Does variability in litter quality determine soil microbial respiration in an Amazonian rainforest?

Tree species-rich tropical rainforests are characterized by a highly variable quality of leaf litter input to the soil at small spatial scales. This diverse plant litter is a major source of energy and nutrients for soil microorganisms, particularly in rainforests developed on old and nutrient-impoverished soils. Here we tested the hypothesis that the variability in leaf litter quality produced by a highly diverse tree community determines the spatial variability of the microbial respiration process in the underlying soil. We analyzed a total of 225 litter-soil pairs from an undisturbed Amazonian rainforest in French Guiana using a hierarchical sampling design. The microbial respiration process was assessed using substrate-induced respiration (SIR) and compared to a wide range of quality parameters of the associated litter layer (litter nutrients, carbon forms, stoichiometry, litter mass and pH). The results show that the variability of both litter quality and SIR rates was more important at large than at small scales. SIR rates varied between 1.1 and 4.0 μg g⁻¹ h⁻¹ and were significantly correlated with litter layer quality (up to 50% of the variability explained by the best mixed linear model). Total litter P content was the individual most important factor explaining the observed spatial variation in soil SIR, with higher rates associated to high litter P. SIR rates also correlated positively with total litter N content and with increasing proportions of labile C compounds. However, contrary to our expectation, SIR rates were not related to litter stoichiometry. These data suggest that in the studied Amazonian rainforest, tree canopy composition is an important driver of the microbial respiration process via leaf litter fall, resulting in potentially strong plant-soil feedbacks.     

Roots from beech (Fagus sylvatica L.) and ash (Fraxinus excelsior L.) differentially affect soil microorganisms and carbon dynamics

Knowledge about the influence of living roots on decomposition processes in soil is scarce but is needed to understand carbon dynamics in soil. We investigated the effect of dominant deciduous tree species of the Central European forest vegetation, European beech (Fagus sylvatica L.) and European ash (Fraxinus excelsior L.), on soil biota and carbon dynamics differentiating between root- and leaf litter-mediated effects. The influence of beech and ash seedlings on carbon and nitrogen flow was investigated using leaf litter enriched in ¹³C and ¹⁵N in double split-root rhizotrons planted with beech and ash seedlings as well as a mixture of both tree species and a control without plants. Stable isotope and compound-specific fatty acid analysis (¹³C-PLFA) were used to follow the incorporation of stable isotopes into microorganisms, soil animals and plants. Further, the bacterial community composition was analyzed using pyrosequencing of 16S rRNA gene amplicons. Although beech root biomass was significantly lower than that of ash only beech significantly decreased soil carbon and nitrogen concentrations after 475 days of incubation. In addition, beech significantly decreased microbial carbon use efficiency as indicated by higher specific respiration. Low soil pH probably increased specific respiration of bacteria suggesting that rhizodeposits of beech roots induced increased microbial respiration and therefore carbon loss from soil. Compared to beech δ¹³C and δ¹⁵N signatures of gamasid mites in ash rhizotrons were significantly higher indicating higher amounts of litter-derived carbon and nitrogen to reach higher trophic levels. Similar δ¹³C signatures of bacteria and fine roots indicate that mainly bacteria incorporated root-derived carbon in beech rhizotrons. The results suggest that beech and ash differentially impact soil processes with beech more strongly affecting the belowground system via root exudates and associated changes in rhizosphere microorganisms and carbon dynamics than ash.     

Succession of collembolan communities during decomposition of leaf and root litter: Effects of litter type and position

Little is known about the collembolan community involved in the decomposition of fine root (≤2.0 mm in diameter) litter, which is largely different from leaves in both litter quality and position. The collembolan communities involved in root and leaf litter decomposition were compared in a litterbag experiment in a coniferous forest of Chamaecyparis obtusa. A two-factor experiment (litter type × litter position) was conducted to evaluate the relative effects of litter quality and position. Litterbags of roots and leaves were each placed at two positions (on the soil surface and in the soil), and were collected at seven different times over three years. Abundance and biomass of Collembola involved in root decomposition in the soil were higher than those involved in leaf decomposition on the soil surface, and the collembolan community composition largely differed between these two types of litterbag. Differences between root and leaf decomposition were mainly caused by litter position, but effects of litter type were also detected at species-level. Species that preferred roots were abundant at an early stage of litter decomposition in the soil. Because the early stage of decomposition in the soil is naturally achieved only by root litter initially deposited in the soil, root litter may function as an essential resource for certain species. The results of this study indicate that root litter contributes to collembolan community organization as a spatially and qualitatively different resource than leaf litter. This also suggests that root litter is decomposed via different soil faunal processes than leaf litter.     

