Effect of CO2 enhancement in an outdoor algal production system usingTetraselmis

One of the objectives of microalgal culture is to provide reliable production technology for important live aquaculture feed organisms. Presented here are the results of experiments designed to provide a better understanding of the relationship between inorganic carbon availability and algal production.Our results suggest that through additions of CO₂ gas we were able to maintain sufficient dissolved carbon to stabilize outdoor algal cultures. Increases in the rate of addition of CO₂ increased levels of dissolved CO₂, total dissolved inorganic carbon (CO₂), and decreased pH in the growth medium. This translated into improved buffering capacity of the culture medium and higher growth rate. A minimum of 2.4 mM CO₂ was found necessary to maintain a maximal growth rate of 0.7 doublings/day. We also found that the increased productivity more than offsets the cost of adding the CO₂.     

Chemical composition and nutrient loading by precipitation in the Trachypogon savannas of the Orinoco llanos, Venezuela

Samples of bulk precipitation were collected in the Trachypogon savanna, Calabozo, Venezuela, during three consecutive years. In the first year, rain samples were taken daily; in the following years the samples were grouped on a monthly basis. In addition, samples of dry deposition were collected during the dry seasons. All samples were analyzed for the following water soluble cations and anions: P0₄-P, S0₄-S, N0₃-N, NH₄-N, Ca⁺², Mg⁺², K⁺, Na⁺ and H⁺. The mean annual input rate of chemical constituents (Kg ha^-1 year^-1) was: PO₄-P (0.42); SO₄-S (2.62); NO₃-N (0.21); NH₄-N (2.03); Ca₊₂ (3.50); Mg⁺² (11.31); K⁺ (3.60); Na⁺ (5.93) and H⁺ (0.03). The total mean input of particulate material to the savanna during the dry season was 2.06 Kg ha^-1 year^-1, with a soluble fraction of 30%. Possible sources of nutrients input were analyzed.     

Involvement of Exported Photosynthetic Products in the CO2Exchange of the Skeletal Shoots of Pine

CO₂exchange in the leafy and skeletal parts of attached shoots of Pinus sylvestrisL. was measured with an infrared gas-analyzer in an open differential system during daylight hours. The ¹⁴CO₂assimilation rates in the leafy parts of shoots and ¹⁴CO₂evolution from current photosynthetic products in the lower skeletal part of shoots were measured in afternoons. Chlorophyll content was measured in the needles of the same shoot. The carbon of exported assimilates contributed only about 4% to CO₂exchange in the heterotrophic tree tissues. Only this component of CO₂evolution from the surface of the skeletal part of the tree was related to the losses of the net primary photosynthetic production (NPP) in the aboveground part of the pine stand during the current growth period.     

Non-phototrophic CO 2 fixation by soil microorganisms

Although soils are generally known to be a net source of CO₂ due to microbial respiration, CO₂ fixation may also be an important process. The non-phototrophic fixation of CO₂ was investigated in a tracer experiment with ¹⁴CO₂ in order to obtain information about the extent and the mechanisms of this process. Soils were incubated for up to 91 days in the dark. In three independent incubation experiments, a significant transfer of radioactivity from ¹⁴CO₂ to soil organic matter was observed. The process was related to microbial activity and could be enhanced by the addition of readily available substrates such as acetate. CO₂ fixation exhibited biphasic kinetics and was linearly related to respiration during the first phase of incubation (about 20–40 days). The fixation amounted to 3–5% of the net respiration. After this phase, the CO₂ fixation decreased to 1–2% of the respiration. The amount of carbon fixed by an agricultural soil corresponded to 0.05% of the organic carbon present in the soil at the beginning of the experiment, and virtually all of the fixed CO₂ was converted to organic compounds. Many autotrophic and heterotrophic biochemical processes result in the fixation of CO₂. However, the enhancement of the fixation by addition of readily available substrates and the linear correlation with respiration suggested that the process is mainly driven by aerobic heterotrophic microorganisms. We conclude that heterotrophic CO₂ fixation represents a significant factor of microbial activity in soils.     

