Determinants of litter mixing effects in a Swedish boreal forest

When the litter of a given species decomposes, it will often break down in the proximity of litters from other species. We investigated the effects of litters of 10 different species in a boreal forest of northern Sweden on each others' decomposition and N release rates; this was done through the use of litterbags containing two compartments separated by single mesh partition. Different litters could be placed on opposite sides of this mesh so that they were in contact with each other. Treatments consisted of all the possible pairwise combinations of the 10 species, with members of each pair placed in different compartments of the same litterbag. Litterbags were harvested after 1, 2 and 4 years in the field. Species differed significantly in their effects on decomposition and N loss rates of associated litters. Generally, litters from feather mosses and lichens showed the greatest promotion of decomposition on associated litters, while some vascular plant species, notably Empetrum hermaphroditum, showed the least. At year four, feather mosses also had the greatest positive effects on N loss from the litters of associated species. There were several instances in which litter of a given species decomposed at different rate when litter from its own species, rather than that of a different species, was placed in the adjacent litterbag compartment. This was particularly apparent in the second year, when across the entire data set, litters decomposed fastest when associated with their own litters. Generally, slowly decomposing litters had the greatest positive effects on decomposition of associated litters. It is proposed that in boreal forests slow decomposing litters (particularly those of feather mosses) may contribute to enhancing moisture attention in the litter layer, which in turn promotes the decomposition and N release of associated litters. Further, while litter mixing effects were clearly demonstrated in our study, they were also shown to be of secondary importance to the effects of species identity on decomposition.     

Leaf litter decomposition in the pure and mixed plantations of <Emphasis Type="Italic">Cunninghamia lanceolata</Emphasis> and <Emphasis Type="Italic">Michelia macclurei</Emphasis> in subtropical China

Leaf litter decomposition of Cunninghamia lanceolata, Michelia macclurei, and their mixture in the corresponding stands in subtropical China was studied using the litterbag method. The objective was to assess the influence of native evergreen broadleaved species on leaf litter decomposition. The hypotheses were: (1) M. macclurei leaf litter with lower C/N ratio and higher initial N concentration decomposed faster than C. lanceolata litter, (2) decomposition rates in litter mixtures could be predicted from single-species decay rates, and (3) litters decomposed more rapidly at the site that contained the same species as in the litterbag. The mass loss of leaf litter was positively correlated with initial N concentration and negatively correlated with C/N ratio. The decomposition rate of M. macclurei leaf litter was significantly higher than that of C. lanceolata needle litter in the pure C. lanceolata stand. Contrary to what would be predicted, the litter mixture decomposed more slowly than expected based on the results from component species decomposing alone. There was no significant difference in litter decomposition rate between different habitats.     

Do invertebrate decomposers affect the disappearance rate of litter mixtures?

We designed an experiment using litter bags with fine and coarse mesh size to analyse interacting effects between invertebrate decomposers and the number of litter species on litter disappearance rates. We used litter of nine broad-leaved tree species to compare disappearance rates of litter from single species with mixtures of two to six species. Species composition of litter and invertebrates interacted strongly in their effects on litter disappearance rate. Contribution of invertebrates to litter disappearance increased with time mainly for litter which disappeared slower in the absence of invertebrates. Disappearance rates were positively correlated with initial N content and negatively correlated to initial C content of litter. These relationships were stronger in the presence of invertebrates, suggesting that their activity is positively related to initial litter chemistry. Number of component litter species, however, had no effect on disappearance rate irrespective of the activity of invertebrates. Using individual rates of disappearance for single species, we calculated the expected rates of disappearance for each of the experimental mixtures of leaf litters. We found that mixtures of several species of leaf litter resulted in significant deviations from the expected values. These deviations showed a significant effect of the number of component litter species. However, this result was caused by a strong negative deviation of one single mixture of six species. The presence of invertebrates resulted in even greater deviations from the expected values, suggesting an important contribution of invertebrates to the effects of litter mixing on litter disappearance rates. Hence, our results underline the importance of idiosyncratic effects of species traits in mixtures. Our results suggest that the influence of invertebrate decomposers interacts with litter chemistry during decomposition, but is not affected by litter species richness per se.     

