Greater accumulation of litter in spruce (Picea abies) compared to beech (Fagus sylvatica) stands is not a consequence of the inherent recalcitrance of needles

Background and aims

Replacement of beech by spruce is associated with changes in soil acidity, soil structure and humus form, which are commonly ascribed to the recalcitrance of spruce needles. It is of practical relevance to know how much beech must be admixed to pure spruce stands in order to increase litter decomposition and associated nutrient cycling. We addressed the impact of tree species mixture within forest stands and within litter on mass loss and nutritional release from litter.

Methods

Litter decomposition was measured in three adjacent stands of pure spruce (Picea abies), mixed beech-spruce and pure beech (Fagus sylvatica) on three nutrient-rich sites and three nutrient-poor sites over a three-year period using the litterbag method (single species and mixed species bags).

Results

Mass loss of beech litter was not higher than mass loss of spruce litter. Mass loss and nutrient release were not affected by litter mixing. Litter decay indicated non-additive patterns, since similar remaining masses under pure beech (47%) and mixed beech-spruce (48%) were significantly lower than under pure spruce stands (67%). Release of the main components of the organic substance (C_org, N_tot, P, S, lignin) and associated K were related to mass loss, while release of other nutrients was not related to mass loss.

Conclusions

In contradiction to the widely held assumption of slow decomposition of spruce needles, we conclude that accumulation of litter in spruce stands is not caused by recalcitrance of spruce needles to decay; rather adverse environmental conditions in spruce stands retard decomposition. Mixed beech-spruce stands appear to be as effective as pure beech stands in counteracting these adverse conditions.     

Climatic effects on litter decomposition from arctic tundra to tropical rainforest

Climatic effects on the decomposition rates of various litter types in different environments must be known to predict how climatic changes would affect key functions of terrestrial ecosystems, such as nutrient and carbon cycling and plant growth. We developed regression models of the climatic effects on the first‐year mass loss of Scots pine needle litter in boreal and temperate forests across Europe (34 sites), and tested the applicability of these models for other litter types in different ecosystems from arctic tundra to tropical rainforest in Canada (average three year mass loss of 11 litter types at 18 sites), the USA and Central America (four litter types at 26 sites). A temperature variable (annual mean temperature, effective temperature sum or its logarithm) combined with a summer drought indicator (precipitation minus potential evapotranspiration between May and September) explained the first‐year mass loss of the Scots pine needle litter across Europe with a higher R² value than actual evapotranspiration (0.68–0.74 vs. 0.51) and with less systematic error for any sub‐region. The model with temperature sum and the summer drought indicator appeared best suited to the other litter types and environments. It predicted the climatic effects on the decomposition rates in North and Central America with least systematic error and highest R² values (0.72–0.80). Compared with Europe, the decomposition rate was significantly less sensitive to annual mean temperature in Canada, and to changes in actual evapotranspiration in the USA and Central America. A simple model distinguishing temperature and drought effects was able to explain the majority of climatic effects on the decomposition rates of the various litter types tested in the varying environments over the large geographical areas. Actual evapotranspiration summarizing the temperature and drought effects was not as general climatic predictor of the decomposition rate.     

Decomposition of <Superscript>15</Superscript>N-labelled beech litter and fate of nitrogen derived from litter in a beech forest

