Fine Mesh Litter Bags Research Articles

Multiple stressors affecting microbial decomposer and litter decomposition in restored urban streams: Assessing effects of salinization, increased temperature, and reduced flow velocity in a field mesocosm experiment

A multitude of anthropogenic stressors impact biological communities and ecosystem processes in urban streams. Prominent among them are salinization, increased temperature, and altered flow regimes, all of which can affect microbial decomposer communities and litter decomposition, a fundamental ecosystem process in streams. Impairments caused by these stressors individually or in combination and recovery of communities and ecosystem processes after release from these stressors are not well understood. To improve our understanding of multiple stressors impacts we performed an outdoor stream mesocosm experiment with 64 experimental units to assess the response of microbial litter decomposers and decomposition. The three stressors we applied in a full-factorial design were increased salinity (NaCl addition, 0.53 mS cm−1 above ambient), elevated temperature (3.5 °C above ambient), and reduced flow velocity (3.5 vs 14.2 cm s−1). After two weeks of stressor exposure (first sampling) and two subsequent weeks of recovery (second sampling), we determined leaf-associated microbial respiration, fungal biomass, and the sporulation activity and community composition of aquatic hyphomycetes in addition to decomposition rates of black alder (Alnus glutinosa) leaves confined in fine-mesh litter bags. Microbial colonization of the litter was accompanied by significant mass loss in all mesocosms. However, there was little indication that mass loss, microbial respiration, fungal biomass, sporulation rate or community composition of aquatic hyphomycetes was strongly affected by either single stressors or their interactions. Two exceptions were temperature effects on sporulation and decomposition rate. Similarly, no notable differences among mesocosms were observed after the recovery phase. These results suggest that microbial decomposers and leaf litter decomposition are either barely impaired by exposure to the tested stressors at the levels applied in our experiment, or that communities in restored urban streams are well adapted to cope with these stressor levels.

High functional breadth of microbial communities decreases home-field advantage of litter decomposition

Home-field advantage (HFA) predicts that litter decomposes faster under its derived plant species. Growing evidence suggests that HFA depends on litter chemical traits and the microbiome's composition, but whether and how their interactions influence HFA effects has rarely been explored. Here, we conducted a reciprocal field transplantation experiment in a temperate forest to examine the effects of litter quality and microbial community composition on decomposition rates. We collected leaf litter from four dominant tree species (Pinus koraiensis, Tilla amurensis, Fraxinus mandshurica, and Acer mono) and utilized two mesh sizes (i.e., 0.5-μm and 35-μm) to manipulate the composition of decomposer communities (i.e., small-sized bacteria only or bacteria + fungi). We assessed the effects of HFA and the ability of decomposers to decay a wide range of litter, i.e., the functional breadth (FB). We found that HFA was stronger for recalcitrant litter in the fine-mesh litterbags. Low litter phosphorus (P) and nitrogen (N) content, explained the positive HFA effects when only small-sized bacteria were present during litter decomposition, but not when both bacteria and fungi were present. Soil parameters such as potassium content and dissolved organic carbon also significantly affected decomposer communities and contributed to the variability in HFA effects. Finally, the FB negatively correlated with HFA in coarse- and fine-mesh litterbags and accounted for 46 and 54% of the observed variation in HFA effects, respectively. Our results highlight that the interactive relationships between litter quality, soil properties, and the microbiome's composition, are central in understanding the functional abilities of microbes during litter decomposition, and suggest a tradeoff between the microbiome's capacity to degrade all litter efficiently and the specialization of decomposers toward their own litter. This further underscores the independent role of decomposers communities in litter decomposition rates, which should be considered when predicting biogeochemical cycling in forest ecosystems.

No Difference in Instream Decomposition Among Upland Agricultural and Forested Streams in Kenya

Expansion of agriculture is particularly worrying in tropical regions of the world, where native forests have been replaced by croplands and grasslands, with severe consequences for biodiversity conservation and ecosystem functioning. However, limited data exist on the effects of agriculture on the functioning of tropical streams. We conducted a leaf litter decomposition experiment in coarse- and fine-mesh litterbags using the three species of leaves (Eucalyptus globulus [non-native], Vernonia myriantha, and Syzygium cordatum [indigenous]) in three forested and agricultural streams to determine the effect of agriculture on instream leaf litter decomposition in headwater stream sites. We also examined the functional composition of macroinvertebrates in the streams through the contents of benthic kick samples. Agricultural streams had a less dense riparian canopy and smaller abundance of coarse organic particulate matter, and higher electric conductivity and suspended solids than forested streams. In terms of the effects of litter quality on decomposition rates, Vernonia had the fastest decomposition rates while Eucalyptus had the slowest in both forested and agricultural sites. Shredder invertebrates were less abundant in agricultural streams, and in both stream types, they were less diverse and abundant than other functional groups. Overall, leaf litter decomposition rates did not respond to agricultural land-use. The hypothesized negative effects of agriculture on organic matter processing were minimal and likely modulated by intact riparian zones along agricultural streams.