Nutrient dynamics in litter mixtures of four Mediterranean maquis species decomposing in situ

In natural conditions, litters shed from different species become mixed with each other, and decompose together. Most studies deal with decomposition of individual species; few studies investigate the influence of litter mixing on decomposition and nutrient dynamics; the results are contradictory as positive, negative, or no effect, of litter mixing have been observed. In this study we test the hypothesis: i) that litter mixing in the Mediterranean maquis, a nutrient poor, high diversity ecosystem, produces non-additive effects on nutrient dynamics; ii) that the effects vary with the composition in species of the mixture and with the relative amount of the species component the mixture. Two types of 3-species mixtures were set up; one contained three sclerophylls, Phillyrea angustifolia, Pistacia lentiscus and Quercus ilex; the other contained the first two species with the mesophyll Cistus. Litterbags, containing monospecific litters and even and uneven mixtures, were incubated under natural condition in situ; even mixtures had the 3 species in equal proportion, whereas uneven mixtures had one of the species as dominant (50%) and the other two species in equal proportion (25%:25%). Litterbags were retrieved after 92, 188 and 403 days; litters from the mixtures were separately analyzed for mass loss and content of nitrogen (N), potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), manganese (Mn), iron (Fe), copper (Cu), and zinc (Zn). Results indicate that mixing influences the dynamics of N, Mn, Ca, Mg, Fe, Cu and Zn, but scarcely affects the dynamics of K and Na. The comparison of observed to expected values for changes of nutrients in litterbags indicates the occurrence of non-additive effects of litter mixing on movements of N, Fe, Cu, and Zn to or from the litterbags containing the mixtures. The effects depend on the composition in species of the mixture, whereas the relative amount of the species component the mixture is not relevant.     

Litter mixture effects on decomposition in tropical montane rainforests vary strongly with time and turn negative at later stages of decay

In a litterbag study in a tropical montane rainforest in Ecuador we assessed the impact of leaf litter species identity and richness on decomposition. We incubated leaf litter of six native tree species in monocultures and all possible two and four species combinations and analysed mass loss over a period of 24 months. Mass loss in monocultures averaged 30.7% after 6 month and differed significantly between species with variations being closely related to initial concentrations of lignin, Mg and P. At later harvests mass loss in monocultures averaged 54.5% but did not vary among leaf litter species and, unexpectedly, did not increase between 12 and 24 months suggesting that litter converged towards an extremely poor common quality retarding decomposition. After 6 months mass loss of leaf litter species was significantly faster in mixtures than in monocultures, resulting in synergistic non-additive mixture effects on decomposition, whereas at later harvests mass loss of component litter species was more variable and leaf litter mixture effects differed with species richness. Mass loss in the two species mixtures did not deviate from those predicted from monocultures, while we found antagonistic non-additive mixture effects in the four species mixtures. This suggests that litter species shared a poor common quality but different chemistry resulting in negative interactions in chemically diverse litter mixtures at later stages of decomposition. Overall, the results suggest that interspecific variations in diversity and composition of structural and secondary litter compounds rather than concentrations of individual litter compounds per se, control long term leaf litter decomposition in tropical montane rainforests. Plant species diversity thus appears to act as a major driver for decomposition processes in tropical montane rainforest ecosystems, highlighting the need for increasing plant conservation efforts to protect ecosystem functioning of this threatened biodiversity hotspot.     

Mass loss and nutrient release during litter decay in peatland: The role of microbial adaptability to litter chemistry

In peatlands the reduced decomposition rate of plant litter is the fundamental mechanism making these peat-accumulating ecosystems effective carbon sinks. A better knowledge of litter decomposition and nutrient cycling is thus crucial to improve our predictions of the effects of anthropogenic perturbation on the capacity of peatlands to continue to behave as carbon sinks. We investigated patterns of plant litter decomposition and nutrient release along a minerotrophic-ombrotrophic gradient in a bog on the south-eastern Alps of Italy. We determined mass loss as well as P, N, K, and C release of seven vascular plant species and four moss species after 1 year in both native and transplanted habitats. Hence, differences in litter decay were supposed to reflect the degree of adaptability of microbial communities to litter quality. Polyphenols/nutrient and C/nutrient quotients appeared as the main parameters accounting for decomposition rates of Sphagnum litter. In particular, litter of minerotrophic Sphagnum species decomposed always faster than litter of ombrotrophic Sphagnum species, both in native and transplanted habitats. Decomposition rates of vascular plant litter in native habitats were always higher than the corresponding mass loss rates of Sphagnum litter. Minerotrophic forbs showed the fastest decomposition both in native and transplanted habitats in accordance with low C/P and C/N litter quotients. On the other hand, C/P quotient seems to play a primary role also in controlling decomposition of graminoids. Decomposition of deciduous and evergreen shrubs was negatively related to their high lignin content. Nitrogen release from Sphagnum litter was primarily controlled by C/N quotient, so that minerotrophic Sphagnum litter released more N than ombrotrophic Sphagnum litter. Overall, we observed slower N release from litter of ombrotrophic vascular plant species compared to minerotrophic vascular plant species. No single chemical parameter could predict the variability associated with different functional groups. The release of K was very high compared to all the other nutrients and rather similar between ombrotrophic and minerotrophic litter types. In Sphagnum litter, a higher C/P quotient was associated with a slower P mineralisation, whereas a faster P release from vascular plant litter seems primarily associated with lower C/P and polyphenols/P quotients.     