Plant litter quality influences the contribution of soil fauna to litter decomposition in humid tropical forests, southwestern China

The aim of this field experiment was to quantify the contribution of soil fauna to plant litter decomposition in three forest sites differing in C/N ratio under natural conditions in Xishuangbanna, southwestern China. We conducted a survey of soil fauna communities, the forest floor litter and investigated mass loss of mixed tree species leaf litter for two years in a tropical secondary forest, an evergreen broad-leaf forest and a tropical rain forest. Exclusion treatments of different sized soil fauna from the leaf litter by using varying mesh size litter bags (2 mm and 0.15 mm) were also performed. Mass loss and C and N concentrations in litter bag leaf materials were determined at monthly intervals. We found that: (1) the three forests differed in floor litter biomass and nutrient contents but not in soil fauna richness and abundance; (2) litter mass loss and decomposition rate were slower when soil macrofauna and most of mesofauna were excluded; and (3) greatest soil fauna contribution to plant litter decomposition occurred in the rain forest, where leaf litter C/N ratio was also highest (41.5% contribution: 54.8 C/N ratio), in comparison to 8.69% in the broad-leaf forest and 19.52% in the secondary forest, both with low leaf litter C/N ratios (<32). Our results suggested that, soil fauna played a more pronounced role in the decomposition of mixed leaf litter in tropical rain forest, and significantly bigger effects from fauna were ascribed to the enhancement of N concentration and decrease of C concentration of the initially high C/N ratio litter in this forest site.     

Microbial properties and soil respiration in submontane forests of Venezuelian Guyana: characteristics and response to fertilizer treatments

The distribution of vegetation types in Venezuelan Guyana (in the ‘Canaima’ National Park) represents a transitional stage in a long term process of savannization, a process considered to be conditioned by a combined chemical and intermittent drought stress. All types of woody vegetation in this environment accumulate large amounts of litter and soil organic carbon (SOC). We hypothesized that this accumulation is caused by low microbial activity. During 1 year we measured microbial biomass carbon (Cmic), microbial respiration and soil respiration of stony Oxisols (Acrohumox) at a tall, a medium and a low forest and with three chemical modifications of site conditions by the addition of NO₃⁻, Ca²⁺ and PO₄³⁻ as possible limiting elements. Due to high SOC contents, mean Cmic was 1 mg g soil⁻¹ in the mineral topsoil and 3 mg g soil⁻¹ in the forest floor. Mean microbial respiration in the mineral topsoil and the forest floor were 165 and 192 μg CO₂-C g soil⁻¹ d⁻¹, respectively. We calculated high mean metabolic quotients (qCO₂) of 200 mg CO₂-C g Cmic⁻¹ d⁻¹ in the litter layer and 166 mg CO₂-C g Cmic⁻¹ d⁻¹ in the mineral topsoil, while the Cmic-to-SOC ratios were as low as 1.0% in the litter layer and 0.8% in the mineral topsoil. Annual soil respiration was 9, 12 and 10 Mg CO₂-C ha⁻¹ yr⁻¹ in the tall, medium and low forest, respectively. CO₂ production was significantly increased by CaHPO₄ fertilization, but no consistent effects were caused by Ca²⁺ and NO₃⁻, fertilization. Our findings indicate that Cmic and microbial respiration are reduced by low nutrient concentrations and low litter and SOC quality. Reduced microbial decomposition may have contributed to SOC accumulation in these forests.     

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.     

Nutrient limitation of litter decomposition with long-term secondary succession: evidence from controlled laboratory experiments

Purpose

Understanding ecosystem processes such as litter decomposition in response to dramatic land-use change is critical for modeling and predicting carbon (C) cycles. However, the patterns of litter decomposition along with long-term secondary succession (over 100 years) are not well reported, especially concerning nutrient limitations on litter decomposition.

Materials and methods

To clarify the response of litter decomposition to changes in soil nutrient availability, we conducted four incubation experiments involving soil and litter and nutrient addition from different successional stages and investigated the changes in microbial respiration and litter mass loss.

