Decomposition of litter produced under elevated CO2: Dependence on plant species and nutrient supply

We investigated the effect of CO₂ concentration and soilnutrient availability during growth on the subsequent decomposition andnitrogen (N) release from litter of four annual grasses that differ inresource requirements and native habitat. Vulpia microstachys isa native grass found on California serpentine soils, whereas Avenafatua, Bromus hordaceus, and Lolium multiflorum areintroduced grasses restricted to more fertile sandstone soils (Hobbs & Mooney 1991). Growth in elevated CO₂ altered litter C:N ratio,decomposition, and N release, but the direction and magnitude of thechanges differed among plant species and nutrient treatments. ElevatedCO₂ had relatively modest effects on C:N ratio of litter,increasing this ratio in Lolium roots (and shoots at high nutrients),but decreasing C:N ratio in Avena shoots. Growth of plants underelevated CO₂ decreased the decomposition rate of Vulpialitter, but increased decomposition of Avena litter from the high-nutrient treatment. The impact of elevated CO₂ on N loss fromlitter also differed among species, with Vulpia litter from high-CO₂ plants releasing N more slowly than ambient-CO₂litter, whereas growth under elevated CO₂ caused increased Nloss from Avena litter. CO₂ effects on N release in Lolium and Bromus depended on the nutrient regime in whichplants were grown. There was no overall relationship between litter C:Nratio and decomposition rate or N release across species and treatments.Based on our study and the literature, we conclude that the effects ofelevated CO₂ on decomposition and N release from litter arehighly species-specific. These results do not support the hypothesis thatCO₂ effects on litter quality consistently lead to decreasednutrient availability in nutrient-limited ecosystems exposed to elevatedCO₂.     

Microbial communities and activities in alpine and subalpine soils

Soil samples were collected along two slopes (south and north) at subalpine (1500–1900 m, under closed vegetation, up to the forest line) and alpine altitudes (2300–2530, under scattered vegetation, above the forest line) in the Grossglockner mountain area (Austrian central Alps). Soils were analyzed for a number of properties, including physical and chemical soil properties, microbial activity and microbial communities that were investigated using culture-dependent (viable heterotrophic bacteria) and culture-independent methods (phospholipid fatty acid analysis, FISH). Alpine soils were characterized by significantly ( P <0.01) colder climate conditions, i.e. lower mean annual air and soil temperatures, more frost and ice days and higher precipitation, compared with subalpine soils. Microbial activity (soil dehydrogenase activity) decreased with altitude; however, dehydrogenase activity was better adapted to cold in alpine soils compared with subalpine soils, as shown by the lower apparent optimum temperature for activity (30 vs. 37 °C) and the significantly ( P <0.01–0.001) higher relative activity in the low-temperature range. With increasing altitude, i.e. in alpine soils, a significant ( P <0.05–0.01) increase in the relative amount of culturable psychrophilic heterotrophic bacteria, in the relative amount of the fungal population and in the relative amount of Gram-negative bacteria was found, which indicates shifts in microbial community composition with altitude.     

Response of soil microbial activity to temperature,moisture, and litter leaching on a wetland transect during seasonal refilling

In nutrient impoverished landscapes in southwest Australia, terrestrial litter appears to be important in phosphorus (P) turnover and in the gradual accumulation of P in wetland systems. Little is known about the fate of P leached from litter during the wet season and the associated effects of soil microclimate on microbial activity. The effects of temperature, moisture, and litter leaching on soil microbial activity were studied on a transect across a seasonal wetland in southwestern Australia, after the onset of the wet season. Heterotrophic respiration (CO₂ efflux) was higher in the dried lakebed and riparian areas than in upland soils, and higher during the day than at night. There were significant variations in CO₂ efflux with time of sampling, largely caused by the effect of temperature. The addition of litter leachate significantly increased CO₂ efflux, more significantly in soils from upland sites, which had lower moisture and nutrient contents. There was a difference in response of microbial respiration between upland soils and wetland sediments to litter leachate and wetter, warmer conditions. In general, the litter leachate enhanced heterotrophic microbial respiration, and more significantly at warmer conditions (31 °C). The relative fungal to bacterial ratio was 2.9 – 3.2 for surface litter and 0.7–1.0 for soils, suggesting a fungal dominance in heterotrophic respiration of surface litter, but increased bacterial dominance in soils, especially in exposed sediments in the lakebed.     

