Productivity, carbon dioxide uptake and net energy return of microalgal bubble column photobioreactors

This work examined the energy return of Chlorella vulgaris and Dunaliella tertiolecta cultivated in a gas-sparged photobioreactor design where the power input for sparging was manipulated (10, 20, and 50 W m⁻³). Dry weight, organic carbon and heating values of the biomass were measured, plus a suite of variables including Fv/Fm and dissolved oxygen. A model for predicting the higher heating value of microalgal biomass was developed and used to measure the energetic performance of batch cultivations. High power inputs enhanced maximum biomass yields, but did not improve the energy return. Cultivation in 10 W m⁻³ showed up to a 39% higher cumulative net energy return than 50 W m⁻³, and increased the cumulative net energy ratio up to fourfold. The highest net energy ratio for power input was 19.3 (D. tertiolecta, 12% CO₂, 10 W m⁻³). These systems may be a sustainable method of biomass production, but their effectiveness is sensitive to operational parameters.     

Unexpected CH4 emission from the Three Gorges Reservoir and its implications

Until the end of last century, scientists began to show their concern about greenhouse gas emission from reservoirs and questioned the “green credential” of hydroelectric dams since then. Through measurements along the channel of the TGR, an unexpectedly low CH₄ emission rate was recently observed from the surface of the TGR, much lower than our assumed estimate before. Moreover, the rate from the TGR is lower than that from many hydroelectric reservoirs. One possible reason for such a low emission rate is that lack of substrates supplied by decomposed vegetation limits the CH₄ production in the sediment of the TGR because of vegetation clearance since 2002 before impounding, whose primary purpose is to conserve the water quality. These results indicated that TGR is not a hotspot of CH₄ emission. On a broader sense, it also indicated that removal of flooded vegetation would help to decrease CH₄ emission from dam reservoirs before impounding, especially in the drawdown area.     

A global experiment suggests climate warming will not accelerate litter decomposition in streams but might reduce carbon sequestration

The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding influence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO(2) production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a significant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback.     

Integrating models with data in ecology and palaeoecology: advances towards a model-data fusion approach

It is increasingly being recognized that global ecological research requires novel methods and strategies in which to combine process-based ecological models and data in cohesive, systematic ways. Model-data fusion (MDF) is an emerging area of research in ecology and palaeoecology. It provides a new quantitative approach that offers a high level of empirical constraint over model predictions based on observations using inverse modelling and data assimilation (DA) techniques. Increasing demands to integrate model and data methods in the past decade has led to MDF utilization in palaeoecology, ecology and earth system sciences. This paper reviews key features and principles of MDF and highlights different approaches with regards to DA. After providing a critical evaluation of the numerous benefits of MDF and its current applications in palaeoecology (i.e., palaeoclimatic reconstruction, palaeovegetation and palaeocarbon storage) and ecology (i.e. parameter and uncertainty estimation, model error identification, remote sensing and ecological forecasting), the paper discusses method limitations, current challenges and future research direction. In the ongoing data-rich era of today's world, MDF could become an important diagnostic and prognostic tool in which to improve our understanding of ecological processes while testing ecological theory and hypotheses and forecasting changes in ecosystem structure, function and services.     

Unexpected CH4 emission from the Three Gorges Reservoir and its implications

Until the end of last century, scientists began to show their concern about greenhouse gas emission from reservoirs and questioned the “green credential” of hydroelectric dams since then. Through measurements along the channel of the TGR, an unexpectedly low CH₄ emission rate was recently observed from the surface of the TGR, much lower than our assumed estimate before. Moreover, the rate from the TGR is lower than that from many hydroelectric reservoirs. One possible reason for such a low emission rate is that lack of substrates supplied by decomposed vegetation limits the CH₄ production in the sediment of the TGR because of vegetation clearance since 2002 before impounding, whose primary purpose is to conserve the water quality. These results indicated that TGR is not a hotspot of CH₄ emission. On a broader sense, it also indicated that removal of flooded vegetation would help to decrease CH₄ emission from dam reservoirs before impounding, especially in the drawdown area.     

Microbial substrate preference and community dynamics during decomposition of Acer saccharum

The Guild Decomposition Model (GDM) hypothesized that temporal shifts in microbial “guilds,” each with distinct substrate preferences, drive decomposition dynamics and regulate soil carbon (C) losses and sequestration. To test this hypothesis, we established a laboratory incubation of Acer saccharum litter and monitored respiration, microbial biomass and enzyme activities, inorganic nutrients and shifts in functional groups of decomposers using phospholipid fatty acid (PLFA) analysis. Biomass and respiration peaked within the first 2 d of incubation, and the Gram negative PLFA biomarker 18:1ω7c predominated during the first 5 d. Hydrolytic enzyme activities and two fungal biomarkers (18:2ω6,9c and 18:3ω6c) increased by 25 d and lignolytic enzyme activity was detected at 68 d. Our results suggest that decomposers preferentially use labile substrates and that shifts in decomposer groups occur in response to changes in available substrates, which supports the GDM.     

