Methanol and other VOC fluxes from a Danish beech forest during late springtime

In-canopy mixing ratio gradients and above-canopy fluxes of several volatile organic compounds (VOCs) were measured using a commercial proton transfer reaction mass spectrometer (PTR-MS) in a European beech (Fagus sylvatica) forest in Denmark. Fluxes of methanol were bidirectional: Emission occurred during both day and night with highest fluxes (0.2 mg C m⁻² h⁻¹) during a warm period; deposition occurred dominantly at daytime. Confirming previous branch-level measurements on beech, the forest’s monoterpene emissions (0–0.5 mg C m⁻² h⁻¹), and in-canopy mixing ratios showed a diurnal cycle consistent with light-dependent emissions; a result contrasting temperature-only driven emissions of most conifer species. Also emitted was acetone, but only at ambient temperatures exceeding 20°C. Slow deposition dominated at lower temperatures. Our in-canopy gradient measurements contrast with earlier results from tropical and pine forest ecosystems in that they did not show this beech ecosystem to be a strong sink for oxygenated VOCs (OVOCs). Instead, their gradients were flat and only small deposition velocities (<0.2 cm s⁻¹) were observed to the onsite soil. However, as methanol soil uptake was consistent and possibly related to soil moisture, more measurements are needed to evaluate its soil sink strength. In turn, as canopy scale fluxes are net fluxes with stomatal emissions from photosynthesizing leaves potentially affecting non-stomatal oxygenated VOC uptake, only independent, controlled laboratory experiments may be successful in separating gross fluxes.     

Physiological differences in the growth of <Emphasis Type="Italic">Sargassum horneri</Emphasis> between the germling and adult stages

The effects of temperature, irradiance, and daylength on Sargassum horneri growth were examined at the germling and adult stages to discern their physiological differences. Temperature–irradiance (10, 15, 20, 25, 30°C × 20, 40, 80 μmol photons m⁻²s⁻¹) and daylength (8, 12, 16, 24 h) experiments were carried out. The germlings and blades of S. horneri grew over a wide range of temperatures (10–25°C), irradiances (20–80 μmol photons m⁻²s⁻¹), and daylengths (8–24 h). At the optimal growth conditions, the relative growth rates (RGR) of the germlings were 21% day⁻¹ (25°C, 20 μmol photons m⁻²s⁻¹) and 13% day⁻¹ (8 h daylength). In contrast, the RGRs of the blade weights were 4% day⁻¹ (15°C, 20 μmol photons m⁻²s⁻¹) and 5% day⁻¹ (12 h daylength). Negative growth rates were found at 20 μmol photons m⁻²s⁻¹ of 20°C and 25°C treatments after 12 days. This phenomenon coincides with the necrosis of S. horneri blades in field populations. In conclusion, we found physiological differences between S. horneri germlings and adults with respect to daylength and temperature optima. The growth of S. horneri germlings could be enhanced at 25°C, 20 μmol photons m⁻²s⁻¹, and 8 h daylength for construction of Sargassum beds and restoration of barren areas.     

N2O, CH4 and CO2 emissions from seasonal tropical rainforests and a rubber plantation in Southwest China