Effects of gamma irradiation on instream leaf litter decomposition

Leaf litter decomposition is a key process in stream ecosystems, the rates of which can vary with changes in litter quality or its colonization by microorganisms. Decomposition in streams is increasingly used to compare ecosystem functioning globally, often requiring the distribution of litter across countries. It is important to understand whether litter sterilization, which is required by some countries, can alter the rates of decomposition and associated processes. We examined whether litter sterilization with gamma irradiation (25 kGy) influenced decomposition rates, litter stoichiometry, and colonization by invertebrates after weeks of instream incubation within coarse-mesh and fine-mesh litterbags. We used nine plant species from three families that varied widely in litter chemistry but found mostly consistent responses, with no differences in decomposition rates or numbers of invertebrates found at the end of the incubation period. However, litter stoichiometry differed between irradiated and control litter, with greater nutrient losses (mostly phosphorus) in the former. Therefore, the effects of irradiation on litter chemistry should be taken into account in studies focused on stoichiometry but not necessarily in those focused on decomposition rates, at least within the experimental timescale considered here.

Decomposing decomposition: isolating direct effects of temperature from other drivers of detrital processing.

Understanding the observed temperature dependence of decomposition (i.e., its "apparent" activation energy) requires separation of direct effects of temperature on consumer metabolism (i.e., the "inherent" activation energy) from those driven by indirect seasonal patterns in phenology and biomass, and by longer-term, climate-driven shifts in acclimation, adaptation, and community assembly. Such parsing is important because studies that relate temperature to decomposition usually involve multi-season data and/or spatial proxies for long-term shifts, and so incorporate these indirect factors. The various effects of such factors can obscure the inherent temperature dependence of detrital processing. Separating the inherent temperature dependence of decomposition from other drivers is important for accurate prediction of the contribution of detritus-sourced greenhouse gases to climate warming and requires novel approaches to data collection and analysis. Here, we present breakdown rates of red maple litter incubated in coarse- and fine-mesh litterbags (the latter excluding macroinvertebrates) for serial approximately one-month increments over one year in nine streams along a natural temperature gradient (mean annual: 12.8°-16.4°C) from north Georgia to central Alabama, USA. We analyzed these data using distance-based redundancy analysis and generalized additive mixed models to parse the dependence of decomposition rates on temperature, seasonality, and shredding macroinvertebrate biomass. Microbial decomposition in fine-mesh bags was significantly influenced by both temperature and seasonality. Accounting for seasonality corrected the temperature dependence of decomposition rate from 0.25 to 0.08eV. Shredder assemblage structure in coarse-mesh bags was related to temperature across both sites and seasons, shifting from "cold" stonefly-dominated communities to "warm" communities dominated by snails or crayfish. Shredder biomass was not a significant predictor of either coarse-mesh or macroinvertebrate-mediated (i.e., coarse- minus fine-mesh) breakdown rates, which were also jointly influenced by temperature and seasonality. Unlike fine-mesh bags, however, temperature dependence of litter breakdown did not differ between models with and without seasonality for either coarse-mesh (0.36eV) or macroinvertebrate-mediated (0.13eV) rates. We conclude that indirect (non-thermal) seasonal and site-level effects play a variable and potentially strong role in shaping the apparent temperature dependence of detrital breakdown. Such effects should be incorporated into studies designed to estimate inherent temperature dependence of slow ecological processes.

