Coarse-mesh Litter Bags Research Articles

Liming of acidified forests changes leaf litter traits but does not improve leaf litter decomposability in forest streams

Liming, the spreading of Ca/MgCO3 on terrestrial ecosystems, is often used as a way to promote the recovery of terrestrial and aquatic ecosystems affected by anthropogenic acidification. When liming is applied to a watershed, surface waters may be improved directly through water chemistry changes but also indirectly through possible changes in the quality of terrestrial organic matter inputs to the stream. The aim of this study was to evaluate the effect of liming on leaf litter chemical and structural traits and their consequences on another trait, leaf litter decomposability, giving the potential for a leaf litter to be decomposed.Beech (Fagus sylvatica, L.) leaf litter was collected in forested sites on acid bedrock (granite and sandstone), limed (9 or 21 years ago) or not (controls), as well as on two calcareous reference sites. Several leaf litter traits were determined for each collection site: carbon (C), nitrogen (N), phosphorous (P), calcium (Ca), magnesium (Mg), potassium (K), lignin and cellulose content, and leaf mass area and toughness. To compare leaf litter decomposability in aquatic ecosystem, leaf litters from all sites were placed in a reference forested headwater stream over a 128-day period, in fine and coarse mesh litter bags, and leaf mass loss dynamic as well as fungal biomass were evaluated.Liming significantly increased leaf litter Ca and Mg contents and decreased K content while other measured traits remained unchanged. Leaf litters collected in limed sites exhibited significantly lower decomposition rates in coarse mesh bags but similar decomposition rates in fine mesh bags. Liming had no significant effect on fungal biomass. In contrast, leaf litter collected on the calcareous, reference sites, showed significantly different trait profiles (e.g. higher Ca, Mg, K and P contents), leading to up to twice higher decomposition rates and fungal development.Our study shows that under our experimental conditions, increases in Ca and Mg leaf litter contents arising from acidic forests liming did not improve leaf litter microbial decomposition and even tended to reduce macroinvertebrate mediated decomposability. Concomitant changes in other leaf litter traits might be necessary to stimulate leaf litter decomposition. Liming with Ca/MgCO3 might thus not improve functional recovery of stream ecosystems in forests affected by acidification through an increase of litter quality.

Effect of sodium amendments on the home-field advantage of litter decomposition in a subtropical forest of China

Sodium (Na) can affect ecosystem function by altering soil fauna and microbial community composition and abundance. Sodium’s potential influences on plant-soil interactions, such as home-field advantage (HFA: enhanced decomposition in a home environment compared to an away environment), in relation to litter decomposition remains unclear. To fill this knowledge gap, we conducted a ten-month reciprocal litter transplant field experiment with Na amendments (0.005% and 0.5% water solution of NaCl) in both a subtropical broad-leaved forest dominated by Quercus variabilis (oak) and a coniferous artificial forest planted by Pinus massoniana (pine) in China. Litterbags with different mesh sizes were used to partition the impact of soil biota groups on the HFA. The results showed that 0.005% Na amendment facilitated litter decomposition as well as carbon (C) and nitrogen (N) mineralization by enhancing soil extracellular enzyme activity and soil faunal abundance. But 0.5% Na amendment inhibited litter decomposition in fine-mesh litterbags by decreasing microbial activity, and promoted litter decomposition in coarse-mesh litterbags by attracting more soil fauna. In the control plots, we observed a negative HFA in coarse-mesh litterbags (with most soil fauna and all soil microorganisms), and a positive HFA in fine-mesh litterbags (with complete exclusion of meso-and macrofauna). Whether the HFA was positive or negative (i.e. regardless of the direction of the HFA), Na amendments significantly decreased the magnitude of HFA by increasing the similarity of microbial community composition between the incubation fields (‘home’ vs. ‘away’) and decreasing selective foraging of soil fauna for different leaf litters. In sum, our results indicate that anthropogenic Na enrichment can decrease the effects of HFA through the modification of soil microbial and faunal communities, and thereby affect C sequestration and N cycling in subtropical forests.