The main focus of this study was to evaluate the effects of soil moisture and temperature on temporal variation of N₂O, CO₂ and CH₄ soil-atmosphere exchange at a primary seasonal tropical rainforest (PF) site in Southwest China and to compare these fluxes with fluxes from a secondary forest (SF) and a rubber plantation (RP) site. Agroforestry systems, such as rubber plantations, are increasingly replacing primary and secondary forest systems in tropical Southwest China and thus effect the N₂O emission in these regions on a landscape level. The mean N₂O emission at site PF was 6.0 ± 0.1 SE μg N m⁻² h⁻¹. Fluxes of N₂O increased from <5 μg N m⁻² h⁻¹ during dry season conditions to up to 24.5 μg N m⁻² h⁻¹ with re-wetting of the soil by the onset of first rainfall events. Comparable fluxes of N₂O were measured in the SF and RP sites, where mean N₂O emissions were 7.3 ± 0.7 SE μg N m⁻² h⁻¹ and 4.1 ± 0.5 SE μg N m⁻² h⁻¹, respectively. The dependency of N₂O fluxes on soil moisture levels was demonstrated in a watering experiment, however, artificial rainfall only influenced the timing of N₂O emission peaks, not the total amount of N₂O emitted. For all sites, significant positive correlations existed between N₂O emissions and both soil moisture and soil temperature. Mean CH₄ uptake rates were highest at the PF site (−29.5 ± 0.3 SE μg C m⁻² h⁻¹), slightly lower at the SF site (−25.6 ± 1.3 SE μg C m⁻² h⁻¹) and lowest for the RP site (−5.7 ± 0.5 SE μg C m⁻² h⁻¹). At all sites, CH₄ uptake rates were negatively correlated with soil moisture, which was also reflected in the lower uptake rates measured in the watering experiment. In contrast to N₂O emissions, CH₄ uptake did not significantly correlate with soil temperature at the SF and RP sites, and only weakly correlated at the PF site. Over the 2 month measurement period, CO₂ emissions at the PF site increased significantly from 50 mg C m⁻² h⁻¹ up to 100 mg C m⁻² h⁻¹ (mean value 68.8 ± 0.8 SE mg C m⁻² h⁻¹), whereas CO₂ emissions at the SF and RP site where quite stable and varied only slightly around mean values of 38.0 ± 1.8 SE mg C m⁻² h⁻¹ (SF) and 34.9 ± 1.1 SE mg C m⁻² h⁻¹ (RP). A dependency of soil CO₂ emissions on changes in soil water content could be demonstrated for all sites, thus, the watering experiment revealed significantly higher CO₂ emissions as compared to control chambers. Correlation of CO₂ emissions with soil temperature was significant at the PF site, but weak at the SF and not evident at the RP site. Even though we demonstrated that N and C trace gas fluxes significantly varied on subdaily and daily scales, weekly measurements would be sufficient if only the sink/ source strength of non-managed tropical forest sites needs to be identified.     

Six-year Stable Annual Uptake of Carbon Dioxide in Intensively Managed Humid Temperate Grassland

Variability and future alterations in regional and global climate patterns may exert a strong control on the carbon dioxide (CO₂) exchange of grassland ecosystems. We used 6 years of eddy-covariance measurements to evaluate the impacts of seasonal and inter-annual variations in environmental conditions on the net ecosystem CO₂ exchange (NEE), gross ecosystem production (GEP), and ecosystem respiration (ER) of an intensively managed grassland in the humid temperate climate of southern Ireland. In all the years of the study period, considerable uptake of atmospheric CO₂ occurred in this grassland with a narrow range in the annual NEE from −245 to −284 g C m⁻² y⁻¹, with the exception of 2008 in which the NEE reached −352 g C m⁻² y⁻¹. None of the measured environmental variables (air temperature (Ta), soil moisture, photosynthetically active radiation, vapor pressure deficit (VPD), precipitation (PPT), and so on) correlated with NEE on a seasonal or annual scale because of the equal responses from the component fluxes GEP and ER to variances in these variables. Pronounced reduction of summer PPT in two out of the six studied years correlated with decreases in both GEP and ER, but not with NEE. Thus, the stable annual NEE was primarily achieved through a strong coupling of ER and GEP on seasonal and annual scales. Limited inter-annual variations in Ta (±0.5°C) and generally sufficient soil moisture availability may have further favored a stable annual NEE. Monthly ecosystem carbon use efficiency (CUE; as the ratio of NEE:GEP) during the main growing season (April 1–September 30) was negatively correlated with temperature and VPD, but positively correlated with soil moisture, whereas the annual CUE correlated negatively with annual NEE. Thus, although drier and warmer summers may mildly reduce the uptake potential, the annual uptake of atmospheric CO₂, in this intensively managed grassland, may be expected to continue even under predicted future climatic changes in the humid temperate climate region.     