On the assessment of root and soil respiration for soils of different textures: interactions with soil moisture contents and soil CO2 concentrations

Estimates of root and soil respiration are becoming increasingly important in agricultural and ecological research, but there is little understanding how soil texture and water content may affect these estimates. We examined the effects of soil texture on (i) estimated rates of root and soil respiration and (ii) soil CO₂ concentrations, during cycles of soil wetting and drying in the citrus rootstock, Volkamer lemon (Citrus volkameriana Tan. and Pasq.). Plants were grown in soil columns filled with three different soil mixtures varying in their sand, silt and clay content. Root and soil respiration rates, soil water content, plant water uptake and soil CO₂ concentrations were measured and dynamic relationships among these variables were developed for each soil texture treatment. We found that although the different soil textures differed in their plant-soil water relations characteristics, plant growth was only slightly affected. Root and soil respiration rates were similar under most soil moisture conditions for soils varying widely in percentages of sand, silt and clay. Only following irrigation did CO₂ efflux from the soil surface vary among soils. That is, efflux of CO₂ from the soil surface was much more restricted after watering (therefore rendering any respiration measurements inaccurate) in finer textured soils than in sandy soils because of reduced porosity in the finer textured soils. Accordingly, CO₂ reached and maintained the highest concentrations in finer textured soils (> 40 mmol CO₂ mol⁻¹). This study revealed that changes in soil moisture can affect interpretations of root and soil measurements based on CO₂ efflux, particularly in fine textured soils. The implications of the present findings for field soil CO₂ flux measurements are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Estimating respiration of roots in soil: Interactions with soil CO2, soil temperature and soil water content

Little information is available on the variability of the dynamics of the actual and observed root respiration rate in relation to abiotic factors. In this study, we describe I) interactions between soil CO₂ concentration, temperature, soil water content and root respiration, and II) the effect of short-term fluctuations of these three environmental factors on the relation between actual and observed root respiration rates. We designed an automated, open, gas-exchange system that allows continuous measurements on 12 chambers with intact roots in soil. By using three distinct chamber designs with each a different path for the air flow, we were able to measure root respiration over a 50-fold range of soil CO₂ concentrations (400 to 25000 ppm) and to separate the effect of irrigation on observed vs. actual root respiration rate. All respiration measurements were made on one-year-old citrus seedlings in sterilized sandy soil with minimal organic material.Root respiration was strongly affected by diurnal fluctuations in temperature (Q₁₀ = 2), which agrees well with the literature. In contrast to earlier findings for Douglas-fir (Qi et al., 1994), root respiration rates of citrus were not affected by soil CO₂ concentrations (400 to 25000 ppm CO₂; pH around 6). Soil CO₂ was strongly affected by soil water content but not by respiration measurements, unless the air flow for root respiration measurements was directed through the soil. The latter method of measuring root respiration reduced soil CO₂ concentration to that of incoming air. Irrigation caused a temporary reduction in CO₂ diffusion, decreasing the observed respiration rates obtained by techniques that depended on diffusion. This apparent drop in respiration rate did not occur if the air flow was directed through the soil. Our dynamic data are used to indicate the optimal method of measuring root respiration in soil, in relation to the objectives and limitations of the experimental conditions.     

