate.     

Studies on decomposition ofCeratophyllum demersum litter under laboratory and field conditions: losses of dry mass and nutrients,qualitative changes in organic compounds and consequences for ambient water and sediments

A study was made of decomposition ofCeratophyllum demersum litter over a 17-day period under controlled conditions of temperature and oxygen (5, 10 and 18 °C; aerobic and anaerobic) and over a 169-day period in the field (Lake Vechten, The Netherlands). Litter, water and sediment were sampled on the 0, 2, 4, 7 and 17th day under controlled conditions and on the 0, 17, 49, 127 and 169th day in the field. The litter was analyzed quantitatively for dry mass, ash, carbon, nitrogen, phosphorus and qualitatively of organic composition by pyrolysis mass spectrometry. The water was analyzed for the elemental concentrations of organic carbon (total and dissolved), nitrogen (total, ammonia and particulate) and phosphorus (total and orthophosphate) and for the concentrations of photosynthetic pigments and bacteria. The sediment was analyzed for the elemental concentrations of nitrogen, carbon and phosphorus, and for bacterial numbers.The pattern of litter mass loss fitted an exponential model fairly well. Mass decreased faster under controlled aerobic than under anaerobic conditions and the decrease was stimulated by increasing temperature, relatively more in the range of 5 to 10 °C (by 20%) than in the range of 10 of 18 °C (by 2%). The residual mass ranged from 73 to 43% of initial under controlled aerobic conditions and from 84 to 65% under anaerobic conditions after 17 days. It decreased far less in the field, to 38% of initial mass in the field after 169 days.The litter initially lost a carbohydrate fraction by leaching in all treatments. The protein content decreased initially as well but increased subsequently at increasing temperature stimulated under anaerobic conditions. The changes in organic composition were correlated with those in nitrogen but not with those in carbon and phosphorus contents. The organic composition of litter incubated in the field differed from that of litter incubated in the laboratory. The field residues contained less proteinaceous material than the laboratory residues.The changes in carbon, nitrogen and phosphorus concentrations in the litter showed different patterns. The carbon concentration generally increased, the nitrogen concentration initially dropped and increased subsequently, and the phosphorus concentration initially dropped and remained relatively constant subsequently. Chemical immobilization of the decomposition process may have occurred in the laboratory, but was unlikely in the field.Carbon, nitrogen and phosphorus left the litter initially largely in particulate form and were recovered in the water. The ratio dissolved: total nutrient concentration was lower under controlled aerobic than under anaerobic conditions. Increasing temperature stimulated bacterial use of dissolved organic carbon and nitrogen. A rapid nutrient flow occurred from macrophyte litter, via water to sediment.The phytoplankton biomass in the water was greatly stimulated by substances freed from the decomposing litter. Diatoms increased generally relatively more than green algae, predominating alternatively with green algae under aerobic conditions and continuously under anaerobic conditions. Bacterial numbers in the water initially increased, partly due to transgression of bacteria from the sediment-water interface to the water and partly due to an actual increase in community biomass. The bacteria returned largely to the sediment-water interface, stimulated by increasing temperature, as most of the substrate readily usable by them had left the litter in the litter-bag and was associated with the upper sediment layers.It is feasible that the annual die-off of theC. demersum population of Lake Vechten barely affects nutrient cycling in the lake, because the contribution to the nutrient pools of the lake when fully mixed is only small. However, small particles originating from decomposingC. demersum litter may influence the lake considerably by decreasing water transparency and serving as a food source for filter-feeders and detritivorous macrofauna.     