Results and discussion

Our results revealed that microbial respiration increased with succession without any litter addition (1.19~1.73 mg C g^?1 soil), and litter addition significantly promoted microbial respiration (16.5~72.9%), especially in the early successional stage (grassland and shrubland). The decomposition rate of the same litter decreased with succession. In addition, nitrogen (N) and phosphorus (P) addition showed significant effects on litter decomposition and microbial respiration; P addition promoted litter decomposition (2.4~15.3%) and microbial respiration (10.1~34.5%) in all successional stages, while N addition promoted litter decomposition (4.0~10.3%) and microbial respiration (5.4~27.2%) in all except the last stage of succession, which showed a negative effect on litter decomposition (??7.5%) and microbial respiration (??6.1%), indicating possible N saturation of litter decomposition and microbial respiration.

Conclusions

This work highlights that soil nutrient availability and successional stages need to be taken into account to predict the changes to litter decomposition in response to global changes.

     

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.     

Dominant effects of litter substrate quality on the difference between leaf and root decomposition process above- and belowground

Initial decomposition rates, changes in organic chemical components (acid-insoluble fraction, holocellulose, polyphenols, soluble carbohydrates) and nutrient dynamics (K, Mg, Ca, P, N) were examined for fine roots and leaves of Japanese cypress (Chamaecyparis obtusa). Litterbag experiments designed to evaluate the relative effects of litter type and position of litter supply in the soil were carried out, considering that root and leaf litter typically occupy different locations and have different substrate qualities. Litterbags of roots and leaves were placed at two positions (on the soil surface and in the humus layer), and collected every 3 months over one year. The mass loss rate and N release were slower during root decomposition in the humus layer than during leaf decomposition on the soil surface. These differences between root and leaf decomposition were mainly caused by the litter type, and the effect of the position on decomposition was relatively small. Root litter was less influenced by position related effects, such as differences in humidity, than leaf litter, and this recalcitrant trait to environmental effects may be responsible for the slower mass loss rate and N release in root decomposition. The results of the present study suggest that fine roots are persistent in the soil and serve an important role in N retention in forest ecosystems because of their litter substrate quality.     

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.     

Leaf litterfall and decomposition of different above- and belowground parts of birch (Betula ermanii) trees and dwarf bamboo (Sasa kurilensis) shrubs in a young secondary forest in Northern Japan

In many Japanese forests, the forest understory is largely dominated by dwarf bamboo (Sasa) species, which compete with overstory vegetation for soil nutrients. We studied the rate of leaf litterfall, and decomposition and mineralization of carbon (C) and nitrogen (N) from various components (leaf, root, wood, and rhizome) of overstory and understory vegetation in a young Betula ermanii forest from 2002 to 2004. Total litterfall was 377 g m⁻² year⁻¹, of which the overstory vegetation contributed about two thirds. A litter decomposition experiment conducted for 770 days indicated that mass loss of different litter components varied significantly, except for Sasa kurilensis wood and rhizome. Relative decomposition rates were significantly greater in the first growth period (June to October) than the dormant period (November to May) in most cases. Rainfall was the most important abiotic variable, explaining 75–80% of the variability in mass loss rates. Concentrations of ethanol soluble substances and N were significantly positively correlated (r=0.77 to 0.97, P<0.05) with mass loss at an early stage (41 days). The ratios of lignin/N and C/N were found to be negatively correlated with mass loss rates at all stages of litter decomposition. C stock loss was similar to that of mass loss, whereas N stock loss was slower, except for S. kurilensis fine root litter. The evergreen understory species S. kurilensis exhibited greater N use efficiency than B. ermanii, suggesting better competitive ability that might favor the production of a high biomass and invasion under tree species like B. ermanii.     

The role of Armadillidium vulgare (Isopoda: Oniscidea) in litter decomposition and soil organic matter stabilization

We studied the effects of the terrestrial isopod Armadillidium vulgare on organic matter decomposition and stabilization in a long-term (65-week) laboratory experiment. We quantified the microbial activity in leaf litter (Acer pseudoplatanus) which did not come into contact with isopods, in A. vulgare feces produced from the same litter, and in unconsumed leftover of this litter. Freshly fallen leaf litter and up to 3 day old feces and leftover of litter were used. All materials were air dried immediately after collection and rewetted 1 day before use. Simultaneously, we measured how microbial activity in litter and feces are affected by fluctuations in humidity and temperature and by the addition of easily decomposed substances (starch and glucose).Microbial respiration was lower in feces than in litter or unconsumed leaf fragments. At the same time, moisture and temperature fluctuations and addition of glucose or starch increased respiration much more in litter than in feces. The results indicate that the processing of litter by A. vulgare reduces microbial respiration and reduces the sensitivity of microbial respiration to environmental fluctuations. ¹³C NMR spectra from feces indicated preferential loss of polysaccharide-carbon and accumulation of lignin with some modification to the aromatic-carbon. TMAH-Py-GC MS showed that lignin content was higher in feces than in litter and that lignin quality differed between the two substrates. Guaiacyl units were depleted in the feces, which indicated breakdown of guaiacyl associated with gut passage. As a conclusion, the results suggest that this common isopod greatly affects leaf litter decomposition. Decomposition of isopod feces in a long-term experiment is lower than litter decomposition which may support stabilization of organic matter in soil. This is caused mainly due to higher content of aromatic carbon in feces, which may cause its considerable resistance to bacterial degradation.     