Response of soil CO2 efflux to water manipulation in a tallgrass prairie ecosystem

Although CO₂ efflux plays a critical role in carbon exchange between the biosphere and atmosphere, our understanding of its regulation by soil moisture is rather limited. This study was designed to examine the relationship between soil CO₂ efflux and soil moisture in a natural ecosystem by taking advantage of the historically long drought period from 29 July to 21 September 2000 in the southern Central Great Plain, USA. At the end of August when soil moisture content at the top 50 mm was reduced to less than 50 g kg^–1 gravimetrically, we applied 8 levels of water treatments (simulated to rainfall of 0, 10, 25, 50, 100, 150, 200, and 300 mm) with three replicates to 24 plots in a Tallgrass Prairie ecosystem in Central Oklahoma, USA. In order to quantify root-free soil CO₂ efflux, we applied the same 8 levels of water treatments to 24 500-mm soil columns using soil from field adjacent to the experimental plots. We characterized dynamic patterns of soil moisture and soil CO₂ efflux over the experimental period of 21 days. Both soil moisture content and CO₂ efflux showed dramatic increases immediately after the water addition, followed by a gradual decline. The time courses in response to water treatments are well described by Y=Y₀+ate^–bt, where Y is either soil moisture or CO₂ efflux, t is time, Y ₀, a, and b are coefficients. Among the 8 water treatments, the maximal soil CO₂ efflux rate occurred at the 50 mm water level in the field and 100 mm in the root-free soil 1 day after the treatment. The maximal soil CO₂ efflux gradually shifted to higher water levels as the experiment continued. We found the relationship between soil CO₂ efflux and soil moisture using the data from the 21-day experiment was highly scattered, suggesting complex mechanisms determining soil CO₂ efflux by soil moisture.     

Evaluation of soil respiration and soil CO2 concentration in a lowland moist forest in Panama

Soil gas exchange was investigated in a lowland moist forest in Panama. Soil water table level and soil redox potentials indicate that the soils are not waterlogged. Substantial microspatial variation exists for soil respiration and soil CO₂ concentration. During the rainy season, soil CO₂ at 40 cm below the surface accumulates to 2.3%–4.6% and is correlated with rainfall during the previous two weeks. Temporal changes in soil CO₂ are rapid, large and share similar trends between sampling points. Possible effects of soil CO₂ changes on plant growth or phenology are discussed.     

Spatial and temporal variation in soil CO2 efflux in an old-growth neotropical rain forest, La Selva, Costa Rica

Our objectives were to quantify and compare soil CO₂ efflux of two dominant soil types in an old-growth neotropical rain forest in the Atlantic zone of Costa Rica, and to evaluate the control of environmental factors on CO₂ release. We measured soil CO₂ efflux from eight permanent soil chambers on six Oxisol sites. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). At the same time we measured soil CO₂ concentrations, soil water content and soil temperature at various depths in 6 soil shafts (3 m deep). Between old alluvium sites, the two-year average CO₂ flux rates ranged from 117.3 to 128.9 mg C m^–2 h^–1. Significantly higher soil CO₂ flux occurred on the residual sites (141.1 to 184.2 mg C m^–2 h^–1). Spatial differences in CO₂ efflux were related to fine root biomass, soil carbon and phosphorus concentration but also to soil water content. Spatial variability in CO₂ storage was high and the amount of CO₂ stored in the upper and lower soil profile was different between old alluvial and residual sites. The major factor identified for explaining temporal variations in soil CO₂ efflux was soil water content. During periods of high soil water content CO₂ emission decreased, probably due to lower diffusion and CO₂ production rates. During the 2-year study period inter-annual variation in soil CO₂ efflux was not detected.     

H2 oxidation,O2 uptake and CO2 fixation in hydrogen treated soils

In many legume nodules, the H₂ produced as a byproduct of N₂ fixation diffuses out of the nodule and is consumed by the soil. To study the fate of this H₂ in soil, a H₂ treatment system was developed that provided a 300 cm³ sample of a soil:silica sand (2:1) mixture with a H₂ exposure rate (147 nmol H₂ cm^–3hr^–1) similar to that calculated exist in soils located within 1–4 cm of nodules (30–254 nmol H₂ cm^–3hr^–1). After 3 weeks of H₂ pretreatment, the treated soils had a K_m and V_max for H₂ uptake (1028 ppm and 836 nmol cm^–3 hr^–1, respectively) much greater than that of control, air-treated soil (40.2 ppm and 4.35 nmol cm^–3 hr^–1, respectively). In the H₂ treated soils, O₂, CO₂ and H₂ exchange rates were measured simultaneously in the presence of various pH₂. With increasing pH₂, a 5-fold increase was observed in O₂ uptake, and CO₂ evolution declined such that net CO₂ fixation was observed in treatments of 680 ppm H₂ or more. At the H₂ exposure rate used to pretreat the soil, 60% of the electrons from H₂ were passed to O₂, and 40% were used to support CO₂ fixation. The effect of H₂ on the energy and C metabolism of soil may account for the well-known effect of legumes in promoting soil C deposition.     

