Litter decomposition of a pine plantation is affected by species evenness and soil nitrogen availability

Background and aims

Litter decomposition is a key process controlling flows of energy and nutrients in ecosystems. Altered biodiversity and nutrient availability may affect litter decomposition. However, little is known about the response of litter decomposition to co-occurring changes in species evenness and soil nutrient availability.

Methods

We used a microcosm experiment to evaluate the simultaneous effects of species evenness (two levels), identity of the dominant species (three species) and soil N availability (control and N addition) on litter decomposition in a Mongolian pine (Pinus sylvestris var. mongolica) plantation in Northeast China. Mongolian pine needles and senesced aboveground materials of two dominant understory species (Setaria viridis and Artemisia scoparia) were used for incubation.

Results

Litter evenness, dominant species identity and N addition significantly affected species interaction and litter decomposition. Higher level of species evenness increased the decomposition rate of litter mixtures and decreased the incidence of antagonistic effects. A. scoparia-dominated litter mixtures decomposed faster than P. sylvestris var. mongolica- and S. viridis-dominated litter mixtures. Notably, N addition increased decomposition rate of both single-species litters and litter mixtures, and meanwhile altered the incidence and direction of non-additive effects during decomposition of litter mixtures. The presence of understory species litters stimulated the decomposition rate of pine litters irrespective of N addition, whereas the presence of pine litters suppressed the mass loss of A. scoparia litters. Moreover, N addition weakened the promoting effects of understory species litters on decomposition of pine litters.

Conclusions

Pine litter retarded the decomposition of understory species litters whereas its own decomposition was accelerated in mixtures. Nitrogen addition and understory species evenness altered species interaction through species-specific responses in litter mixtures and thus affected litter decomposition in Mongolian pine forests, which could produce a potential influence on ecosystem C budget and nutrient cycling.     

Decomposition and nutrient dynamics in mixed litter of Mediterranean species

In the last decade a great research effort addressed the effects of litter diversity on ecosystem functions, reporting both synergistic and antagonistic effects for decomposition dynamics. Four coexisting Mediterranean species, representing a range of litter quality, were used to arrange litter mixtures at three diversity levels for a litterbag decomposition experiment. Species identity appeared as the major determinant for litter mass loss (Coronilla emerus～Hedera helix>Festuca drymeia>Quercus ilex) and nutrient release, with rates for all leaf litter types following the sequence K>N>Mg≥Ca>>Fe. Additive diversity effects were prevalent pooling together all data but also for nutrients separately. Antagonistic interactions were more common than synergistic in the cases of mass loss, N and Ca contents, but not for K, Mg and Fe dynamics. The number of species in the litterbag significantly affected the outcome of non-additive interactions, which were mostly antagonistic for two-species mixtures, and synergistic for the combined 4 species. Litter quality appears to be the most important factor affecting mass loss and nutrient dynamics, while litter diversity, influencing the rates of these processes, plays an important role in reducing their variability, thus suggesting a greater stability of ecosystems properties in presence of mixed litter.     

Litter evenness influences short-term peatland decomposition processes

There is concern that changes in climate and land use could increase rates of decomposition in peatlands, leading to release of stored C to the atmosphere. Rates of decomposition are driven by abiotic factors such as temperature and moisture, but also by biotic factors such as changes in litter quality resulting from vegetation change. While effects of litter species identity and diversity on decomposition processes are well studied, the impact of changes in relative abundance (evenness) of species has received less attention. In this study we investigated effects of changes in short-term peatland plant species evenness on decomposition in mixed litter assemblages, measured as litter weight loss, respired CO₂ and leachate C and N. We found that over the 307-day incubation period, higher levels of species evenness increased rates of decomposition in mixed litters, measured as weight loss and leachate dissolved organic N. We also found that the identity of the dominant species influenced rates of decomposition, measured as weight loss, CO₂ flux and leachate N. Greatest rates of decomposition were when the dwarf shrub Calluna vulgaris dominated litter mixtures, and lowest rates when the bryophyte Pleurozium schreberi dominated. Interactions between evenness and dominant species identity were also detected for litter weight loss and leachate N. In addition, positive non-additive effects of mixing litter were observed for litter weight loss. Our findings highlight the importance of changes in the evenness of plant community composition for short-term decomposition processes in UK peatlands.     