Characteristics of soil CO2 efflux variability in an aseasonal tropical rainforest in Borneo Island

Although soil carbon dioxide (CO₂) efflux from tropical forests may play an important role in global carbon (C) balance, our knowledge of the fluctuations and factors controlling soil CO₂ efflux in the Asian tropics is still poor. This study characterizes the temporal and spatial variability in soil CO₂ efflux in relation to temperature/moisture content and estimates annual efflux from the forest floor in an aseasonal intact tropical rainforest in Sarawak, Malaysia. Soil CO₂ efflux varied widely in space; the range of variation averaged 17.4 μmol m⁻² s⁻¹ in total. While most CO₂ flux rates were under 10 μmol m⁻² s⁻¹, exceptionally high fluxes were observed sporadically at several sampling points. Semivariogram analysis revealed little spatial dependence in soil CO₂ efflux. Temperature explained nearly half of the spatial heterogeneity, but the effect varied with time. Seasonal variation in CO₂ efflux had no fixed pattern, but was significantly correlated with soil moisture content. The correlation coefficient with soil moisture content (SMC) at 30 and 60 cm depth was higher than at 10 cm depths. The annual soil CO₂ efflux, estimated from the relationship between CO₂ efflux and SMC at 30 cm depth, was 165 mol m⁻² year⁻¹ (1,986 g C m⁻² year⁻¹). As this area is known to suffer severe drought every 4–5 years caused by the El Nino-Southern Oscillation, the results suggest that an unpredictable dry period might affect soil CO₂ efflux, leading an annual variation in soil C balance.     

Increase in isoprene and monoterpene emissions after re-watering of droughted Quercus ilex seedlings

We followed the diurnal cycles of isoprenoid emissions from Quercus ilex seedlings under drought and after re-watering. We found that Quercus ilex, generally considered a non-isoprene emitter, also emitted isoprene although at low rates. The emission rates of isoprene reached 0.37 ± 0.02 nmol m⁻² s⁻¹ in controls, 0.15 ± 0.03 nmol m⁻² s⁻¹ under drought and 0.35 ± 0.04 nmol m⁻² s⁻¹ after re-watering, while emission rates of monoterpenes reached 11.0 ± 3.0, 7.0 ± 1.0 and 23.0 ± 5.0 nmol m⁻² s⁻¹, respectively. Emission rates recovered faster after re-watering than photosynthetic rate and followed diurnal changes in irradiance in controls and under drought, but in leaf temperature after re-watering.     

Subsurface CO<Subscript>2</Subscript> Dynamics in Temperate Beech and Spruce Forest Stands

Rates of soil respiration (CO₂ effluxes), subsurface pore gas CO₂/O₂ concentrations, soil temperature and soil water content were measured for 15 months in two temperate and contrasting Danish forest ecosystems: beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.). Soil CO₂ effluxes showed a distinct seasonal trend in the range of 0.48–3.3 μmol CO₂ m⁻² s⁻¹ for beech and 0.50–2.92 μmol CO₂ m⁻² s⁻¹ for spruce and were well-correlated with near-surface soil temperatures. The soil organic C-stock (upper 1 m including the O-horizon) was higher in the spruce stand (184±23 Mg C ha⁻¹) compared to the beech stand (93±19 Mg C ha⁻¹) and resulted in a faster turnover time as calculated by mass/flux in soil beneath the beech stand (28 years) compared to spruce stand (60 years). Observed soil CO₂ concentrations and effluxes were simulated using a Fickian diffusion-reaction model based on vertical CO₂ production rates and soil diffusivity. Temporal trends were simulated on the basis of observed trends in the distribution of soil water, temperature, and live roots as well as temperature and water content sensitivity functions. These functions were established based on controlled laboratory incubation experiments. The model was successfully validated against observed soil CO₂ effluxes and concentrations and revealed that temporal trends generally could be linked to variations in subsurface CO₂ production rates and diffusion over time and with depths. However, periods with exceptionally high CO₂ effluxes (> 20 μmol CO₂ m⁻² s⁻¹) were noted in March 2000 in relation to drying after heavy rain and after the removal of snow from collars. Both cases were considered non-steady state and could not be simulated.     

