Increase of atmospheric CO2 promotes phytoplankton productivity

It is usually thought that unlike terrestrial plants, phytoplankton will not show a significant response to an increase of atmospheric CO₂. Here we suggest that this view may be biased by a neglect of the effects of carbon (C) assimilation on the pH and the dissociation of the C species. We show that under eutrophic conditions, productivity may double as a result of doubling of the atmospheric CO₂ concentration. Although in practice productivity increase will usually be less, we still predict a productivity increase of up to 40% in marine species with a low affinity for bicarbonate. In eutrophic freshwater systems doubling of atmospheric CO₂ may result in an increase of the productivity of more than 50%. Freshwaters with low alkalinity appeared to be very sensitive to atmospheric CO₂ elevation. Our results suggest that the aquatic C sink may increase more than expected, and that nuisance phytoplankton blooms may be aggravated at elevated atmospheric CO₂ concentrations.     

Direct and indirect effects of elevated CO2 on transpiration from Quercus myrtifolia in a scrub-oak ecosystem

Elevated atmospheric carbon dioxide (C_a) usually reduces stomatal conductance, but the effects on plant transpiration in the field are not well understood. Using constant‐power sap flow gauges, we measured transpiration from Quercus myrtifolia Willd., the dominant species of the Florida scrub‐oak ecosystem, which had been exposed in situ to elevated C_a (350 µmol mol ^{? 1} above ambient) in open‐top chambers since May 1996. Elevated C_a reduced average transpiration per unit leaf area by 37%, 48% and 49% in March, May and October 2000, respectively. Temporarily reversing the C_a treatments showed that at least part of the reduction in transpiration was an immediate, reversible response to elevated C_a. However, there was also an apparent indirect effect of C_a on transpiration: when transpiration in all plants was measured under common C_a, transpiration in elevated C_a‐grown plants was lower than that in plants grown in normal ambient C_a. Results from measurements of stomatal conductance (g_s), leaf area index (LAI), canopy light interception and correlation between light and g_s indicated that the direct, reversible C_a effect on transpiration was due to changes in g_s caused by C_a, and the indirect effect was caused mainly by greater self‐shading resulting from enhanced LAI, not from stomatal acclimation. By reducing light penetration through the canopy, the enhanced self‐shading at elevated C_a decreased stomatal conductance and transpiration of leaves at the middle and bottom of canopy. This self‐shading mechanism is likely to be important in ecosystems where LAI increases in response to elevated C_a.     

Land use induced changes of organic carbon storage in soils of China

Using the data compiled from China's second national soil survey and an improved method of soil carbon bulk density, we have estimated the changes of soil organic carbon due to land use, and compared the spatial distribution and storage of soil organic carbon (SOC) in cultivated soils and noncultivated soils in China. The results reveal that ～ 57% of the cultivated soil subgroups ( ～ 31% of the total soil surface) have experienced a significant carbon loss, ranging from 40% to 10% relative to their noncultivated counterparts. The most significant carbon loss is observed for the non‐irrigated soils (dry farmland) within a semiarid/semihumid belt from northeastern to southwestern China, with the maximum loss occurring in northeast China. On the contrary, SOC has increased in the paddy and irrigated soils in northwest China. No significant change is observed for forest soils in southern China, grassland and desert soils in northwest China, as well as irrigated soils in eastern China. The SOC storage and density under noncultivated conditions in China are estimated to ～ 77.4 Pg (10¹⁵ g) and ～ 8.8 kg C m^?2, respectively, compared to a SOC storage of ～ 70.3 Pg and an average SOC density of ～ 8.0 kg C m^?2 under the present‐day conditions. This suggests a loss of ～ 7.1 Pg SOC and a decrease of ～ 0.8 kg C m^?2 SOC density due to increasing human activities, in which the loss in organic horizons has contributed to ～ 77%. This total loss of SOC in China induced by land use represents ～ 9.5% of the world's SOC decrease. This amount is equivalent to ～ 3.5 ppmv of the atmospheric CO₂ increase. Since ～ 78% of the currently cultivated soils in China have been degraded to a low/medium productivities and are responsible for most of the SOC loss, an improved land management, such as the development of irrigated and paddy land uses, would have a considerable potential in restoring the SOC storage. Assuming a restoration of ～ 50% of the lost SOC during the next 20–50 years, the soils in China would absorb ～ 3.5 Pg of carbon from the atmosphere.     