Effect of N addition,moisture, and temperature on soil microbial respiration and microbial biomass in forest soil at different stages of litter decomposition

Purpose

The objective of the present study was to investigate the interactive effects of nitrogen (N) addition, temperature, and moisture on soil microbial respiration, microbial biomass, and metabolic quotient (qCO₂) at different decomposition stages of different tree leaf litters.

Materials and methods

A laboratory incubation experiment with and without litter addition was conducted for 80 days at two temperatures (15 and 25 °C), two wetting intensities (35 and 50 % water-filled porosity space (WFPS)) and two doses of N addition (0 and 4.5 g N m^?2, as NH₄NO₃). The tree leaf litters included three types of broadleaf litters, a needle litter, and a mixed litter of them. Soil microbial respiration, microbial biomass, and qCO₂ along with other soil properties were measured at two decomposition stages of tree leaf litters.

Results and discussion

The increase in soil cumulative carbon dioxide (CO₂) flux and microbial biomass during the incubation depended on types of tree leaf litters, N addition, and hydrothermal conditions. Soil microbial biomass carbon (C) and N and qCO₂ were significantly greater in all litter-amended than in non-amended soils. However, the difference in the qCO₂ became smaller during the late period of incubation, especially at 25 °C. The interactive effect of temperature with soil moisture and N addition was significant for affecting the cumulative litter-derived CO₂-C flux at the early and late stages of litter decomposition. Furthermore, the interactive effect of soil moisture and N addition was significant for affecting the cumulative CO₂ flux at the late stage of litter decomposition but not early in the experiment.

Conclusions

This present study indicated that the effects of addition of N and hydrothermal conditions on soil microbial respiration, qCO₂, and concentrations of labile C and N depended on types of tree leaf litters and the development of litter decomposition. The results highlight the importance of N availability and hydrothermal conditions in interactively regulating soil microbial respiration and microbial C utilization during litter decomposition under forest ecosystems.

     

Ectomycorrhizal‐fungal colonization induces physio‐morphological changes in Quercus serrata leaves and roots

To determine how colonization by different ectomycorrhizal (ECM)‐fungal species affects the physiology and morphology of Quercus serrata seedlings, we assessed the net photosynthetic rate, the respiration rate of the lateral roots, leaf and root nitrogen (N) concentrations, specific leaf area, and specific root length in 9‐month‐old Q. serrata seedlings inoculated with Pisolithus tinctorius, Scleroderma citrinum, Laccaria amethystea, and Astraeus hygrometricus. While uninoculated control plants showed no colonization, the percentage of ECM colonization of root tips attained 35% with P. tinctorius and about 86% with the other three ECM species. Similar to ECM root colonization, the photosynthetic as well as the root‐respiration rates were higher in seedlings with S. citrinum, L. amethystea, and A. hygrometricus than that in the control and those with P. tinctorius. Both the photosynthetic and root‐respiration rates were positively correlated with ECM‐fungal colonization. Similar trends were observed for the N concentration, specific leaf area, and specific root length, which differed significantly among ECM‐fungal species and were related with ECM‐fungal colonization. The results suggest that both physiological and morphological traits are specific to ECM‐fungal species. As Q. serrata seedlings with high colonization can exhibit better resource‐acquisition ability, the identification of strongly colonizing ECM‐fungal species is essential. Comparisons with high‐ and low‐colonizing ECM‐fungal species improve our understanding of source–sink relationships in carbon allocation of forest tree species.     