Soil carbon storage, litterfall and CO2 efflux in fertilized and unfertilized larch (Larix leptolepis) plantations

This study evaluated the effects of forest fertilization on the forest carbon (C) dynamics in a 36-year-old larch (Larix leptolepis) plantation in Korea. Above- and below-ground C storage, litterfall, root decomposition and soil CO₂ efflux rates after fertilization were measured for 2 years. Fertilizers were applied to the forest floor at rates of 112 kg N ha⁻¹ year⁻¹, 75 kg P ha⁻¹ year⁻¹ and 37 kg K ha⁻¹ year⁻¹ for 2 years (May 2002, 2003). There was no significant difference in the above-ground C storage between fertilized (41.20 Mg C ha⁻¹) and unfertilized (42.25 Mg C ha⁻¹) plots, and the C increment was similar between the fertilized (1.65 Mg C ha⁻¹ year⁻¹) and unfertilized (1.52 Mg C ha⁻¹ year⁻¹) plots. There was no significant difference in the soil C storage between the fertilized and unfertilized plots at each soil depth (0–15, 15–30 and 30–50 cm). The organic C inputs due to litterfall ranged from 1.57 Mg C ha⁻¹ year⁻¹ for fertilized to 1.68 Mg C ha⁻¹ year⁻¹ for unfertilized plots. There was no significant difference in the needle litter decomposition rates between the fertilized and unfertilized plots, while the decomposition of roots with 1–2 mm diameters increased significantly with the fertilization relative to the unfertilized plots. The mean annual soil CO₂ efflux rates for the 2 years were similar between the fertilized (0.38 g CO₂ m⁻² h⁻¹) and unfertilized (0.40 g CO₂ m⁻² h⁻¹) plots, which corresponded with the similar fluctuation in the organic carbon (litterfall, needle and root decomposition) and soil environmental parameters (soil temperature and soil water content). These results indicate that little effect on the C dynamics of the larch plantation could be attributed to the 2-year short-term fertilization trials and/or the soil fertility in the mature coniferous plantation used in this study.     

The influence of temperature and nitrogen source on growth and nitrogen uptake of two polar-desert species,Saxifraga caespitosa and Cerastium alpinum

Polar-desert plants experience low average air temperatures during their short growing season (4–8 °C mean July temperature). In addition, low availability of inorganic nitrogen in the soil may also limit plant growth. Our goals were to elucidate which N sources can be acquired by polar-desert plants, and how growth and N-uptake are affected by low growth temperatures. We compared rates of N-uptake and increases in mass and leaf area of two polar-desert species (Cerastium alpinum L. and Saxifraga caespitosa L.) over a period of 3 weeks when grown at two temperatures (6 °C vs. 15 °C) and supplied with either glycine, NH₄ ⁺ or NO₃ ⁻. At 15 °C, plants at least doubled their leaf area, whereas there was no change in leaf area at 6 °C. Measured mean N-uptake rates varied between 0.5 nmol g⁻¹ root DM s⁻¹ on glycine at 15 °C and 7.5 nmol g⁻¹ root DM s⁻¹ on NH₄ ⁺ at 15 °C. Uptake rates based upon increases in mass and tissue N concentrations showed that plants had a lower N-uptake rate at 6 °C, regardless of N source or species. We conclude that these polar-desert plants can use all three N sources to increase their leaf area and support flowering when grown at 15 °C. Based upon short-term (8 h) uptake experiments, we also conclude that the short-term capacity to take up inorganic or organic N is not reduced by low temperature (6 °C). However, net N-uptake integrated over a three-week period is severely reduced at 6 °C. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of light and temperature on the cyanobacterium <Emphasis Type="Italic">Arthronema africanum</Emphasis> - a prospective phycobiliprotein-producing strain