Soil fauna effect on Dryas octopetala litter decomposition in an Alpine tundra of the Changbai Mountains, China

Soil fauna are critical for litter decomposition via physical fragmentation, chemical digestion, and changing activity of microorganisms, yet a few studies have been performed regarding the effects of soil fauna on alpine tundra litter decomposition. To better understand the effects of soil fauna on alpine tundra litter decomposition, we set up a litterbag experiment to determine the characteristics of the Dryas octopetala decomposition, and the diversity of the soil fauna in the litterbags, as well as the influence of the soil fauna on the decomposition in the tundra of the Changbai Mountains over a 36-month period. We found that the decomposition rate of the coarse mesh (2 mm) litterbags was faster than that of the fine mesh (0.01 mm) litterbags. The percentage of the mass lass of litter in the coarse mesh litterbags (2 mm) was 47.60%, while that in the fine mesh (0.01 mm) litterbags was 34.11% at the end of the experimental period (36th month of decomposition), and the contribution of soil fauna to the litter decomposition was confirmed to be 30.50%. The characteristics of litter decomposition exhibited some seasonal and annual differences. In addition, the diversity of the soil fauna in the litterbags was different during each of the years of the experiment. However, there were no significant differences observed during the same year. The effect of soil fauna on the litter decomposition was not obvious at the beginning of the experiment, and soil fauna contribution had a significant negative relationship with mass loss of litter. Our results provide experimental evidence that soil fauna can promote the decomposition of Dryas octopetala litter, but soil fauna contribution decreased with litter decomposition in the alpine tundra ecosystem.

Effect of N addition on home-field advantage of litter decomposition in subtropical forests

The ‘home-field advantage’ (HFA) postulates that litter decompose faster in its home habitat than in other habitats. However, the HFA of litter decomposition appears to be highly variable, and the effects of environmental conditions on HFA have rarely been investigated. Thus, in this study, we performed a reciprocal litter transplant experiment using coarse and fine mesh litterbags under nitrogen (N) addition treatments in a subtropical coniferous forest dominated by Pinus massoniana and a broad-leaved forest dominated by Quercus variabilis. Results showed no significant difference in decomposition between the two dominant litters in the fine-mesh litterbags at home and away habitats under control and N addition plots. P. massoniana litter in the coarse-mesh litterbags decomposed twice as fast at home than in away habitats under N addition. The result suggests a positive HFA effect in the coniferous forest under N addition, but no significant HFA effect was observed in the control plots. N addition did not enhance Q. variabilis litter decomposition in the home habitat. The positive HFA effect of P. massoniana litter in the coarse-mesh litterbags in N addition plots was associated with more abundant soil fauna than in the control plots. However, N addition had no significant effect on the activity of most soil enzyme during litter decomposition. Moreover, soil microbial biomass showed no relationship with the HFA of litter decomposition. Our findings suggest that N addition likely enhances the feeding activity of soil fauna by increasing fauna abundance. This further reinforces the habitat specificity of soil mesofauna in coniferous forests, resulting in a positive HFA of P. massoniana litter decomposition. C and N cycling in coniferous forest may be enhanced by N addition, and coniferous forest management should be suitable for this change.

Structural and functional recovery of macroinvertebrate communities and leaf litter decomposition after a marked drought: Does vegetation type matter?

Climate change and anthropogenic disturbances are expected to lead to more intense and frequent droughts, with potentially severe effects on structure and function of perennial temperate streams. However, more information is required on whether streams flowing through basins already affected by exotic plantations will respond to droughts in the same way as streams under native forests. The recolonisation dynamics of benthic macroinvertebrate communities and leaf litter decomposition rates were examined in nine streams of oceanic-temperate climate that differed in catchment vegetation (three streams draining native deciduous forest, three in pine plantations and three in eucalypt plantations) after a marked drought. In each stream, five benthic samples were collected three times (ca. 1.5months between sampling dates) after flow recovery, and the taxonomic and functional trait compositions of the macroinvertebrate communities were analysed. The decomposition rate of Alnus glutinosa was measured in fine- and coarse-mesh litter bags. Benthic macroinvertebrate density, richness and diversity increased with time after flow recovery but only richness and diversity differed among stream types, with eucalypt streams showing the lowest values. Both the taxonomic and functional compositions of the macroinvertebrate community were dependent on vegetation type and time, with the differences among stream types diminishing over time. While leaf-litter decomposition rate did not depend on catchment vegetation after drought, detritivore activity was the lowest under eucalypt streams and it was positively correlated to benthic shredder density. Our results indicated that in these perennial temperate streams the catchment vegetation influenced the recovery of benthic macroinvertebrate communities after a period of drought, although the decomposition rate of leaf litter was not strongly affected. Greater understanding of the structural and functional responses of stream ecosystems to different stressors is required before the effects of expected more intense and frequent hydrological changes caused by climate change can be adequately forecast.