The decomposition and the fate of ¹⁵N- labelled beech litter was monitored in a beech forest (Vosges mountains, France) over 3 years. Circular plots around beech trees were isolated from neighbouring tree roots by soil trenching. After removal of the litter layer, ¹⁵N-labelled litter was distributed on the soil. Samples [labelled litter, soil (0–15 cm depths], fine roots, mycorrhizal root tips, leaves) were collected during the subsequent vegetation periods and analysed for total N and ¹⁵N concentration. Mass loss of the ¹⁵N-labelled litter was estimated using mass loss data from a litterbag experiment set up at the field site. An initial and rapid release of soluble N from the decomposing litter was balanced by the incorporation of exogenous N into the litter. Fungal N accounted for approximately 35% of the N incorporation. Over 2 years, litter N was continuously released and rates of N and mass loss were equivalent, while litter N was preferentially lost during the 3rd year. Released ¹⁵N accumulated essentially at the soil surface. ¹⁵N from the decomposing litter was rapidly (i.e. in 6 months) detected in roots and beech leaves and its level increased regularly and linearly over the course of the labelling experiment. After 3 years, about 2% of the original litter N had accumulated in the trees. ¹⁵N budgets indicated that soluble N was the main source for soil microbial biomass. Nitrogen accumulated in storage compounds was the main source of leaf N, while soil organic N was the main source of mycorrhizal N. Use of ¹⁵N-labelled beech litter as decomposing substrate allowed assessment of the fate of litter N in the soil and tree N pools in a beech forest on different time scales. Received: 3 May 1999 / Accepted: 3 January 2000     

Microbial Colonization of Beech and Spruce Litter—Influence of Decomposition Site and Plant Litter Species on the Diversity of Microbial Community

The present study was conducted to investigate the effect of decomposition site and plant litter species on the colonizing microbial communities. For this, litter bag technique using beech and spruce litter was combined with RNA-based fingerprinting and cloning. Litter bags were incubated for 2 and 8 weeks in the A_h horizon of beech and beech–spruce mixed forest sites. Although sugars and starch were rapidly lost, lignin content increased by more than 40% for beech and more than doubled for spruce litter at both soil sites at the end of the experiment. Denaturing gradient gel electrophoresis analysis of 16S and 18S rRNA RT–PCR products was used for screening of differences between bacterial and fungal communities colonizing the two litter types. Development of the microbial community over time was observed to be specific for each litter type and decomposition site. RT–PCR products from both litter types incubated in beech–spruce mixed forest site were also cloned to identify the bacterial and fungal colonizers. The 16S rRNA clone libraries of beech litter were dominated by γ-proteobacterial members, whereas spruce libraries were mainly composed of α-, β-, and γ-proteobacterial members. Ascomycota members dominated the 18S rRNA clone libraries. Clones similar to Zygomycota were absent from spruce, whereas those similar to Basidiomycota and Glomeromycota were absent from beech libraries. Selective effects of litter quality were observed after 8 weeks. The study provides an insight into the bacterial and fungal communities colonizing beech and spruce litter, and the importance of litter quality and decomposition site as key factors in their development and succession.     

Regional Contingencies in the Relationship between Aboveground Biomass and Litter in the World’s Grasslands

Based on regional-scale studies, aboveground production and litter decomposition are thought to positively covary, because they are driven by shared biotic and climatic factors. Until now we have been unable to test whether production and decomposition are generally coupled across climatically dissimilar regions, because we lacked replicated data collected within a single vegetation type across multiple regions, obfuscating the drivers and generality of the association between production and decomposition. Furthermore, our understanding of the relationships between production and decomposition rests heavily on separate meta-analyses of each response, because no studies have simultaneously measured production and the accumulation or decomposition of litter using consistent methods at globally relevant scales. Here, we use a multi-country grassland dataset collected using a standardized protocol to show that live plant biomass (an estimate of aboveground net primary production) and litter disappearance (represented by mass loss of aboveground litter) do not strongly covary. Live biomass and litter disappearance varied at different spatial scales. There was substantial variation in live biomass among continents, sites and plots whereas among continent differences accounted for most of the variation in litter disappearance rates. Although there were strong associations among aboveground biomass, litter disappearance and climatic factors in some regions (e.g. U.S. Great Plains), these relationships were inconsistent within and among the regions represented by this study. These results highlight the importance of replication among regions and continents when characterizing the correlations between ecosystem processes and interpreting their global-scale implications for carbon flux. We must exercise caution in parameterizing litter decomposition and aboveground production in future regional and global carbon models as their relationship is complex.     