Land cover affects the breakdown of Pinus elliottii needles litter by microorganisms in soil and stream systems of subtropical riparian zones

Riparian zones are important interface areas between soil and stream systems. Few studies carried out in tropical and subtropical regions evaluate litter decomposition in both stream water and riparian soils. Herein, we assessed the effects of land cover on microbial activity on the decomposition of an exotic litter (Pinus elliottii pine needles only) in water and soils of a subtropical riparian zone. Leaf litter breakdown rates (k in d−1) were estimated for different land covers (Grassland without riparian vegetation, Grassland with riparian vegetation, Forest, and Silviculture). To assess the microbial influence on k, we used fine mesh litter bags with monospecific leaf litter of senescent pine needles. Streams in Silviculture land use showed high k values and orthophosphate, dissolved oxygen and water velocity accelerated the leaf litter breakdown in the stream system. The soil system of Silviculture, Forest, and Grassland with riparian vegetation land covers showed high k values due to the high moisture and litter stock on riparian soil. Only a minor difference between stream and soil systems highlights the significant changes and the negative effects of silviculture on subtropical riparian zones.

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.

Effect of Soil Fauna on Home-Field Advantages of Litter Mass Loss and Nutrient Release in Different Temperate Broad-Leaved Forests

The home-field advantage (HFA) of litter decomposition dynamics has been investigated intensively in different ecosystems with a wide variety of plant types. HFA mainly occurs due to the specialization of a soil organism. However, for the HFA, the linkages between litter mass loss, nutrient release, and soil faunal community are not fully understood. Thus, in this study, we performed a reciprocal litter transplant experiment using coarse and fine mesh litterbags in a Quercus mongolica Fisch. ex Ledeb. forest dominated by Q. mongolica (QM) and Acer pseudosieboldianum (Pax) Komarov (AP) and miscellaneous wood forests dominated by Juglans mandshurica Maxim. (JM) and Ulmus laciniata (Trautv.) Mayr. (UL). Results showed that the A. pseudosieboldianum litter displayed a significantly higher total abundance of Oribatida, Tomoceridae, and Entomobryidae at home than away from home after 7 months. However, all litters showed no significant difference in the HFA between the coarse mesh and fine mesh sizes during the 12-month experiment. A. pseudosieboldianum and J. mandshurica litters showed a significantly higher positive HFA for the C release in the coarse mesh than in the fine mesh litterbags after 7 months. Q. mongolica and J. mandshurica litters showed a significantly higher positive HFA for N release in the coarse mesh than in the fine mesh litterbags after 7 months. The A. pseudosieboldianum litter showed a significantly higher positive HFA for N release in the coarse mesh than in the fine mesh litterbags after 12 months. Q. mongolica and A. pseudosieboldianum litters showed a significantly higher positive HFA for S release in the coarse mesh than in the fine mesh litterbags after 7 and 12 months, respectively. However, A. pseudosieboldianum and Q. mongolica litters showed a significantly higher negative HFA for S release in the coarse mesh than in the fine mesh litterbags after 7 and 12 months, respectively. Our results illustrated that soil faunal specialization was found in the A. pseudosieboldianum litter only at home after 7 months. Soil fauna had a weak effect on the HFA of the litter mass losses during the 12-month experiment. Soil fauna drove the positive HFA for the N release of both the high- and low-quality litters. Soil fauna have a positive and negative HFA for S release in the low-quality litter.

Decomposition processes in coastal lagoons and their implications for the assessment of ecological health

Abstract Aside from structural indicators, functional indicators have been suggested as a tool for assessing the health of transitional water ecosystems. The aim of this study was to assess the potential for the use of plant litter decomposition as a functional indicator of ecosystem health in coastal lagoons. A field experiment was conducted to assess the decomposition rate of Ruppia maritima enclosed in coarse and fine mesh litterbags and incubated in 10 Mediterranean coastal lagoons that conform to a gradient of ecological status produced by anthropogenic impacts as determined by a structural quality index. The microbial decomposition rate in fine mesh litterbags increased linearly along the ecological gradient. However, this relationship was not observed for the overall decomposition rate in coarse mesh bags. Decomposition rates in coarse mesh bags were negatively related to macroinvertebrate diversity, indicating that, in transitional habitats, such as coastal lagoons, high biodiversity is not a prerequisite for successful ecosystem functioning. The results suggest that functional indicators, such as the microbial litter decomposition process, should be used in addition to structural ones to detect anthropogenic‐derived environmental perturbations in coastal lagoons.

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.