Descomposición de hojas y colonización de macroinvertebrados acuáticos en dos microcuencas tropicales (Manizales, Colombia)

Nutrient fluxes in low-order streams of both tropical and temperate regions depend on leaves and tree branches originated from their riparian forests. This substratum suffers physical, chemical and biological breakdown, and can be colonized by several benthic macroinvertebrates. However, the effect of different levels of disturbance and degradation operating on these forested streams such as livestock in cattle-dominated landscapes, remains poorly understood, this is particularly noticeable in many mountain areas of Colombia. Goals. To evaluate the breakdown rates and colonization dynamics of benthic macroinvertebrates on leaves of Eucalyptus sp. and Tibouchina lepidota in tropical streams immersed into livestock matrices. Methods. During March to May 2014, we conducted a field experiment using single and mixed (1:1) leaf treatments of both plant species into fine- and coarse-mesh litterbags, in two contrasting streams (reference [La Elvira]/ disturbed [Chuscales]) of the Chinchina River in central Colombia (Manizales, Caldas). Results. Leaf mass losses were significantly higher in Eucalyptus sp., followed by the mixed (1:1) and T. lepidota treatment into coarse vs. fine mesh bags. However, the higher decomposition rates (k days-1, mean ± SE) were registered at La Elvira for Eucalyptus sp. (fine mesh, k = - 0.0145 ± 0.0026; coarse mesh, k = - 0.0143 ± 0.0019) and, Chuscales for the mixed treatment (fine, k = - 0.0181 ± 0.0053; coarse, k = - 0.0153 ± 0.0046), comparatively. Contrary, macroinvertebrate abundance and density were generally higher in reference than in disturbed stream, and concomitantly to the diversity. However, these variables changed over the sampled period, with higher abundance values registered at 30 days (1129 individuals) and 45 days (958 individuals) in the reference and disturbed stream, respectively. Chironomids and the collectors’ functional feeding group were the most abundant during all sampling. Conclusions. The results showed that breakdown rate differences between streams cannot be attributed specifically to changes in macroinvertebrate community composition, structure or density and that mesh size more than the livestock effect can influence macroinvertebrate communities colonizing in-stream organic matter; however, further studies are necessary to confirm these results.

Heterogeneity in leaf litter decomposition in a temporary Mediterranean stream during flow fragmentation

In temporary Mediterranean streams, flow fragmentation during summer droughts originates an ephemeral mosaic of terrestrial and aquatic habitat types. The heterogeneity of habitat types implies a particular ecosystem functioning in temporary streams that is still poorly understood. We assessed the initial phases of leaf litter decomposition in selected habitat types: running waters, isolated pools and moist and dry streambed sediments. We used coarse-mesh litter bags containing Populus nigra leaves to examine decomposition rates, microbial biomass, macroinvertebrate abundance and dissolved organic carbon (DOC) release rates in each habitat type over an 11-day period in late summer. We detected faster decomposition rates in aquatic (running waters and isolated pools) than in terrestrial habitats (moist and dry streambed sediments). Under aquatic conditions, decomposition was characterized by intense leaching and early microbial colonization, which swiftly started to decompose litter. Microbial colonization in isolated pools was primarily dominated by bacteria, whereas in running waters fungal biomass predominated. Under terrestrial conditions, leaves were most often affected by abiotic processes that resulted in small mass losses. We found a substantial decrease in DOC release rates in both aquatic habitats within the first days of the study, whereas DOC release rates remained relatively stable in the moist and dry sediments. This suggests that leaves play different roles as a DOC source during and after flow fragmentation. Overall, our results revealed that leaf decomposition is heterogeneous during flow fragmentation, which has implications related to DOC utilization that should be considered in future regional carbon budgets.