Decomposability of C3 and C4 grass litter sampled under different concentrations of atmospheric carbon dioxide at a natural CO2 spring

Elevated concentrations of atmospheric CO₂ can influence the relative proportions, biomass and chemical composition of plant species in an ecosystem and, thereby, the input of litter nutrients to soil. Plant growth under elevated CO₂ appears to have no consistent effect on rates of litter decomposition; decomposition can, however, differ in C3 and C4 plant material from the same CO₂ environment. We here describe the decomposability of leaf litter of two grass species – the C3 Holcus lanatus L. (Yorkshire fog) and C4 Pennisetum clandestinum Hochst. (kikuyu) - from an unfertilized, ungrazed grassland at a cold CO₂ spring in Northland, New Zealand. Decomposability was measured by net CO₂–C production from litter incubated for 56 days at 25 °C in a gley soil from the site; net mineral-N production from litter was also determined. Both litter and soils were sampled under `low' and `high' concentrations of atmospheric CO₂. Decomposition of H. lanatus litter was greater than that of P. clandestinum litter throughout the 56-day incubation. Decomposition tended to be greater in `high-CO<sub>2</sub>' than in `low-CO₂' H. lanatus litter, but lower in `high-CO<sub>2</sub>' than `low-CO₂' P. clandestinum litter; differences were, however, non-significant after 28 days. Overall, litter decomposition was greater in the `low-CO<sub>2</sub>' than `high-CO₂' soil. Differences in decomposition rates were related negatively to litter N concentrations and positively to C:N ratios, but were not predictable from lignin:total N ratios. Net mineral-N production from litter decomposition did not differ significantly in `high-CO<sub>2</sub>' and `low-CO₂' samples incubated in `low-CO<sub>2</sub>' soil; in `high-CO₂' soil some net immobilization was observed. Overall, results indicate the likely complexity of litter decomposition in the field but, nevertheless, strongly suggest that rates of decomposition will not necessarily decline in a `high-CO₂' environment.     

A comparative analysis of photosynthetic characteristics of hulless barley at two altitudes on the Tibetan Plateau

To determine the photosynthetic characteristics of C₃ plants and their sensitivity to CO₂ at different altitudes on the Tibetan Plateau, hulless barley (Hordeum vulgare L. ssp. vulgare) was grown at altitudes of 4,333 m and 3,688 m. Using gas-exchange measurements, photosynthetic parameters were simulated, including the maximum net photosynthesis (P _max) and the apparent quantum efficiency (α). Plants growing at higher altitude had higher net photosynthetic rates (P _N), photosynthesis parameters (P _max and α) and sensitivities to CO₂ enhancement than plants growing at lower altitude on the Tibetan Plateau. The enhancements of P _N, P _max, and α for plants growing at higher altitude, corresponding with 10 μmol(CO₂) mol⁻¹ increments, were approximately 0.20∼0.45%, 0.05∼0.20% and 0.12∼0.36% greater, respectively, than for plants growing at lower altitude, respectively, where CO₂ levels rose from 10 to 170 μmol(CO₂) mol⁻¹. Therefore, on the Tibetan Plateau, the changes in the photosynthetic capacities and the photosynthetic sensitivities to CO₂ observed in the C₃ plants grown above 3,688 m are likely to increase with altitude despite the decreasing CO₂ partial pressure.     

Adaptation of microbial activities to the environmental conditions in alpine soils

Summary The adaptation of soil microorganisms to different environmental conditions was investigated in the Austrian Central Alps (Hohe. Tauern). The floristic composition of the soil fungi at different sites was determined and the CO₂-release from soils taken from different altitudes was measured at different temperatures. The results showed a decreasing diversity of soil fungi with increasing altitude and a change in the dominating species at different altitudes and/or with vegetation patterns. The relative rates of CO₂-release from soils from different altitudes did not differ at different incubation temperatures. It was concluded that, among soil fungi the selection of species is a more effective mechanism for the adaption to changed environmental conditions than metabolic adaptations.This study was supported in part by the Österreichische MaB-Hochgebirgsprogramm Hohe TauernDedicated to Dr. K.F. Springer     

Leaf and root litter decomposition is discontinued at high altitude tropical montane rainforests contributing to carbon sequestration