Effects of rainfall events on soil CO2 flux in a cool temperate deciduous broad-leaved forest

The effects of rainfall events on soil CO₂ fluxes were examined in a cool temperate Quercus/Betula forest in Japan. The soil CO₂ fluxes were measured using an open-flow gas exchange system with an infrared gas analyzer in the snow-free season from August 1999 to November 2000. Soil CO₂ flux showed no significant diurnal trend on days without rain. In contrast, rainfall events caused a significant increase in soil CO₂ flux. To determine the effect of rainfall events and to evaluate more precisely the daily and annual soil carbon flux, we constructed a multiple polynomial regression model that included two variables, soil temperature and soil water content, using the soil CO₂ flux data recorded on sunny days. Daily soil carbon fluxes on sunny days calculated by the model were almost the same as those determined by the field measurements. On the contrary, the fluxes measured on rainy days were significantly higher than those calculated daily from the soil carbon fluxes by the model. Annual soil carbon fluxes in 1999 and 2000 were estimated using models that both do and do not take rainfall effects into consideration. The result indicates that post-rainfall increases in soil CO₂ flux represent approximately 16–21% of the annual soil carbon flux in this cool temperate deciduous forest.     

Precipitation pulse size effects on Sonoran Desert soil microbial crusts

Deserts are characterized by low productivity and substantial unvegetated space, which is often covered by soil microbial crust communities. Microbial crusts are important for nitrogen fixation, soil stabilization and water infiltration, but their role in ecosystem production is not well understood. This study addresses the following questions: what are the CO₂ exchange responses of crusts to pulses of water, does the contribution of crusts to ecosystem flux differ from the soil respiratory flux, and is this contribution pulse size dependent? Following water application to crusts and soils, CO₂ exchange was measured and respiration was partitioned through mixing model analysis of Keeling plots across treatments. Following small precipitation pulse sizes, crusts contributed 80% of soil-level CO₂ fluxes to the atmosphere. However, following a large pulse event, roots and soil microbes contributed nearly 100% of the soil-level flux. Rainfall events in southern Arizona are dominated by small pulse sizes, suggesting that crusts may frequently contribute to ecosystem production. Carbon cycle studies of arid land systems should consider crusts as important contributors because of their dynamic responses to different pulse sizes as compared to the remaining ecosystem components.     

Diurnal cycle of carbon isotope ratio in soil CO2 in various ecosystems

Our investigations of diurnal variations of the ¹³C/¹²C ratio and CO₂ content in soil air were carried out in three environments during periods of high biosphere activity. It has been observed that diurnal variation of CO₂ concentration is negatively correlated ¹³. Particularly great variations occurred at shallow soil depths (10–30 cm) when the plant cover activity was high while the soil temperature was rather low. Under such conditions the ¹³ variations had the magnitude of 4, while the CO₂ concentration varied more than doubly. The maximum of the ¹³C/¹²C ratlo and the minimum of the CO₂ concentration in a cultivated field with winter wheat took place in the afternoon, whereas in deciduous forest similar patterns were observed at dawn. In these cases soil temperatures at 10 cm depths varied less than 2°C. Hence, under wheat the variation in root respiration rate seem to be the main reason of the recorded varations. In an uncultivated grass-field during the hottest period in summer we did not measure any distinct variations of CO₂ properties in spite of the fact that soil temperature varied up to 5°C. This might be due to dominant microbial respiration at the high soil temperature, which exceeded 20°C.     