Effects of tree species and topography on soil chemistry, litter quality, and decomposition in Northeast Turkey

Leaf litters from beech (Fagus orientalis Lipsky.) and oak (Quercus robur L.), and needle litters from fir (Abies nordmanniana Spach.) and pine (Pinus sylvestris L.) trees were collected from north-facing site and south-facing site and at three slope positions (top, middle and bottom) on each aspect that varied in soil chemical characteristics (soil pH, cation exchange capacity and base saturation). The litters were analysed for initial total carbon, nitrogen, acid detergent fibre, lignin and cellulose concentrations. Nitrogen, acid detergent fibre and lignin concentrations and carbon:nitrogen and lignin:nitrogen ratios varied significantly within and between species according to soil chemical characteristics on aspects and slope positions. Litter decomposition was studied in the field using the litterbag technique. The litters were placed on two aspects and at three slopes on each aspect in October 2001, and were sampled every 6-month for 2 years. The main effects of aspect, species and slope position on decomposition rates were all statistically significant. Oak leaf litter showed highest decomposition rates, followed by pine, fir and beech litter, and the litters placed on north-facing site decomposed faster than those on the south-facing site. The litters placed at the top slope position decomposed slower than at those at either the bottom or middle positions. Initial lignin concentrations explained most of the variation in decomposition rates between species, and within species for the aspects and the slope positions, but the explained variance showed differences between aspects and slope positions. This result illustrates the important point that litter quality may define the potential rates of microbial decomposition but these are significantly influenced by the biotic and abiotic environment in which decomposition takes place.     

Contrasting decomposition rates and nutrient release patterns in mixed vs singular species litter in agroforestry systems

Purpose

The rate of litter decomposition can be affected by a suite of factors, including the diversity of litter type in the environment. The effect of mixing different litter types on decomposition rates is increasingly being studied but is still poorly understood. We investigated the effect of mixing either litter material with high nitrogen (N) and phosphorus (P) concentrations or those with low N and P concentrations on litter decomposition and nutrient release in the context of agroforestry systems.

Materials and methods

Poplar leaf litter, wheat straw, peanut leaf, peanut straw, and mixtures of poplar leaf litter-wheat straw, poplar leaf litter-peanut leaf, and poplar leaf litter-peanut straw litter samples were placed in litter bags, and their rates of decomposition and changes in nutrient concentrations were studied for 12 months in poplar-based agroforestry systems at two sites with contrasting soil textures (clay loam vs silt loam).

Results and discussion

Mixing of different litter types increased the decomposition rate of litter, more so for the site with a clay loam soil texture, representing site differences, and in mixtures that included litter with high N and P concentrations (i.e., peanut leaf). The decomposition rate was highest in the peanut leaf that had the highest N and P concentrations among the tested litter materials. Initial N and P immobilization may have occurred in litter of high carbon (C) to N or C to P ratios, with net mineralization occurring in the later stage of the decomposition process. For litter materials with a low C to N or P ratios, net mineralization and nutrient release may occur quickly over the course of the litter decomposition.

Conclusions

Non-additive effects were clearly demonstrated for decomposition rates and nutrient release when different types of litter were mixed, and such effects were moderated by site differences. The implications from this study are that it may be possible to manage plant species composition to affect litter decomposition and nutrient biogeochemistry; mixed species agroforestry systems can be used to enhance nutrient cycling, soil fertility, and site productivity in land-use systems.     