Macro- and micronutrient effects on decomposition of leaf litter from two tropical tree species: inferences from a short-term laboratory incubation

While a large number of studies have investigated the effects of macronutrients such as nitrogen (N) or phosphorus (P) on litter decomposition, recent studies suggest that micronutrients including zinc (Zn) may also limit decomposition rates. Our goal was to compare the effects of nutrient addition on decomposition of two leaf litter types from tropical dry forest trees in a short-term laboratory microcosm experiment. Single nutrients (N, P, Zn, potassium, magnesium, and nickel) were applied to leaf litter in solution at low or high concentrations (to mimic in situ availability or to alleviate nutrient limitation, respectively), and decomposition was assessed as final mass remaining and carbon dioxide mineralization. Both mass remaining and CO₂ mineralization were affected by nutrient identity and concentration, and these effects varied by species. In general, P and Zn addition increased decomposition, Mg and N inhibited it, and K and Ni had no significant effects. Future studies should consider the interactions between decomposition processes, decomposer communities, and a wider range of macro- and micronutrients.     

九寨沟国家自然保护区4个典型树种叶片凋落物在林下及高山湖泊中的分解及养分释放特征

叶片凋落物分解对生态系统的养分循环和生产力有着重要意义。该文利用网袋分解法对九寨沟国家自然保护区内黄果冷杉(Abies ernestii)、油松(Pinus tabulaeformis)、红桦(Betula albo-sinensis)和高山柳(Salix cupularis) 4个典型树种叶片凋落物在林下及高山湖泊中的分解及养分释放特征进行了对比研究。结果表明: 1)叶片凋落物分解质量损失规律符合Olson的负指数衰减模型(r > 0.93, p < 0.01), 4个树种叶片在林下完全分解(99%)的时间依次为: 高山柳(6.80 a) <红桦(10.34 a) <黄果冷杉(18.88 a) <油松(27.21 a), 且分别是其在水体中分解的1.48倍、1.55倍、1.80倍和1.65倍。2)分解1年后凋落物质量剩余率(MR)和氮素剩余率(NR)均与叶片初始N含量极显著负相关, 而与叶片初始C:N值极显著正相关。3)不同树种间叶片N和P释放特征差异明显, 且在林下和水体间的释放模式也存在差异; 高山柳叶片凋落物在林下和水体分解过程中N元素从分解初期便开始释放, 而其他树种叶片凋落物N元素释放前存在明显的富集过程; 各树种叶片凋落物P元素释放模式为释放—富集—释放。研究表明: 叶片凋落物分解是一个受其自身性质和外界环境因素共同作用的复杂过程, 而凋落物在高山湖泊中的快速分解将对保护区现有的水体景观产生潜在影响。     

Evaluation of macrofaunal effects on leaf litter breakdown rates in aquatic and terrestrial habitats

Abstract Decomposition of the organic matter is a key process in the functioning of aquatic and terrestrial ecosystems, although different factors influence processing rates between and within these habitats. Most patterns were described for temperate regions, with fewer studies in tropical, warmer sites. In this study, we carried out a factorial experiment to compare processing rates of mixed species of leaf litter between terrestrial and aquatic habitats at a tropical site, using ?ne and coarse mesh cages to allow or prevent colonization by macroinvertebrates. The experiment was followed for 10 weeks, and loss of leaf litter mass through time was evaluated using exponential models. We found no interaction between habitat and mesh size and leaf litter breakdown rates did not differ between ?ne and coarse mesh cages, suggesting that macroinvertebrates do not influence leaf litter decomposition in either habitat at our studied site. Leaf breakdown rates were faster in aquatic than in terrestrial habitats and the magnitude of these differences were comparable to studies in temperate regions, suggesting that equivalent factors can influence between‐habitat differences detected in our study.     

Shedding light on plant litter decomposition: advances, implications and new directions in understanding the role of photodegradation

Litter decomposition contributes to one of the largest fluxes of carbon (C) in the terrestrial biosphere and is a primary control on nutrient cycling. The inability of models using climate and litter chemistry to predict decomposition in dry environments has stimulated investigation of non-traditional drivers of decomposition, including photodegradation, the abiotic decomposition of organic matter via exposure to solar radiation. Recent work in this developing field shows that photodegradation may substantially influence terrestrial C fluxes, including abiotic production of carbon dioxide, carbon monoxide and methane, especially in arid and semi-arid regions. Research has also produced contradictory results regarding controls on photodegradation. Here we summarize the state of knowledge about the role of photodegradation in litter decomposition and C cycling and investigate drivers of photodegradation across experiments using a meta-analysis. Overall, increasing litter exposure to solar radiation increased mass loss by 23% with large variation in photodegradation rates among and within ecosystems. This variation was tied to both litter and environmental characteristics. Photodegradation increased with litter C to nitrogen (N) ratio, but not with lignin content, suggesting that we do not yet fully understand the underlying mechanisms. Photodegradation also increased with factors that increased solar radiation exposure (latitude and litter area to mass ratio) and decreased with mean annual precipitation. The impact of photodegradation on C (and potentially N) cycling fundamentally reshapes our thinking of decomposition as a solely biological process and requires that we define the mechanisms driving photodegradation before we can accurately represent photodegradation in global C and N models.     