We investigated how altitude affects the decomposition of leaf and root litter in the Andean tropical montane rainforest of southern Ecuador, that is, through changes in the litter quality between altitudes or other site‐specific differences in microenvironmental conditions. Leaf litter from three abundant tree species and roots of different diameter from sites at 1,000, 2,000, and 3,000 m were placed in litterbags and incubated for 6, 12, 24, 36, and 48 months. Environmental conditions at the three altitudes and the sampling time were the main factors driving litter decomposition, while origin, and therefore quality of the litter, was of minor importance. At 2,000 and 3,000 m decomposition of litter declined for 12 months reaching a limit value of ~50% of initial and not decomposing further for about 24 months. After 36 months, decomposition commenced at low rates resulting in an average of 37.9% and 44.4% of initial remaining after 48 months. In contrast, at 1,000 m decomposition continued for 48 months until only 10.9% of the initial litter mass remained. Changes in decomposition rates were paralleled by changes in microorganisms with microbial biomass decreasing after 24 months at 2,000 and 3,000 m, while varying little at 1,000 m. The results show that, irrespective of litter origin (1,000, 2,000, 3,000 m) and type (leaves, roots), unfavorable microenvironmental conditions at high altitudes inhibit decomposition processes resulting in the sequestration of carbon in thick organic layers.     

青海云杉林凋落物分解早期微生物生物量时空动态变化

凋落物分解过程中的微生物生物量动态对于深入了解森林生态系统凋落物分解机理具有重要意义。为了解高寒山地森林典型树种凋落物分解过程中的微生物生物量动态特征,采用凋落物袋野外原位分解法,研究了海拔2850 m、2950 m、3050 m、3150 m和3250 m树冠中心(CC)与树冠边缘(CE)青海云杉(Picea crassifolia)叶凋落物分解过程中凋落物和土壤中的微生物生物量碳(MBC)、微生物生物量氮(MBN)和微生物生物量磷(MBP)时空动态变化。时间尺度上,不同海拔CC与CE凋落物中的MBC、MBN和MBP在生长季节初期上升,生长季节后期下降;但土壤中的MBC、MBN和MBP却在生长季节初期下降,生长季节后期上升。然而,3050 m海拔CE和3250 m海拔土壤中的MBP含量呈“先下降-后上升-再下降”的倒“N”型变化格局。空间尺度上,凋落物中的MBC和MBN含量高于土壤中的,但MBP出现相反情况;CE处的MBC、MBN和MBP含量均显著(P<0.05)高于CC;总体而言,MBC、MBN和MBP含量以中海拔显著最高(P<0.05),其次为低海拔,高海拔显著最低(...     

海拔和郁闭度对祁连山青海云杉林叶凋落物分解的影响

为了探究海拔和郁闭度对青海云杉林叶凋落物分解的影响,本文选择海拔为2850 m,3050 m,3250 m和3450 m四个梯度和高、中、低三个林分郁闭度,采用分解网袋法,研究青海云杉叶凋落物分解速率及分解过程中N、P元素变化。结果表明,质量损失率随时间在波动增大。分解速率先减小后增大,不同海拔下分解速率为K₃₄₅₀ > K₃₀₅₀ > K₃₂₅₀ > K₂₈₅₀,不同郁闭度下分解速率为K_低 > K_中 > K_高,青海云杉叶枯落物分解50%和95%所需时间约为5.3 a和22.7 a。枯落物分解过程中,N、P含量和累积系数在不同海拔和郁闭度下的变化不同,与季节变化有关。研究结果为祁连山森林生态系统地球化学循环奠定基础。     

Independent, Interactive, and Species-Specific Responses of Leaf Litter Decomposition to Elevated CO2 and O3 in a Northern Hardwood Forest