Impact of litter species diversity on decomposition processes and communities of soil organisms

The effect of plant species diversity and the C/N ratio of litter on soil processes were analysed in mesocosms in a three year field experiment. Plots 0.5 × 0.5 m with a depth of 15 cm containing sand mixed with loam were used to compare five natural and one artificial litter type (polypropylene string). Natural litters were either monospecific (I: Dactylis glomerata; II: Festuca rubra and III: Trifolium pratense) or were species mixtures (IV: mixture of three species I, II and III; V: mixture of twelve species, IV and nine other meadow plants). Differences among treatments in the litter decomposition rate, humic acid content and nematode density depended on the litter quality (C:N ratio) in most cases. By contrast, most of the differences among treatments found in the substratum below the litter cover resulted from litter diversity. The largest increase of carbon and nitrogen amount during growing season was found under litter mixtures (IV, V) and the highest fulvic acid content under the most diverse litter (V). Similarly, the production of algae in the substratum also significantly increased with litter diversity. Higher taxonomic diversity of Nematoda and Collembola and the most mature community of nematodes were observed under the most complex litter. Epigeic macrofauna, both dwelling in and penetrating the litter, did not differ significantly between experimental treatments. However, the highest share of predators was found in the treatment with the richest plant species diversity. In general, the results suggest that the decomposition of diverse plant litter enhances humus acid accumulation in soils. It is likely that algae participate in the process of humus formation.     

Influence of elevated UV-B radiation on leaf litter chemistry and subsequent decomposition in humid subtropical China

Purpose

Exposure to elevated ultraviolet B (UV-B) radiation during plant growth may influence plant tissue chemistry and subsequent decomposition. We conducted a 22-month decomposition experiment to evaluate the effects of UV-B radiation on litter chemistry and subsequent decomposition in humid subtropical forest systems.

Materials and methods

Leaf litters were derived from five native tree species, including Cunninghamia lanceolata, Cinnamomum camphora, Schima superba, Cyclobalanopsis glauca, and Elaeocarpus sylvestris, which grew under ambient and elevated UV-B radiation treatments for 1 year.

Result and discussion

UV-B treatment significantly altered the original C, N, P, K, and lignin content and ratios of C/N, lignin/N, and C/P of leaf litter of five species but just slightly accelerated decomposition at variable degree from 2 % to 13 %. Statistical analyses showed litter species, but not UV-B treatment, had significant effect on decomposition. Only initial lignin content was significantly related to the decay rate. Abundant precipitation and warm temperature in subtropical China maybe weaken or even mask the importance of litter chemistry change resulted from UV-B radiation to decomposition especially in early decomposition stage.

Conclusions

Exposure to supplemental UV-B level induced significant changes of the initial leaf litter chemistry but did not accelerate significantly subsequent decomposition of each species in humid subtropical areas of China at least in the early phase. The interspecific differences in litter chemistry of the five species showed greater effect on decomposition than elevated UV-B radiation at the early decomposition stage.     

Carbon and nitrogen transfer in leaf litter mixtures

The decomposition rate of litter mixtures can differ from that expected on the basis of the decomposition rate of the individual components. This difference may be linked to nitrogen (N) transfer from high-N to low-N components. Transfer of N is probably also associated with transfer of C, but the extent and direction of this C transfer are unknown. This study examined transfer and loss in laboratory microcosms of C and N from two mixed litter species (Scots pine, Pinus sylvestris L. and maize, Zea mays L.), which have natural isotopic differences in ¹³C. Half the material was ¹⁵N-labelled and the plants were fertilised or unfertilised. Substantial bidirectional transfer of C and N occurred between the litters, with net transfer of C from pine to maize litter and net transfer of N from high-N to low-N litter. Mixtures of fertilised and unfertilised plant litter showed higher than expected C losses and net transfer of N. Mixtures with litters from the same fertilisation treatment had small or insignificant net transfer of N and their C losses did not differ from values estimated using the decomposition rates of the pure litters.     