Evaluating litter decomposition in earth system models with long‐term litterbag experiments: an example using the Community Land Model version 4 (CLM4)

Decomposition is a large term in the global carbon budget, but models of the earth system that simulate carbon cycle‐climate feedbacks are largely untested with respect to litter decomposition. We tested the litter decomposition parameterization of the community land model version 4 (CLM4), the terrestrial component of the community earth system model, with data from the long‐term intersite decomposition experiment team (LIDET). The LIDET dataset is a 10‐year study of litter decomposition at multiple sites across North America and Central America. We performed 10‐year litter decomposition simulations comparable with LIDET for 9 litter types and 20 sites in tundra, grassland, and boreal, conifer, deciduous, and tropical forest biomes using the LIDET‐provided climatic decomposition index to constrain temperature and moisture effects on decomposition. We performed additional simulations with DAYCENT, a version of the CENTURY model, to ask how well an established ecosystem model matches the observations. The results show large discrepancy between the laboratory microcosm studies used to parameterize the CLM4 litter decomposition and the LIDET field study. Simulated carbon loss is more rapid than the observations across all sites, and nitrogen immobilization is biased high. Closer agreement with the observations requires much lower decomposition rates, obtained with the assumption that soil mineral nitrogen severely limits decomposition. DAYCENT better replicates the observations, for both carbon mass remaining and nitrogen, independent of nitrogen limitation. CLM4 has low soil carbon in global earth system simulations. These results suggest that this bias arises, in part, from too rapid litter decomposition. More broadly, the terrestrial biogeochemistry of earth system models must be critically tested with observations, and the consequences of particular model choices must be documented. Long‐term litter decomposition experiments such as LIDET provide a real‐world process‐oriented benchmark to evaluate models.     

Effects of Calcium on the Rate and Extent of Litter Decomposition in a Northern Hardwood Forest

Cross-site syntheses of litter decomposition studies have shown that litter calcium (Ca) concentration may have a role in controlling the extent of decomposition of tree foliage. We used an ongoing watershed CaSiO₃ addition experiment at the Hubbard Brook Experimental Forest in New Hampshire, USA, to test the hypotheses that increased Ca in litter would have no effect on the initial rates of litter decay but would increase the extent or completeness (limit value) of foliar litter decomposition. We tested these hypotheses with a 6-year litter decomposition experiment using foliar litter of four tree species that are prominent at this site and in the Northern Hardwood forest type of North America: sugar maple (Acer saccharum Marsh), American beech (Fagus grandifolia Ehrh.), yellow birch (Betula alleghaniensis Britt.), and white ash (Fraxinus americana L.). The experiment used a reciprocal transplant design with the Ca-treated watershed and a control site providing two sources of litter and two placement sites. The litter from the Ca-treated site was 10–92% higher in Ca concentration, depending on species, than the litter from the control site. After about 3 years of decomposition, the Ca concentrations in the litter reflected the placement of the litter (that is, the site in which it was incubated) rather than the source of the litter. The source of the litter had no significant effect on measures of initial decomposition rate, cumulative mass loss (6 years), or limit value. However, the placement of the litter had a highly significant effect on extent of decomposition. Some litter types responded more than others; in particular, beech litter placed in the Ca-treated site had a significantly higher limit value, indicating more complete decomposition, and maple litter in the Ca-treated site had a marginally higher limit value. These results indicate that Ca may influence the extent of litter decomposition, but it is the Ca at the incubation site rather than the initial litter Ca that matters most. The results also suggest that loss of Ca from the soil due to decades of acid deposition at this site may have impeded late-stage litter decomposition, possibly leading to greater soil C storage, especially in forest stands with a substantial component of beech. Likewise, de-acidification may lead to a reduction in soil C.     

Litter decomposition rates in Canadian forests

The effect of litter quality and climate on the rate of decomposition of plant tissues was examined by the measurement of mass remaining after 3 years’ exposure of 11 litter types placed at 18 forest sites across Canada. Amongst sites, mass remaining was strongly related to mean annual temperature and precipitation and amongst litter types the ratio of Klason lignin to nitrogen in the initial tissue was the most important litter quality variable. When combined into a multiple regression, mean annual temperature, mean annual precipitation and Klason lignin:nitrogen ratio explained 73% of the variance in mass remaining for all sites and tissues. Using three doubled CO₂ GCM climate change scenarios for four Canadian regions, these relationships were used to predict increases in decomposition rate of 4–7% of contemporary rates (based on mass remaining after 3 years), because of increased temperature and precipitation. This increase may be partially offset by evidence that plants growing under elevated atmospheric CO₂ concentrations produce litter with high lignin:nitrogen ratios which slows the rate of decomposition, but this change will be small compared to the increased rate of decomposition derived from climatic changes.     