The catchment and climate regulation of pCO2 in boreal lakes

The regulation of surface water pCO₂ was studied in a set of 33 unproductive boreal lakes of different humic content, situated along a latitudinal gradient (57°N to 64°N) in Sweden. The lakes were sampled four times during one year, and analyzed on a wide variety of water chemistry parameters. With only one exception, all lakes were supersaturated with CO₂ with respect to the atmosphere at all sampling occasions. pCO₂ was closely related to the DOC concentration in lakes, which in turn was mainly regulated by catchment characteristics. This pattern was similar along the latitudinal gradient and at different seasons of the year, indicating that it is valid for a variety of climatic conditions within the boreal forest zone. We suggest that landscape characteristics determine the accumulation and subsequent supply of allochthonous organic matter from boreal catchments to lakes, which in turn results in boreal lakes becoming net sources of atmospheric CO₂.     

Soil-surface carbon dioxide efflux and microbial biomass in relation to tree density 13 years after a stand replacing fire in a lodgepole pine ecosystem

The effects of fire on soil‐surface carbon dioxide (CO₂) efflux, F_S, and microbial biomass carbon, C_mic, were studied in a wildland setting by examining 13‐year‐old postfire stands of lodgepole pine differing in tree density (< 500 to > 500 000 trees ha^?1) in Yellowstone National Park (YNP). In addition, young stands were compared to mature lodgepole pine stands (～110‐year‐old) in order to estimate ecosystem recovery 13 years after a stand replacing fire. Growing season F_S increased with tree density in young stands (1.0 µmol CO₂ m^?2 s^?1 in low‐density stands, 1.8 µmol CO₂ m^?2 s^?1 in moderate‐density stands and 2.1 µmol CO₂ m^?2 s^?1 in high‐density stands) and with stand age (2.7 µmol CO₂ m^?2 s^?1 in mature stands). Microbial biomass carbon in young stands did not differ with tree density and ranged from 0.2 to 0.5 mg C g^?1 dry soil over the growing season; C_mic was significantly greater in mature stands (0.5–0.8 mg C g^?1 dry soil). Soil‐surface CO₂ efflux in young stands was correlated with biotic variables (above‐ground, below‐ground and microbial biomass), but not with abiotic variables (litter and mineral soil C and N content, bulk density and soil texture). Microbial biomass carbon was correlated with below‐ground plant biomass and not with soil carbon and nitrogen, indicating that plant activity controls not only root respiration, but C_mic pools and overall F_S rates as well. These findings support recent studies that have demonstrated the prevailing importance of plants in controlling rates of F_S and suggest that decomposition of older, recalcitrant soil C pools in this ecosystem is relatively unimportant 13 years after a stand replacing fire. Our results also indicate that realistic predictions and modeling of terrestrial C cycling must account for the variability in tree density and stand age that exists across the landscape as a result of natural disturbances.     

Antioxidant and Pigment Composition during Autumnal Leaf Senescence in Woody Deciduous Species Differing in their Ecological Traits

Abstract: Photoprotection mechanisms have been studied during autumnal senescence in sun and shade leaves of woody plants with different ecological characteristics and senescence patterns. Three of them belonging to the same family, Betulaceae: the shade‐intolerant and early successional species (Betula alba L.), the shade‐tolerant and late successional species (Corylus avellana L.), and an N‐fixing tree with low N resorption efficiency (Alnus glutinosa L.). The other two species: a shade‐intolerant (Populus tremula L.) and a shade‐tolerant (Cornus sanguinea L.), were chosen because of their ability to accumulate anthocyanins during autumnal leaf senescence. The study of plants with different ecological strategies allowed us to establish general trends in photoprotection mechanisms during autumnal senescence, when nutrient remobilisation occurs, but also during whole leaf ontogeny. We have not found a clear relationship between shade tolerance and the level of photoprotection; the main difference between both groups of species being the presence of α‐carotene in shade leaves of shade‐tolerant species. Preceding autumn, nitrogen resorption started in mid‐summer and occurred in parallel with a slight and continuous ascorbate, chlorophyll and carotenoid degradation. However, the ascorbate pool remained highly reduced and lipid oxidation did not increase at this time. Contrasting with ascorbate, α‐tocopherol accumulated progressively in all species. Only during the last stages of senescence was chlorophyll preferentially degraded with respect to carotenoids, leading to the yellowing of leaves, except in A. glutinosa in which a large retention of chlorophyll and N took place. Senescing leaves were characterised, except in C. sanguinea, by a relative increase in the proportion of de‐epoxidised xanthophylls: zeaxanthin, antheraxanthin and lutein. The light‐induced accumulation of anthocyanins in C. sanguinea could play an additional protective role, compensating for the low retention of de‐epoxidised xanthophylls. These different strategies among deciduous species are consistent with a role for photoprotective compounds in enhancing nitrogen remobilization and storage for the next growing season.     

