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.     

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.     

Organic chemical and nutrient dynamics in decomposing beech leaf litter in relation to fungal ingrowth and succession during 3-year decomposition processes in a cool temperate deciduous forest in Japan

Decomposition processes of beech leaf litter were studied over a 3-year period in a cool temperate deciduous forest in Japan. Organic chemical and nutrient dynamics, fungal biomass and succession were followed on upper (Moder) and lower (Mull) of a forest slope. Litter decomposition rates were similar between the sites. Nutrient dynamics of the decomposing litter was categorized into two types: weight changes in nitrogen and phosphorus showed two phases, the immobilization (0–21 months) and the mobilization phase (21–35 months), while those in potassium, calcium and magnesium showed only the mobilization phase. The rate of loss of organic chemical constituents was lignin < holocellulose < soluble carbohydrate < polyphenol in order. The changes in lignocellulose index (LCI), the ratio of holocellulose in lignin and holocellulose, were significantly correlated to the changes in nitrogen and phosphorus concentrations during the decomposition. During the immobilization phase, increase in total fungal biomass contributed to the immobilization of nitrogen and phosphorus. The percentage of clamp-bearing fungal biomass (biomass of the Basidiomycota) to total fungal biomass increased as the decomposition proceeded and was significantly correlated to LCI. Two species in the xylariaceous Ascomycota were dominantly isolated by the surface sterilization method from decomposing litter collected in the 11th month. The organic chemical, nitrogen and phosphorus dynamics during the decomposition were suggested to be related to the ingrowth, substrate utilization and succession of the Xylariaceae and the Basidiomycota. Twenty-one species in the other Ascomycota and the Zygomycota isolated by the washing method were classified into three groups based on their occurrence patterns: primary saprophytes, litter inhabitants and secondary sugar fungi. These species showed different responses to LCI and soluble carbohydrate concentration of the litter between the groups.     

The Estimated Impact of Fungi on Nutrient Dynamics During Decomposition of Phragmites australis Leaf Sheaths and Stems

Decomposition of culms (sheaths and stems) of the emergent macrophyte Phragmites australis (common reed) was followed for 16 months in the litter layer of a brackish tidal marsh along the river Scheldt (the Netherlands). Stems and leaf sheaths were separately analyzed for mass loss, litter-associated fungal biomass (ergosterol), nutrient (N and P), and cell wall polymer concentrations (cellulose and lignin). The role of fungal biomass in litter nutrient dynamics was evaluated by estimating nutrient incorporation within the living fungal mass. After 1 year of standing stem decay, substantial fungal colonization was found. This corresponded to an overall fungal biomass of 49 ± 8.7 mg g⁻¹ dry mass. A vertical pattern of fungal colonization on stems in the canopy is suggested. The litter bag experiment showed that mass loss of stems was negligible during the first 6 months, whereas leaf sheaths lost almost 50% of their initial mass during that time. Exponential breakdown rates were −0.0039 ± 0.0004 and −0.0026 ± 0.0003 day⁻¹ for leaf sheaths and stems, respectively (excluding the initial lag period). In contrast to the stem tissue—which had no fungal colonization—leaf sheaths were heavily colonized by fungi (93 ± 10 mg fungal biomass g⁻¹ dry mass) prior to placement in the litter layer. Once being on the sediment surface, 30% of leaf sheath's associated fungal biomass was lost, but ergosterol concentrations recovered the following months. In the stems, fungal biomass increased steadily after an initial lag period to reach a maximal biomass of about 120 mg fungal biomass g⁻¹ dry mass for both plant parts at the end of the experiment. Fungal colonizers are considered to contain an important fraction of nutrients within the decaying plant matter. Fungal N incorporation was estimated to be 64 ± 13 and 102 ± 15% of total available N pool during decomposition for leaf sheaths and stems, respectively. Fungal P incorporation was estimated to be 37 ± 9 and 52 ± 15% of total available P during decomposition for leaf sheaths and stems, respectively. Furthermore, within the stem tissue, fungi are suggested to be active immobilizers of nutrients from the external environment because fungi were often estimated to contain more than 100% of the original nutrient stock.     