Effects of flow scarcity on leaf-litter processing under oceanic climate conditions in calcareous streams

Although temporary streams represent a high proportion of the total number and length of running waters, historically the study of intermittent streams has received less attention than that of perennial ones. The goal of the present study was to assess the effects of flow cessation on litter decomposition in calcareous streams under oceanic climate conditions. For this, leaf litter of alder was incubated in four streams (S1, S2, S3 and S4) with different flow regimes (S3 and S4 with zero-flow periods) from northern Spain. To distinguish the relative importance and contribution of decomposers and detritivores, fine- and coarse-mesh litter bags were used. We determined processing rates, leaf-C, -N and -P concentrations, invertebrate colonization in coarse bags and benthic invertebrates. Decomposition rates in fine bags were similar among streams. In coarse bags, only one of the intermittent streams, S4, showed a lower rate than that in the other ones as a consequence of lower invertebrate colonization. The material incubated in fine bags presented higher leaf-N and -P concentrations than those in the coarse ones, except in S4, pointing out that the decomposition in this stream was driven mainly by microorganisms. Benthic macroinvertebrate and shredder density and biomass were lower in intermittent streams than those in perennial ones. However, the bags in S3 presented a greater amount of total macroinvertebrates and shredders comparing with the benthos. The most suitable explanation is that the fauna find a food substrate in bags less affected by calcite precipitation, which is common in the streambed at this site. Decomposition rate in coarse bags was positively related to associated shredder biomass. Thus, droughts in streams under oceanic climate conditions affect mainly the macroinvertebrate detritivore activity, although macroinvertebrates may show distinct behavior imposed by the physicochemical properties of water, mainly travertine precipitation, which can override the flow intermittence effects.

Leaf litter breakdown of <i>Phyllostachys heterocycla</i>, <i>Lithocarpus glabra</i> and <i>Lindera glauca</i> in a headwater stream of Xitiaoxi watershed, upper reaches of Taihu Basin, China

In order to explore the impact of the change of the dominant leaf litter source in stream from broad leaf trees to a bamboo tree of Phyllostachys heterocycla on headwater stream ecosystem and their associated benthic macroinvertebrates,we compared the leaf physical and chemical characters of Phyllostachys heterocycla,Lithocarpus glabra and Lindera glauca,and then evaluated the leaf litter breakdown of above three species using coarse-mesh litter bags in a headwater stream of Xitiaoxi watershed,Zhejiang Province.The concentrations of nitrogen,phosphorus and the thickness of the three categories of leaves varied significantly.The nitrogen concentration of bamboo tree leaf(30.23 g/kg) was much higher than that of L.glabra(20.98 g/kg) and L.glauca(9.69 g/kg).Moreover,the leaves of P.heterocycla(k=0.00592 d-1) decomposed faster than those of L.glabra(0.00297 d-1) and L.glauca(0.00212 d-1).No significant differences were observed in macroinvertebrates abundance and biomass among the three leaf types,however,the macroinvertebrate abundance and biomass varied significantly during the sampling period.Proportions of the functional feeding groups showed that shredders was the most abundant group,which represented 40.3% of individuals and 41.6% of the macroinvertebrate biomass,indicating it was an important component of fauna associated with litter breakdown.The dominant shredder taxon is Lepidostoma,which constructs nests based on broadleaves,and accounts for 14% of the total macroinvertebrate individuals.Because bamboo leaves are narrower,with higher concentrations of nitrogen and phosphorus,and defoliation occurs every two years.We concluded that when the leaf litter source of headwater streams dominated by bamboo leaf,it might change the concentrations of nitrogen and phosphorus in the stream water,the annual amount of leaf litter into stream,the retention time,and the macrobenthos community structure.