Biological decomposition efficiency in different woodland soils

The decomposition (meaning disappearance) of different leaf types and artificial leaves made from cellulose hydrate foil was studied in three forests — an alluvial forest (Ulmetum), a beech forest on limestone soil (Melico-Fagetum), and a spruce forest in soil overlying limestone bedrock. Fine, medium, and coarse mesh litter bags of special design were used to investigate the roles of abiotic factors, microorganisms, and meso- and macrofauna in effecting decomposition in the three habitats. Additionally, the experimental design was carefully arranged so as to provide information about the effects on decomposition processes of the duration of exposure and the date or moment of exposure.

1. Exposure of litter samples oor 12 months showed:
2. Litter enclosed in fine mesh bags decomposed to some 40–44% of the initial amount placed in each of the three forests. Most of this decomposition can be attributed to abiotic factors and microoganisms.
3. Litter placed in medium mesh litter bags reduced by ca. 60% in alluvial forest, ca. 50% in beech forest and ca. 44% in spruce forest.
4. Litter enclosed in coarse mesh litter bags was reduced by 71% of the initial weights exposed in alluvial and beech forests; in the spruce forest decomposition was no greater than observed with fine and medium mesh litter bags. Clearly, in spruce forest the macrofauna has little or no part to play in effecting decomposition.
5. Sequential month by month exposure of hazel leaves and cellulose hydrate foil in coarse mesh litter bags in all three forests showed that one month of exposure led to only slight material losses, they did occur smallest between March and May, and largest between June and October/November.
6. Coarse mesh litter bags containing either hazel or artificial leaves of cellulose hydrate foil were exposed to natural decomposition processes in December 1977 and subsampled monthly over a period of one year, this series constituted the From-sequence of experiments. Each of the From-sequence samples removed was immediately replaced by a fresh litter bag which was left in place until December 1978, this series constituted the To-sequence of experiments. The results arising from the designated From- and To-sequences showed:
7. During the course of one year hazel leaves decomposed completely in alluvial forest, almost completely in beech forest but to only 50% of the initial value in spruce forest.
8. Duration of exposure and not the date of exposure is the major controlling influence on decomposition in alluvial forest, a characteristic reflected in the mirror-image courses of the From- and To-sequences curves with respect to the abscissa or time axis. Conversely the date of exposure and not the duration of exposure is the major controlling influence on decomposition in the spruce forest, a characteristic reflected in the mirror-image courses of the From-and To-sequences with respect to the ordinate or axis of percentage decomposition.
9. Leaf powder amendment increased the decomposition rate of the hazel and cellulose hydrate leaves in the spruce forest but had no significant effect on their decomposition rate in alluvial and beech forests. It is concluded from this, and other evidence, that litter amendment by leaf fragments of phytophage frass in sites of low biological decomposition activity (eg. spruce) enhances decomposition processes.
10. The time course of hazel leaf decomposition in both alluvial and beech forest is sigmoidal. Three s-phases are distinguished and correspond to the activity of microflora/microfauna, mesofauna/macrofauna, and then microflora/microfauna again. In general, the sigmoidal pattern of the curve can be considered valid for all decomposition processes occurring in terrestrial situations. It is contended that no decomposition (=disappearance) curve actually follows an e-type exponential function. A logarithmic linear regression can be constructed from the sigmoid curve data and although this facilitates inter-system comparisons it does not clearly express the dynamics of decomposition.
11. The course of the curve constructed from information about the standard deviations of means derived from the From- and To-sequence data does reflect the dynamics of litter decomposition. The three s-phases can be recognised and by comparing the actual From-sequence deviation curve with a mirror inversion representation of the To-sequence curve it is possible to determine whether decomposition is primarily controlled by the duration of exposure or the date of exposure. As is the case for hazel leaf decomposition in beech forest intermediate conditions can be readily recognised.