Nitrogen dynamics and mineralization in degraded agricultural soil mulched with fresh grass

Understanding mulching influences on nitrogen (N) availability is important for developing N management strategies in plantations at the upland sites of the southwestern China. Dynamics of biomass loss and nutrient release of mulching material, N availability in the soil and N mineralization in situ were evaluated for the treatments with different mulch quantity in degraded agricultural soil. The time taken for 95% decomposition of the initial biomass of Cogon grass (Imperata cylindrical L. Beauv. var. major) was 17 months with a half-life (t _1/2) of about 4.8 months. During the first 4 months about 55.2% of N was released, and after 1-year decomposition about 71.6% of N was released from the mulch material. The fresh grass mulch increased the available N in the soil as they decomposed. Compared to no mulch treatment, mulch treatments with 2.5, 5.0 and 7.5 kg m⁻² mulching grass increased available N by about 13.1, 40.8 and 56.4% in the top soil (0–5 cm), and about 23.6, 78.0 and 139.3% in the middle layer (5–20 cm), respectively. The mean annual net N mineralization in the mulched plots had 9.0–40.9% higher cumulative rate than that in no-mulch plots, and the majority of the accumulated N in the incubated soils existed as NO₃–N. There was a positive relationship between the rate of N mineralization and the available N in both the top soil and the middle layer. Mulch improves soil nutrients and this improvement increased with increasing mulching quantity. The increment of net N mineralization was approximately 69, 161 and 322 kg N ha⁻¹ year⁻¹ in the soil of 0–20 cm depth for the 2.5, 5.0 and 7.5 kg m⁻² grass mulch treatments, respectively. The results from this study will provide a basis to optimize mulching techniques for poplar plantations in degraded agricultural soils of southwestern China.     

A 5 year study of litter decomposition processes in aChamaecyparis obtusa Endl. forest

Decomposition of needle litter in aChamaecyparis obtusa forest was studied over a 5 year period using a litter bag method. Organic matter, nitrogen and carbon mass and faunal abundance were monitored. The pattern of weight loss was represented by three phases: the initial leaching of carbon and nitrogen (0–3 months), nitrogen immobilization (3–15 months), and nitrogen mobilization (15–60 months). The decomposition rate of needle litter was expressed by Olson's decomposition constant (k) which was−0.113 over a 5 year period. The decomposition rate decreased with the advance of decomposition processes. The role of soil fauna in the decomposition process was assessed by comparing decomposition rates between the control and defaunated plots. In the leaching phase, soil animals had no significant role in the decomposition processes. During the immobilization phase, soil animals contributed to the immobilization processes through their grazing activities, and there were significant differences in weight loss between the control and defaunated plots. In the mobilization phase, saprovorous soil animals such as Collembola and Acari contributed to the mobilization processes by feeding on decomposing litter. Decomposition processes observed in this study were compared with other similar studies.     

Whole-stream nitrate addition affects litter decomposition and associated fungi but not invertebrates

We assessed the effect of whole-stream nitrate enrichment on decomposition of three substrates differing in nutrient quality (alder and oak leaves and balsa veneers) and associated fungi and invertebrates. During the 3-month nitrate enrichment of a headwater stream in central Portugal, litter was incubated in the reference site (mean NO₃-N 82 μg l⁻¹) and four enriched sites along the nitrate gradient (214–983 μg NO₃-N l⁻¹). A similar decomposition experiment was also carried out in the same sites at ambient nutrient conditions the following year (33–104 μg NO₃-N l⁻¹). Decomposition rates and sporulation of aquatic hyphomycetes associated with litter were determined in both experiments, whereas N and P content of litter, associated fungal biomass and invertebrates were followed only during the nitrate addition experiment. Nitrate enrichment stimulated decomposition of oak leaves and balsa veneers, fungal biomass accrual on alder leaves and balsa veneers and sporulation of aquatic hyphomycetes on all substrates. Nitrate concentration in stream water showed a strong asymptotic relationship (Michaelis–Menten-type saturation model) with temperature-adjusted decomposition rates and percentage initial litter mass converted into aquatic hyphomycete conidia for all substrates. Fungal communities did not differ significantly among sites but some species showed substrate preferences. Nevertheless, certain species were sensitive to nitrogen concentration in water by increasing or decreasing their sporulation rate accordingly. N and P content of litter and abundances or richness of litter-associated invertebrates were not affected by nitrate addition. It appears that microbial nitrogen demands can be met at relatively low levels of dissolved nitrate, suggesting that even minor increases in nitrogen in streams due to, e.g., anthropogenic eutrophication may lead to significant shifts in microbial dynamics and ecosystem functioning. Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Nitrogen sequestration capacity of two salt marshes from the Tagus estuary

