Litter Breakdown Research Articles

Trophic position and periphyton carbon sources for macroinvertebrates in the litter patches of forested streams, with special reference to predatory species

Abstract Most research on lotic food webs has focused on reach-scale webs, whereas research on the structure and system openness of lotic food webs has not focused on the microhabitat scale. In forested streams, plant litter from riparian vegetation accumulates as discrete patches, forming a microhabitat for macroinvertebrates. We primarily aimed to determine the relative trophic position and contribution of periphyton to the assimilated diet of macroinvertebrate species in pool litter patches via stable isotope analysis. We specifically focused on predatory species in the context of their indirect effects on litter breakdown and estimated the range of their absolute trophic positions. The mean δ15N values of the 19 macroinvertebrate predator species considered ranged from 0.0 ‰ to 5.3 ‰. This range was greater than the overall mean enrichment factor of δ15N (Δ15N = 1.8 ‰) estimated from aquatic invertebrate predators based on five existing data sources, indicating that the absolute trophic position of predator species can extend across more than one level. The estimated periphyton contributions to the diets of predator species were almost half or more, indicating that the food webs of pool litter patches are closely connected with external periphyton-based webs. Our findings suggest that the top-down effect of predatory macroinvertebrates in litter patches is likely to be dissipated by the shift to the grazing food chains instead of cascading down to the litter.

Litter decomposition and nutrient dynamics in a subtropical ecosystem: A comparison of natural and plantation forests (Duabanga grandiflora) in Nagaland, North-East India

Litter decomposition and nutrient dynamics in a subtropical ecosystem: A comparison of natural and plantation forests (Duabanga grandiflora) in Nagaland, North-East India

Ground beetle trophic interactions alter available nitrogen in forest soil

It is generally held that microbes exert primary control over nitrogen availability in temperate forests. Yet the role of soil and litter‐dwelling invertebrates to provide additional control via the breakdown of organic matter is an area of current exploration. Through trophic interactions within soil food webs, predators may indirectly affect prey with cascading effects on litter breakdown and nitrogen availability. The importance of these interactions, however, may be context‐dependent, varying with the stage of forest development and associated decomposer species composition given that young and old forests have vast differences in nitrogen availability, vegetation litter, soil properties and invertebrate functional groups. We examined ground beetle control over soil nitrogen and soil properties using a 68‐day mesocosm experiment that manipulated trophic structure (omnivore + predator beetles, predator beetles, and no beetles) in a young and old forest stand in the northeastern United States. In the young forest, net nitrogen mineralization decreased under predator + omnivore and the bulk soil C:N ratio in the old forest. However, we found no response in either forest context to the predator only treatment. Our study demonstrates the potential for ground beetles to strongly impact nitrogen availability and soil properties in forest ecosystems. Therefore, animal trophic interactions and their contexts must be included in our paradigm of nutrient cycles in temperate forests.

Functional effects of subsidies and stressors on benthic microbial communities along freshwater to marine gradients.

Leaf litter in coastal wetlands lays the foundation for carbon storage, and the creation of coastal wetland soils. As climate change alters the biogeochemical conditions and macrophyte composition of coastal wetlands, a better understanding of the interactions between microbial communities, changing chemistry, and leaf litter is required to understand the dynamics of coastal litter breakdown in changing wetlands. Coastal wetlands are dynamic systems with shifting biogeochemical conditions, with both tidal and seasonal redox fluctuations, and marine subsidies to inland habitats. Here, we investigated gene expression associated with various microbial redox pathways to understand how changing conditions are affecting the benthic microbial communities responsible for litter breakdown in coastal wetlands. We performed a reciprocal transplant of leaf litter from four distinct plant species along freshwater-to-marine gradients in the Florida Coastal Everglades, tracking changes in environmental and litter biogeochemistry, as well as benthic microbial gene expression associated with varying redox conditions, carbon degradation, and phosphorus acquisition. Early litter breakdown varied primarily by species, with highest breakdown in coastal species, regardless of the site they were at during breakdown, while microbial gene expression showed a strong seasonal relationship between sulfate cycling and salinity, and was not correlated with breakdown rates. The effect of salinity is likely a combination of direct effects, and indirect effects from associated marine subsidies. We found a positive correlation between sulfate uptake and salinity during January with higher freshwater inputs to coastal areas. However, we found a peak of dissimilatory sulfate reduction at intermediate salinity during April when freshwater inputs to coastal sites are lower. The combination of these two results suggests that sulfate acquisition is limiting to microbes when freshwater inputs are high, but that when marine influence increases and sulfate becomes more available, dissimilatory sulfate reduction becomes a key microbial process. As marine influence in coastal wetlands increases with climate change, our study suggests that sulfate dynamics will become increasingly important to microbial communities colonizing decomposing leaf litter.