Effects of experimental fires on litter decomposition in a seasonally dry Amazonian forest

Abstract:Litter decomposition is a fundamental process for nutrient cycling but we have a limited understanding of this process in disturbed tropical forests. We studied litter decomposition over a 10-mo period in a seasonally dry Amazon forest in Mato Grosso, Brazil. The study plots (50 ha each) included unburned forest (UF), once-burned (BF1) and forest burned annually for 3 y (BF3). We measured understorey density, litter depth, canopy openness, temperature and relative humidity in the plots. Decomposition experiments took place using 720 litterbags filled with approximately 10 g of natural abscised oven-dried leaves. To test the effects of fire on soil meso- and macrofauna, the litterbags had either a fine (2 mm) or coarse (with 1-cm holes in side) mesh size. Litterbags were collected and reweighed 2, 4, 6 and 8 mo after being placed on the forest floor. All forest structure variables were significantly different across plots: BF3 was hotter, less humid, had the highest degree of canopy openness, lowest understorey density and the shallowest litter depth. Litter decomposition (mass loss) was similar in the once-burned and unburned plots, but declined more slowly in BF3. In addition, decomposition was slower in fine-mesh litterbags than coarse-mesh litterbags in BF3, but there was no difference between mesh sizes in BF1 and UF. It is likely that changes in forest structure and microclimate explain the lower decomposition rates in BF3. These results show the importance of recurrent fires, but suggest that single understorey fires may not have long-term negative effects on some ecological processes in seasonally dry Amazonian forests.

Decomposition in tropical forests: a pan‐tropical study of the effects of litter type, litter placement and mesofaunal exclusion across a precipitation gradient

Summary 1. Litter decomposition recycles nutrients and causes large fluxes of carbon dioxide into the atmosphere. It is typically assumed that climate, litter quality and decomposer communities determine litter decay rates, yet few comparative studies have examined their relative contributions in tropical forests. 2. We used a short-term litterbag experiment to quantify the effects of litter quality, placement and mesofaunal exclusion on decomposition in 23 tropical forests in 14 countries. Annual precipitation varied among sites (760‐5797 mm). At each site, two standard substrates ( Raphia farinifera and Laurus nobilis ) were decomposed in fine- and coarse-mesh litterbags both above and below ground for approximately 1 year.

Functional leaf traits and biodiversity effects on litter decomposition in a stream

Rapid loss of biodiversity worldwide has raised concerns about the consequences to ecosystem functioning, including processes such as litter decomposition. Consequent experiments with litter mixtures to assess effects of changing tree composition and diversity on decomposition have given mixed results, but the causes are not clear. Reasoning that such conflicting accounts reported in the literature may be reconciled by considering differences in functional litter traits, we conducted a field experiment in a stream with leaf litter from nine deciduous tree species mixed in a total of 40 combinations. Fine-mesh and coarse-mesh litter bags were used to distinguish between potential effects mediated by microbial decomposers and detritivores. We hypothesized that diversity effects would emerge in species mixtures containing functionally dissimilar leaves but be absent in mixtures of functionally similar litter as assessed by determining proximate lignin, nitrogen, and phosphorus contents of the litter. Mean decomposition rates of litter mixtures did not lend support to our hypothesis for either microbial decomposition (as inferred from mass loss in fine-mesh litter bags) or decomposition caused by both microbes and detritivores (mass loss from coarse-mesh bags). Decomposition rates were largely controlled by litter lignin content, whereas P and especially N were not important. In line with our hypothesis, the most recalcitrant (slowly decomposing) and most labile (rapidly decomposing) species decomposed slower and faster, respectively, in litter mixtures comprising different decay categories than in homogenous mixtures composed of a single decay category or in single-species litter bags. However, across decay categories, evidence was weak in support of the idea that functional litter dissimilarity matters, in spite of plausible mechanisms that could cause such effects.