The future capacity of forest ecosystems to sequester atmospheric carbon is likely to be influenced by CO₂-mediated shifts in nutrient cycling through changes in litter chemistry, and by interactions with pollutants like O₃. We evaluated the independent and interactive effects of elevated CO₂ (560 μl l⁻¹) and O₃ (55 nl l l⁻¹) on leaf litter decomposition in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) at the Aspen free air CO₂ enrichment (FACE) site (Wisconsin, USA). Fumigation treatments consisted of replicated ambient, +CO₂, +O₃, and +CO₂ + O₃ FACE rings. We followed mass loss and litter chemistry over 23 months, using reciprocally transplanted litterbags to separate substrate quality from environment effects. Aspen decayed more slowly than birch across all treatment conditions, and changes in decomposition dynamics of both species were driven by shifts in substrate quality rather than by fumigation environment. Aspen litter produced under elevated CO₂ decayed more slowly than litter produced under ambient CO₂, and this effect was exacerbated by elevated O₃. Similarly, birch litter produced under elevated CO₂ also decayed more slowly than litter produced under ambient CO₂. In contrast to results for aspen, however, elevated O₃ accelerated birch decay under ambient CO₂, but decelerated decay under enriched CO₂. Changes in decomposition rates (k-values) were due to CO₂- and O₃-mediated shifts in litter quality, particularly levels of carbohydrates, nitrogen, and tannins. These results suggest that in early-successional forests of the future, elevated concentrations of CO₂ will likely reduce leaf litter decomposition, although the magnitude of effect will vary among species and in response to interactions with tropospheric O₃.     

Chemical Composition and Decomposition of Silver Birch Leaf Litter Produced under Elevated CO2 and O3

Two field-growing silver birch (Betula pendula Roth) clones (clone 4 and 80) were exposed to elevated CO₂ and O₃ over three growing seasons (1999–2001). In each year, the nutrients and cell wall chemistry of naturally abscised leaf litter were analyzed in order to determine the possible CO₂- and O₃-induced changes in the litter quality. Also CO₂ and O₃ effects on the early leaf litter decomposition dynamics (i.e. decomposition before the lignin decay has started) were studied with litter-bag experiments (Incubation 1 with 1999 leaf litter, Incubation 2 with 2000 leaf litter, and Incubation 3 with 2001 leaf litter) in a nearby silver birch forest. Elevated CO₂ decreased N, S, C:P and α-cellulose concentrations, but increased P, hemicellulose and lignin+polyphenolic concentrations, C:N and lignin+polyphenolic:N in both clones. CO₂ enrichment decreased the subsequent decomposition of leaves of clone 4 transiently (in Incubations 1 and 2), whereas elevated CO₂ effects on the subsequent leaf decomposition of clone 80 were inconsistent. In contrast to CO₂, O₃ decreased P concentrations and increased C:P, but both of these trends were visible in elevated O₃ treatment only. O₃-induced decreases in Mn, Zn and B concentrations were observed also, but O₃ effects on the cell wall chemistry of leaf litter were minor. Some O₃-induced changes either became more consistent in leaf litter collected during 2001 (decrease in B concentrations) or appeared only in this litter lot (decrease in N concentrations, decrease in decomposition at the end of Incubation 3). In conclusion, in northern birch forests elevated CO₂ and O₃ levels have the potential to affect leaf litter quality, but consistent CO₂ and O₃ effects on the decomposition process remain to be validated.     

CO2 and water vapour exchange in four alpine herbs at two altitudes and under varying light and temperature conditions

CO₂ and water vapour exchange rates of four alpine herbs namely: Rheum emodi, R. moorcroftianum, Megacarpaea polyandra and Rumex nepalensis were studied under field conditions at 3600 m (natural habitat) and 550 m altitudes. The effect of light and temperature on CO₂ and water vapour exchange was studied in the plants grown at lower altitude. In R. moorcroftianum and R. nepalensis, the average photosynthesis rates were found to be about three times higher at 550 m as compared to that under their natural habitat. However, in M. polyandra, the CO₂ exchange rates were two times higher at 3600 m than at 550 m but in R. emodi, there were virtually no differences at the two altitudes. These results indicate the variations in the CO₂ exchange rates are species specific. The change in growth altitude does not affect this process uniformly.The transpiration rates in R. emodi and M. polyandra were found to be very high at 3600 m compared to 550 m and are attributed to overall higher stomatal conductance in plants of these species, grown at higher altitude. The mid-day closure of stomata and therefore, restriction of transpirational losses of water were observed in all the species at 550 m altitude. In addition to the effect of temperature and relative humidity, the data also indicate some endogenous rhythmic control of stomatal conductance.The temperature optima for photosynthesis was close to 30°C in M. polyandra and around 20°C in the rest of the three species. High temperature and high light intensity, as well as low temperature and high light intensity, adversely affect the net rate of photosynthesis in these species.Both light compensation point and dark respiration rate increased with increasing temperature.The effect of light was more prominent on photosynthesis than the effect of temperature, however, on transpiration the effect of temperature was more prominent than the effect of light intensity.No definite trends were found in stomatal conductance with respect to light and temperature. Generally, the stomatal conductance was highest at 20°C.The study reveals that all these species can easily be cultivated at relatively lower altitudes. However, proper agronomical methodology will need to be developed for better yields.     