Litter species traits, but not richness, contribute to carbon and nitrogen dynamics in an alpine meadow on the Tibetan Plateau

Aims

Litter, as afterlife of plants, plays an important role in driving belowground decomposition processes. Here we tested effects of litter species identity and diversity on carbon (C) and nitrogen (N) dynamics during litter decomposition in N-limited alpine meadow soil from the Qinghai–Tibet Plateau.

Methods

We incubated litters of four meadow species, a sedge (“S”, Kobresia humilis), a grass (“G”, Elymus nutans), a herb (“H”, Saussurea superba), and a legume (“L”, Oxytropis falcata), in monoculture and in mixture with meadow soil. CO₂ release was measured 21 times during the incubation, and soil available N and microbial biomass C and N were measured before and after the experiment.

Results

The organic C decay rate did not differ much among soils amended with monocultures or mixtures of litter, except in the H, S, L, and S+H treatments, which had much higher decay rates. Potential decomposable C pools were lowest in the control, highest in the L treatment, and intermediate in the S treatment. Mineralized N was completely immobilized by soil microbes in all treatments except the control, S+L, and S+G+L treatments. Litter mixtures had both additive and non-additive effects on CO₂-C emission (mainly antagonistic effects), net N mineralization (mainly synergistic), and microbial biomass C and N (both). Overall, these parameters were not significantly correlated with litter species richness. Similarly, microbial C or N was not significantly correlated with litter N content or C/N. However, cumulative CO₂-C emission and net N mineralization were positively correlated with litter N content and negatively correlated with litter C/N.

Conclusions

Litter N content and C/N rather than litter species richness drove the release of CO₂-C and net available N in this ecosystem. The antagonistic effects of litter mixtures contributed to a modest release of CO₂-C, but their synergistic effects enhanced net available N. We suggest that in alpine meadow communities, balancing species with high and low N contents will benefit soil carbon sequestration and plant competition for available N with soil microbes.     

Do physical plant litter traits explain non‐additivity in litter mixtures? A test of the improved microenvironmental conditions theory

The decomposition rates of plant litter mixtures often deviate from the averaged rates of monocultures of their component litter species. The mechanisms behind these non‐additive effects in decomposition of litter mixtures are lively debated. One plausible explanation for non‐additive effects is given by the improved microenvironmental condition (IMC) theory. According to this theory, plant litter species, whose physical characteristics improve the microclimatic conditions for decomposers, will promote the decomposition of their co‐occurring litter species. We tested the IMC theory in relation to leaf litter and soil moisture in two contrasting moisture conditions in a dry subarctic mountain birch forest with vascular plant leaf litters of poor and high quality. The non‐additive effects in mass loss of litter mixtures increased when moisture conditions in litter and soil became more favourable for plant litter decomposition. The sign of this increase (antagonistic or synergistic) in non‐additive effects was more predictable for litter mixtures of poor litter quality. Although the specific mechanisms underlying the IMC theory depended on the litter quality of the litter mixtures, a standardized water holding capacity (WHC) was the litter trait most closely related to the non‐additive effects in mixtures of both poor and high quality litter types. Furthermore, we found that higher dissimilarity in WHC traits between the component litter species in a mixture increased synergistic effects in litter mixtures under limiting moisture conditions. However, under improved moisture conditions, increased antagonistic effects were observed. Thus, we found clear support for the IMC theory and showed that climatic conditions and leaf litter physical traits determine whether the non‐additive effects in litter mixtures are antagonistic or synergistic. Our study emphasizes the need to include litter physical traits into predictive models of mixing effects on plant litter decomposition and in general suggests climate specificity into these models.     