DIEL CHANGES IN THE COMPOSITION OF PHOTOSYNTHETIC PIGMENTS AND CELLULAR CARBON AND NITROGEN IN CHATTONELLA ANTIQUA (RAPHIDOPHYCEAE)1

An axenic clonal culture of Chattonella antiqua (Hada) Ono was grown on a 12: 12 h LD cycle in a laboratory culture tank containing 1 m³ of f/2 medium. Diel changes in mean cell volume, cellular carbon (carbon content per cell), C/N ratio, cellular Chl a, Chl a/c ratio and carotenoid composition were observed. Mean cell volume and cellular C, N and pigments increased during the light period as a result of photosynthesis and decreased with increase of cell concentration by phased cell division during the dark period. These changes indicated that carbon assimilation and pigment synthesis occurred together during the light period. However, the patterns of increase were not the same since different diel patterns were also found in the ratios of C/N and chl a/c. Photosynthetic pigments were analyzed by reversed-phase high-performance liquid chromatography with ion-pairing solution. This analysis showed that the dominant carotenoids in C. antiqua were fucoxanthin, violaxanthin and β-carotene. Diel patterns of Chls a and c were similar to that of fucoxanthin but different from those of violaxanthin and β-carotene. The cellular contents of Chl a, fucoxanthin and carbon increased in a parallel manner during the light period. On the other hand, the increase of violaxanthin was restricted to only a few hours at the beginning of the light period during cell division cycles.     

Restoration of eutrophied shallow softwater lakes based upon carbon and phosphorus limitation

Plant communities from oligotrophic, poorly buffered waters are seriously threatened by both, acidification and eutrophication/alkalinization. Acidification is mainly caused by atmospheric deposition of acidifying substances while eutrophication is often the result of inlet of nutrient enriched, calcareous brook- or groundwater. The plant production in very soft waters is often limited by low levels of inorganic carbon, nitrogen and/or phosphorus. This paper deals with the possibilities for restoration of formerly oligotrophic but now eutrophied and alkalinized softwater systems. Restoration based upon nitrogen limitation is not likely to be successful as the atmospheric deposition of nitrogen in The Netherlands is very high. Phosphorus limitation can also be a problem. One can stop the input of phosphorus and remove the mud layer, but the problem remains that also the deeper mineral sandy sediments are saturated with phosphate. A possible remedy, however, is a combination of carbon- and phosphorus limitation. Many plants from eutrophic environments never occur in very soft waters, probably as a result of carbon limitation. In addition, mobilisation of phosphate is much lower in waters with very low bicarbonate levels. Restoration of a former oligotrophic softwater lake by reducing the inlet of calcareous surface water, in combination with removal of the organic sediment layer, appeared to be very successful. Many endangered plant species such asIsoetes echinospora, Luronium natans, Deschampsia setacea andEchinodorus repens developed spontaneously from the still viable seedbank.     

Carbon flux and diversity of nematodes and termites in Cameroon forest soils

Theoretically, there are three principal ways in which ecosystem processes might respond to reductions in species richness. These theories are reviewed, and then considered in the context of a study of the diversity of soil nematodes and termites in near-primary forest sites at Mbalmayo, Cameroon, and the contribution made by these two taxa to carbon fluxes (CO₂ and CH₄) from the forest floor. Nematode abundances average 2.04 × 10⁶ m^-2, and termites between 2933 and 6957 m^-2. The site is the most species-rich yet investigated for both groups anywhere in the world, so that a very large number of species contribute to carbon fluxes. We speculate about how much redundancy might be built into the functioning of both assemblages, and point out the enormous difficulties of resolving such questions, and of producing such detailed species-inventories.     

黄土高原部分地区180－73万年间孢粉植物群和环境演变初探

根据黄土高原３个剖面的孢粉资料及测年数据,恢复了本区１８０－７３万年期间的植被面貌。在此基础上,结合中国和全球其它地区的孢粉、构造、地层、水系演化和火山活动等特征。将此期间植被、气候和环境的演化划分为３个主旋回,６个主要演化阶段,１０个亚阶段和５个事件。初步认识到,该时间段的气候环境演化有较大波动,且这些变化不限于局部地区,而是区域性的,是多种因素（内外动力）作用的结果,且有规律可循。