Hyperhydricity in shoot cultures of Scrophularia yoshimurae can be effectively reduced by ventilation of culture vessels

An effective procedure for obtaining healthy shoots from nodal segments of Scrophularia yoshimurae is described. Nodal segments cultured on Murashige and Skoog's (MS) basal medium supplemented with 1.0 mg L(-1) benzyladenine (BA) and 0.2 mg L(-1) alpha-naphthaleneacetic acid (NAA) induced multiple shoots in conical flasks that differed in the way they were closed and sealed. Hermitically sealed culture vessels resulted in high hyperhydricity/vitrification. High concentrations of ethylene and CO2 were found to accumulate in these vessels. The hyperhydricity of the shoot cultures could be decreased by progressively ventilating the vessels. Exchange of gases was achieved by removing the Parafilm sealing without affecting sterility. This reduced the hyperhydricity rate and gave a good recovery of vitrified shoots, but resulted in decreased proliferation and a dehydration of proliferating nodal segments and the culture medium. The best number of normal shoots was observed when the parafilm was removed for gaseous exchange after four weeks of culture incubation. The results show that hyperhydricity in shoot cultures of S. yoshimurae could be prevented by sufficient gas exchange during culture.     

Interpreting,measuring, and modeling soil respiration

This paper reviews the role of soil respiration in determining ecosystem carbon balance, and the conceptual basis for measuring and modeling soil respiration. We developed it to provide background and context for this special issue on soil respiration and to synthesize the presentations and discussions at the workshop. Soil respiration is the largest component of ecosystem respiration. Because autotrophic and heterotrophic activity belowground is controlled by substrate availability, soil respiration is strongly linked to plant metabolism, photosynthesis and litterfall. This link dominates both base rates and short-term fluctuations in soil respiration and suggests many roles for soil respiration as an indicator of ecosystem metabolism. However, the strong links between above and belowground processes complicate using soil respiration to understand changes in ecosystem carbon storage. Root and associated mycorrhizal respiration produce roughly half of soil respiration, with much of the remainder derived from decomposition of recently produced root and leaf litter. Changes in the carbon stored in the soil generally contribute little to soil respiration, but these changes, together with shifts in plant carbon allocation, determine ecosystem carbon storage belowground and its exchange with the atmosphere. Identifying the small signal from changes in large, slow carbon pools in flux dominated by decomposition of recent material and autotrophic and mycorrhizal respiration is a significant challenge. A mechanistic understanding of the belowground carbon cycle and of the response of different components to the environment will aid in identifying this signal. Our workshop identified information needs to help build that understanding: (1) the mechanisms that control the coupling of canopy and belowground processes; (2) the responses of root and heterotrophic respiration to environment; (3) plant carbon allocation patterns, particularly in different forest developmental stages, and in response to treatments (warming, CO₂, nitrogen additions); and (4) coupling measurements of soil respiration with aboveground processes and changes in soil carbon. Multi-factor experiments need to be sufficiently long to allow the systems to adjust to the treatments. New technologies will be necessary to reduce uncertainty in estimates of carbon allocation, soil carbon pool sizes, and different responses of roots and microbes to environmental conditions.     

Seasonal patterns in soil surface CO<Subscript>2</Subscript> flux under snow cover in 50 and 300 year old subalpine forests

Soil CO₂ flux can contribute as much as 60–80% of total ecosystem respiration in forests. Although considerable research has focused on quantifying this flux during the growing season, comparatively little effort has focused on non-growing season fluxes. We measured soil CO₂ efflux through snow in 50 and ~300 year old subalpine forest stands near Fraser CO. Our objectives were to quantify seasonal patterns in wintertime soil CO₂ flux; determine if differences in soil CO₂ flux between the two forest ages during the growing season persist during winter; and to quantify the sample size necessary to discern treatment differences. Soil CO₂ flux during the 2002–2003 and 2003–2004 snow season averaged 0.31 and 0.35 μmols m⁻² s⁻¹ for the young and old forests respectively; similar to the relative difference observed during summer. There was a significant seasonal pattern of soil CO₂ flux during the winter with fluxes averaging 0.22 μmols m⁻² s⁻¹ in December and January and increasing to an average of 0.61 μmols m⁻² s⁻¹ in May. Within-plot variability for measurements used in calculating flux was low. The coefficients of variation (CV) for CO₂ concentration, snowpack density, and snow depth were 17, 8 and 14%, respectively, yielding a CV for flux measurements within-plot of 29%. A within plot CV of 29% requires 8 sub-samples per plot to estimate the mean flux with a standard error of ±10% of the mean. Variability in CO₂ flux estimates among plots (size = 400 m²) was similar to that within plot and was also low (CV = ~28%). With a CV of 28% among plots, ten plots per treatment would have a 50% probability of detecting a 25% difference in treatment means for α = 0.05.