The role of salt marshes as nitrogen sink is examined taking into consideration the seasonal variation of above and belowground biomass of Spartina martima and Halimione portulacoides in two marshes from Tagus estuary, Pancas and Corroios, and the degradation rates of belowground litter. Total nitrogen was determined in plant components, decomposing litter and sediment. Biomass was higher in Corroios, the saltier marsh, with 7190 g m⁻² y⁻¹ dw of S. maritima and 6593 g m⁻² y⁻¹ dw of H. portulacoides and the belowground component contributed to 96% and 90% of total biomass, respectively. In the other marsh, Pancas, belowground biomass contributed to 56% and 76% of total biomass for S. maritima and H. portulacoides, respectively. Litterbag experiment showed that between 25% and 50% of nitrogen is lost within the first month and remained relatively constant in the next four months. Slower decomposition is observed in sediments with higher nitrogen concentration (max. 0.7% N in the saltier marsh). Higher concentrations of N were found in the sediment upper layers. Considering the sediment-root system, most of the nitrogen is stored in the sediment compartment and only about 1–4% of the total N was found in the roots. Considering these results, Tagus salt marshes act as a sink for nitrogen.     

Litter Decomposition in a California Annual Grassland: Interactions Between Photodegradation and Litter Layer Thickness

In annual grasslands that experience a mediterranean-type climate, the synchrony between plant senescence and peak solar radiation over summer results in high litter sun exposure. We examined the decomposition of both shaded and sun-exposed litter over summer and inferred the effects of photodegradation from changes in mass loss and litter chemistry. The carry-over effects of summer litter exposure on wet season decomposition were also assessed, and the attenuation of photodegradation with litter layer thickness was used to estimate the proportion of grass litter lignin susceptible to photodegradation under different treatments of a factorial global change experiment. Over summer, mass loss from grass and forb litter exposed to ambient sunlight ranged from 8% to 10%, whereas lignin decreased in grass litter by approximately 20%. After one year of decomposition, mass losses from grass leaves exposed to sunlight over summer were more than double the mass losses from summer-shaded leaves. When shade litter layer thickness was varied, mass losses over summer for all treatments were also approximately 8%; however, lignin decreased significantly only in the low shade treatments (0–64 g m⁻² of shade litter). Aboveground production of annual grasses nearly quadrupled in response to the combined effects of N addition, elevated atmospheric CO₂, increased precipitation and warming. The estimated proportion of grass litter lignin experiencing full photodegradation ranged from 100% under ambient conditions to 31–62% in plots receiving the combined global change treatments. These results reveal an important role of sun exposure over summer in accelerating litter decomposition in these grasslands and provide evidence that future changes in the quantity of litter deposition may modulate the influence of photodegradation integrated across the litter layer.     

Decomposition of roots of black alder and hybrid poplar in short-rotation plantings: Nitrogen and lignin control

The decomposition of the roots (0–2 mm, 2–5 mm and 5–10 mm) of black alder (Alnus glutinosa (L.) Gaertn.) and hybrid poplar (Populus nigra L. X Populus trichocarpa Torr & Gray) was followed over a 462-day period in pure and mixed plantings in southern Quebec. Small roots of alder had the highest initial concentrations of nitrogen and lignin, and lost 9 and 10% less mass than medium and large roots, respectively. Large roots of poplar had the highest lignin-to-nitrogen ratio and showed the smallest loss of mass over the total incubation period. Slow root decomposition of black alder and hybrid poplar was characterized by a greater proportion of initial root nitrogen immobilized per unit of carbon respired. Lignin concentration in roots of alder and poplar increased rapidly at the beginning of the incubation. Our results suggest that high levels of nitrogen in roots of alder could contribute in slowing the rate of decomposition by allowing the formation of nitrogen-lignin derivatives and low levels of nitrogen in roots of poplar may limit the growth of microorganisms and the rate of root decomposition. A multiple regression was developed using initial nitrogen, lignin concentration and the ratio of lignin to nitrogen to produce an index of the rate of root decomposition. The correlation between the index values and the percentage of residual root mass was significant (r=0.98, p<0.01).     