Responses of soil biota to elevated atmospheric carbon dioxide

Increasing concentrations of atmospheric CO₂ could have dramatic effects upon terrestrial ecosystems including changes in ecosystem structure, nutrient cycling rates, net primary production, C source-sink relationships and successional patterns. All of these potential changes will be constrained to some degree by below ground processes and mediated by responses of soil biota to indirect effects of CO₂ enrichment. A review of our current state of knowledge regarding responses of soil biota is presented, covering responses of mycorrhizae, N-fixing bacteria and actinomycetes, soil microbiota, plant pathogens, and soil fauna. Emphasis will be placed on consequences to biota of increasing C input through the rhizosphere and resulting feedbacks to above ground systems. Rising CO₂ may also result in altered nutrient concentrations of plant litter, potentially changing decomposition rates through indirect effects upon decomposer communities. Thus, this review will also cover current information on decomposition of litter produced at elevated CO₂. Summary Predictably, the responses of soil biota to CO₂ enrichment and the degree of experimental emphasis on them increase with proximity to, and intimacy with, roots. Symbiotic associations are all stimulated to some degree. Total plant mycorrhization increases with elevated CO₂. VAM fungi increase proportionately with fine root length/mass increase. ECM fungi, however, exhibit greater colonization per unit root length/mass at elevated CO₂ than at current atmospheric levels. Total N-fixation per plant increases in all species examined, although the mechanisms of increase, as well as the eventual benefit to the host relative to N uptake may vary. Microbial responses are unclear. The assumption that changes in root exudation will drive increased mineralization and facilitate nutrient uptake should be examined experimentally, in light of recent models. Microbial results to date suggest that metabolic activity (measured as changes in process rates) is stimulated by root C input, rather than population size (measured by cell or colony counts). Insufficient evidence exists to predict responses of either soil-borne plant pathogens or soil fauna (i.e., food web responses). These are areas requiring attention, the first for its potential to limit ecosystem production through disease and the second because of its importance to nutrient cycling processes. Preliminary data on foliar litter decomposition suggests that neither nutrient ratios nor decomposition rates will be affected by rising CO₂. This is another important area that may be better understood as the number of longer term studies with more realistic CO₂ exposures increase. Evidence continues to mount that C fixation increases with CO₂ enrichment and that the bulk of this C enters the belowground component of ecosystems. The global fate and effects of this additional C may affect all hierarchical levels, from organisms to ecosystems, and will be largely determined by responses of soil biota.     

Decomposition of tree leaf litters grown under elevated CO2: Effect of litter quality

Ash (Fraxinus excelsior L.), birch (Betula pubescens Ehrh.), sycamore (Acer pseudoplatanus L.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) leaf litters were monitored for decomposition rates and nutrient release in a laboratory microcosm experiment. Litters were derived from solar domes where plants had been exposed to two different CO₂ regimes: ambient (350 L L^-1 CO₂) and enriched (600 L L^-1 CO₂).Elevated CO₂ significantly affected some of the major litter quality parameters, with lower N, higher lignin concentrations and higher ratios of C/N and lignin/N for litters derived from enriched CO₂. Respiration rates of the deciduous species were significantly decreased for litters grown under elevated CO₂, and reductions in mass loss at the end of the experiment were generally observed in litters derived from the 600 ppm CO₂ treatment. Nutrient mineralization, dissolved organic carbon, and pH in microcosm leachates did not differ significantly between the two CO₂ treatments for any of the species studied. Litter quality parameters were examined for correlations with cumulative respiration and decomposition rates: N concentration, C/N and lignin/N ratios showed the highest correlations, with differences between litter types. The results indicate that higher C storage will occur in soil as a consequence of litter quality changes resulting from higher atmospheric concentrations of CO₂.Abbreviations CHO soluble carbohydrates - DOC dissolved organic carbon - HCel holocellulose - WTREM weight remaining     

Species control variation in litter decomposition in a pine forest exposed to elevated CO2