Congregations of the leaf-shredding insect Lepidostoma togatum mediate exceptionally rapid mass loss from leaf litter in Nova Scotia rivers

In shallow, rocky-bottomed river systems in Nova Scotia, Canada, decomposition rates of autumn-fallen red maple (Acer rubrum) and speckled alder (Alnus incana) leaf litter were determined in spring and early summer using litter bags. In most shaded upstream tributaries, decomposition followed a typical exponential curve (k = −0.013 to −0.032 day−1). In unshaded, downstream reaches, and sometimes in low-order tributaries, decomposition was often exceptionally rapid (2–5% day−1) and followed a linear pattern, associated with dense congregations of the caddisfly Lepidostoma togatum. Linear mass loss persisted until litter bags were empty, after 35–50 days. Colonization of litter bags by L. togatum was advanced after 1 week, and peak densities (40–120 animals/bag) could be reached in 2–3 weeks. In a feeding experiment, alder litter in fine mesh litter bags containing 30 L. togatum larvae lost ~2.1% mass day−1, compared with only 0.1% day−1 without L. togatum. The L. togatum congregations were a distinctly seasonal phenomenon, beginning in early June and ending in mid-July when the last of the larvae (one or two cohorts) pupated. Severe resource limitation among detritivores in these degraded river systems apparently leads to dense colonization of added litter and extremely fast, shredder-mediated decomposition.

Litter processing and shredder distribution as indicators of riparian and catchment influences on ecological health of tropical streams

Terrestrial plant litter is the main source of energy for food webs in forest headwater streams. Leaf litter quality often changes when native tree species are replaced by exotic ones and land use change in the watershed can alter physico-chemistry and functional composition of invertebrate communities, ultimately impairing associated ecosystem processes. We used the composition of invertebrate functional feeding groups (FFGs) and the ecosystem process of leaf breakdown as structural and functional indicators, respectively, of ecosystem health in upland Kenyan streams. During dry and wet conditions, invertebrates were sampled in 24 streams within forest (10), mixed (7) and agriculture (7) catchments. Five forest and five agriculture streams were subsequently used for leaf litter breakdown experiments using two native (Croton macrostachyus and Syzygium cordatum) and one exotic (Eucalyptus globulus) species differing in quality. Coarse- and fine-mesh litterbags were used to compare microbial (fine-mesh) with shredder+microbial (coarse-mesh) breakdown rates, and by extension, determine the role of shredders in litter processing in these streams. Seasonal influences on water quality were observed across catchment land uses. Total suspended sediments, turbidity and total dissolved nitrogen were consistently higher during the wet than dry season. However, seasonal influences on FFGs were inconsistent. Catchment land use influenced invertebrate functional composition: 21 taxa, including eight shredders, were restricted to forest streams, but abundance was a poor discriminator of disturbance. Breakdown rates were generally higher in coarse- compared with fine-mesh litterbags for the native leaf species and the relative differences in breakdown rates among leaf species remained unaltered in both agriculture and forest streams. Shredder and microbial breakdown of leaf litter displayed contrasting responses with shredders relatively more important at forest compared with agriculture streams. However, these patterns were inconsistent across leaf species over the dry and wet seasons. Overall, shredder mediated leaf litter breakdown was dependent on leaf species, and was highest for C. macrostachyus and lowest for E. globulus. This suggests that replacement of indigenous riparian vegetation with poorer quality Eucalyptus species has the potential to reduce nutrient cycling in streams, with foodwebs becoming more reliant on microbial processing of leaf litter, which cannot support diverse consumers and complex food webs.

Preconditioning effects of intermittent stream flow on leaf litter decomposition

Autumnal input of leaf litter is a pivotal energy source in most headwater streams. In temporary streams, however, water stress may lead to a seasonal shift in leaf abscission. Leaves accumulate at the surface of the dry streambed or in residual pools and are subject to physicochemical preconditioning before decomposition starts after flow recovery. In this study, we experimentally tested the effect of photodegradation on sunlit streambeds and anaerobic fermentation in anoxic pools on leaf decomposition during the subsequent flowing phase. To mimic field preconditioning, we exposed Populus tremula leaves to UV–VIS irradiation and wet-anoxic conditions in the laboratory. Subsequently, we quantified leaf mass loss of preconditioned leaves and the associated decomposer community in five low-order temporary streams using coarse and fine mesh litter bags. On average, mass loss after approximately 45 days was 4 and 7% lower when leaves were preconditioned by irradiation and anoxic conditions, respectively. We found a lower chemical quality and lower ergosterol content (a proxy for living fungal biomass) in leaves from the anoxic preconditioning, but no effects on macroinvertebrate assemblages were detected for any preconditioning treatment. Overall, results from this study suggest a reduced processing efficiency of organic matter in temporary streams due to preconditioning during intermittence of flow leading to reduced substrate quality and repressed decomposer activity. These preconditioning effects may become more relevant in the future given the expected worldwide increase in the geographical extent of intermittent flow as a consequence of global change.