Temperature dependence of leaf breakdown in streams differs between organismal groups and leaf species.

Increased temperatures are altering rates of organic matter (OM) breakdown in stream ecosystems with implications for carbon (C) cycling in the face of global change. The metabolic theory of ecology (MTE) provides a framework for predicting temperature effects on OM breakdown, but differences in the temperature dependence of breakdown driven by different organismal groups (i.e., microorganisms vs. invertebrate detritivores) and litter species remain unresolved. Over two years, we conducted 12 60-day leaf litterbag incubations in 20 headwater streams in the southern Appalachian Mountains (USA). We compared temperature dependence (as activation energy, Ea) between microbial and detritivore-mediated breakdown, and between a highly recalcitrant (Rhododendron maximum) and a relatively labile (Acer rubrum) leaf species. Detritivore-mediated breakdown had a higher Ea than microbial breakdown for both leaf species (Rhododendron: 1.48 > 0.56 eV; Acer: 0.97 > 0.29 eV), and Rhododendron breakdown had a higher Ea than Acer breakdown for both organismal groups. Similarly, the Ea of total (coarse-mesh) Rhododendron breakdown was higher than the Ea of total Acer breakdown (0.89 > 0.52 eV). These effects for total breakdown were large, implying that the number of days to 95% mass loss would decline by 40% for Rhododendron and 26% for Acer between 12°C (our mean temperature value) and 16°C (+4°C, reflecting projected increases in global surface temperature due to climate change). Despite patterns in Ea, overall breakdown rates were higher for microbes than detritivores, and for Acer than Rhododendron over most of our temperature gradient. Additionally, the Ea for a subset of the microbial breakdown data declined from 0.40 to 0.22 eV when fungal biomass was included as a model predictor, highlighting the key role of fungi in determining the temperature dependence of litter breakdown. Our results imply that, as streams warm, routing of leaf litter C to detritivore-mediated fates will increase faster than predicted by previous studies and MTE, especially for labile litter. As temperatures rise, earlier depletion of autumn-shed, labile leaf litter combined with rapid breakdown rates of recalcitrant litter could exacerbate seasonal resource limitation and alter carbon storage and transport dynamics in temperate headwater stream networks.

Effects of riparian forest and annual variation in stream environmental conditions on leaf litter breakdown and invertebrate communities in highland grassland streams

Effects of riparian forest and annual variation in stream environmental conditions on leaf litter breakdown and invertebrate communities in highland grassland streams

Intraspecific variation in crayfish behavioral traits affects leaf litter breakdown in streams.

Although intraspecific trait variation is increasingly recognized as affecting ecosystem processes, few studies have examined the ecological significance of among-population variation in behavioral traits in natural ecosystems. In freshwater habitats, crayfish are consumers that can influence ecosystem structure (e.g., macroinvertebrate communities) and function (e.g., leaf litter breakdown). To test whether crayfish behavioral traits (activity, boldness, and foraging voracity) are major contributors of leaf litter breakdown rates in the field, we collected rusty crayfish (Faxonius rusticus) from eight streams across the midwestern USA and measured behaviors using laboratory assays. At the same streams, we measured breakdown rates of leaf packs that were accessible or inaccessible to crayfish. Our results provide evidence that among-population variation in crayfish boldness and foraging voracity was a strong predictor of leaf litter breakdown rates, even after accounting for commonly appreciated environmental drivers (water temperature and human land use). Our results suggest that less bold rusty populations (i.e., emerged from shelter more slowly) had greater direct impacts on leaf litter breakdown than bold populations (P = 0.001, r2 = 0.85), potentially because leaf packs can be both a shelter and food resource to crayfish. Additionally, we found that foraging voracity was negatively related to breakdown rates in leaf packs that were inaccessible to crayfish (P = 0.025, r2 = 0.60), potentially due to a trophic cascade from crayfish preying on other invertebrates that consume leaf litter. Overall, our results add to the growing evidence that trait variation in animals may be important for understanding freshwater ecosystem functioning.

