Effects of Soil Arthropods on Non-Leaf Litter Decomposition: A Meta-Analysis
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According to the widely accepted triangle model, global litter decomposition is collectively controlled by climate, litter initial quality, and decomposers. However, the specific contribution of soil arthropods to litter, especially the non-leaf litter, the decomposition of terrestrial ecosystems and its drivers are still unclear. We conducted a global meta-analysis based on 268 pairs of data to determine the contribution and pattern of soil arthropods to branch, stem, and root litter decomposition in farmlands, forests, and grasslands and analyzed the relationship of soil arthropods’ decomposition effect and potential drivers. Our results showed that: (1) soil arthropods increased global non-leaf litter mass loss by 32.3%; (2) the contribution varied with climate zone and ecosystem type, with a value of subtropical (53.3%) > temperate (18.7%) > tropical (14.7%) and of farmlands (40.6%) > grasslands (34.3%) > forests (0.6%), respectively; (3) the soil arthropods’ decomposition effect gradually decreased with decomposition time, and it was higher in litterbags with a mesh size of 1–2 mm (65.4%) and >2 mm (49.8%) than that of 0.5–1 mm (13.6%); (4) the soil arthropods’ decomposition effects were negatively correlated with the litter initial C/N ratio, mean annual precipitation (MAP; p < 0.001), and elevation and was positively correlated with litter weight. In conclusion, soil arthropod promoted global non-leaf litter decomposition, and the contribution varied with climate zone, ecosystem type, and decomposition time as well as litterbag mesh size. Overall, this study improves the understanding of soil arthropods driving global non-leaf litter decomposition.Keywords:
Decomposer
Litter
Plant litter
Terrestrial ecosystem
Abstract Small streams and their riparian vegetation are closely linked ecosystems. Thus, the invasion of native riparian forests with non‐native species can impact stream ecosystems. We assessed the effects of the invasion of broadleaf deciduous forests by evergreen, nitrogen‐fixing Acacia species on seasonal variation of relevant instream environmental variables, litterfall in the riparian area, aquatic decomposers, and leaf litter decomposition, by comparing three streams flowing through native forests ( native streams ) and three streams flowing through invaded forests ( invaded streams ) in central Portugal. Invaded streams flow through forests composed (almost) of monospecific stands of Acacia trees. Litterfall in the riparian area was sampled with fabric traps and sorted into five categories: leaf (including phyllodes), flower, fruit and seed, wood litter, and other materials. Aquatic hyphomycete conidia suspended in water were sampled to assess conidia concentration and community composition. Leaf litter of Quercus robur was enclosed in coarse‐mesh bags and incubated in streams to assess decomposition rates and associated macroinvertebrate density and community composition. Samples from each variable were collected monthly from streams over 1 year. Aquatic hyphomycete conidia concentration was higher in invaded streams in spring/summer when litter inputs, water temperature, and aquatic nutrient concentrations were higher. In contrast, conidia concentration was lower in invaded streams in autumn/winter as they received less native deciduous leaf litter in autumn than native streams. Aquatic hyphomycete community structure changed, and species richness was lower in invaded streams because aquatic nutrient concentrations were higher and leaf litter species richness was lower. Macroinvertebrate and shredder density in decomposing leaf litter did not differ between native and invaded streams, but litter bags may have artificially increased densities by providing high quality food and/or refuges in streams with poor‐quality resources. Nevertheless, macroinvertebrate community structure changed, and family richness was lower in invaded streams. Finally, decomposition rates of Q. robur leaf litter in coarse‐mesh bags were similar between stream types, despite differences in aquatic decomposer communities. Overall, Acacia invasion changed water quality, litterfall seasonality and composition, and aquatic decomposer communities (especially aquatic hyphomycetes). However, Acacia effects on macroinvertebrate density and leaf litter decomposition rates were less pronounced, suggesting that higher trophic levels may be more resilient to invasion than basal levels, or the invasion time/extent in our invaded streams was not strong enough to affect macroinvertebrates and associated processes. Instream invasion effects on aquatic hyphomycete communities were more strongly mediated by changes in litter inputs rather than increases in aquatic nutrient concentrations because they remained oligotrophic in invaded streams. Simplification of riparian and aquatic communities may render them less efficient in coping with additional environmental changes. Acacia effects might be mitigated by the maintenance of a riparian corridor composed of native vegetation. The protection of non‐invaded riparian galleries and restoration of invaded ones could protect and restore stream ecosystems.