     

Diversity and Decomposing Ability of Saprophytic Fungi from Temperate Forest Litter

This study was designed to examine saprophytic fungi diversity under different tree species situated in the same ecological context. Further, the link between the diversity and decomposition rate of two broadleaved, two coniferous and two mixed broadleaved-coniferous litter types was targeted. Litter material was decomposed in litter bags for 4 and 24 months to target both early and late stages of the decomposition. Fungal diversity of L and F layers were also investigated as a parallel to the litter bag method. Temperature gradient gel electrophoresis fingerprinting was used to assess fungal diversity in the samples. Mass loss values and organic and nutrient composition of the litter were also measured. The results showed that the species richness was not strongly affected by the change of the tree species. Nevertheless, the community compositions differed within tree species and decomposition stages. The most important shift was found in the mixed litters from the litter bag treatment for both variables. Both mixed litters displayed the highest species richness (13.3 species both) and the most different community composition as compared to pure litters (6.3–10.7 species) after 24 months. The mass loss after 24 months was similar or greater in the mixed litter (70.5% beech–spruce, 76.2% oak–Douglas-fir litter) than in both original pure litter types. This was probably due to higher niche variability and to the synergistic effect of nutrient transfer between litter types. Concerning pure litter, mass loss values were the highest in oak and beech litter (72.8% and 69.8%) compared to spruce and D. fir (59.4% and 66.5%, respectively). That was probably caused by a more favourable microclimate and litter composition in broadleaved than in coniferous plantations. These variables also seemed to be more important to pure litter decomposition rates than were fungal species richness or community structure.     

Aboveground decomposition dynamics in riparian depression and slope wetlands of central Pennsylvania

We examined two types of groundwater-fed wetlands (riparian depressions and slopes) classified using the hydrogeomorphic (HGM) system. These wetland types had previously been shown to differ hydrologically. Our first objective was to determine if HGM was a useful structuring variable when examining aboveground decomposition dynamics (rate of mass loss and rate of nitrogen loss). Our second objective was to determine what soil variables were related to any differences in aboveground decomposition dynamics we might find regardless of HGM subclass. We used the litterbag field bioassay technique, and employed a standard litter type (Phalaris arundinacea) across all wetlands. Our results indicated that HGM would not readily serve as an adequate structuring variable for aboveground decomposition in riparian depressions and slope wetlands of central Pennsylvania. Discriminant analysis and classification and regression tree (CART) modeling found soil cation exchange capacity, soil pH, soil organic matter, and soil % nitrogen to be potentially important soil variables related to mass loss, and soil % nitrogen and soil pH to be potentially important variables related to nitrogen loss rate.     

沈阳城市和城郊油松凋落叶的分解动态

为了检测分解地点和凋落物类型对分解的影响,采用交互分解实验分析了油松凋落叶在城市林分和城郊林分中分解时不同立地条件(城市和城郊)以及不同油松凋落叶类型(城市油松凋落物和城郊油松凋落物)对凋落物的分解速率以及N、P残留率的相对作用。结果表明:不同的立地条件对油松凋落叶分解速率有显著影响(P<0.05):城郊林分中的油松凋落叶比城市林分中分解快,说明外在环境条件对分解的影响显著;在同一林分里,凋落物类型对凋落叶分解速率也有显著影响(P<0.05):城市凋落物分解快于城郊凋落物,说明凋落物内在特性对分解的影响显著。分解地点对于N、P残留率没有显著的影响,然而凋落物类型对N、P残留率有显著影响:城郊凋落物由于具有较高的C/N以及C/P初始值,比城市凋落物更容易富集N、P。     

Foliage litter turnover and earthworm populations in three beech forests of contrasting soil and vegetation types