Leaf litter breakdown phenology in headwater stream networks is modulated by groundwater thermal regimes and litter type

AbstractLeaf litter dominates particulate organic carbon inputs to forest streams. Using data‐informed simulations, we explored how litter type (slow‐ vs. fast‐decomposing species), pulsed autumn litter inputs, groundwater‐mediated temperature regimes, and climate warming affect litter breakdown in a 3rd‐order stream network. We found that the time‐dependent interactions of these variables govern network‐scale litter breakdown phenology, with greater thermal sensitivity of slow‐decomposing litter for both current and future scenarios. Groundwater thermal inputs modified litter breakdown phenology by reducing spring and summer and elevating winter litter breakdown fluxes. Under future warming scenarios, the source depth of contributing groundwater influenced summer detrital resources; shallow groundwater‐fed streams had reduced summer resources compared to deep groundwater‐fed streams. Our results demonstrate that predicting in‐stream carbon cycling requires explicit consideration of the phenology of resource inputs and the seasonal timing of environmental factors, notably stream thermal regimes.

Leaf quality and macrofauna are more important than the presence of trees and shrubs in riparian vegetation for leaf litter breakdown in subtropical highland grassland soil systems

Leaf quality and macrofauna are more important than the presence of trees and shrubs in riparian vegetation for leaf litter breakdown in subtropical highland grassland soil systems

Land-use effects on leaf-litter breakdown in streams in a tropical lowland catchment

The expansion of oil palm and cattle grazing in the tropics continues to alter numerous ecosystem functions. The generated land-use change is potentially impacting stream leaf-litter breakdown, a fundamental process for freshwater ecosystems. To assess the effect of land-use change, we studied breakdown rates of forest (Pachira aquatica, Pouroma aspera, Sloanea ampla, and Hippocratea volubilis) and oil palm (Elaeis guineensis) leaves after a 26-day immersion in streams surrounded by rainforests, grazing lands, or oil palm plantations with and without riparian buffers. In addition, we assessed breakdown drivers by deploying litter bags (248) of two mesh sizes (15 mm and 0.5 mm) allowing or restricting macroinvertebrates’ access (134 coarse-mesh bags and 114 fine-mesh bags). Forest leaf breakdown by microbes (fine-mesh) was lower by 55% in the plantations compared to forests; while microbial oil palm-leaf breakdown was lower by 32% in the unbuffered plantations compared to forests (p < 0.05). Total litter breakdown was lower (p > 0.05) in the plantations but not when these preserved riparian buffers. Litter breakdown was driven primarily by microbes in all land uses except in the buffered plantations, possibly due to increased shredder biomass. These results suggest that oil palm agriculture may decrease microbial stream leaf-litter breakdown, especially in streams with no riparian buffers.

Diversity of fungi species that decompose Ceriops tagal leaf litter on Pulau Sembilan, Langkat Regency

One of the processes in mangrove ecosystems that contributes greatly to water fertility is the decomposition or breakdown of mangrove litter. Fungi, as decomposers in the process of decomposing litter, have an important role in the ecosystem and also play a role in regulating nutrient availability and carbon cycling.This research was conducted to determine the number of species and diversity of fungi found in decomposed C. tagal leaf litter, to measure the rate of decomposition of C. tagal leaf litter and to determine the levels of carbohydrates and proteins during the decomposition process.The research was carried out at the Forest Cultivation Laboratory, Faculty of Forestry, Universitas Sumatera Utara. The sampling location of C. tagal leaf litter was in the mangrove ecosystem on Pulau Sembilan, PangkalanSusu District, Langkat Regency.The determination of carbohydrate and protein content was carried out at the Medan Industrial Research and Standardization Center.The research was conducted from June 2022 to December 2022.The average rate of litter decomposition was obtained using the formula: ln (Xt/X0) = -kt, and analysis of carbohydrate and protein content using the SNI 01-2891-1992 method.The results of this research showed that there were 5 types of fungi found in the decomposition process of C. tagal leaf litter, which were Aspergillus sp. 1, Aspergillus sp. 2, Penicillium sp., Trichoderma sp. and Culvularia sp. The average value of fungal diversity is 1.19, which is included in the medium category. The decomposition rate of C. tagal leaf litter was 7.55 per year. On day 90, the lowest carbohydrate content was 3.62% and the highest protein content was 5.4%.