Net primary production and the flux of dry matter and nutrients from vegetation to soils has increased following four years of exposure to elevated CO₂ in a southern pine forest in NC, USA. This has increased the demand for nutrients to support enhanced rates of NPP and altered the conditions for litter decomposition on the forest floor. We quantified the chemistry and decomposition dynamics of leaf litter produced by five of the most abundant tree species in this ecosystem during the third and fourth growing seasons under elevated CO₂. The objectives of this study were to determine (i) if there were systemic or species‐specific changes in leaf litter chemistry associated with a sustained enhancement of plant growth under elevated CO₂; and (ii) whether the process of litter decomposition was altered by increased inputs of energy and nutrients to the forest floor in the plots under elevated CO₂. Leaf litter chemistry, including various C fractions and N concentration, was virtually unchanged by elevated CO₂. With few exceptions, plant litter produced under elevated CO₂ lost mass or N at the same relative rate as that produced under ambient CO₂. The relationship between initial litter chemistry and decomposition was not altered by elevated CO₂. The greater forest floor mass and nutrient content in the plots under elevated CO₂ had no consistent or long‐term effect on litter decomposition. Thus, we found no evidence that plant and microbial processes under elevated CO₂ resulted in systemic changes in mass loss or N dynamics during decomposition. In contrast to the limited effects of elevated CO₂ on litter chemistry and decomposition, there were large differences among species in initial litter chemistry, mass loss and N dynamics during decomposition. If the species composition of this forest community is altered by elevated CO₂, the indirect effect of a change in species composition will exert greater control over the long‐term rate of nutrient cycling than the direct effect of elevated CO₂ on litter chemistry and decomposition dynamics alone.     

Free and total thyroid hormones in humans at extreme altitude

Alterations in circulatory levels of total T₄ (TT₄), total T₃ (TT₃), free T₄ (FT₄), free T₃ (FT₃), thyrotropin (TSH) and T₃ uptake (T₃U) were studied in male and female sea-level residents (SLR) at sea level, in Armed forces personnel staying at high altitude (3750 m) for prolonged duration (acclimatized lowlanders, ALL) and in high-altitude natives (HAN). Identical studies were also performed on male ALL who trekked to an extreme altitude of 5080 m and stayed at an altitude of more than 6300 m for about 6 months. The total as well as free thyroid hormones were found to be significantly higher in ALL and HAN as compared to SLR values. Both male as well as female HAN had higher levels of thyroid hormones. The rise in hormone levels in different ALL ethnic groups drawn from amongst the southern and northern parts of the country was more or less identical. In both HAN and ALL a decline in FT₃ and FT₄ occurred when these subjects trekked at subzero temperatures to extreme altitude of 5080 m but the levels were found to be higher in ALL who stayed at 6300 m for a prolonged duration. Plasma TSH did not show any appreciable change at lower altitudes but was found to be decreased at extreme altitude. The increase in thyroid hormones at high altitude was not due to an increase in hormone binding proteins, since T₃U was found to be higher at high altitudes. A decline in TSH and hormone binding proteins and an increase in the free moiety of the hormones is indicative of a subtle degree of tissue hyperthyroidism which may be playing an important role in combating the extreme cold and hypoxic environment of high altitudes.     

Decomposition of secondary compounds from needle litter of Scots pine grown under elevated CO2 and O3

Elevated atmospheric carbon dioxide (CO₂) and ozone (O₃) concentrations have both been shown to affect plant tissue quality, which in turn could affect litter decomposition and carbon (C) and nutrient cycling. In order to evaluate effects of climate change on litter chemistry, needle litter was collected from Scots pine (Pinus sylvestris L.) saplings exposed to elevated CO₂ or O₃ concentration and their combination over three growing seasons in open‐top chambers. The decomposition of needle litter was followed for 19 months in a pine forest. During decomposition, needle samples for secondary compound analysis were collected and the mass loss of needles was followed. Main nutrients and total phenolics were analysed from litter in the beginning and at the end of the experiment. After 19‐month decomposition, the accumulated mass loss was about 34%; however, no significant differences were found in decomposition rates of needle litter between various treatments. Concentrations of total monoterpenes were about 4%, total resin acids 21% and total phenolics 14% of the initial concentrations in litter after 19‐month decomposition. In the beginning of litter decomposition, concentrations of individual monoterpenes –α‐pinene and β‐pinene – were significantly higher in needle litter grown under elevated CO₂. However, concentrations of total monoterpenes during the whole decomposition period were not significantly affected by CO₂ or O₃ treatments. Concentrations of some individual and total resin acids were higher in needle litter grown under elevated CO₂ or O₃ than under ambient air. There were no significant differences in concentrations of total phenolics as well as nitrogen (N) and the main nutrient concentrations between treatments during decomposition. High concentrations of monoterpenes and resin acids in needles might slightly delay C recycling in forest soils. It is concluded that elevated CO₂ and O₃ concentrations do not have remarkable impacts on litter decomposition processes in Scots pine forests.     