Review of mixed forest litter decomposition researches

The litter plays an important role in forest ecosystems. Decomposition of mixed leaf litters has recently become an active research area because it mimics the natural state of leaf litters in most of forests. Many studies reported effects of mixing litters on their decomposition, ranging from positive, negative to neutral. In this paper decomposition mechanisms of mixed litters concluded by researchers were summarized. Firstly, plant litter quality had been recognized as an important factor to affect decomposition rate. Some studies showed a positive significant correlation between initial N, P concentration and non-additive effect in litter mixture decomposition. Secondly, it has been suggested that litter mixture could increase abundance and diversity of fauna and microbial decomposers, especially fungi. Thirdly, compared with single litter decomposition, the nutrient exchange between different litter species is often considered as one of main non-additive effects observed in litter mixture. Some results showed that the active transport of nutrients by fungal hyphae derived positive effect on the decomposition of litter mixture. The multiple factors such as, leaf litter species, investigation method and plot, were also analyzed. In conclusion, it is necessary to enhance a further research on factors in mixed litter decomposition and an interaction between various factors due to the complex relationship. We are looking forward to using these theories of mixed litter decomposition to direct practical forest management.     

Review of mixed forest litter decomposition researches

The litter plays an important role in forest ecosystems. Decomposition of mixed leaf litters has recently become an active research area because it mimics the natural state of leaf litters in most of forests. Many studies reported effects of mixing litters on their decomposition, ranging from positive, negative to neutral. In this paper decomposition mechanisms of mixed litters concluded by researchers were summarized. Firstly, plant litter quality had been recognized as an important factor to affect decomposition rate. Some studies showed a positive significant correlation between initial N, P concentration and non-additive effect in litter mixture decomposition. Secondly, it has been suggested that litter mixture could increase abundance and diversity of fauna and microbial decomposers, especially fungi. Thirdly, compared with single litter decomposition, the nutrient exchange between different litter species is often considered as one of main non-additive effects observed in litter mixture. Some results showed that the active transport of nutrients by fungal hyphae derived positive effect on the decomposition of litter mixture. The multiple factors such as, leaf litter species, investigation method and plot, were also analyzed. In conclusion, it is necessary to enhance a further research on factors in mixed litter decomposition and an interaction between various factors due to the complex relationship. We are looking forward to using these theories of mixed litter decomposition to direct practical forest management.     

The effects of leaf litter evenness on decomposition depend on which plant functional group is dominant

Background and aims

Climbing plants are increasing in dominance in the subtropical forests of South China and other areas around the world, altering patterns of plant dominance and evenness in community. We investigated how changes in species’ identity and patterns of leaf litter evenness affected decomposition of litter mixtures.

Methods

We used litter-bag method to study the influence of different relative abundance mixtures (75 % : 25 %; 50 % : 50 %; 25 % : 75 %) of plant litter from two functional groups (climbing plants and trees) on decomposition rates in a subtropical forest in Guangdong, China.

Results

We found negative non-additive effects of mixing litter overall and species composition affected decomposition rates the most. In addition, when climbing plants were dominant, even mixtures decomposed slower significantly than uneven mixtures. Evenness did not affect decomposition rates, however, when trees were dominant. The magnitude of antagonistic effects increased with increasing dominance of climbing plants but decreased with time, suggesting a strong negative feedback between litter proportion of climbing plant and decomposition rates at the initial stage.

Conclusion

The evenness in leaf litter composition affects rates of decomposition, but these effects depend on which plant functional group is dominant. Thus, we should pay more attention to shifts in identity of dominant species and patterns of community evenness.     

Stream carbon and nitrogen supplements during leaf litter decomposition: contrasting patterns for two foundation species

Leaf litter decomposition plays a major role in nutrient dynamics in forested streams. The chemical composition of litter affects its processing by microorganisms, which obtain nutrients from litter and from the water column. The balance of these fluxes is not well known, because they occur simultaneously and thus are difficult to quantify separately. Here, we examined C and N flow from streamwater and leaf litter to microbial biofilms during decomposition. We used isotopically enriched leaves (¹³C and ¹⁵N) from two riparian foundation tree species: fast-decomposing Populus fremontii and slow-decomposing Populus angustifolia, which differed in their concentration of recalcitrant compounds. We adapted the isotope pool dilution method to estimate gross elemental fluxes into litter microbes. Three key findings emerged: litter type strongly affected biomass and stoichiometry of microbial assemblages growing on litter; the proportion of C and N in microorganisms derived from the streamwater, as opposed to the litter, did not differ between litter types, but increased throughout decomposition; gross immobilization of N from the streamwater was higher for P. fremontii compared to P. angustifolia, probably as a consequence of the higher microbial biomass on P. fremontii. In contrast, gross immobilization of C from the streamwater was higher for P. angustifolia, suggesting that dissolved organic C in streamwater was used as an additional energy source by microbial assemblages growing on slow-decomposing litter. These results indicate that biofilms on decomposing litter have specific element requirements driven by litter characteristics, which might have implications for whole-stream nutrient retention.     