Site of leaf origin affects how mixed litter decomposes

Recent studies have demonstrated that mass loss, nutrient dynamics, and decomposer associations in leaf litter from a given plant species can differ when leaves of that species decay alone compared to when they decay mixed with other species’ leaves. Results of litter-mix experiments have been variable, however, making predictions of decomposition in mixtures difficult. It is not known, for example, whether interactions among litter types in litter mixes are similar across sites, even for litter mixtures containing the same plant species. To address this issue, we used reciprocal transplants of litter in compartmentalized litterbags to study decomposition of equal-mass litter mixtures of sugar maple (Acer saccharum Marshall) and red oak (Quercus rubra L.) at four forest sites in northwestern Connecticut. These species differ significantly in litter quality. Red oak always has higher lignin concentrations than maple, and here C:N is lower in oak leaves and litter, a pattern often observed when oak coexists with maple. Overall, we observed less mass loss and lower N accumulation in sugar maple and red oak litter mixtures than we predicted from observed dynamics in single-species litterbags. Whether these differences were significant or not depended on the site of origin of the leaves (P<0.02), but there was no significant interaction between sites of decay and the differences in observed and predicted decomposition (P>0.2) . Mixing of leaf litter types could have significant impacts on nutrient cycling in forests, but the extent of the impacts can vary among sites and depends on the origin of mixed leaves even when the species composition of mixes is constant.     

Litter-fall and litter decomposition in a low Mediterranean shrubland

Annual production of litter by Cistus incanus (L.) and Myrtus communis (L.) and decomposition dynamics of leaf litter of these species was studied in a Mediterranean shrubland. Myrtus and Cistus produced 472 and 429 g dry weight litter m^-2 year^-1, respectively. Leaves were the predominant litter component for both species. The average decay constant of Myrtus and Cistus litters enclosed in litter bags, calculated over the whole study period (38 months), was 0.71 year^-1 and 0.31 year^-1 respectively. In green leaves the N content differed during growth seasons for both species, whereas the content of Ca, Mg, P, K, and Na did not show significant changes. Abscised leaves had lower N, P and K contents than green leaves, evidencing that a nutrient translocation before abscission occurred from senescent leaves. The nutrient contents of the leaves at abscission time, generally higher in Cistus than in Myrtus, allowed us to estimate the annual nutrient input to the soil. Phosphorus and K more than N were rapidly released by the decomposing litters after exposure. Nutrient limitation, in particular P, might be considered the main growth limiting factor for Myrtus and Cistus. Both species were adapted to recovery and rapidly recycle P more than N and K in the living biomass through retranslocation from green leaves before abscission and/or a high release rate from the decomposing litter. The former strategy was better used by Cistus, the latter by Myrtus.     

Effect of earthworm addition on soil nitrogen availability,microbial biomass and litter decomposition in mesocosms

The aim of the study was to determine the effect of adding two tropical earthworm species, Rhinodrilus contortus and Pontoscolex corethrurus, to mesocosms on the availability of mineral N (NH₄ ⁺ and NO₃ ^– concentrations), soil microbial biomass (bio-N), and the decomposition rates of three contrasting leaf litter species, in a glasshouse experiment. The mesocosms were filled with forest soil and covered with a layer of leaf litter differing in nutritional quality: (1) Hevea brasiliensis (C/N=27); (2) Carapa guianensis (C/N=32); (3) Vismia sp., the dominant tree species in the second growth forest (control, C/N= 42); and, (4) a mixture of the former three leaf species, in equal proportions (C/N=34). At the end of the 97-day experiment, the soil mineral N concentrations, bio-N, and leaf litter weight loss were determined. Both earthworm species showed significant effects on the concentrations of soil NO₃ ^– (p<0.01) and NH₄ ⁺ (p<0.05). Bio-N was always greater in the mesocosms with earthworms (especially with R. contortus) and in the mesocosms with leaf litter of H. brasiliensis (6 µg N g^–1 soil), the faster decomposing species, than in the other treatments (0.1–1.6 µg N g^–1). Thus, earthworm activity increased soil mineral-N concentrations, possibly due to the consumption of soil microbial biomass, which can speed turnover and mineralization of microbial tissues. No significant differences in decomposition rate were found between the mesocosms with and without earthworms, suggesting that experiments lasting longer are needed to determine the effect of earthworms on litter decomposition rates.     

Litter-type specific microbial responses to the transformation of leaf litter into millipede feces

The transformation of leaf litter into fecal pellets by saprophagous macroarthropods has long been suggested to play an important role in litter decomposition by altering microbial processes. However, conflicting results are reported in the literature, and it is currently not clear to what extent varying initial litter quality contributes to distinct microbial responses to the transformation of litter into feces. Here we performed a screening test using a wide range of distinct leaf litter from 26 tree species. We fed these litters to the macroarthropod species Glomeris marginata during one week under controlled conditions, and compared microbial responses in uningested leaf litter with that of feces produced from the 26 different leaf litter types. We assessed substrate induced respiration (SIR) as an integrative measure of microbial responses. We found that litter SIR was highly variable across species and well related to initial litter quality. However, variability in feces SIR was strongly reduced and only weakly related to initial litter quality. Moreover, the difference between feces and litter SIR decreased with increasing litter SIR as a result of higher microbial stimulation in litter with low associated litter SIR. Our data clearly showed that the direction and magnitude of microbial stimulation in feces depend strongly on the litter type. Therefore, the consequence of litter transformation into macroarthropod fecal pellets for microbial decomposers and possibly for subsequent decomposition of feces is specific to litter species.     