Effects of enhanced atmospheric CO2 and nutrient supply on the quality and subsequent decomposition of fine roots of Betula pendula Roth. and Picea sitchensis (Bong.) Carr.

Fine root litter derived from birch (Betula pendula Roth.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) plants grown under two CO₂ atmospheric concentrations (350 ppm and 600 ppm) and two nutrient regimes was used for decomposition studies in laboratory microcosms. Although there were interactions between litter type, CO₂/fertiliser treatments and decomposition rates, in general, an increase in the C/N ratio of the root tissue was observed for roots of both species grown under elevated CO₂ in unfertilized soil. Both weight loss and respiration of decomposing birch roots were significantly reduced in materials derived from enriched CO₂, whilst the decomposition of spruce roots showed no such effect. A parallel experiment was performed using Betula pendula root litter grown under different N regimes, in order to test the relationship between C/N ratio of litter and root decomposition rate. A highly significant (p<0.001) negative correlation between C/N ratio and root litter respiration was found, with an r²=0.97. The results suggest that the increased C/N ratio of plant tissues induced by elevated CO₂ can result in a reduction of decomposition rate, with a resulting increase in forest soil C stores.     

Elevated CO2 reduces field decomposition rates of Betula pendula (Roth.) leaf litter

The effect of elevated atmospheric CO₂ and nutrient supply on elemental composition and decomposition rates of tree leaf litter was studied using litters derived from birch (Betula pendula Roth.) plants grown under two levels of atmospheric CO₂ (ambient and ambient +250 ppm) and two nutrient regimes in solar domes. CO₂ and nutrient treatments affected the chemical composition of leaves, both independently and interactively. The elevated CO₂ and unfertilized soil regime significantly enhanced lignin/N and C/N ratios of birch leaves. Decomposition was studied using field litter-bags, and marked differences were observed in the decomposition rates of litters derived from the two treatments, with the highest weight remaining being associated with litter derived from the enhanced CO₂ and unfertilized regime. Highly significant correlations were shown between birch litter decomposition rates and lignin/N and C/N ratios. It can be concluded, from this study, that at levels of atmospheric CO₂ predicted for the middle of the next century a deterioration of litter quality will result in decreased decomposition rates, leading to reduction of nutrient mineralization and increased C storage in forest ecosystems. However, such conclusions are difficult to generalize, since tree responses to elevated CO₂ depend on soil nutritional status.     

川西亚高山/高山森林土壤氧化还原酶活性及其对季节性冻融的响应

川西亚高山/高山森林土壤通常具有明显的季节性冻融特征。为深入了解川西亚高山/高山森林冬季土壤生态过程,于2008年11月-2009年10月,在土壤初冻期、冻结期和融化期及生长季节,研究了不同海拔(3582 m、3292 m和3023 m)岷江冷杉林的土壤氧化还原酶活性及其对土壤冻融的响应。土壤冻结时间和冻融循环次数随海拔的增加而增加。冻融格局显著影响了土壤氧化还原酶活性,但不同土壤酶在不同海拔表现出明显差异。土壤过氧化物酶和脱氢酶活性受初冻期冻融循环和温度降低影响显著下降,而过氧化氢酶活性明显上升。3种土壤氧化还原酶活性在土壤温度相对稳定的冻结期变化不显著,但在融化期随着土壤温度急剧增加经历一个明显的活性高峰后快速降低,且冻结时间最长和冻融循环次数最多的3582 m变化更为显著。此外,海拔和土层的交互作用显著影响了过氧化物物活性,但对脱氢酶和过氧化氢酶活性不显著。脱氢酶活性与土壤温度极显著相关,但过氧化物酶和过氧化氢酶活性与土壤温度的相关性随海拔差异而不同。这些结果表明川西亚高山/高山森林冬季土壤氧化还原酶仍然具有较高的活性,但受到季节性冻融及其变化的显著影响。