Impact of grazing intensity on seasonal variations in soil organic carbon and soil CO2 efflux in two semiarid grasslands in southern Botswana

Biological soil crusts (BSCs) are an important source of organic carbon, and affect a range of ecosystem functions in arid and semiarid environments. Yet the impact of grazing disturbance on crust properties and soil CO₂ efflux remain poorly studied, particularly in African ecosystems. The effects of burial under wind-blown sand, disaggregation and removal of BSCs on seasonal variations in soil CO₂ efflux, soil organic carbon, chlorophyll a and scytonemin were investigated at two sites in the Kalahari of southern Botswana. Field experiments were employed to isolate CO₂ efflux originating from BSCs in order to estimate the C exchange within the crust. Organic carbon was not evenly distributed through the soil profile but concentrated in the BSC. Soil CO₂ efflux was higher in Kalahari Sand than in calcrete soils, but rates varied significantly with seasonal changes in moisture and temperature. BSCs at both sites were a small net sink of C to the soil. Soil CO₂ efflux was significantly higher in sand soils where the BSC was removed, and on calcrete where the BSC was buried under sand. The BSC removal and burial under sand also significantly reduced chlorophyll a, organic carbon and scytonemin. Disaggregation of the soil crust, however, led to increases in chlorophyll a and organic carbon. The data confirm the importance of BSCs for C cycling in drylands and indicate intensive grazing, which destroys BSCs through trampling and burial, will adversely affect C sequestration and storage. Managed grazing, where soil surfaces are only lightly disturbed, would help maintain a positive carbon balance in African drylands.     

The effect of plant material grown under elevated CO2 on soil respiratory activity

The biodegradability of aerial material from a C4 plant, sorghum grown under ambient (345 µmol mol^–1) and elevated (700 µmol mol^–1) atmospheric CO₂ concentrations were compared by measuring soil respiratory activity. Initial daily respiratory activity (measured over 10 h per day) increased four fold from 110 to 440 cm³ CO₂ 100g dry weight soil^–1 in soils amended with sorghum grown under either elevated or ambient CO₂. Although soil respiratory activity decreased over the following 30 days, respiration remained significantly higher (t-test;p>0.05) in soils amended with sorghum grown under elevated CO₂ concentrations. Analysis of the plant material revealed no significant differences in C:N ratios between sorghum grown under elevated or ambient CO₂. The reason for the differences in soil respiratory activity have yet to be elucidated. However if this trend is repeated in natural ecosystems, this may have important implications for C and N cycling.     

Influence of carbon availability on the production of NO, N2O, N2 and CO2 by soil cores during anaerobic incubation

Net productions of permanent soil atmosphere gases (N₂, CO₂, O₂) and temporary gases (N₂O, NO) were monitored in soil cores using a non-interfering, fully automated measuring technique allowing highly time resolved measurements over prolonged periods. The influence of changes in available organic carbon on CO₂, N₂O, NO and N₂ production was studied by changing the soil carbon content through aerobic preincubations of different length, up to 21 days.The aerobic preincubation caused an increase in NO₃ ^- concentration and a decrease in available carbon content. Available carbon content dominated both CO₂ and total N gas (N₂+N₂O+NO) production during anaerobiosis. Both CO₂ and total N gas production rates decreased with increasing length of the previous aerobic preincubation, this in spite of the higher initial NO₃ ^- concentration.Total denitrification rates were closely related to the anaerobic CO₂ production rates. No relation was found between water soluble carbon content and total denitrification. The N₂O/N₂ ratio could be explained by an interaction of carbon availability, NO₃ ^- concentration and enzyme status. Net N₂O consumption was monitored. The balance between cumulative total N gas production and NO₃ ^- consumption varied according to the different treatments. Cumulative N₂O production exceeded cumulative N₂ production for 0 up to 5 days.     