The effect of light intensity (50–300 μmol photons m⁻² s⁻¹) and temperature (15–50°C) on chlorophyll a, carotenoid and phycobiliprotein content in Arthronema africanum biomass was studied. Maximum growth rate was measured at 300 μmol photons m⁻² s⁻¹ and 36°C after 96 h of cultivation. The chlorophyll a content increased along with the increase in light intensity and temperature and reached 2.4% of dry weight at 150 μmol photons m⁻² s⁻¹ and 36°C, but it decreased at higher temperatures. The level of carotenoids did not change significantly under temperature changes at illumination of 50 and 100 μmol photons m⁻² s⁻¹. Carotenoids were about 1% of the dry weight at higher light intensities: 150 and 300 μmol photons m⁻² s⁻¹. Arthronema africanum contained C-phycocyanin and allophycocyanin but no phycoerythrin. The total phycobiliprotein content was extremely high, more than 30% of the dry algal biomass, thus the cyanobacterium could be deemed an alternative producer of C-phycocyanin. A highest total of phycobiliproteins was reached at light intensity of 150 μmol photons m⁻² s⁻¹ and temperature of 36°C, C-phycocyanin and allophycocyanin amounting, respectively, to 23% and 12% of the dry algal biomass. Extremely low (<15°C) and high temperatures (>47°C) decreased phycobiliprotein content regardless of light intensity.     

Pools and fluxes of carbon in three Norway spruce ecosystems along a climatic gradient in Sweden

This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001–2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190–233 g C m⁻² year⁻¹). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from −8 g C m⁻² year⁻¹ in the north to +9 g C m⁻² year⁻¹ in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m⁻² year⁻¹. An erratum to this article can be found at     

Variations in daytime net carbon and water exchange in a montane shrubland ecosystem in southeast Spain

Carbon and water fluxes in a semiarid shrubland ecosystem located in the southeast of Spain (province of Almería) were measured continuously over one year using the eddy covariance technique. We examined the influence of environmental variables on daytime (photosynthetically active photons, F _P >10 μmol m⁻² s⁻¹) ecosystem gas exchange and tested the ability of an empirical eco-physiological model based on F _P to estimate carbon fluxes over the whole year. The daytime ecosystem fluxes showed strong seasonality. During two solstitial periods, summer with warm temperatures (>15 °C) and sufficient soil moisture (>10 % vol.) and winter with mild temperatures (>5 °C) and high soil moisture contents (>15 % vol.), the photosynthetic rate was higher than the daytime respiration rate and mean daytime CO₂ fluxes were ca. −1.75 and −0.60 μmol m⁻² s⁻¹, respectively. Daytime evapotranspiration fluxes averaged ca. 2.20 and 0.24 mmol m⁻² s⁻¹, respectively. By contrast, in summer and early autumn with warm daytime temperatures (>10 °C) and dry soil (<10 % vol.), and also in mid-winter with near-freezing daytime temperatures the shrubland behaved as a net carbon source (mean daytime CO₂ release of ca. 0.60 and 0.20 μmol m⁻² s⁻¹, respectively). Furthermore, the comparison of water and carbon fluxes over a week in June 2004 and June 2005 suggests that the timing—rather than amount—of spring rainfall may be crucial in determining growing season water and carbon exchange. Due to strongly limiting environmental variables other than F _P, the model applied here failed to describe daytime carbon exchange only as a function of F _P and could not be used over most of the year to fill gaps in the data.     