Determinism of nonadditive litter mixture effect on decomposition: Role of the moisture content of litters

The mechanisms behind the plant litter mixture effect on decomposition are still difficult to disentangle. To tackle this issue, we used a model that specifically addresses the role of the litter moisture content. Our model predicts that when two litters interact in terms of water flow, the difference of evaporation rate between two litters can trigger a nonadditive mixture effect on decomposition. Water flows from the wettest to the driest litter, changing the reaction rates without changing the overall litter water content. The reaction rate of the litter receiving the water increases relatively more than the decrease in the reaction rate of the litter supplying the water, leading to a synergistic effect. Such water flow can keep the microbial biomass of both litter in a water content domain suitable to maintain decomposition activity. When applied to experimental data (Sphagnum rubellum and Molinia caerulea litters), the model is able to assess whether any nonadditive effect originates from water content variation alone or whether other factors have to be taken into account.     

Litter composition rather than plant presence affects decomposition of tropical litter mixtures

Litter decomposition is strongly controlled by litter quality, but the composition of litter mixtures and potential interactions with live plants through root activity may also influence decomposers. In a greenhouse experiment in French Guiana we studied the combined effects of the presence of tropical tree seedlings and of distinct litter composition on mass and nitrogen (N) loss from decomposing litter and on microbial biomass. Different litter mixtures decomposed for 435 days in pots filled with sand and containing an individual seedling from one of four different tree species. We found both additive and negative non-additive effects (NAE) of litter mixing on mass loss, whereas N loss showed negative and positive NAE of litter mixing. If litter from the two tree species, Platonia insignis and Goupia glabra were present, litter mixtures showed more positive and more negative NAE on N loss, respectively. Overall, decomposition, and in particular non-additive effects, were only weakly affected by the presence of tree seedlings. Litter mass loss weakly yet significantly decreased with increasing fine root biomass in presence of Goupia seedlings, but not in the presence of seedlings of any other tree species. Our results showed strong litter composition effects and also clear, mostly negative, non-additive effects on mass loss and N loss. Species identity of tree seedlings can modify litter decomposition, but these live plant effects remain quantitatively inferior to litter composition effects.     

Do non‐additive effects on decomposition in litter‐mix experiments result from differences in resource quality between litters?

Differences in resource quality between litter species have been postulated to explain why litter-mixtures may decompose at a different rate to that which would be predicted from single species litters (termed 'non-additive effects'). In particular, positive, non-additive effects of litter-mixing on decomposition have been explained by differences in initial nitrogen concentration between litter species. This interpretation is confounded because litter species that differ in nitrogen content also differ by a number of other resource quality attributes. Thus, to investigate whether initial nitrogen concentration does account for positive, non-additive effects of litter-mixing, we mixed grass litters that differed in initial nitrogen concentration but not species or structural plant part identity, and then followed mass loss from the litter-mixes over time. We used the litterbag technique and three grass species for which a gradient of four distinct initial nitrogen concentrations had been generated. We produced all no- to four-mix compositions of litter qualities for each species. Litter from different species was never mixed.
Contrary to what would be predicted, we found that when litters of the same species but with different initial nitrogen concentrations were mixed, that negative, non-additive effects on decomposition were generally observed. In addition, we found that once mixed, increasing litter quality richness from two to four mixtures had no significant, non-additive effect on decomposition. Litter quality composition explained little of the experimental variation when compared to litter quality richness, and different compositions generally behaved in the same manner. Our findings challenge the commonly held assumption that differences in nitrogen concentration between plant species are responsible for positive, non-additive effects of litter-mixing on decomposition.     