Above- and belowground response of Populus grandidentata to elevated atmospheric CO2 and soil N availability

Soil N availability may play an important role in regulating the long-term responses of plants to rising atmospheric CO₂ partial pressure. To further examine the linkage between above- and belowground C and N cycles at elevated CO₂, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO₂ in open bottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralization in local dry oak forests. Nitrogen added during August was enriched with ¹⁵N to trace the flux of N within the plant-soil system. Above-and belowground growth, CO₂ assimilation, and leaf N content were measured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and ¹⁵N. There was no CO₂ treatment effect on leaf area, root length, or net assimilation prior to the completion of N addition. Following the N addition, leaf N content increased in both CO₂ treatments, but net assimilation showed a sustained increase only in elevated CO₂ grown plants. Root relative extension rate was greater at elevated CO₂, both before and after the N addition. Although final root biomass was greater at elevated CO₂, there was no CO₂ effect on plant N uptake or allocation. While low soil N availability severely inhibited CO₂ responses, high CO₂ grown plants were more responsive to N. This differential behavior must be considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests.     

CO2 transported in xylem sap affects CO2 efflux from Liquidambar styraciflua and Platanus occidentalis stems, and contributes to observed wound respiration phenomena

The [CO₂] in the xylem of tree stems is typically two to three orders of magnitude greater than atmospheric [CO₂]. In this study, xylem [CO₂] was experimentally manipulated in saplings of sycamore (Platanus occidentalis L.) and sweetgum (Liquidambar styraciflua L.) by allowing shoots severed from their root systems to absorb water containing [CO₂] ranging from 0.04% to 14%. The effect of xylem [CO₂] on CO₂ efflux to the atmosphere from uninjured and mechanically injured, i.e., wounded, stems was examined. In both wounded and unwounded stems, and in both species, CO₂ efflux was directly proportional to xylem [CO₂], and increased 5-fold across the range of xylem [CO₂] produced by the [CO₂] treatment. Xylem [CO₂] explained 76–77% of the variation in pre-wound efflux. After wounding, CO₂ efflux increased substantially but remained directly proportional to internal stem [CO₂]. These experiments substantiated our previous finding that stem CO₂ efflux was directly related to internal xylem [CO₂] and expanded our observations to two new species. We conclude that CO₂ transported in the xylem may confound measurements of respiration based on CO₂ efflux to the atmosphere. This study also provided evidence that the rapid increase in CO₂ efflux observed after tissues are excised or injured is likely the result of the rapid diffusion of CO₂ from the xylem, rather than an actual increase in the rate of respiration of wounded tissues.     

Soil properties differently influence estimates of soil CO2 efflux from three chamber-based measurement systems

Soil CO₂ efflux is a major component of net ecosystem productivity (NEP) of forest systems. Combining data from multiple researchers for larger-scale modeling and assessment will only be valid if their methodologies provide directly comparable results. We conducted a series of laboratory and field tests to assess the presence and magnitude of soil CO₂ efflux measurement system × environment interactions. Laboratory comparisons were made with a dynamic, steady-state CO₂ flux generation apparatus, wherein gas diffusion drove flux without creating pressure differentials through three artificial soil media of varying air-filled porosity. Under these conditions, two closed systems (Li-6400-09 and SRC-1) exhibited errors that were dependent on physical properties of the artificial media. The open system (ACES) underestimated CO₂ flux. However, unlike the two other systems, the ACES results could be corrected with a single calibration equation that was unaffected by physical differences in artificial media. Both scale and rank changes occurred among the measurement systems across four sites. Our work clearly shows that soil CO₂ efflux measurement system × environment interactions do occur and can substantially impact estimates of soil CO₂ efflux. Until reliable calibration techniques are developed and applied, such interactions make direct comparison of published rates, and C budgets estimated using such rates, difficult.