Predicting responses of photosynthesis and root fraction to elevated [CO2]a: interactions among carbon,nitrogen, and growth*

At elevated atmospheric CO₂ concentrations ([CO₂]_a), photosynthetic capacity (A_max) and root fraction (η_R, the ratio of root to plant dry mass) increased in some studies and decreased in others. Here, we have explored possible causes of this, focusing on the relative magnitudes of the effects of elevated [CO₂]_a on specific leaf (n_m) and plant (n_p) nitrogen concentrations, leaf mass per unit area (h), and plant nitrogen productivity (α). In our survey of 39 studies with 35 species, we found that elevated [CO₂]_a led to decreased n_m and n_p in all the studies and to increased h and α in most of the studies. The magnitudes of these changes varied with species and with experimental conditions. Based on a model that integrated [CO₂]_a-induced changes in leaf nitrogen into a biochemically based model of leaf photosynthesis, we predicted that, to a first approximation, photosynthesis will be upregulated (A_max will increase) when growth at increased [CO₂]_a leads to increases in h that are larger than decreases in n_m. Photosynthesis will be downregulated (A_max will decrease) when increases in h are smaller than decreases in n_m. The model suggests that photosynthetic capacity increases at elevated [CO₂]_a only when additional leaf mesophyll more than compensates the effects of nitrogen dilution. We considered two kinds of regulatory paradigms that could lead to varying responses of η_R to elevated [CO₂]_a, and compared the predictions of each with the data. A simple static model based on the functional balance concept predicts that η_R should increase when neither n_p nor h is very responsive to elevated [CO₂]_a. The quantitative and qualitative agreement of the predictions with data from the literature, however, is poor. A model that predicts η_R from the relative sensitivities of photosynthesis and relative growth rate to elevated [CO₂]_a corresponds much more closely to the observations. In general, root fraction increases if the response of photosynthesis to [CO₂]_a is greater than that of relative growth rate.     

Speculations on carbon dioxide starvation, Late Tertiary evolution of stomatal regulation and floristic modernization

Ambient atmospheric CO₂ concentration ([CO₂]_a) has apparently declined from values above 200μmol mol⁻¹ to values below 200μmol mol⁻¹ within the last several million years. The lower end of this range is marginal for C₃ plants. I hypothesize that: (1) declining [CO₂]_a imposed a physiological strain on plants, and plant taxa evolving under declining [CO₂]_a tended to develop compensating mechanisms, including increased stomatal efficiency; (2) angiosperms were better able to adjust to declining [CO₂]_a than were gymnosperms and pteridophytes; and (3) angiosperm adjustment has been uneven. Fast-evolving taxa (e.g. grasses and herbs) have been better able to adapt to CO₂ starvation. If these propositions are true, stomatal adjustment mechanisms should show patterned variation, and a single pattern of stomatal regulation cannot be assumed.     

The effects of UV-B radiation on loblolly pine. 3. Interaction with CO2 enhancement

Projected depletions in the stratospheric ozone layer will result in increases in solar ultraviolet-B radiation (290–320 nm) reaching the earth's surface, These increases will likely occur in concert with other environmental changes such as increases in atmospheric carbon dioxide concentrations. Currently very little information is available on the effectiveness of UV-B radiation within a CO₂-enriched atmosphere, and this is especially true for trees. Loblolly pine (Pinus taeda L.) seedlings were grown in a factorial experiment at the Duke University Phytotron with either 0, 8.8 or 13.8 kJ m⁻² of biologically effective UV-B radiation (UV-B_BE). The CO₂ concentrations used were 350 and 650 μmol mol⁻¹. Measurements of chlorophyll fluorescence were made at 5-week intervals and photosynthetic oxygen evolution and leaf pigments were measured after 22 weeks, prior to harvest. The results of this study demonstrated a clear growth response to CO₂ enrichment but neither photosynthetic capacity nor quantum efficiency were altered by CO₂. The higher UV-B irradiance reduced total biomass by about 12% at both CO₂ levels but biomass partitioning was altered by the interaction of CO₂ and UV-B radiation. Dry matter was preferentially allocated to shoot components by UV-B radiation at 350 μmol mol⁻¹ CO₂ and towards root components at 650 μmol mol⁻¹ CO₂. These subtle effects on biomass allocation could be important in the future to seedling establishment and competitive interactions in natural as well as agricultural communities.     

Carbon dioxide consumption during soil development

Carbon is sequestered in soils by accumulation of recalcitrant organic matter and by bicarbonate weathering of silicate minerals. Carbon fixation by ecosystems helps drive weathering processes in soils and that in turn diverts carbon from annual photosynthesis-soil respiration cycling into the long-term geological carbon cycle. To quantify rates of carbon transfer during soil development in moist temperate grassland and desert scrubland ecosystems, we measured organic and inorganic residues derived from the interaction of soil biota and silicate mineral weathering for twenty-two soil profiles in arkosic sediments of differing ages. In moist temperate grasslands, net annual removal of carbon from the atmosphere by organic carbon accumulation and silicate weathering ranges from about 8.5 g m^–2 yr^–1 for young soils to 0.7 g M^–2 yr^–1 for old soils. In desert scrublands, net annual carbon removal is about 0.2 g m^–2 yr^–1 for young soils and 0.01 g m^–2 yr^–1 for old soils. In soils of both ecosystems, organic carbon accumulation exceeds CO₂ removal by weathering, however, as soils age, rates of CO₂ consumption by weathering accounts for greater amounts of carbon sequestration, increasing from 2% to 8% in the grassland soils and from 2% to 40% in the scrubland soils. In soils of desert scrublands, carbonate accumulation far outstrips organic carbon accumulation, but about 90% of this mass is derived from aerosolic sources that do not contribute to long-term sequestration of atmospheric carbon dioxide.     