Fine root vertical distribution and temporal dynamics in mature stands of two enset (Enset ventricosum Welw Cheesman) clones

Quantification of the role of fine roots in the biological cycle of nutrients necessitates understanding root distribution, estimating root biomass, turnover rate and nutrient concentrations, and the dynamics of these parameters in perennial systems. Temporal dynamics, vertical distribution, annual production and turnover, and nitrogen use of fine roots (≤2 mm in diameter) were studied in mature (5-year-old) stands of two enset (Ensete ventricosum) clones using the in-growth bag technique. Live fine root mass generally decreased with increasing depth across all seasons except the dry period. Except for the dry period, more than 70% of the fine root mass was in the above 0-20 cm depth, and the fine root mass in the upper 0–10 cm depth was significantly higher than in the lowest depth (20–30 cm). Live fine root mass showed a seasonal peak at the end of the major rainy season but fell to its lowest value during the dry or short rainy season. The difference between the peak and low periods were significant (p ≤ 0.05). Fine root nitrogen (N) use showed significant seasonal variation where the mean monthly fine root N use was highest during the major rainy season. There were significant effects on N use due to depths and in-growth periods, but not due to clones. Enset fine root production and turnover ranged from 2,339 to 2,451 kg ha⁻¹ year⁻¹ and from 1.55 to 1.80 year⁻¹, respectively. Root N return, calculated from fine root turnover, was estimated at 64–65 kg ha⁻¹ year⁻¹. Fine root production, vertical distribution and temporal dynamics may be related to moisture variations and nutrient (N) fluxes among seasons and along the soil depth. The study showed that fine root production and turnover can contribute considerably to the carbon and nitrogen economy of mature enset plots.     

Effects of nitrogen amendment on annual plants in the Chihuahuan Desert

Effects of nitrogen amendments on spring annual plant distribution, primary production, and species diversity in a semi-arid environment were studied. The ecological responses of spring annual plant species to nitrogen differed between species, and between sites. The distribution of each species in a control transect was wider than in a nitrogen-treated transect. Annual plant species diversity at each station in the control transect was higher than that of the nitrogen-treated transect. The lower site of the Lower Basin Slope areas had the highest species diversity (0.94 for the control and 0.84 for the nitrogen-treated), and the Upper Basin Slope area, shrub vegetation zone, had the lowest species diversity (0.27 for the control and 0.05 for the nitrogen-treated) in both transects. Inorganic nitrogen in the nitrogen-treated transect soils was consistently higher than that in the control transect soils; however, the former showed more fluctuation from station to station than the latter. Above-ground biomass of spring annual plants in the nitrogen-treated transect was consistently higher than that in the control at each station. The maximum above-ground biomass in the control and nitrogen-treated transect was 24.4±4.4 gm⁻² and 61.2±10.6 gm⁻², respectively. Variations in above-ground biomass along the transect did not parallel with the variation in inorganic nitrogen in soils and species diversity.     

Estimation of fungal biomass in the decaying cones ofPinus densiflora

Fungal biomass in the decaying cones ofPinus densiflora was investigated. Leaching, immobilization and mobilization phases were recognized in the decomposition process of cones. Fungal biomass was estimated by the agar-film technique, using a conversion factor of 0.62 mg dry wt. mm⁻³ of hyphal volume to biomass and a factor of 2.5 for in-efficiencency of homogenization. The fungal biomass was 4.9±2.1 (mean±S.D.) mg dry wt. g⁻¹ dry matter in the cones on the tree, 11±6 mg g⁻¹ in the leaching phase, 19±7 mg g⁻¹ in the immobilization phase and 30±15 mg g⁻¹ in the mobilization phase. It significantly increased after cones had lain on the forest floor, and also in the immobilization phase. The latter result suggests that the fungal biomass contributed to the immobilization of nitrogen in the decomposition process. The ratio of ergosterol content to fungal biomass in the cones was 2.9–8.8 μg mg⁻¹ dry wt., lying in the range of 2–16 μg mg⁻¹ reported for mycelia. This suggested that the estimate of fungal biomass was reasonable. Reduction in this ratio with the dry weight loss in the cones suggested that the proportion of relatively active fungal biomass decreased with the progress of decomposition.     