Species effects on earthworm density in tropical tree plantations in Hawaii

Summary Tree species differ in the quantity and quality of litter produced, and these differences may significantly affect ecosystem structure and function. I examined the importance of tree species in determining earthworm densities in replicated stands of Eucalyptus saligna Sm. and Albizia falcataria (L.) Fosberg, and in mixed stands (25% albizia and 75% eucalyptus). Mean earthworm densities ranged from 92 m^-2 in the pure eucalyptus, to 281 m^-2 in the mixture, and a maximum of 469 m^-2 in the pure albizia stands. Only two earthworm species were present, Pontoscolex corethrurus and Amynthas gracilis. Leaf biomass on the forest floor was highest in the pure eucalyptus and lowest in the pure albizia stands, whereas the annual fine litterfall production was lowest in the pure eucalyptus and highest in the albizia stands. The N content of fine litterfall was correlated positively with earthworm density, and the fine litterfall biomass: N ratio was correlated negatively with earthworm density. Greater leaf biomass on the forest floor under eucalyptus stands despite lower rates of litterfall suggests that litter quality, rather than litter quantity, was primarily responsible for the greater earthworm density in the albizia stands. Some biogeochemical effects of tree species in the tropics may be mediated through effects on earthworm populations.     

Effects of tree identity dominate over tree diversity on the soil microbial community structure

This study investigated the possible effects of tree species diversity and identity on the soil microbial community in a species-rich temperate broad-leaved forest. For the first time, we separated the effects of tree identity and tree species diversity on the link between above and belowground communities in a near-natural forest. We established 100 tree clusters consisting of each three tree individuals represented by beech (Fagus sylvatica L.), ash (Fraxinus excelsior L.), hornbeam (Carpinus betulus L.), maple (Acer pseudoplatanus L.), or lime (Tilia spec.) at two different sites in the Hainich National Park (Thuringia, Germany). The tree clusters included one, two or three species forming a diversity gradient. We investigated the microbial community structure, using phospholipid fatty acid (PLFA) profiles, in mineral soil samples (0–10 cm) collected in the centre of each cluster.The lowest total PLFA amounts were found in the pure beech clusters (79.0 ± 23.5 nmol g⁻¹ soil dw), the highest PLFA amounts existed in the pure ash clusters (287.3 ± 211.3 nmol g⁻¹ soil dw). Using principle components analyses (PCA) and redundancy analyses (RDA), we found only for the variables ‘relative proportion of beech trees’ and ‘living lime fine root tips associated with ectomycorrhiza’ a significant effect on the PLFA composition. The microbial community structure was mainly determined by abiotic environmental parameters such as soil pH or clay content. The different species richness levels in the clusters did not significantly differ in their total PLFA amounts and their PLFA composition. We observed a tendency that the PLFA profiles of the microbial communities in more tree species-rich clusters were less influenced by individual PLFAs (more homogenous) than those from species-poor clusters.We concluded that tree species identity and site conditions were more important factors determining the soil microbial community structure than tree species diversity per se.     

Factors causing temporal and spatial variation in heterotrophic and rhizospheric components of soil respiration in afforested organic soil croplands in Finland

Partitioning soil respiration (SR) into its components, heterotrophic and rhizospheric respiration, is an important step for understanding and modelling carbon (C) cycling in organic soils. However, no partitioning studies on afforested organic soil croplands exist. We separated soil respiration originating from the decomposition of peat (SR_P), and aboveground litter (SR_L) and root respiration (SR_R) in six afforested organic soil croplands in Finland with varying tree species and stand ages using the trenching method. Across the sites temporal variation in SR was primarily related to changes in soil surface temperature (?5 cm), which explained 71–96% of variation in SR rates. Decomposition of peat and litter was not related to changes in water table level, whereas a minor increase in root respiration was observed with the increase in water table depth. Temperature sensitivity of SR varied between the different respiration components: SR_P was less sensitive to changes in soil surface temperature than SR_L or SR_R. Factors explaining spatial variation in SR differed between different respiration components. Annual SR_P correlated positively with peat ash content while that of SR_L was found to correlate positively with the amount of litter on the forest floor, separately for each tree species. Root respiration correlated positively with the biomass of ground vegetation. From the total soil respiration peat decomposition comprised a major share of 42%; the proportion of autotrophic respiration being 41% and aboveground litter 17%. Afforestation lowered peat decomposition rates. Nevertheless the effect of agricultural history can be seen in peat properties for decades and due to high peat decomposition rates these soils still loose carbon to the atmosphere.     