Leaf Decomposition in a Mountain Stream in the Sultanate of Oman

AbstractDecomposition of Juglans regia leaves was studied in fine and coarse mesh bags in a permanent mountain stream in Oman. A rapid initial mass loss, attributed to leaching, was followed by a more gradual decline. Daily exponential decay rates (k) calculated over 32 days were 0.011 (fine mesh litter bags) and 0.014 (coarse mesh litter bags). The difference between bag types was not significant, suggesting limited impact of leaf‐shredding invertebrates. Ergosterol levels on leaves from fine mesh bags peaked at 0.3 mg g–1 AFDM after 16 days of stream exposure.During the experimental period, which followed the annual leaf fall, the concentration of aquatic hyphomycete conidia in the stream varied between 82 and 1362 l–1. Based on the morphology of conidia found in the water column or released from leaves, we identified 14 species of aquatic hyphomycetes. Tetracladium apiense was the most common taxon (62.2% of conidia in water column during the field experiment). Three other Tetracladium species contributed another 8%. Plating out leaf particles yielded common epiphytic taxa such as Alternaria sp., Aureobasidium pullulans and Phoma sp.The measured metrics of leaf decay in this desert stream fall within the range of values observed in temperate and tropical streams, with clear evidence for an early leaching phase, and no evidence of a strong impact of leaf shredders. The community of aquatic hyphomycetes appears impoverished. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ecological process of leaf litter decomposition in tropical rainforest in Xishuangbanna, southwest China. III. Enzyme dynamics

We tested the dynamics of nine enzymes during leaf litter decomposition in Xishuangbanna tropical rain-forest both in the field and laboratory to explore the response of enzyme dynamics to decomposition under different food-web structures. We used coarse and fine (1 mm and 100 μm mesh size, respectively) litterbags in the field to create different food-web structures during litter decomposition. Most soil macrofauna such as nematodes could access only the coarse mesh litterbags, leaving only microbiota, such as mites, in the fine mesh litterbags. In the laboratory, sterilization and inoculation were adopted to investigate different enzyme dynamics with nematodes or only microbiota participating in litter decomposition. Invertase and amylase increased more for shorter food webs at the early stages of decomposition, while activities of endocellulase, β-glucosidase, xylanase and polyphenoloxydase increased to their maxima at the later stages, but greater increase occurred with extended food webs. Invertase and amylase had negative relationships and endocellulase, β-glucosidase, xylanase and polyphenoloxydase had positive relationships with litter decomposition (mass loss). The activities of enzymes responded to the process of litter decomposition. Invertase and amylase played key roles for microbiota utilizing the substrates at early stages of decomposition, while endocellulase, β-glucosidase, xylanase and polyphenoloxydase worked on the further decay of recalcitrant compounds at later stages. All enzymes related to carbon decay acted as effective indicators of litter decomposition. The decomposition of plant organic matter was essentially an enzymatic process.

Litter decomposition in earthworm-invaded northern hardwood forests: Role of invasion degree and litter chemistry

The effects of invasive earthworms on decomposition are little known, and the controls of their effect on decomposition may be different than those of microbes. Sugar maple—dominated forests previously devoid of earthworms in the western Great Lakes region (USA) exhibit different degrees of earthworm invasion, presenting a natural experiment to study its effects on litter decomposition. We hypothesized that litter decomposition would depend on the degree of earthworm invasion, the presence or absence of worms of differing sizes, and initial litter chemistry. We established an experiment using fine- and coarse-mesh litterbags (to allow access by different-sized worms) to study decomposition of 3 different litters in mixture under different degrees of earthworm invasion in 12 Minnesota sites. The effect of earthworm invasion degree on litter decomposition varied by identity of the litter, mesh size, and time of litter collection. Decomposition of Quercus rubra, the litter with the lowest initial calcium concentration and highest lignin:nitrogen, was not significantly influenced by earthworm invasion degree. In contrast, decomposition of Tilia americana, the litter with the highest calcium concentration and lowest lignin:nitrogen, was fastest in coarse-mesh litterbags. After 15 months the mass of the highest-quality litters was highest in the fine-mesh bags of heavily invaded plots, suggesting that microbially mediated decomposition slowed where earthworms had removed the forest floor.