Microplastics alter the leaf litter breakdown rates and the decomposer community in subtropical lentic microhabitats

Microplastics, pervasive pollutants in aquatic environments, have been primarily studied for their impact on marine ecosystems. However, their effects on freshwater systems, particularly in forested phytotelmata habitats, remain understudied in Subtropical systems. This research examines the influence of varying microplastic concentrations (0.0, 200, 2,000, 20,000, and 200,000ppm) on leaf litter breakdown of Inga vera (in bags of 10 and 0.05mm mesh) and the naturally associated invertebrate community occurring in forested phytotelmata. The study employs an experimental design with microplastic concentration treatments in artificial microcosms (buckets with 800mL of rainwater) arranged in an area of Atlantic Rain Forest native vegetation of Subtropical systems. The results indicate that elevated concentrations of microplastics may enhance leaf litter breakdown (6-8%), irrespective of the bag mesh, attributed to heightened decomposer activity and biofilm formation. Consequently, this contributes to increased invertebrate richness (33-37%) and greater shredder abundance (21-37%). Indicator analysis revealed that Culicidae, Stratiomyidae, Chironomidae, Empididae, Planorbidae, and Ceratopogonidae were indicative of some microplastic concentrations. These findings underscore the significance of accounting for microplastics when evaluating the taxonomic and trophic characteristics of invertebrate communities, as well as the leaf breakdown process in Subtropical systems.

Intrinsic and extrinsic drivers of organic matter processing along phosphorus and salinity gradients in coastal wetlands

Abstract Climate change is accelerating sea‐level rise and saltwater intrusion in coastal regions world‐wide and interacting with large‐scale changes in species composition in coastal wetlands. Quantifying macrophyte litter breakdown along freshwater‐to‐marine coastal gradients is needed to predict how carbon stores will respond to shifts in both macrophyte communities and water chemistry under changing environmental conditions. To test the interactive drivers of changing species identity and water chemistry, we performed a reciprocal transplant of four macrophyte litter species in seven sites along freshwater‐to‐marine gradients in the Florida Coastal Everglades. We measured surface water chemistry (dissolved organic carbon, total nitrogen and total phosphorus), litter chemistry (% nitrogen, % phosphorus, change in N:P molar ratio, % cellulose and % lignin as proxies for recalcitrance) and litter breakdown rates (k/degree‐day). Direct effects of salinity and surface water nutrients were the strongest drivers of k, but unexpectedly, litter chemistry did not correlate with litter k. However, salinity strongly correlated with changes in litter chemistry, whereby litter incubated in brackish and marine wetlands was more labile and gained more phosphorus compared with litter in freshwater marshes. Our results suggest that litter k in coastal wetlands is explained by species‐specific interactions among water and litter chemistries. Water nutrient availability was an important predictor of breakdown rates across species, but breakdown rates were only explained by the carbon recalcitrance of litter in the species with the slowest breakdown (Cladium jamaicense), indicating the importance of carbon structure, and species identity on breakdown rates. Synthesis. In oligotrophic ecosystems, nutrients are often the primary driver of organic matter breakdown. However, we found that variation in macrophyte breakdown rates in oligotrophic coastal wetlands was also explained by salinity and associated seawater chemistry, emphasising the need to understand how saltwater intrusion will alter organic matter processing in wetlands. Our results suggest that marine subsidies associated with sea‐level rise have the potential to accelerate leaf litter breakdown. The increase in breakdown rates could either be buffered or increase further as sea‐level rise also shifts macrophyte community composition to more or less recalcitrant species.