桂西北喀斯特区原生林与次生林凋落物量及其降解

凋落物量及其降解速率在养分循环尤其碳循环中起着至关重要的作用。本文比较了桂西北喀斯特区3种原生林(圆果化香、大叶蚊母树和青檀)与3种次生林(圆叶乌桕、八角枫和黄荆)凋落物量及其降解特征。结果发现,在所有原生林与次生林中,叶凋落量占总凋落物量的80%左右,且与降雨量呈负相关,降雨量最少的旱期凋落物量最大。除大叶蚊母树外,次生林的总凋落物量及养分归还量大于原生林。凋落物降解90d后,大叶蚊母树的质量损失最小,有87.6%未分解。经过1年的降解,质量损失最大的为圆果化香,仅剩余49.5%未降解。圆叶乌桕凋落物含N量最高,降解最快;大叶蚊母树凋落物量最高,降解最慢;而圆叶乌桕凋落物养分含量高,降解速率快。因此,圆叶乌桕是喀斯特区退化土地植被重建与土壤养分修复较好的选择。     

水生植物荇菜和菹草分解对物种混合的响应研究

为探讨水生植物混合的分解效应, 研究了浮叶植物荇菜(Nymphoides peltatum)、沉水植物菹草(Potamogeton crispus)及两物种混合的分解速率和养分动态。结果显示: (1)两单物种的分解速率与初始N含量呈显著正相关关系(P 0.05, r=0.862), 荇菜和菹草分解90d后的干重剩余率分别为24.74%和44.91%。物种混合干重剩余率在分解初期阶段的实测值比期望值高6.63% (P 0.05), 表明物种混合对分解速率具有拮抗效应, 但在随后的分解时间里无显著的混合效应, 分解90d后干重剩余率为30.39%; (2)在分解初期的N、P释放阶段, 物种混合的N、P剩余率实测值比其期望值分别高14.36%和12.88% (P 0.05), 表明物种混合对初期N、P元素释放具有拮抗效应, 在随后的分解过程中对N元素无显著的混合效应, 但分解后期P剩余率实测值比期望值低4.26% (P 0.05), 表现为协同效应; (3)物种混合N、P动态在分解初期呈一个快速释放的过程, 但在随后的分解阶段N元素释放或积累, P元素持续释放, 最终N、P均表现为净释放, 与两单物种分解的N、P动态的规律基本一致。另外, 总酚在物种混合分解初期释放迅速, 随后释放缓慢。研究结果表明, 荇菜和菹草混合分解存在非加和效应, 即单物种的分解速率和营养动态变化不能用来预测两物种混合的分解速率和营养动态变化。物种混合在分解的不同阶段其分解效应不同, 这说明混合效应具出一定的时间依赖性。此外, 混合效应与浮叶植物和沉水植物其初始质量特征有较密切的关系。       