Effects of temperature on photosynthesis of two morphologically contrasting plant species along an altitudinal gradient in the tropical high Andes

The effects of temperature on photosynthesis of a rosette plant growing at ground level, Acaena cylindrostachya R. et P., and an herb that grows 20–50 cm above ground level, Senecio formosus H.B.K., were studied along an altitudinal gradient in the Venezuelan Andes. These species were chosen in order to determine – in the field and in the laboratory – how differences in leaf temperature, determined by plant form and microenvironmental conditions, affect their photosynthetic capacity. CO₂ assimilation rates (A) for both species decreased with increasing altitude. For Acaena leaves at 2900 m, A reached maximum values above 9 μmol m⁻² s⁻¹, nearly twice as high as maximum A found at 3550 m (5.2) or at 4200 m (3.9). For Senecio leaves, maximum rates of CO₂ uptake were 7.5, 5.8 and 3.6 μmol m⁻² s⁻¹ for plants at 2900, 3550 and 4200 m, respectively. Net photosynthesis-leaf temperature relations showed differences in optimum temperature for photosynthesis (A _o.t.) for both species along the altitudinal gradient. Acaena showed similar A _o.t. for the two lower altitudes, with 19.1°C at 2900 m and 19.6°C at 3550 m, while it increased to 21.7°C at 4200 m. Maximum A for this species at each altitude was similar, between 5.5 and 6.0 μmol m⁻² s⁻¹. For the taller Senecio, A _o.t. was more closely related to air temperatures and decreased from 21.7°C at 2900 m, to 19.7°C at 3550 m and 15.5°C at 4200 m. In this species, maximum A was lower with increasing altitude (from 6.0 at 2900 m to 3.5 μmol m⁻² s⁻¹ at 4200 m). High temperature compensation points for Acaena were similar at the three altitudes, c. 35°C, but varied in Senecio from 37°C at 2900 m, to 39°C at 3550 m and 28°C at 4200 m. Our results show how photosynthetic characteristics change along the altitudinal gradient for two morphologically contrasting species influenced by soil or air temperatures. Received: 5 July 1997 / Accepted: 25 October 1997     

Effects of light and temperature on the attachment and development of <Emphasis Type="BoldItalic">Gloiopeltis tenax</Emphasis> and <Emphasis Type="BoldItalic">Gloiopeltis furcata</Emphasis> tetraspores

Two 60-day experiments were conducted to study the influence of photon flux density (PFD) and temperature on the attachment and development of Gloiopeltis tenax and Gloiopeltis furcata tetraspores. In the first experiment, tetraspores of the two Gloiopeltis species were incubated at five temperature ranges (8°C, 12°C, 16°C, 20°C, 24°C) under a constant PFD of 80 μmol photons m⁻² s⁻¹ with a photoperiod of 12:12. In a second experiment, tetraspores were incubated under five PFD gradients (30, 55, 80, 105, 130 μmol photons m⁻² s⁻¹) at a constant temperature of 16°C with a photoperiod of 12:12. Maximum density of attached tetraspores was observed at 16°C for both species. Maximum per cent of spore germinating into disc was recorded at 12–16°C for G. tenax and 8–12°C for G. furcata. Maximum per cent of discs producing erect axes for G. tenax and G. furcata were recorded at 24°C and 20°C, respectively. Light had no significant effect on tetraspore attachment and developing into disc, but it affected the growth, sprouting and survival of its discs. Under 30–55 μmol photons m⁻² s⁻¹, the discs of the two species of Gloiopeltis did not form thallus until the end of the experiment. Optimum PFD range for G. tenax discs was 80–105 μmol photons m⁻² s⁻¹, whilst it was 80–130 μmol photons m⁻² s⁻¹ for G. furcata. Results presented in this study are expected to assist the progress of artificial seeding of Gloiopeltis.     