Effects of exotic detritus input on native litter breakdown in a eutrophic lake: investigating the home-field advantage

Effects of exotic detritus input on native litter breakdown in a eutrophic lake: investigating the home-field advantage

Interrelationships among litter chemistry, plant species diversity, and litter decomposition in tropical stream environments: a review

Factors that may accelerate decomposition are important for ecosystem functioning since plant litter decomposition is essential for carbon and nutrient cycling, but it is a generally slow process, which can take weeks up to years. In this context, studies have demonstrated that the chemical characteristics of litter mixtures can accelerate decomposition through several mechanisms. Tropical riparian forests are known for their high diversity of tree species, which leads to a wide array of litter types in tropical streams, each with distinct chemical properties. This underscores the key role of litter chemistry in significantly influencing the litter breakdown rate within of these streams. Here, we explore the interplay among litter chemistry, plant species diversity, and litter decomposition in tropical streams. We highlight the importance of litter physical and chemical characteristics for decomposition, as well as of the preservation of the natural floristic composition of tropical riparian forests. In this sense, more attention must be paid to the influence that the environment and phylogeny may have on the phytochemical characteristics of riparian forest plant species in different tropic biomes, and how the insertion of different exotic species interferes with the decomposition process. Furthermore, we emphasize the need for additional research into the consequences of the loss of rare plant species with unique functional characteristics to decomposition in tropical ecosystems.

Impact of a set of environmental variables on the leaf litter breakdown rate in natural streams of the equatorial forest in Cameroon

This study assessed the environmental factors underlying the leaf litter decomposition rate in streams in the equatorial rainforest of Cameroon. To reach this goal we used the litterbag method and dead leaves of Funtumia africana (Benth) Stapf (Apocynaceae)in seven natural streams. Concomitantly, we measured biological (fungi and macroinvertebrates) and environmental parameters to highlight those that control the leaf litter breakdown rates. The breakdown rates ranged from 0.035 to 0.056 with an average of 0.042 ± 0.006 in the coarse-mesh litterbags (Kc) and from 0.018 to 0.059 with an average of 0.037 ± 0.01 in the fine-mesh litterbags (Kf). No significant difference was observed between seasons or sites, except for Kf.. As in other tropical rainforests in South America and Asia, the breakdown rates are mainly resulted from microbial activity; the contribution of shredders was negligible, as confirmed by the Kc to Kf ratio and the litter fragmentation rate λF. Among environmental factors, only the distance from the source and the pH were positively correlated with the leaf litter breakdown rates.

Do magnesium and chloride ameliorate high sodium bicarbonate concentrations? A comparison between laboratory and mesocosm toxicity experiments

Increasing salinity is a concern for biodiversity in many freshwater ecosystems globally. Single species laboratory toxicity tests show major differences in freshwater organism survival depending on the specific ions that comprise salinity types and/or their ion ratios. Toxicity has been shown to be reduced by altering ionic composition, despite increasing (total) salinity. For insistence, single species tests show the toxicity of sodium bicarbonate (NaHCO3, which commonly is a large proportion of the salts from coalbeds) to freshwater invertebrates is reduced by adding magnesium (Mg2+) or chloride (Cl-). However, it is uncertain whether reductions in mortality observed in single-species laboratory tests predict effects within populations, communities and to ecosystem processes in more complex multi-species systems both natural and semi-natural. Here we report the results of an outdoor multi-species mesocosm experiment to determine if the effects of NaHCO3 are reduced by increasing the concentrations of Mg2+ or Cl- on: a) stream macroinvertebrate populations and communities; b) benthic chlorophyll-a and; c) the ecosystem process of leaf litter decomposition. We found a large effect of a high NaHCO3 concentration (≈4.45 mS/cm) with reduced abundances of multiple taxa, reduced emergence of adult insects and reduced species richness, altered community structure and increased leaf litter breakdown rates but no effect on benthic chlorophyll-a. However, despite predictions based on laboratory findings, we found no evidence that the addition of either Mg2+ or Cl- altered the effect of NaHCO3. In semi-natural environments such as mesocosms, and natural environments, organisms are subject to varying temperature and habitat factors, while also interacting with other species and trophic levels (e.g. predation, competition, facilitation), which are absent in single species laboratory tests. Thus, it should not be assumed single-species tests are good predictors of the effects of changing ionic compositions on stream biota in more natural environments.

Relative contribution of photodegradation to litter breakdown in Australian grasslands.