The decomposition of aquatic macrophytes: bioassays versus in situ experiments

In aquatic sciences, the agreement between laboratory and field observations remains a challenge. Using kinetic modeling, this research aims to compare the decomposition in laboratory and in situ conditions. In the in situ incubations, the mass decreases of the aquatic macrophytes (Echinodorus tenellus, Hydrocotyle verticillata, Najas microcarpa and Pontederia parviflora) were described using a litter bag technique and in the laboratory their decomposition was maintained under controlled conditions. The plants and water samples were collected from a tropical reservoir (Brazil). To describe the particulate organic carbon (POC) decay we adopted a two stage kinetic model that considered the heterogeneity of resources. The released organic carbon (i.e., losses related to mineralization, dissolution and sedimentation of smaller particles than the litter bag mesh) were used to compare the results derived from the field and laboratory incubations. Despite the methodological differences, the results show equivalence among the POC decay. The decomposition measured by litter bags method was 1.32 faster, owing to the effects of losses by sedimentation of the smaller particles, abrasion, action of decomposer organisms (e.g., fragmentation and enzymatic attack) and synergy among these factors. From a mathematical modeling approach, the results validate the use of decomposition data obtained under controlled conditions providing estimations of energy and matter fluxes within aquatic ecosystems. However, it is necessary to adopt a coefficient to acquire the similarity (e.g., 1.32).     

Spatial patterns of litter decomposition in the littoral zone of boreal lakes

1. We studied the patterns of litter decomposition in lake littoral habitats and investigated whether decay rates, as an integrating proxy for environmental conditions in the sediment, would co‐vary with net carbon dioxide (CO₂) exchange and methane (CH₄) efflux. These gas fluxes are known to be sensitive to environmental conditions. Losses in the mass of cellulose, root, rhizome and moss litter were measured during 2 years in boreal littoral wetlands in Finland and compared with published data on concurrently measured gas fluxes. Four study sites covered a range of sediment types and hydrological conditions. 2. Decomposition was not linearly related to the duration of flooding but depended on sediment type. Readily decomposable litter fractions, such as cellulose and rhizome litter, lost mass at a faster rate in marshes with a longer period of flooding but wide water level fluctuations that hinder establishment of a Sphagnum cover, than in peat‐forming fens. In marshes, the mean first‐year mass losses were 83–99% and 19–62% for cellulose and rhizomes, respectively. In fens, the respective losses were 40–53% and 33%. In the first year, the loss in the mass of the more recalcitrant root litter did not differ between sites (mean 19–30%) and moss litter lost no mass. 3. The estimated first‐year carbon loss from belowground litter was about 0.1–0.3 times ecosystem respiration and roughly similar to net carbon gas (CO₂, CH₄) efflux, suggesting that vascular plants and recent plant residues contribute substantially to ecosystem release of carbon gases. On the other hand, at least 40% of the mass of the belowground litter remained on a littoral site after the first 2 years of decomposition. Slow decomposition may indicate the accumulation of organic‐rich sediments. The accumulated carbon could explain the excess CO₂ release found in most littoral sites. In continuously inundated sites decomposition rates were similar to those in periodically flooded sites, but ecosystem‐atmosphere CO₂ exchange fell to close to zero. This discrepancy implies that the released CO₂ is dissolved in water and may be exported into the pelagic zone of the lake.     

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.     

Decomposition of Carex and Sphagnum litter in two mesotrophic fens differing in dominant plant species

Peatlands can be classified into fens and bogs based on their hydrology. Development of fens to bogs is accompanied by the invasion of Sphagnum species. The purpose of this study was to determine how the decomposition process in fens is influenced by the transition from a vascular plant-dominated system to a Sphagnum -dominated system. We carried out a reciprocal litter bag experiment, using litter of Carex diandra , C. lasiocarpa , Sphagnum papillosum and S. squarrosum in a fen dominated by Sphagnum species and a fen without Sphagnum . Decomposition rate and nitrogen and phosphorus dynamics of the plant litter were measured in a field experiment for two years. Decomposition rate was highest for the Carex litter types and lowest for the Sphagnum litter types. Surprisingly, decomposition rates hardly differed between the two sites. Nutrient dynamics, however, showed a clear site-effect: In the Sphagnum site net mineralization was observed for all litter types whereas in the Carex site net immobilization was observed. These results show that carbon and nutrient cycles were coupled in a different way in a Sphagnum -dominated and a Carex -dominated site, respectively. Nutrient availability and adaptation of the microbial community to nutritional and other environmental conditions may be the main regulators of carbon and nutrient cycles in these peatlands.