Soil organic matter dynamics under soybean exposed to elevated [CO2]

It is unclear how changing atmospheric composition will influence the plant–soil interactions that determine soil organic matter (SOM) levels in fertile agricultural soils. Positive effects of CO₂ fertilization on plant productivity and residue returns should increase SOM stocks unless mineralization or biomass removal rates increase in proportion to offset gains. Our objectives were to quantify changes in SOM stocks and labile fractions in prime farmland supporting a conventionally managed corn–soybean system and the seasonal dynamics of labile C and N in soybean in plots exposed to elevated [CO₂] (550 ppm) under free-air concentration enrichment (FACE) conditions. Changes in SOM stocks including reduced C/N ratios and labile N stocks suggest that SOM declined slightly and became more decomposed in all plots after 3 years. Plant available N (>273 mg N kg⁻¹) and other nutrients (Bray P, 22–50 ppm; extractable K, 157–237 ppm; Ca, 2,378–2,730 ppm; Mg, 245–317 ppm) were in the high to medium range. Exposure to elevated [CO₂] failed to increase particulate organic matter C (POM-C) and increased POM-N concentrations slightly in the surface depth despite known increases (≈30%) in root biomass. This, and elevated CO₂ efflux rates indicate accelerated decay rates in fumigated plots (2001: elevated [CO₂]: 10.5 ± 1.2 μmol CO₂ m⁻² s⁻¹ vs. ambient: 8.9 ± 1.0 μmol CO₂ m⁻² s⁻¹). There were no treatment-based differences in the within-season dynamics of SOM. Soil POM-C and POM-N contents were slightly greater in the surface depth of elevated than ambient plots. Most studies attribute limited ability of fumigated soils to accumulate SOM to N limitation and/or limited plant response to CO₂ fertilization. In this study, SOM turnover appears to be accelerated under elevated [CO₂] even though soil moisture and nutrients are non-limiting and plant productivity is consistently increased. Accelerated SOM turnover rates may have long-term implications for soil’s productive potential and calls for deeper investigation into C and N dynamics in highly-productive row crop systems.     

CO2 exchange of the endolithic lichen Verrucaria baldensis from karst habitats in northern Italy

CO₂ exchange of the endolithic lichen Verrucaria baldensis was measured in the laboratory under different conditions of water content, temperature, light, and CO₂ concentration. The species had low CO₂ exchange rates (maximum net photosynthesis: c. 0.45 μmol CO₂ m⁻² s⁻¹; maximum dark respiration: c. 0.3 μmol CO₂ m⁻² s⁻¹) and a very low light compensation point (7 μmol photons m⁻² s⁻¹ at 8°C). The net photosynthesis/respiration quotient reached a maximum at 9–15°C. Photosynthetic activity was affected only after very severe desiccation, when high resaturation respiratory rates were measured. Microclimatic data were recorded under different weather conditions in an abyss of the Trieste Karst (northeast Italy), where the species was particularly abundant. Low photosynthetically active radiation (normally below 40 μmol photons m⁻² s⁻¹), very high humidities (over 80%), and low, constant temperatures were measured. Thallus water contents sufficient for CO₂ assimilation were often measured in the absence of condensation phenomena. Received: 22 September 1996 / Accepted: 26 April 1997     

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]     

Microphytobenthos of Arctic Kongsfjorden (Svalbard, Norway): biomass and potential primary production along the shore line

During summer 2007, Arctic microphytobenthic potential primary production was measured at several stations around the coastline of Kongsfjorden (Svalbard, Norway) at ≤5 m water depth and at two stations at five different water depths (5, 10, 15, 20, 30 m). Oxygen planar optode sensor spots were used ex situ to determine oxygen exchange in the overlying water of intact sediment cores under controlled light (ca. 100 μmol photons m⁻² s⁻¹) and temperature (2–4°C) conditions. Patches of microalgae (mainly diatoms) covering sandy sediments at water depths down to 30 m showed high biomass of up to 317 mg chl a m⁻². In spite of increasing water depth, no significant trend in “photoautotrophic active biomass” (chl a, ratio living/dead cells, cell sizes) and, thus, in primary production was measured at both stations. All sites from ≤5 to 30 m water depth exhibited variable rates of net production from −19 to +40 mg O₂ m⁻² h⁻¹ (−168 to +360 mg C m⁻² day⁻¹) and gross production of about 2–62 mg O₂ m⁻² h⁻¹ (17–554 mg C m⁻² day⁻¹), which is comparable to other polar as well as temperate regions. No relation between photoautotrophic biomass and gross/net production values was found. Microphytobenthos demonstrated significant rates of primary production that is comparable to pelagic production of Kongsfjorden and, hence, emphasised the importance as C source for the zoobenthos.     