Grassy ecosystems cover ~40% of the global land surface and are an integral component of the global carbon (C) cycle. Grass litter decomposes via a combination of photodegradation (which returns C to the atmosphere rapidly) and biological decomposition (a slower C pathway). As such, decomposition and C storage in grasslands may vary with climate and exposure to solar radiation. We investigated rates of grass litter decomposition in Australian temperate grasslands along a climate gradient to uncouple the relative importance of photodegradation and climate on decomposition. Litterbags containing leaf litter from two common native grass species (Poa labillardierei, Themeda triandra) were deployed at six grassland sites across a precipitation gradient (380-890 mm) in south-eastern Australia. Bags were retrieved over 39 weeks to measure mass loss from decomposition. We used shade treatments on the litter of one species (T. triandra) to partition photodegradation from biological decomposition. The shade treatment reduced the rate of decomposition of T. triandra relative to the full-sun treatment at all sites, by an average of 38% at 39 weeks; the effect size of the shade treatment was not correlated with site productivity. The rate of decomposition in both species was positively correlated with rainfall midway through the experiment, but there were no significant differences in total decomposition among sites after 39 weeks. By week 39, total decomposition of T. triandra was significantly greater than for P. labillardierei. In general, we observed relatively linear decomposition rather than the strong negative exponential decay observed in many global litter decomposition studies. Synthesis: We found that solar radiation exposure was a strong contributor to litter decomposition in temperate Australian grasslands across a broad climate gradient, which may be related to a period of photopriming prior to further biotic decomposition. This study highlights the importance of litter composition and solar radiation exposure in our understanding of how decomposition patterns contribute to global C cycling.

Functional diversity of shredders, not species richness, drives the decomposition rate of leaf litter in ponds

Leaf litter decomposition is a critical ecosystem-level process in many freshwater habitats. Although ponds are likely to derive a large proportion of their energy from riparian vegetation, allochthonous organic matter decomposition in these water bodies has received little attention. We studied the breakdown rates of black alder (Alnus glutinosa (L.) Gaertn.) litter in ponds and provide the first evidence of the role of the taxonomic and functional diversity of pond-dwelling shredders in this ecosystem process. Despite a strong connection to riparian zones, the litter breakdown rates observed in ponds were generally lower than those reported in headwater streams. It seems that ponds provide less favorable conditions for shredder communities than headwaters. The rate of organic matter decomposition in ponds was significantly positively related to functional diversity, represented by the variability of shredder body size, while shredder species richness did not appear to be a reliable proxy for this ecosystem function. This finding is consistent with theoretical predictions that functional complementarity among species has a systematic effect on ecosystem processes. It also emphasizes that body size is a crucial functional trait mediating the effects of shredder diversity on litter decomposition in ponds.

Stoichiometric imbalances between soil microorganisms and their resources regulate litter decomposition

Abstract Litter decomposition is dependent on the requirements of decomposer communities and their ability to acquire energy and nutrients from their substrates (i.e. litter) and the surrounding environment (i.e. soil). However, knowledge about whether and how stoichiometric imbalance (i.e. the differences in C:N:P ratios between microorganisms and their substrates) regulate litter decomposition rates and whether it can be compensated by soil resources have rarely been evaluated, and even less across different decomposition stages over time. In this study, we conducted a reciprocal litter transplantation experiment using a stoichiometric gradient along the forest‐steppe ecotone to evaluate mechanisms underlying litter‐microbe‐soil interactions at different moments during litter breakdown. We measured the C:N:P stoichiometry of litter, soil, microbes, enzyme ratios and soil microbial community composition (via metabarcoding) after 6 and 12 months of litter decomposition. We found that the stoichiometric imbalances between soil microorganisms and litter substrate controlled decomposition rates directly during the early phase of decomposition. In contrast, the stoichiometric imbalances between soil microorganisms and soil substrate regulated decomposition rates during the later phase of decomposition, but this was an indirect effect mediated via shifts in the saprophytic fungal community composition and enzyme allocation. These findings highlight that the stoichiometric imbalance between soil microorganisms and litter substrates can be partly compensated by the local soil resources over the course of the decomposition process. We conclude that the stoichiometric imbalance between soil microorganisms and their resources is a key mechanism that should not be ignored when predicting soil C and nutrient cycling in terrestrial ecosystems. Read the free Plain Language Summary for this article on the Journal blog.