Seasonal dynamics of fine root biomass, root length density, specific root length, and soil resource availability in a Larix gmelinii plantation

Fine root turnover is a major pathway for carbon and nutrient cycling in terrestrial ecosystems and is most likely sensitive to many global change factors. Despite the importance of fine root turnover in plant C allocation and nutrient cycling dynamics and the tremendous research efforts in the past, our understanding of it remains limited. This is because the dynamics processes associated with soil resources availability are still poorly understood. Soil moisture, temperature, and available nitrogen are the most important soil characteristics that impact fine root growth and mortality at both the individual root branch and at the ecosystem level. In temperate forest ecosystems, seasonal changes of soil resource availability will alter the pattern of carbon allocation to belowground. Therefore, fine root biomass, root length density (RLD) and specific root length (SRL) vary during the growing season. Studying seasonal changes of fine root biomass, RLD, and SRL associated with soil resource availability will help us understand the mechanistic controls of carbon to fine root longevity and turnover. The objective of this study was to understand whether seasonal variations of fine root biomass, RLD and SRL were associated with soil resource availability, such as moisture, temperature, and nitrogen, and to understand how these soil components impact fine root dynamics in Larix gmelinii plantation. We used a soil coring method to obtain fine root samples (⩽2 mm in diameter) every month from May to October in 2002 from a 17-year-old L. gmelinii plantation in Maoershan Experiment Station, Northeast Forestry University, China. Seventy-two soil cores (inside diameter 60 mm; depth intervals: 0–10 cm, 10–20 cm, 20–30 cm) were sampled randomly from three replicates 25 m × 30 m plots to estimate fine root biomass (live and dead), and calculate RLD and SRL. Soil moisture, temperature, and nitrogen (ammonia and nitrates) at three depth intervals were also analyzed in these plots. Results showed that the average standing fine root biomass (live and dead) was 189.1 g·m⁻²·a⁻¹, 50% (95.4 g·m⁻²·a⁻¹) in the surface soil layer (0–10 cm), 33% (61.5 g·m⁻²·a⁻¹), 17% (32.2 g·m⁻²·a⁻¹) in the middle (10–20 cm) and deep layer (20–30cm), respectively. Live and dead fine root biomass was the highest from May to July and in September, but lower in August and October. The live fine root biomass decreased and dead biomass increased during the growing season. Mean RLD (7,411.56 m·m⁻³·a⁻¹) and SRL (10.83 m·g⁻¹·a⁻¹) in the surface layer were higher than RLD (1 474.68 m·m⁻³·a⁻¹) and SRL (8.56 m·g⁻¹·a⁻¹) in the deep soil layer. RLD and SRL in May were the highest (10 621.45 m·m⁻³ and 14.83m·g⁻¹) compared with those in the other months, and RLD was the lowest in September (2 198.20 m·m⁻³) and SRL in October (3.77 m·g⁻¹). Seasonal dynamics of fine root biomass, RLD, and SRL showed a close relationship with changes in soil moisture, temperature, and nitrogen availability. To a lesser extent, the temperature could be determined by regression analysis. Fine roots in the upper soil layer have a function of absorbing moisture and nutrients, while the main function of deeper soil may be moisture uptake rather than nutrient acquisition. Therefore, carbon allocation to roots in the upper soil layer and deeper soil layer was different. Multiple regression analysis showed that variation in soil resource availability could explain 71–73% of the seasonal variation of RLD and SRL and 58% of the variation in fine root biomass. These results suggested a greater metabolic activity of fine roots living in soil with higher resource availability, which resulted in an increased allocation of carbohydrate to these roots, but a lower allocation of carbohydrate to those in soil with lower resource availability. __________ Translated from Acta Phytoecologica Sinica, 2005, 29(3): 403–410 [译自: 植物生态学报, 2005, 29(3): 403–410]     