Impact of soil vertical water movement on the energy balance of different land surfaces

The soil heat flux determination method proposed by Gao (Boundary-Layer Meteorol 114:165–178, 2005) is discussed for (1) dry surfaces, (2) bare soil or sparse short-grass lands, and (3) dense-grass surfaces or forest. Our analysis shows that, when neglecting the contribution of soil vertical water movement to soil heat flux, the energy components measured independently will (1) still achieve balance over dry surfaces, and (2) be significantly in imbalance over bare soil or sparse short-grass lands. The mean of bare ground evaporation modeled by SiB2 is 1.58 × 10⁻⁵ m³ s⁻¹ m⁻², and the mean of soil water flux obtained by the method of Gao is 1.22 × 10⁻⁵ m³ s⁻¹ m⁻² for the Naqu site in the summer of 1998. Comparison of the bare ground evaporation with the mean of soil water flux shows a difference, the causes of which are investigated. Physically, the bare ground evaporation is equal to the sum of soil water flux and water content change in the soil surface layer. Because the bare ground evaporation is very limited for the dense-grass surfaces or forest, our analysis implies that the energy imbalance encountered over the dense-grass or forest is not caused by the fact that previous researchers neglected soil water movements in their energy budget analyses.     

Response of soil respiration to simulated N deposition in a disturbed and a rehabilitated tropical forest in southern China

Responses of soil respiration (CO₂ emission) to simulated N deposition were studied in a disturbed (reforested forest with previous understory and litter harvesting) and a rehabilitated (reforested forest with no understory and litter harvesting) tropical forest in southern China from October 2005 to September 2006. The objectives of the study were to test the following hypotheses: (1) soil respiration is higher in rehabilitated forest than in disturbed forest; (2) soil respiration in both rehabilitated and disturbed tropical forests is stimulated by N additions; and (3) soil respiration is more sensitive to N addition in disturbed forest than in rehabilitated forest due to relatively low soil nutrient status in the former, resulting from different previous human disturbance. Static chamber and gas chromatography techniques were employed to quantify the soil respiration, following different N treatments (Control, no N addition; Low-N, 5 g N m⁻² year⁻¹; Medium-N, 10 g N m⁻² year⁻¹), which had been applied continuously for 26 months before the respiration measurement. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates observed in the hot and wet growing season (April–September) and the lowest rates in winter (December–February) in both rehabilitated and disturbed forests. Soil respiration rates exhibited significant positive exponential relationship with soil temperature and significant positive linear relationship with soil moisture. Soil respiration was also significantly higher in the rehabilitated forest than in the disturbed forest. Annual mean soil respiration rate in the rehabilitated forest was 20% lower in low-N plots (71 ± 4 mg CO₂-C m⁻² h⁻¹) and 10% lower in medium-N plots (80 ± 4 mg CO₂-C m⁻² h⁻¹) than in the control plots (89 ± 5 mg CO₂-C m⁻² h⁻¹), and the differences between the control and low-N or medium-N treatments were statistically significant. In disturbed forest, annual mean soil respiration rate was 5% lower in low-N plots (63 ± 3 mg CO₂-C m⁻² h⁻¹) and 8% lower in medium-N plots (61 ± 3 mg CO₂-C m⁻² h⁻¹) than in the control plots (66 ± 4 mg CO₂-C m⁻² h⁻¹), but the differences among treatments were not significant. The depressed effects of experimental N deposition occurred mostly in the hot and wet growing season. Our results suggest that response of soil respiration to elevated N deposition in the reforested tropical forests may vary depending on the status of human disturbance. Responsible Editor: Hans Lambers.