Invertebrates Colonization on Native Tree Leaves in a Neotropical Stream (Brazil)

AbstractWe evaluate the composition and structure of invertebrate assemblages during leaf breakdown of five native tree species (Myrcia guyanensis, Ocotea sp., Miconia chartacea, Protium brasiliense and Protium heptaphyllum) and a mixture of them in a headwater stream, Southeastern Brazil. Coarse mesh litter bags were used over a 120‐day period. P. heptaphyllum showed the slowest breakdown rate, while Myrcia showed the fastest. Total densities and biomass values of associated invertebrates were different among leaf types and incubation periods. The highest values of total abundance were found in P. heptaphyllum, while Ocotea sp. and Miconia were few colonized. Gathering‐collectors feeding group dominated all leaf types, followed by predators and shredders. Shredders were more abundant in Myrcia than in Miconia and the mixed pack. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of stream restoration on ecosystem functioning: detritus retentiveness and decomposition

Summary Increasing degradation of ecological conditions in streams because of human activities has prompted widespread restoration attempts; however, the ecological consequences of restoration remain poorly understood. We explored the effects of restoration through placement of boulders into the channel in the Ume River catchment in northern Sweden, where tributary streams were extensively channelized to facilitate the transport of timber in the 19th and early 20th centuries. Retentiveness and breakdown of coarse particulate organic matter (CPOM), two key ecological functions in low‐order streams most likely to be affected by channelization, were compared between restored, channelized and unimpacted reference stream sites. Artificial leaves were used to assess short‐term CPOM retentiveness, while CPOM breakdown was estimated as the mass loss of alder (Alnus spp.) leaf packs placed in coarse mesh litter bags. Also, the taxonomic richness, abundance, biomass and evenness of the leaf‐eating invertebrates (shredders) on the retrieved leaf material were quantified. Detailed field measurements were carried out to identify geomorphological and hydraulic controls of CPOM retentiveness and breakdown at the study sites. CPOM retentiveness reflected most strongly the density of boulders and submerged woody debris at the study sites. Restored sites were on average twice as retentive as channelized sites and significantly more retentive than reference sites when discharge was controlled. Current velocity at bank‐full flow was the single most important predictor of CPOM mass loss, implying that mechanical fragmentation was substantial during high flows; other apparent controls of CPOM breakdown included water temperature and shredder abundance. CPOM mass loss was similar between restored and reference sites. However, breakdown was slightly faster at most channelized sites, consistent with higher hydraulic stress during high flow conditions. The shredder assemblages that colonized the litter bags were similar in richness, abundance, biomass and evenness between treatments. Synthesis and applications. In channelized forest streams, low retentiveness and fast mechanical fragmentation during high flows contribute to the rapid depletion of benthic CPOM following leaf abscission in autumn, thereby weakening the heterotrophic energy pathways that probably support much of the biological production in these systems. Our results illustrate that restoration by replacement of boulders can successfully reverse these impacts of channelization and thus contribute to the efficient ecological functioning of impacted streams.

Leaf Litter Decomposition and Monodominance in the Peltogyne Forest of Maracá Island, Brazil1

The forest type dominated by Peltogyne gracilipes (Caesalpiniaceae) on the riverine Maraca Island is the least species-rich of any recorded for Brazilian Amazonia. Because the forest has high soil and foliar Mg concentrations, and Mg is known to be toxic to plant growth at high concentrations, this study tested the hypothesis that dominance by Peltogyne is related to Mg leaf litter amounts and decomposition. We predicted that decomposition of Peltogyne leaves would differ from that of other species, and that their decomposition would result in a pulse of Mg release. Three plots (50 ;ts 50 m) were established in each of three forest types: Peltogyne-rich forest (PRF; dominated by P. gracilipes), Peltogyne-poor forest (PPF), and forest without Peltogyne (FWP). Three leaf litter decomposition experiments tested if decomposition of mixed leaf litter in coarse- mesh (CM) litterbags differed among forests (experiment 1); whether or not decomposition and nutrient release of Ecclinusa guianensis, Lueheopsis duckeana, and Peltogyne in CM litterbags differed among forests and species (experiment 2); and using fine-mesh (FM) litterbags, investigated the differences in the influence of faunal activity on Ecclinusa and Peltogyne decomposition (experiment 3). Decomposition was independent of the presence and dominance of Peltogyne, since decomposition rates in both PRF and FWP were in general lower than in PPF. These differences appeared to be related to faunal activity. The decomposition of Peltogyne leaves was lower than that of the other species tested and was more affected by microbial and physical action. It is possible that the monodominance of Peltogyne is related to its deciduousness and faster decomposition in the dry season, which coincides with a large leaf fall. Magnesium was lost quickly from the Peltogyne leaves and the resultant pulses of Mg into the soil during the heavy rains at the beginning of the wet season may be deleterious for other species that are not adapted to high solution Mg concentrations. Results obtained were consistent with the hypothesis that Peltogyne dominance is related to the pattern of its leaf decomposition and the seasonal pulses of toxic Mg.