Non‐additive effects of invasive tree litter shift seasonal N release: a potential invasion feedback

Many invasive plant species strongly alter ecosystem processes by producing leaf litter that decomposes faster and releases N more quickly than that of native species. However, while most studies of invasive species litter impacts have only considered the decomposition of species in monoculture, forest litter layers typically contain litter from many species. Many litter mixtures decompose in a non‐additive manner, in which the mixture decomposes more quickly (synergistic effect) or more slowly (antagonistic effect) than would be expected based on decomposition of the component species’ litters in isolation. We investigated the potential for non‐additive effects of invasive species’ litter by conducting a one‐year litter bag experiment in which we mixed the litters of four native tree species with each of four invasive species. Litter mixtures frequently lost mass at non‐additive rates, although not at every loading ratio, and the presence, sign, and strength of effects depended on species composition. Non‐additive effects on N loss occurred in more litter combinations, and were almost always antagonistic at 90 days and synergistic at 365 days. Invasive species litter with lower C:N led to more strongly synergistic N loss with time. During the growing season, non‐additive patterns of N loss almost always resulted in increased N release – up to six times greater than would be expected based on single‐species decomposition. Consequently, we suggest that invasive species may further synchronize N release from the litter layer with plant N demand, enhancing any positive litter feedback to invasion. These results highlight the need to consider non‐additive effects of litter mixing in invaded forest communities, and suggest that estimates of invasive species’ impacts on ecosystem processes would be improved by considering these effects.     

陆地生态系统混合凋落物分解研究进展

凋落物分解在陆地生态系统养分循环与能量流动中具有重要作用,是碳、氮及其他重要矿质养分在生态系统生命组分间循环与平衡的核心生态过程。自然生态系统中,植物群落大多具有较高的物种丰富度和多样性,其混合凋落物在分解过程中也更有可能发生养分传递、化学抑制等种间互作,形成多样化的分解生境,多样性较高的分解者类群以及复杂的级联效应分解,这些因素和过程均对研究混合凋落物分解过程、揭示其内在机制形成了极大的挑战。从构成混合凋落物物种丰富度和多样性对分解生境、分解者多样性及其营养级联效应的影响等方面,综合阐述混合凋落物对陆地生态系统凋落物分解的影响,探讨生物多样性在凋落物分解中的作用。通过综述近些年的研究发现,有超过60%的混合凋落物对其分解速率的影响存在正向或负向的效应。养分含量有差异的凋落物混合分解过程中,分解者优先利用高质量凋落物,使低质量的凋落物反而具有了较高的养分有效性,引起低质量凋落物分解加快并最终使混合凋落物整体分解速率加快;而凋落物物种丰富度对土壤动物群落总多度有轻微的影响或几乎没有影响,但是对线虫和大型土壤动物的群落组成和多样性有显著影响,并随着分解阶段呈现一定动态变化;混合凋落物改变土壤微生物生存的理化环境,为微生物提供更多丰富的分解底物和养分,优化微生物种群数量和群落结构及其分泌酶的活性,并进一步促进了混合凋落物的分解。这些基于植物-土壤-分解者系统的动态分解过程的研究,表明混合凋落物分解作用不只是经由凋落物自身质量的改变,更会通过逐级影响分解者多样性水平而进一步改变分解速率和养分释放动态,说明生物多样性确实在一定程度上调控凋落物分解及其养分释放过程。     

Leaf Litter Mixtures Alter Microbial Community Development: Mechanisms for Non-Additive Effects in Litter Decomposition

To what extent microbial community composition can explain variability in ecosystem processes remains an open question in ecology. Microbial decomposer communities can change during litter decomposition due to biotic interactions and shifting substrate availability. Though relative abundance of decomposers may change due to mixing leaf litter, linking these shifts to the non-additive patterns often recorded in mixed species litter decomposition rates has been elusive, and links community composition to ecosystem function. We extracted phospholipid fatty acids (PLFAs) from single species and mixed species leaf litterbags after 10 and 27 months of decomposition in a mixed conifer forest. Total PLFA concentrations were 70% higher on litter mixtures than single litter types after 10 months, but were only 20% higher after 27 months. Similarly, fungal-to-bacterial ratios differed between mixed and single litter types after 10 months of decomposition, but equalized over time. Microbial community composition, as indicated by principal components analyses, differed due to both litter mixing and stage of litter decomposition. PLFA biomarkers a15∶0 and cy17∶0, which indicate gram-positive and gram-negative bacteria respectively, in particular drove these shifts. Total PLFA correlated significantly with single litter mass loss early in decomposition but not at later stages. We conclude that litter mixing alters microbial community development, which can contribute to synergisms in litter decomposition. These findings advance our understanding of how changing forest biodiversity can alter microbial communities and the ecosystem processes they mediate.     