Effects of nitrogen addition and litter properties on litter decomposition and enzyme activities of individual fungi

Litter decomposition is an important process of C and N cycling in the soil. Variation in the response of litter decomposition to nitrogen (N) addition (positive, negative or neutral) has been observed in many field studies. However, mechanism about variability in individual fungal species response to N addition has not yet been well demonstrated in the literature. Therefore, the objective of this study was to investigate the effects of N addition and litter chemistry properties on litter decomposition and enzyme activities of individual fungi. Three fungal species (Penicillium, Aspergillus, and Trichoderma) were isolated from a subtropical mixed forest soil. An incubation experiment was conducted using the individual fungi with two types of litter (leaf of Pinus massoniana and needle of Cryptocarya chinensis) and different N addition levels (0, 50 and 100 for N-deficient treatments, and 500 and 1000 μg N for N-excessive treatments). Cumulative CO₂-C, enzyme activities, and lignin and cellulose loss were measured during the incubation period of 60 days. Litter decomposition and enzyme activities significantly varied with the fungal species, while the N addition and litter types greatly affected fungal enzyme activities. The N treatments significantly increased lignin-rich needle decomposition by lignocellulose decomposers (Penicillium and Aspergillus) but did not affect their leaf decomposition. On the contrary, The N treatments stimulated leaf decomposition by cellulolytic species (Trichoderma) but did not affect its needle decomposition. Correlation analysis showed that lignin in the litter was the key component to affect litter decomposition. Activities of N-acetyl-β-glucosaminidase and phenol oxidase were both positively correlated to litter decomposition. The fungi (Penicillium and Aspergillus) with higher production of N-acetyl-β-glucosaminidase showed higher litter decomposition ability. The low N addition levels stimulated Penicillium and Aspergillus litter decomposition, but they still required more N source (e.g., litter N source) to support decomposition. Depressed fungal litter N uptake (lower N-acetyl-β-glucosaminidase activities) only occurred at the highest N addition level. Litter decomposition of Trichoderma depended more on external N and its litter decomposition capability was the lowest among the three species.     

Effects of fungal inocula on the decomposition of lignin and structural polysaccharides in Pinus sylvestris litter

 Litter bags containing sterile Scots pine (Pinus sylvestris) needles (19.8% lignin, 26.5% cellulose and 0.34% N) were inoculated with two species of fungi in the laboratory and then placed in the litter layer of a pine plantation. Marasmius androsaceus, which can degrade lignocellulose, was initially displaced by other fungal colonisers and was not detected in the litter after 2–3 months; but was re-isolated from the needles after 12 months. Trichoderma viride, which is a cellulolytic species and also antagonistic to other fungi, dominated the litter throughout the experiment. The control litter was naturally colonised by litter fungi. After 12 months, mass losses were similar at 52% for M. androsaceus and 48% for T. viride, compared with 36% for the control litter colonised by a more complex fungal community. Lignin concentrations increased with time in control litter and with T. viride because mass losses of carbohydrates were greater than those of lignin. Litter inoculated with M. androsaceus showed significant lignin decomposition throughout the experiment but cellulose concentrations showed a proportional increase in the first 6 months, suggesting that the fungus was preferentially exploiting hemicellulose and non-structural carbohydrates. Analysis of TFA-extractable sugars (mainly from hemicellulose) and CuO-derived phenylpropanoid moieties from lignin confirmed the differential patterns of resource decomposition which were not evident from total mass losses. During the initial stages of decomposition, T. viride was as effective in utilising structural polysaccharides as the complex fungal community in the control litter. Furthermore, M. androsaceus not only exhibited unexpectedly low cellulolytic activity but also facilitated lignin depolymerisation after the fungus was no longer detectable in the litter. The pre-inoculation of litter with these two fungal species therefore affected the overall dynamics of decomposition at a biochemical level. This study illustrates the importance of understanding the effects and interactions of specific fungi, rather than assumptions about the functional competence of diverse communities, on the processes of litter decomposition. Received: 5 July 2000     