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Abstract The home-field advantage (HFA) hypothesis establishes that plant litter decomposes faster at ‘home’ sites than in ‘away’ sites due to more specialized decomposers acting at home sites. This hypothesis has predominantly been tested through ‘yes or no’ transplanting experiments, where the litter decomposition of a focal species is quantified near and away from their conspecifics. Herein, we evaluated the occurrence and magnitude of home-field effects on the leaf litter decomposition of Myrcia ramuliflora (O.Berg) N. Silveira (Myrtaceae) along a natural gradient of conspecific litterfall input and also if home-field effects are affected by litter and soil traits. Litter decomposition of M. ramuliflora was assessed through litterbags placed in 39 plots in a tropical heath vegetation over a period of 12 months. We also characterized abiotic factors, litter layer traits, and litter diversity. Our results indicated the occurrence of positive (i.e. Home-field advantage) and negative (i.e. Home-field disadvantage) effects in more than half of the plots. Positive and negative effects occurred in a similar frequency and magnitude. Among all predictors tested, only the community weighted mean C/N ratio of the litterfall input was associated with home-field effects. Our results reinforce the lack of generality for home-field effects found in the literature and thus challenge the understanding of litter-decomposer interaction in tropical ecosystems.
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Soil organisms influence organic matter turnover and nutrient cycling via processing of organic matter entering the soil as litter and root-derived resources. Plant species differ enormously in the quality and quantity of litter and roots that they produce, and this diversity strongly modifies decomposition of litter by decomposer organisms. Higher plant diversity is generally assumed to improve habitat conditions and availability of resources, thereby improving the abundance and activity of decomposer organisms. Tropical Andean montane rainforest ecosystems harbor an exceptional diversity of plant and animal species. However, little is known on how the huge diversity of plants and root resources affect the activity of soil communities and the overall decomposition rates, particularly during early stages of decomposition. This thesis aims to contribute to our understanding of the effects of leaf litter diversity and root resources on microorganisms and decomposer microarthropods during the early stages of litter decomposition in Andean tropical montane rainforest ecosystems. The studies were performed as field experiments at 2000 m (Chapter 2 and 4) and along an altitudinal gradient from 1000 to 2000 to 3000 m (Chapter 3) in a tropical montane rainforest in Southern Ecuador. Chapter 2 investigates the effect of leaf litter diversity and identity on microbial functions and microarthropod abundance. The results suggest that decomposition and microbial parameters in litter vary with litter diversity as well as litter identity, while microarthropods respond only to litter identity. The results show that higher levels of diversity detrimentally affect soil microbial biomass and result in a decline in litter decomposition. Further, the results indicate that the differential response of soil biota was mostly due to differences in the initial chemical composition of litter species. However, the results also highlight the importance of leaf litter physical traits, particularly on the abundance of decomposer invertebrates. Overall, the results indicate that litter species identity functions as major driver of the abundance and activity of soil organisms and thereby exerts distinct effects on ecosystem processes such as decomposition and nutrient mobilization. Chapter 3 investigates the contribution of soil microbes and decomposer microarthropods to the decomposition of leaf and root litter along an altitudinal gradient of the studied tropical rainforests. The results suggest that the decomposition of both leaf and root litter in montane rainforests is mainly due to microorganisms, whereas the effect of microarthropods is minor along the altitudinal gradient. However, at higher altitudes soil microarthropods accelerate the decomposition of low-quality litter, such as root litter. Further, the study suggests that the abundance of microorganisms as food is of minor importance in structuring decomposer microarthropod communities, underscoring the role of litter quality. Overall, our findings highlight that resource quality or local interspecific variation in litter quality has stronger effects on decomposer organisms regardless climatic variations associated to altitude, at least during early stages of decomposition. Chapter 4 investigates the response of arbuscular mycorrhizal (AM) fungi, microorganisms and microarthropods to the rotation of hyphal-ingrowth cores, defaunation and nitrogen addition. The results suggest that in the study site AM fungi are closely associated with living roots and do not form extensive extraradical hyphae that can be cut by rotation of the cores. Nonetheless, the results suggest that on top of the litter layer, AM fungi likely compete with saprotrophic microorganisms for litter-derived resources, with mycorrhizal fungi suppressing the activity of saprotrophic microorganisms. While in the soil layer interactions of mycorrhizal fungi with other soil biota are restricted to the close vicinity of roots. Nitrogen addition increased the quality of litter material produced by plants and beneficially affected microbial activity, highlighting that decomposition processes in the studied montane rainforests are strongly limited by nutrient availability and microorganisms in these forests even respond to moderate increase in nitrogen. The results also document a restricted recovery of microorganisms and microarthropods after defaunation of the rotated cores, highlighting the importance of root-derived resources for fueling soil food webs. Chapter 5 presents a discussion and conclusions on the contribution of the research chapters to the overall state of knowledge. Generally, the results of this thesis suggest that during early stages of decomposition the abundance, diversity and activity of soil organisms are strongly associated with the quality and availability of the litter resources. Overall, the results suggest that decomposition processes in montane rainforests at early stages are mainly driven by microorganisms, whereas the contribution of microarthropods is of minor importance. Further, the results also highlight the importance of root-derived resources for fueling soil microarthropod abundance during early stages of decomposition. In addition, the results point to AM fungi as an important player for determining the abundance and activity of microbial communities during early stages of decomposition in tropical montane rainforests.