Mixtures with grass litter may hasten shrub litter decomposition after shrub encroachment into mountain grasslands

Aims

Shrub encroachment in mesic grasslands alters the identity and quality of litters entering the system. As litter from shrubs and grasses can differ in their quality, this can lead to differences in litter decomposition by the direct effect of quality, but also to litter interaction during decomposition. The objective of this study was to examine the occurrence of non-additive effects of litter mixtures on the decomposition rates of legume shrub litter (poor in P) or conifer shrub litter (poor in N) and grass litter.

Methods

In addition to single litter type litterbags for the three species, we mixed litters of each pair of possible combinations to determine the influence of each species on mass loss. Litterbags were placed in the field and collected after 1, 6, 8, 12 and 24 months. In each collection, litter of each species remaining in mixed bags was separated, dry weighed and analyzed for C, N and P.

Results

With respect to shrub litter decomposing alone, mass loss of shrub litter when mixed with grass showed a 9–10 % increase in decomposition rate for conifer and a 3 % increase for legume litter. These litter mixture effects varied with time and they were detected after a decomposition period of 1 year in legume litter and of 2 years in conifer litter.

Conclusions

Grass litter hastened conifer and legume litter decomposition in leaf litter mixtures, at least during the first stages of the process. The potential consequences of this result to alter litter accumulation patterns and thus carbon sequestration rates after shrub encroachment into grasslands will depend on whether the observed trends are maintained in the advanced decomposition stages.     

Nitrogen transfer in litter mixture enhances decomposition rate,temperature sensitivity,and C quality changes

Background and aims

Litter decomposition is a critical process in terrestrial ecosystems and, since in natural conditions plant litter occurs in mixtures, understanding the interactive effects of mixed litter is of great ecological relevance. In this context, we test the hypothesis that N transfer between high quality litter to N-poor substrates are at the base of synergistic interactions, positively affecting litter decay rate, temperature sensitivity, and changes of organic C quality.

Methods

We carried out a manipulative experiment using four organic substrates, encompassing a wide range of biochemical quality (Hedera helix and Quercus ilex leaf litter, cellulose strips and woody sticks), each decomposing either separately or in matched pair mixtures for 360 days. Organic substrates were characterized for mass loss, C and N content and by ¹³C CPMAS NMR to assess biochemical quality changes.

Results

Litter response to mixing was related to the biochemical quality of the components in the mixture: additive when substrates with similarly high (H. helix and Q. ilex) or low (cellulose and wood) N content were paired, but synergistic when substrates with contrasting N content were associated (either of the two leaf litters with either cellulose or wood). Overall, no antagonist effects were observed in this experiment. Interestingly, decomposition of cellulose and wood showed an higher temperature sensitivity, compared to monospecific substrates, when paired with N rich materials. Significant N transfer was found from N rich litter to N poor substrates and ¹³C CPMAS NMR showed rapid changes of C quality of cellulose and wood sticks only when paired with N rich litter.

Conclusions

Our findings support the hypothesis that mixing litters of different quality, with quality expressed in terms of C/N ratio and N content, increases decomposition rate and temperature sensitivity of the lower quality substrates.     

马尾松与乡土阔叶树种凋落叶混合分解过程中全碳释放的动态变化

为了调整低山丘陵区低效林林分结构,探明马尾松（Pinus massoniana Lamb.）与乡土阔叶树种凋落叶混合分解过程中的全碳（C）释放规律。本研究以华南广泛分布的马尾松、檫木（Sassafras tzumu（Hemsl.） Hemsl）、香樟（Cinnamomum camphora（Linn） Presl）以及香椿（Toona sinensis（A. Juss.） Roem.）凋落叶为研究对象,将这4个树种凋落叶按照不同树种搭配以及混合比例组合为35个处理后进行野外分解实验,探讨C释放最佳的凋落叶树种组合以及混合比例。研究发现：4个单一树种凋落叶之间,香椿凋落叶的C释放最快,檫木和香樟凋落叶次之,马尾松凋落叶最慢。31个混合凋落叶中,C释放的非加和效应随着分解时间的延长表现出先升增强后减弱的趋势,且相对于其他季节,凋落叶在秋季的非加和效应有所减弱。一针一阔树种组合中,香樟凋落叶占比≥30%的处理：PC73和PC64的协同效应较强;一针两阔和一针三阔组合中,阔叶占比≥30%且含有香椿凋落叶的处理：PST613和712、PCT631和613、PSCT7111和6121的协同效应较强。     