Mass loss and nitrogen dynamics in decomposing acid forest litter in the Netherlands at increased nitrogen deposition

Litterbag experiments were carried out in five forest ecosystems in the Netherlands to study weight loss and nitrogen dynamics during the first two years of decomposition of leaf and needle litter. All forests were characterized by a relatively high atmospheric nitrogen input by throughfall, ranging from 22–55 kg N ha^–1 yr^–1.Correlation analysis of all seven leaf and needle litters revealed no significant relation between the measured litter quality indices (nitrogen and lignin concentration, lignin-to-nitrogen ratio) and the decomposition rate. A significant linear relation was found between initial lignin-to-nitrogen ratio and critical nitrogen concentration, suggesting an effect of litter quality on nitrogen dynamics.Comparison of the decomposition of oak leaves in a nitrogen-limited and a nitrogen-saturated forest suggested an increased nitrogen availability. The differences in capacities to retain atmospheric nitrogen inputs between these two sites could be explained by differences in net nitrogen immobilization in first year decomposing oak leaves: in the nitrogen-limited oak forest a major part (55%) of the nitrogen input by throughfall was immobilized in the first year oak leaf litter.The three coniferous forests consisted of two monocultures of Douglas fir and a mixed stand of Douglas fir and Scots pine. Despite comparable litter quality in the Douglas fir needles in all sites, completely different nitrogen dynamics were found.     

氮添加对中亚热带杜鹃灌丛凋落物生产和叶分解的影响

凋落物的生产和分解是生态系统养分循环的重要过程,受到大气氮沉降的深刻影响。但目前相关研究主要集中于森林和草地生态系统,氮沉降对灌丛生态系统凋落物养分归还的影响规律尚不清楚。因此选择亚热带分布广泛的杜鹃灌丛为研究对象,进行了为期两年的模拟氮沉降试验。试验设置4个处理:对照(CK, 0 g m-2 a-1)、低氮(LN, 2 g m-2 a-1)、中氮(MN, 5 g m-2 a-1)和高氮(HN, 10 g m-2 a-1)。结果显示:CK、LN、MN和HN 4种处理下,群落年平均凋落物量分别为(1936.54±358.9)、(2541.89±112.5)、(2342.97±519.8)、(2087.22±391.8) kg/hm²,LN、MN和HN处理样地的凋落量分别比对照样地高出32.68%、21.16%和7.93%;凋落叶、花果、凋落枝和其他组分占总凋落量的比例分别为75.75%、15.09%、7.70%和1.45%,不同浓度氮处理下各组分的凋落量均高于对照样地;凋落物组分表现出明显的季节动态:凋落叶在10—11月份达到峰值,凋落枝在10月份达到峰值,花果凋落物则在5月份凋落量最高,不同氮处理下凋落物的季节动态基本一致;白檀凋落叶分解速率显著高于杜鹃,二者分解95%所需时间分别为5.08—11.11 a和7.69—17.65 a,施氮使白檀凋落叶分解周期比对照样地缩短18.18%—54.28%;凋落叶分解过程中,N元素表现为富集-释放模式,P元素表现为富集模式。研究表明,氮添加能够促进群落中白檀凋落叶分解及N、P元素的释放,说明施氮可以调节凋落叶养分释放模式,对灌丛生态系统的养分循环具有调控作用。     

Estimation of autotrophic and heterotrophic components of soil respiration by trenching is sensitive to corrections for root decomposition and changes in soil water content