Summary Leaf litter decomposition, levels of accumulated litter as well as the abundance and biomass of earthworms were measured in three mature beech forests in southern Sweden: one mor site, one poor mull site, and one rich mull site. The disappearance rate of beech litter, measured with litter bags, increased with increasing soil fertility. On the rich mull site, the disappearance rate was much higher than in the two other forests, due to the combined effects of higher earthworm activity, more favouable soil moisture conditions, and higher litter quality. Incubating the litter in finely meshed bags (1-mm mesh) to exclude macrofauna had a great effect on litter mass loss in the rich mull site, but it had only a minor effect in the other sites. Simultaneous incubations of local and transplanted leaf litter on the three study sites showed that the substrate quality of the litter increased in the order: mor site — poor mull site — rich mull site. Lignin, N, and P concentrations of the leaf litter failed to explain the observed differences in decomposition rates, and acid/base properties are suggested to be more important. Earthworm numbers per m² were 2.5 (1 species) in the mor, 40 (6 species) in the poor mull and 220 (9 species) in the rich mull forest. Soil chemical conditions, notably pH, were suggested as the main factors determining the inter-site differences in abundance and species composition of earthworms. The role of litter decomposition and earthworm activity in the accumulation of organic matter in the forest floor in different types of beech woodlands are discussed.     

Decomposition and nitrogen dynamics of <Emphasis Type="Italic">Rhynchospora asperula</Emphasis> in floating soils of Esteros del Iberá, Argentina

In floating soils, organic matter accumulation is the result of the imbalance between decomposition rate and macrophytes’ production, and it can limit nutrient availability. In this study, we determined the percentage of litter that is added to the floating soil in one year and the nitrogen dynamics of Rhynchospora asperula (Nees) Steud (Cyperaceae), an abundant species in Esteros del Iberá, a South American wetland with extended areas of floating soils. According to the decomposition rate determined (k = 0.0032 day⁻¹), the annual percentage of mass lost was 69%. Conditions of the floating soil were simulated in a 146-day field experiment. The results show that the decomposition rate was higher when the litter was in water contact, and the mass loss in the field sampling at the beginning of the decomposition was similar to that of the treatments that simulated this condition. The nitrogen concentration in the aboveground biomass was almost constant, and the results indicate that there was translocation from the senescent leaves, but not a preferential nitrogen translocation from the rhizomes and roots. During summer the maximum biomass and the low nitrogen concentration in the floating soil coincide, but the nitrogen intake by the aboveground biomass was only 4% of the total nitrogen content of the floating soil. Nitrogen concentration in the litter increased and, though immobilization cannot be ruled out, there was net mineralization. The nitrogen mineralized in the first decomposition year was 30% of the nitrogen added to aboveground biomass during the study period.     

Do soil organisms affect aboveground litter decomposition in the semiarid Patagonian steppe, Argentina?

Surface litter decomposition in arid and semiarid ecosystems is often faster than predicted by climatic parameters such as annual precipitation or evapotranspiration, or based on standard indices of litter quality such as lignin or nitrogen concentrations. Abiotic photodegradation has been demonstrated to be an important factor controlling aboveground litter decomposition in aridland ecosystems, but soil fauna, particularly macrofauna such as termites and ants, have also been identified as key players affecting litter mass loss in warm deserts. Our objective was to quantify the importance of soil organisms on surface litter decomposition in the Patagonian steppe in the absence of photodegradative effects, to establish the relative importance of soil organisms on rates of mass loss and nitrogen release. We estimated the relative contribution of soil fauna and microbes to litter decomposition of a dominant grass using litterboxes with variable mesh sizes that excluded groups of soil fauna based on size class (10, 2, and 0.01 mm), which were placed beneath shrub canopies. We also employed chemical repellents (naphthalene and fungicide). The exclusion of macro- and mesofauna had no effect on litter mass loss over 3 years (P = 0.36), as litter decomposition was similar in all soil fauna exclusions and naphthalene-treated litter. In contrast, reduction of fungal activity significantly inhibited litter decomposition (P < 0.001). Although soil fauna have been mentioned as a key control of litter decomposition in warm deserts, biogeographic legacies and temperature limitation may constrain the importance of these organisms in temperate aridlands, particularly in the southern hemisphere.     