Non‐additive effects of litter mixing are suppressed in a nutrient‐enriched stream

Investigations of how species compositional changes interact with other aspects of global change, such as nutrient mobilization, to affect ecosystem processes are currently lacking. Many studies have shown that mixed species plant litters exhibit non‐additive effects on ecosystem functions in terrestrial and aquatic systems. Using a full‐factorial design of three leaf litter species with distinct initial chemistries (carbon:nitrogen; C:N) and breakdown rates (Liriodendron tulipifera, Acer rubrum and Rhododendron maximum), we tested for additive and non‐additive effects of litter species mixing on breakdown in southeastern US streams with and without added nutrients (N and phosphorus). We found a non‐additive (antagonistic) effect of litter mixing on breakdown rates under reference conditions but not when nutrient levels were elevated. Differential responses among single‐species litters to nutrient enrichment contributed to this result. Antagonistic litter mixing effects on breakdown were consistent with trends in litter C:N, which were higher for mixtures than for single species, suggesting lower microbial colonization on mixtures. Nutrient enrichment lowered C:N and had the greatest effect on the lowest‐ (R. maximum) and the least effect on the highest‐quality litter species (L. tulipifera), resulting in lower interspecific variation in C:N. Detritivore abundance was correlated with litter C:N in the reference stream, potentially contributing to variation in breakdown rates. In the nutrient‐enriched stream, detritivore abundance was higher for all litter and was unrelated to C:N. Thus, non‐additive effects of litter mixing were suppressed by elevated streamwater nutrients, which increased nutrient content of all litter, reduced variation in C:N among litter species and increased detritivore abundance. Nutrients reduced interspecific variation among plant litters, the base of important food web pathways in aquatic ecosystems, affecting predicted mixed‐species breakdown rates. More generally, world‐wide mobilization of nutrients may similarly modify other effects of biodiversity on ecosystem processes.     

How to manage degraded monoculture plantations in South China: a perspective from reciprocal litter transplant experiment

Litter decomposition, an important component of nutrient cycling, is often one of the limiting factors for the development of monoculture tree plantations for restoration, and how to improve the litter decomposition rate remains as a major challenge. To help resolve this issue, we developed a mixed-litter transplantation approach to improve the litter decomposition and nutrient cycling in Schima superba, Cunninghamia lanceolata, Eucalyptus urophylla, and Acacia mangium monoculture plantations in China. The monospecific leaf litters of the four species were collected and their possible two-, three- and four-species combinations were transplanted between plantations. We examined the influences of home/away field, litter species richness, and litter composition on litter decomposition during 24 months treatment. A significant effect of litter composition on litter decomposition (Duration?×?Composition effect) was detected in E. urophylla plantation. The influence of litter richness on litter decomposition was significant in A. mangium plantation (Duration?×?Richness effect). The litter of C. lanceolata and A. mangium had a distinct home-field advantage, while the litter of S. superba had a distinct away-field advantage in decomposition. We observed a positive relationship between richness and litter decomposition in C. lanceolate plantation. The effect of Duration?×?Species Interaction on litter decomposition, was significant in E. urophylla plantation, indicating a non-additive effect. Litter decomposition in E. urophylla plantation could be explained by idiosyncratic model, and the rivet model may be appropriate to illustrate the litter decomposition in A. mangium plantation. Finally, since the litter decomposition in degraded A. mangium plantations had a distinct home-field advantage and was significantly affected by litter richness, transplanting mixed litters of neighboring plantations may be beneficial to improve its litter decomposition rate. Transplanting of S. superba litters due to the distinct home-field advantage to neighboring plantations such as E. urophylla plantation whose litter decomposition is significantly affected by litter composition, may be an effective management method for improving litters decomposition.