The influence of plant litter diversity on decomposer abundance and diversity

Although there has been much recent interest in the effect of litter mixing on decomposition processes, much remains unknown about how litter mixing and diversity affects the abundance and diversity of decomposer organisms. We conducted a litter mixing experiment using litterbags in a New Zealand rainforest, in which treatments consisted of litter monocultures of each of 8 forest canopy and understory plant species, as well as mixtures of 2, 4 and 8 species. We found litter mixing to have little effect on net decomposition rates after either 279 or 658 days, and for each species decomposition rates in mixture treatments were the same as in monoculture. Litter species identity had important effects on litter microfauna, mesofauna and macrofauna, with different litter types promoting different subsets of the fauna. Litter mixing had few effects on densities of mesofauna and macrofauna, but did have some important effects on components of the microfauna, notably microbe-feeding and predatory nematodes. At day 279, litter mixing also consistently reduced the ratio of bacterial-feeding to microbe-feeding (bacterial-feeding+fungal-feeding) nematodes, pointing to mixing causing a significant switch from the bacterial-based to the fungal-based energy channel. Litter mixing sometimes influenced the community composition and diversity of nematodes and macrofauna, but effects of litter mixing on diversity were not necessarily positive, and were much weaker than effects of litter species identity on diversity. We conclude that litter mixing effects on the abundance and diversity of decomposer biota, when they occur, are likely to be of secondary and generally minor significance when compared to the effects of litter species identity and composition.     

Estimating N transfers between N<Subscript>2</Subscript>-fixing actinorhizal species and the non-N<Subscript>2</Subscript>-fixing<Emphasis Type="Italic"> Prunus avium</Emphasis> under partially controlled conditions

Two methods of N transfer between plants—by litter decomposition and root-to-root exchange—were examined in mixed plantations of N-fixing and non-fixing trees. Nitrogen transfers from decaying litters were measured by placing ¹⁵N-labelled litters from four actinorhizal tree species around shoots of containerized Prunus avium. Nitrogen transfers by root-to-root exchanges were measured after foliar NO₃-¹⁵N fertilization of Alnus subcordata and Elaeagnus angustifolia growing in containers in association with P. avium. During the first 2 years of litter decomposition, from 5–20% of the N, depending on the litter identity, was released and taken up by P. avium. N availability in the different litters was strongly correlated with the amount of water-soluble N, which was highest in leaves of E. angustifolia. In the association between fixing and non-fixing plants, 7.5% of the A. subcordata N and 25% of E. angustifolia N was transferred to P. avium by root exchange. These results showed that the magnitude of N transfers by root exchange depended on the associated N₂-fixing species. Among the species investigated, E. angustifolia displayed the highest capacity for exudating N from roots as well as for releasing N from litters. These qualities make this tree a promising species for enhancing wood yields in mixed stands.     

Interactions between litter quality, decomposition and soil fertility: a laboratory study

Leaf litters from beech (Fagus sylvatica L.) and oak (Quercus robur L.) trees were collected from mixed, deciduous woodlands growing on three soil types that varied in mineral nutrient concentrations and N mineralisation potential. Litter quality, including %N, %Mn, %P, acid detergent fibre, cellulose, Klason lignin, phenylpropanoid constituents of lignin, hexose and pentose sugar (mainly from hemicelluloses) varied within species according to soil type. However, oak and beech showed the opposite responses to soil nutrient status for most of these variables. The litters were incubated in the laboratory for 12 months (at 18 °C and constant moisture) on beds of forest floor material from two soils of contrasting high nutrient material (HNM) or low nutrient material (LNM) nutrient status to investigate litter quality and substrate interactions. At 4, 8 and 12 months there were significant differences in mass losses from oak and beech litters from all sites, and for each litter type exposed to the HNM and LMN soils. At 12 months mean mass losses were higher for HNM treatment (38.7% oak, 27.8% beech) than for the LNM treatment (30.6% oak, 25.5% beech). However, the beech and oak litters from the different sites consistently responded in opposite ways on the same soil treatment reflecting site-related effects on litter quality. Initial concentration of Klason lignin was the best predictor for mass losses from litter species and litter types. Intra-specific variation in rates of litter decomposition of beech and oak litters from different sites, and differences in their interactions with the two forest floor materials, illustrate the complexities of proximate controls on decomposition that are often masked in system-level studies.