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.

Litter decomposition across multiple spatial scales in stream networks

Spatial scale is a critical consideration for understanding ecological patterns and controls of ecological processes, yet very little is known about how rates of fundamental ecosystem processes vary across spatial scales. We assessed litter decomposition in stream networks whose inherent hierarchical nature makes them a suitable model system to evaluate variation in decay rates across multiple spatial scales. Our hypotheses were (1) that increasing spatial extent adds significant variability at each hierarchical level, and (2) that stream size is an important source of variability among streams. To test these hypotheses we let litter decompose in four riffles in each of twelve 3rd-order streams evenly distributed across four 4th-order watersheds, and in a second experiment determined variation in decomposition rate along a stream-size gradient ranging from orders 1 to 4. Differences in decay rates between coarse-mesh and fine-mesh litter bags accounted for much of the overall variability in the data sets, and were remarkably consistent across spatial scales and stream sizes. In particular, variation across watersheds was minor. Differences among streams and among riffles were statistically significant, though relatively small, leaving most of the total variance (51%) statistically unexplained. This result suggests that variability was generated mainly within riffles, decreasing successively with increasing scale. A broad range of physical and chemical attributes measured at the study sites explained little of the variance in decomposition rate. This, together with the strong mesh-size effect and greater variability among coarse-mesh bags, suggests that detritivores account, at least partly, for the unexplained variance. These findings contrast with the widespread perception that variability of ecosystem characteristics, including process rates, invariably increases (1) with spatial extent and (2), in stream networks, when analyses encompass headwaters of various size. An important practical implication is that natural variability need not compromise litter decomposition assays as a means of assessing functional ecosystem integrity.

Aquatic fungi associated with decomposingFicussp. leaf litter in a neotropical stream

Aquatic fungi are important decomposers of plant litter in temperate streams. However, these microorganisms have been poorly studied in tropical streams. We assessed the dynamics of aquatic hyphomycetes during the decomposition of Ficus sp. (Moraceae) leaves in an intermittent lowland stream of northwestern Venezuela. Our goals were to: 1) determine the composition of aquatic hyphomycetes during leaf decomposition, 2) assess the effect of different mesh sizes (i.e., coarse vs fine mesh) on assemblage composition and activity, and 3) assess the role of habitat (i.e., pools vs riffles) on fungal assemblages. Coarse- and fine-mesh litter bags were incubated in a pool and a riffle for 31 d. For each habitat and treatment, we determined physicochemical variables, sporulation activity, and the structure and composition of fungal assemblages. Leaf litter decomposition rates ranged from 0.058/d in fine-mesh bags in the riffle to 0.031/d in coarse-mesh bags in the pool. Decomposition rates were significantly influenced by habitat type, with the highest decomposition rates found in riffle habitats. Sporulation rates were significantly different between habitats but not between mesh sizes. The highest sporulation rate (399 spores mg−1 leaf dry mass d−1) and fungal diversity (16 spp.) was found in coarse-mesh bags in the riffle. Twenty-one fungal species were identified during the study. Anguillospora longissima and Lunuluspora curvula contributed up to 75% of the conidia produced in the coarse- and fine-mesh bags in the riffle, whereas Filosporella aquatica and Lemonniera terrestris accounted for 65 to 75% of the conidia produced in the coarse- and fine-mesh bags in the pool. In ordination space based on species composition, fungal assemblages formed 2 clear groups that corresponded to habitat type. A temporal change in fungal species composition was observed in riffle samples. Early succession assemblages (2–14 d) were characterized by the presence of A. longissima, L. curvula, and F. aquatica, whereas late assemblages (24–31 d) were dominated by Pyramidospora fluminea, Pyramidospora casuarinae, and unidentified sp 4. Overall, habitat type (pool or riffle) influenced sporulation rates and assemblage composition of aquatic hyphomycetes. Our study suggests that environmental factors associated with hydrology (e.g., stream velocity, width, depth) regulate fungal composition and activity and, hence, leaf litter decomposition in this tropical stream.

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)