Long-term dynamics of pine and hardwood litter in contrasting environments: toward a global model of decomposition

We analysed data on mass loss after five years of decomposition in the field from both fine root and leaf litters from two highly contrasting trees, Drypetes glauca, a tropical hardwood tree from Puerto Rico, and pine species from North America as part of the Long‐Term Intersite Decomposition Experiment (LIDET). LIDET is a reciprocal litterbag study involving the transplanting of litter from 27 species across 28 sites in North and Central America reflecting a wide variety of natural and managed ecosystems and climates, from Arctic tundra to tropical rainforest. After 5 years, estimated k‐values ranged from 0.032 to 3.734, lengths of Phase I (to 20% mass remaining) from 0.49 to 47.92 years, and fractional mass remaining from 0 to 0.81. Pine litter decomposed more slowly than Drypetes litter, supporting the notion of strong control of substrate quality over decomposition rates. Climate exerted strong and consistent effects on decomposition. Neither mean annual temperature or precipitation alone explained the global pattern of decomposition; variables including both moisture availability and temperature (i.e. actual evapotranspiration and DEFAC from the CENTURY model) were generally more robust than single variables. Across the LIDET range, decomposition of fine roots exhibited a Q₁₀ of 2 and was more predictable than that of leaves, which had a higher Q₁₀ and greater variability. Roots generally decomposed more slowly than leaves, regardless of genus, but the ratio of above‐ to belowground decomposition rates differed sharply across ecosystem types. Finally, Drypetes litter decomposed much more rapidly than pine litter in ‘broadleaved habitats’ than in ‘conifer habitats’, evidence for a ‘home‐field advantage’ for this litter. These results collectively suggest that relatively simple models can predict decomposition based on litter quality and regional climate, but that ecosystem‐specific problems may add complications.     

Simple three‐pool model accurately describes patterns of long‐term litter decomposition in diverse climates

As atmospheric CO₂ increases, ecosystem carbon sequestration will largely depend on how global changes in climate will alter the balance between net primary production and decomposition. The response of primary production to climatic change has been examined using well‐validated mechanistic models, but the same is not true for decomposition, a primary source of atmospheric CO₂. We used the Long‐term Intersite Decomposition Experiment Team (LIDET) dataset and model‐selection techniques to choose and parameterize a model that describes global patterns of litter decomposition. Mass loss was best represented by a three‐pool negative exponential model, with a rapidly decomposing labile pool, an intermediate pool representing cellulose, and a recalcitrant pool. The initial litter lignin/nitrogen ratio defined the size of labile and intermediate pools. Lignin content determined the size of the recalcitrant pool. The decomposition rate of all pools was modified by climate, but the intermediate pool's decomposition rate was also controlled by relative amounts of litter cellulose and lignin (indicative of lignin‐encrusted cellulose). The effect of climate on decomposition was best represented by a composite variable that multiplied a water‐stress function by the Lloyd and Taylor variable Q₁₀ temperature function. Although our model explained nearly 70% of the variation in LIDET data, we observed systematic deviations from model predictions. Below‐ and aboveground material decomposed at notably different rates, depending on the decomposition stage. Decomposition in certain ecosystem‐specific environmental conditions was not well represented by our model; this included roots in very wet and cold soils, and aboveground litter in N‐rich and arid sites. Despite these limitations, our model may still be extremely useful for global modeling efforts, because it accurately (R²=0.6804) described general patterns of long‐term global decomposition for a wide array of litter types, using relatively minimal climatic and litter quality data.     