This study aims to assess the effects of corrections for disturbances such as an increased amount of dead roots and an increase in volumetric soil water content on the calculation of soil CO₂ efflux partitioning. Soil CO₂ efflux, soil temperature and superficial soil water content were monitored in two young beech sites (H1 and H2) during a trenching experiment. Trenching induced a significant input of dead root mass that participated in soil CO₂ efflux and reduced the soil dissolved organic carbon content, while it increased superficial soil water content within the trenched plot. Annual soil CO₂ efflux in control plots was 528 g C m⁻² year⁻¹ at H1 and 527 g C m⁻² year⁻¹ at H2. The annual soil CO₂ efflux in trenched plots was 353 g C m⁻² year⁻¹ at H1 and 425 g C m⁻² year⁻¹ at H2. By taking into account annual CO₂ efflux from decaying trenched roots, the autotrophic contribution to total soil CO₂ efflux reached 69% at H1 and 54% at H2. The partitioning calculation was highly sensitive to the initial root mass estimated within the trenched plots. Uncertainties in the remaining root mass, the fraction of root C that is incorporated into soil organic matter during root decomposition, and the root decomposition rate constant had a limited impact on the partitioning calculation. Corrections for differences in superficial soil water content had a significant impact on annual respired CO₂ despite a limited effect on partitioning.     

Zooplankton of two arms in the Morava River floodplain in Slovakia

The species composition, seasonal dynamic of biomass and density of zooplankton were studied in two arms with a different hydrological regime. The samples were collected in two hydrologically different years — extremely wet in 2002 and extremely dry in 2003. In the first arm the mean annual chlorophyll-a concentration was 31.6 μg L⁻¹ (2002) and relatively high 64.7 μg L⁻¹ during 2003. Mean seasonal zooplankton wet biomass was low and varied: 11.6 g m⁻³ (2002) and 2.93 g m⁻³ (2003). Total zooplankton density was high (7,370 N L⁻¹) in 2002, when rotifers predominated in an open water zone and contributed up to 81% of the total zooplankton biomass and 83% of the total zooplankton density. In medial and littoral zone, in total, 22 cladoceran and 15 copepod species were identified. In the second arm the mean annual concentration of chlorophyll-a was high: 74.8 μg L⁻¹ (2002) and 61.4 μg L⁻¹ (2003). Mean seasonal zooplankton wet biomass varied from 92.5 g m⁻³ (2002) and 44.10 g m⁻³ (2003). In 2002 the planktonic crustaceans predominated; their mean biomass was 87.1 g m⁻³ and B. longirostris formed more than 91% of this value. In 2003, the zooplankton density was high (15,687 N L⁻¹), when rotifers contributed up to 94% of this value. The boom of rotifers (58,740 N L⁻¹) was recorded in June 2003. In total, 45 cladoceran and 14 copepod species were recorded in the medial and littoral zones. During observation we concluded that the structure of zooplankton, particularly species composition, abundance, biomass and seasonal dynamics are affected by the fluctuation of water levels in the arms of the rivers’ inundation areas. This unstable hydrological regime prevented the development of planktonic crustaceans.     

Response of Litter Decomposition to Simulated N Deposition in Disturbed, Rehabilitated and Mature Forests in Subtropical China

The response of decomposition of litter for the dominant tree species in disturbed (pine), rehabilitated (pine and broadleaf mixed) and mature (monsoon evergreen broadleaf) forests in subtropical China to simulated N deposition was studied to address the following hypothesis: (1) litter decomposition is faster in mature forest (high soil N availability) than in rehabilitated/disturbed forests (low soil N availability); (2) litter decomposition is stimulated by N addition in rehabilitated and disturbed forests due to their low soil N availability; (3) N addition has little effect on litter decomposition in mature forest due to its high soil N availability. The litterbag method (a total of 2880 litterbags) and N treatments: Control-no N addition, Low-N: −5 g N m⁻² y⁻¹, Medium-N: −10 g N m⁻² y⁻¹, and High-N: −15 g N m⁻² y⁻¹, were employed to evaluate decomposition_. Results indicated that mature forest, which has likely been N saturated due to both long-term high N deposition in the region and the age of the ecosystem, had the highest litter decomposition rate, and exhibited no significant positive and even some negative response to nitrogen additions. However, both disturbed and rehabilitated forests, which are still N limited due to previous land use history, exhibited slower litter decomposition rates with significant positive effects from nitrogen additions. These results suggest that litter decomposition and its responses to N addition in subtropical forests of China vary depending on the nitrogen status of the ecosystem.