Effects of sulfuric,nitric, and mixed acid rain on litter decomposition,soil microbial biomass,and enzyme activities in subtropical forests of China

Acid rain pollution is changing gradually from sulfuric acid rain (SAR) to mixed acid rain (MAR) and then to nitric acid rain (NAR) with the rapidly growing number of motor vehicles. The influences of changed acid rain types on ecosystem functions, particularly on litter decomposition, remain unclear. Two dominant litter types from a coniferous forest and a broad-leaved forest were incubated in microcosms with original forest soils and treated by five types of acid rain with different SO₄²⁻ to NO₃⁻ ratios (1:0, 5:1, 1:1, 1:5, and 0:1). During a six-month incubation period, litter mass losses, soil microbial biomass, and enzyme activities were investigated. Results showed that various acid treatments inhibited litter decomposition, soil microbial biomass, and most enzyme activities, and the inhibitory effects of NAR were more significant than those of SAR and MAR. The resistance to external acid of microbial communities in broad-leaved forest was higher than that in coniferous forest. NAR and MAR treatments slowed down soil carbon (C), nitrogen (N), and phosphorus (P) mineralization by attenuating the correlations between litter mass losses and the enzymes involved in C, N, and P cycling. Results reveal that the ratio of SO₄²⁻ to NO₃⁻ in acid rain is an important factor which profoundly influences litter decomposition process. In the future, a decreasing ratio of SO₄²⁻ to NO₃⁻ in acid rain will be observed in subtropical forests. Thus, soil C would accumulate as a consequence of future acid precipitation, and this may seriously affect the balance of ecosystem C, N flux.     

Soil organisms and carbon, nitrogen and phosphorus mineralisation in Norway spruce and mixed Norway spruce – Birch stands

 We examined how soil organisms and C, N and P mineralisation are affected by admixing deciduous tree species, silver birch (Betula pendula) and woollen birch (B. pubescens), in managed Norway spruce (Picea abies) stands. Pure spruce and mixed spruce–birch stands were examined at four sites in southern and central Sweden. Soil macroarthropods and enchytraeids were sampled in litter and soil. In the uppermost 5 cm of soil humus we determined microbial biomass and microbial respiration; we estimated the rate of C, N and P mineralisation under laboratory conditions. The densities of Coleoptera, Diptera and Collembola were larger in mixed stands than in spruce stands. Soil fauna composition differed between mixed and spruce stands (as revealed by redundancy analysis). Staphyliniidae, Elateridae, Cecidiomyidae larvae and Onychiuridae were the families that increased most strongly in mixed stands. There were no differences in microbial biomass and microbial respiration, nor in the C, N and P mineralisation rates, between mixed and spruce stands. However, within mixed stands microbial biomass, microbial activity and C mineralisation were approximately 15% higher under birch trees than under spruce trees. We propose that the presence of birch leaf litter was likely to be the most important factor causing differences in soil fauna composition. Birch may also influence the quality and the decomposition rate of humus in mixed stands. However, when the proportion of birch trees is low, the short-term (decades) effect of this species on decomposition is likely to be small in mixed stands on acid forest soils. Received: 20 February 1998     

Decomposition of leaf litter from chitinase transgenic silver birch (Betula pendula) and effects on decomposer populations in a field trial

Although the area under cultivation of genetically modified plants (GMPs) has substantially increased during the last decade, the effects of transgenic organisms on ecosystem processes (such as litter decomposition and nutrient cycling) largely remain unknown. In this study, the decomposition of leaf litter from transgenic birch trees (Betula pendula) expressing sugar beet chitinase IV gene was studied in a field experiment. Eight chitinase transgenic lines and a non-transgenic control were included in the study. The decomposition of these litters was investigated by studying: (i) litter mass loss, (ii) fungal (litter ergosterol content) and total microbial biomass (SIR) and their activity (basal respiration), and (iii) the effects of transgenic litter on microbial-feeding soil fauna (number of nematodes and abundance of different functional groups). At the end of the study (8 and 11 months after establishment) mass loss of chitinase transgenic leaf litter did not differ from that of non-transgenic control trees. Similarly, no differences in either the fungal or total microbial biomass between the treatments were recorded. A single transgenic line showing high chitinase IV expression differed significantly from the controls in the mean number of nematodes. The nematode populations in this litter showed distinct temporal dynamics compared to the controls, thus indirectly indicating microbial differences in the litter. The results of this study indicate that conceivable changes, possibly derived from pleiotropic effects due to gene modification, in the litter quality due to gene transformation are either absent or too weak to affect the decomposability of the litter in the soil.