Isopod effects on decomposition of litter produced under elevated CO2, N deposition and different soil types

The performance of Oniscus asellus (Isopoda) and its influence on litter mass loss and mineralization was assessed in a microcosm experiment, using beech (Fagus sylvatica) leaf litter that was produced on different soil types, contrasting atmospheric CO₂ concentrations, and different nitrogen deposition rates. Litter quality was significantly altered by these treatments, and many of the CO₂ and N effects differed between soil types. Litter quality affected subsequent litter mass loss rates, microbial respiration, and leaching of dissolved organic carbon (DOC) and nitrate. These effects were largely independent of the presence of isopods, even though isopods highly accelerated litter mass loss, stimulated microbial respiration by 37%, and increased nitrate leaching by 50%. Isopods did not change their relative rates of litter consumption and growth in response to litter quality. Isopod mortality, however, increased with increasing litter lignin/N ratios, and was significantly different between soil types, which may indicate long‐term effects on litter decomposition through altered isopod densities. Having the choice among the litter of three different species [maple (Acer pseudoplatanus), beech (Fagus sylvatica) and oak (Quercus robur)] grown at either ambient or elevated CO₂, isopods preferred maple to beech when all the litter was produced under elevated CO₂. This suggests that beyond changes in consumption, an altered food selection by isopods in a CO₂‐enriched atmosphere may affect the temporal and spatial composition of the litter layer in temperate forests. In contrast to previous findings, we conclude that isopods do not always increase their consumption rates, and hence do not differentially affect microbial decomposition in litter of poorer quality. Nevertheless changes in animal densities and/or shifts in their food preferences, could result in altered decomposition and carbon and nutrient turnover rates.     

Changes in the decomposer community when converting spruce monocultures to mixed spruce/beech stands

The impact of the admixture of beech in spruce monocultures on structure and function of the decomposer community was studies in the Ore mountains (Saxony, Germany) on Dystric Cambisols between 2000 and 2002. The study sites represented four stages of forest conversion from a mature spruce stand to a mature spruce/beech stand. There, the functional profile of the nematodes, enchytraeids, lumbricids, and dipterans was analysed on the basis of ecological guilds, and their metabolic equivalences were calculated to characterize the decomposition potential of the invertebrate decomposer community. Because of the acidic parent soil the coenoses at all study sites were dominated by the enchytraeids with increasing importance of lumbricids and dipterans in progress of forest conversion. Gradual changes with rising coverage of beech culminated in intense differences of entire biomasses and metabolic equivalences between the mature stands, indicating a higher decomposition potential of the invertebrate decomposer community by the admixture of beeches in spruce forests. The quality of the beech litter is likely to be the important factors for these changes. To prove this assumption further investigations of the saprovore food chain are necessary, taking microbial parameters into account.     

Fate of organic carbon during decomposition of different litter types in Japan

Carbon dynamics during litter decomposition have been described in a variety of forest ecosystems and provided insights into carbon flow in soils. To quantitatively assess how decomposition processes vary between litter types, solid-state ¹³C cross-polarization and magic-angle spinning nuclear magnetic resonance (CPMAS NMR) technique was applied to analyze conifer (cedar, cypress) and hardwood (chinquapin, beech, oak, birch) litter which had degraded during a 3 year litterbag experiment throughout Japan. The results were used to identify compositional changes and estimate decomposition constants (k values) in exponential equations. Total litter and carbon type mass losses during decomposition varied significantly between litter types, being affected by the initial physicochemical litter quality. Concomitant increases and decreases in carbonyl and O/N-alkyl C compositions, respectively, were observed for all litter types, but aromatic and aliphatic C dynamics were less consistent. In hardwoods, [aromatic/aliphatic C ratio] was generally stable during decomposition, suggesting that, in hardwoods, the decomposabilities of aromatic and aliphatic C were similar. In the conifers, an increasing [aromatic/aliphatic C ratio] during decomposition suggested that aromatic C was more recalcitrant than aliphatic C. These results suggest that different decomposition processes between litter types might be related to different aromatic and aliphatic C behaviors, as affected by lignin stability and lipid leachability and biosynthesis. Variations in the k values for total litter and carbon types were not obvious between litter types, although the mass loss patterns differed significantly. The k values estimated in this study may contribute to predictions of soil carbon dynamics and the validation of carbon compartment models in forest ecosystems.