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Tropical rain forest
Nutrient cycle
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Abstract 1. Forested headwater streams are generally considered to be light‐limited ecosystems where primary production is reduced, and the main source of energy and nutrients is composed of allochthonous detritus. We hypothesised that in these ecosystems, the development of primary producers might also be limited by (1) competition for nutrients with leaf‐litter decomposers (e.g. bacteria and fungi), and (2) leaf‐litter leachates or allelopathic compounds produced by aquatic fungi. 2. To test these hypotheses, a 48‐day mesocosm experiment was performed in 12 artificial streams containing stream water inoculated with epilithic biofilm suspensions collected from a forested headwater stream. Three different treatments were applied: control without leaf litter (C), microbially conditioned leaf litter added at the beginning of the experiment and left to decompose throughout the experiment (L), or leaf litter renewed three times during the experiment (RL). 3. We predicted that (1) the presence of litter, through microbial nutrient immobilisation and allelopathy, would reduce primary production and that (2) this effect would be amplified by litter renewal. We also predicted that nutrient competition would mean that (3) leaf‐litter decomposers will alter primary producer community composition and physiology. These predictions were tested by analysing biofilm development, physiology, stoichiometry, and benthic algal community structure. To distinguish between the effects of nutrient immobilisation and allelopathy, the biofilm responses to leaf‐litter leachates collected after different microbial conditioning durations were also measured in a parallel laboratory experiment. 4. Contrary to our expectations, by day 28, primary producer growth was higher in the mesocosms containing leaf litter (L and RL) despite the rapid decrease in dissolved nutrients when leaf litter was present. After 48 days, the lowest phototrophic biofilm development was observed when leaf litter was renewed (RL), whereas phototrophic biofilm development was similar in the C and L treatments. Biofilm stoichiometry indicated that this effect was most probably related to greater nitrogen limitation in the RL treatment. The presence of leaf litter also affected primary producers' photophysiology, which could be attributed to changes in taxonomic composition and to physiological adjustments of primary producers. 5. Laboratory measurements showed that despite a strong inhibition of primary producer growth by unconditioned leaf‐litter leachates, microbially conditioned leaf litter had either low or no effects on the development of primary producers. 6. These results reveal that leaf‐litter decomposers can have both positive and negative effects on primary producers underlining the need to consider microbial interactions when investigating the functioning of forested headwater streams.
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Background Plant litter decomposition is a key process in carbon and nutrient cycling. Among the factors determining litter decomposition rates, the role of soil biota in the decomposition of different plant litter types and its modification by variations in climatic conditions is not well understood. Methods In this study, we used litterbags with different mesh sizes (45 µm, 1 mm and 4 mm) to investigate the effect of microorganisms and decomposer microarthropods on leaf and root litter decomposition along an altitudinal gradient of tropical montane rainforests in Ecuador. We examined decomposition rates, litter C and N concentrations, microbial biomass and activity, as well as decomposer microarthropod abundance over one year of exposure at three different altitudes (1,000, 2,000 and 3,000 m). Results Leaf litter mass loss did not differ between the 1,000 and 2,000 m sites, while root litter mass loss decreased with increasing altitude. Changes in microbial biomass and activity paralleled the changes in litter decomposition rates. Access of microarthropods to litterbags only increased root litter mass loss significantly at 3,000 m. The results suggest that the impacts of climatic conditions differentially affect the decomposition of leaf and root litter, and these modifications are modulated by the quality of the local litter material. The findings also highlight litter quality as the dominant force structuring detritivore communities. Overall, the results support the view that microorganisms mostly drive decomposition processes in tropical montane rainforests with soil microarthropods playing a more important role in decomposing low-quality litter material.
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Abstract Litter fragment size and quality can have profound effects on ecosystem functioning and global biogeochemical cycling due to differential utilization by decomposers. Here we study the influence of these factors on decomposers from two guilds found in a tropical savannah stream: invertebrate shredders of the genus Phylloicus and microorganisms. Containers (16 × 16 × 12 cm, ~ 3L) with either Phylloicus (cases removed; N = 16) or stream water containing microorganisms ( N = 16) were supplied with litter from the species Inga laurina, Maprounea guianensis, and Richeria grandis , and cut into disks of 18.7, 13.2, and 8.1 mm in diameter (3 sizes × 3 species = 9 disks per container). Relative decomposition was greater for smaller leaf disks and disks of higher quality in microbial‐only cultures. Phylloicus preferentially harvested large fragments for case building, also preferring the leaves of M. guianensis and R. grandis , likely due increased robustness for case formation. Microbial decomposition resulted in ~20% litter mass loss compared to 30% in Phylloicus (of which 8% was used for case building and 24% for food). Thus, changes to input litter size, such as a decrease in leaf size after drought, may alter microbial decomposition and potentially affect shredder populations by limiting the availability of casing material.
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Biogeochemical Cycle
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Detritivore
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