Microbial biomass, growth, and respiration associated with submerged litter of Phragmites australis decomposing in a littoral reed stand of a large lake

This study examined the microbial dynamics associated with decomposing litter of the widespread emergent macrophyte Phragmites australis in a littoral reed stand of a large lake. Stand- ing dead leaf and stem litter were collected, placed into fine and coarse mesh litter bags, and sub- merged in the reed stand. Litter bags were retrieved periodically and analyzed for fungal and bacte- rial biomass, fungal growth rates and production, rates of microbial respiration, litter mass loss, nutrient concentrations (N and P), and rates of dissolved organic carbon (DOC) release. Microbial biomass associated with both leaf and stem litter (12 to 85 mg C g -1 detrital C) was predominantly fungal (always ♢90% of the total biomass), even though bacterial biomass (0.13 to 5.6 mg C g -1 detri- tal C) increased and fungal biomass decreased or remained constant as litter decay proceeded. Although rates of fungal growth (0.02 to 0.08% h -1 ) and production (leaves only; 3 to 51 µg C g -1 detrital C h -1 ), and rates of microbial respiration (11 to 257 µg C g -1 detrital C h -1 ) decreased follow- ing litter submergence, fungi continued to be metabolically active in both leaf and stem litter. Signif- icant differences in fungal and bacterial biomass, fungal production rates, and rates of respiration were observed between leaf and stem material, with leaves often having 5 times higher values than corresponding stems. Rates of mass loss differed significantly between leaf litter in fine and coarse mesh bags, with less than 10% of the initial mass remaining in coarse mesh bags after 86 d, versus nearly 60% remaining in fine mesh bags. Nitrogen and P concentrations of leaf litter enclosed in fine mesh bags increased during litter decay, whereas N concentrations of leaf litter in coarse mesh bags remained unchanged and P concentrations decreased. Both N and P concentrations of stem litter were similar among litter bags and varied little throughout the study period. Results obtained in this study indicate that significant changes in microbial colonization and activity associated with P. aus- tralis litter can occur following the collapse of standing dead plant matter to the water. Furthermore, these findings suggest that fungi are active on submerged litter and thus play a vital role in the decomposition of P. australis litter in the aquatic environment.

Soil mesofauna dynamics, wheat residue decomposition and nitrogen mineralization in buried litterbags

The effect of soil microarthropods and enchytraeids on the decomposition of wheat straw in buried litterbags was studied by selective admission and exclusion. Litterbags with 20 μm mesh size admitted nematodes, but excluded microarthropods, although temporarily. After 27 weeks of incubation part of these litterbags were colonized, probably through egg-deposition of mainly fungivorous Collembola and mites. When litterbags with a complete microarthropod community (1.5 mm mesh size) were compared to litterbags with strongly reduced microarthropod numbers (20 μm mesh size), no differences between decomposition rates were found. However, in colonized 20-μm mesh bags, we found reduced decomposition rates compared to the coarse mesh litterbags, probably due to overgrazing of the fungal population by large numbers of fungivorous microarthropods. These large numbers might be caused by the absence of predators. Extraction of microarthropods as well as enchytraeids and nematodes from the coarse mesh litterbags showed a distinct succession during decomposition. The decomposition process was dominated in the first phase by bacterivorous nematodes, nematophagous and bacterivorous mites, and in the later phase by fungivorous nematodes, fungivorous and omnivorous mites and Collembola, and predatory mites. This succession is indicative of a sequence from bacterial to fungal dominated decomposition of the buried organic matter. The results indicate that the decomposition rate is predator controlled.