CHARACTERIZATION OF TWO CHLORELLA VULGARIS (CHLOROPHYCEAE) STRAINS ISOLATED FROM WASTEWATER OXIDATION PONDS1

We report here on the characterization and isolation of two ecotypes of Chlorella vulgaris Beyerinck that coexist in wastewater reservoirs. One ecotype (C1) contains high amounts of chlorophyll b, is capable of autotrophic growth, and can utilize only a few organic solutes for growth. The second ecotype (C2) contains low amounts of chlorophyll b, requires vitamin B₁₂, and can support its growth with a broad range of organic compounds. Of the two ecotypes, the latter showed slower growth rates when light was the sole source of energy. Cells of C2-type Chlorella attained higher photosynthetic activities than C1-type cells at saturating irradiances. However, their low chlorophyll b content and lower light utilization efficiency suggest that C2-type Chlorella contains relatively low amounts of light-harvesting antennae, a disadvantage in severely light-limited ecosystems like wastewater reservoirs. We hypothesize that the two Chlorella types coexist by adopting different lifestyles: C1-type cells rely largely on their photosynthetic potential for energy conservation and growth, whereas C2-type cells may exploit their heterotrophic properties for this purpose.     

Soil pCO2, soil respiration,and root activity in CO2-fumigated and nitrogen-fertilized ponderosa pine

The purpose of this paper is to describe the effects of CO₂ and N treatments on soil pCO₂, calculated CO₂ efflux, root biomass and soil carbon in open-top chambers planted with Pinus ponderosa seedlings. Based upon the literature, it was hypothesized that both elevated CO₂ and N would cause increased root biomass which would in turn cause increases in both total soil CO₂ efflux and microbial respiration. This hypothesis was only supported in part: both CO₂ and N treatments caused significant increases in root biomass, soil pCO₂, and calculated CO₂ efflux, but there were no differences in soil microbial respiration measured in the laboratory. Both correlative and quantitative comparisons of CO₂ efflux rates indicated that microbial respiration contributes little to total soil CO₂ efflux in the field. Measurements of soil pCO₂ and calculated CO₂ efflux provided inexpensive, non-invasive, and relatively sensitive indices of belowground response to CO₂ and N treatments.     

Changes in carbon storage in temperate humic loamy soils after forest clearing and continuous corn cropping in France

Soil samples from forest and agricultural sites in three areas of southwest France were collected to determine the effect of forest conversion to continuous intensive corn cropping with no organic matter management on soil organic carbon (C) content. Soils were humic loamy soils and site characteristics that may affect soil C were as uniform as possible (slope, elevation, texture, soil type, vegetation). Three areas were selected, with adjacent sites of various ages of cultivation (3 to 35 yr), and paired control forest sites. The ploughed horizon (0-Dt cm) and the Dt-50 cm layer were collected at each agricultural site. In forest sites, each 10 cm layer was collected systematically down to 1 meter depth. Carbon concentrations were converted to total content to a given depth as the product of concentration, depth of sample and bulk density, and expressed in units of kg m^-2. For each site and each sampled layer, the mineral mass of soil was calculated, in order to base comparisons on the same soil mass rather than the same depth. The pattern of C accumulation in forest soils showed an exponential decrease with depth. Results suggested that soil organic carbon declined rapidly during the first years of cultivation, and at a slower rate thereafter. This pattern of decrease can be fitted by a bi-exponential model assuming that initial soil organic carbon can be separated into two parts, a very labile pool reduced during the first rapid decline and more refractory fractions oxidizing at a slower rate. Sampling to shallow depths (0-Dt cm) resulted in over-estimation of the rate of carbon release in proportion to the initial amount of C, and in under-estimation of the total loss of C with age. The results for the 0–50 cm horizon indicated that losses of total carbon average about 50% in these soils, ranging in initial carbon content from 19 to 32.5 kg m^-2. Carbon release to the atmosphere averaged 0.8 kg m^-2 yr^-1 to 50 cm depth during the first 10 years of cultivation. The results demonstrate that temperate soils may also be an important source of atmospheric carbon, when they are initially high in carbon content and then cultivated intensively with no organic matter management.     

Extractable sulphate and organic sulphur in soils and their availability to plants

Ten soils collected from the major arable areas in Britain were used to assess the availability of soil sulphur (S) to spring wheat in a pot experiment. Soils were extracted with various reagents and the extractable inorganic SO₄-S and total soluble S(SO₄-S plus a fraction of organic S) were determined using ion chromatography (IC) or inductively-coupled plasma atomic emission spectrometry (ICP-AES), respectively. Water, 0.016 M KH₂PO₄, 0.01 M CaCl₂ and 0.01 M Ca(H₂PO₄)₂ extracted similar amounts of SO₄-S, as measured by IC, which were consistently smaller than the total extractable S as measured by ICP-AES. The amounts of organic S extracted varied widely between different extractants, with 0.5 M NaHCO₃ (pH 8.5) giving the largest amounts and 0.01 M CaCl₂ the least. Organic S accounted for approximately 30–60% of total S extracted with 0.016 M KH₂PO₄ and the organic C:S ratios in this extract varied typically between 50 and 70. The concentrations of this S fraction decreased in all soils without added S after two months growth of spring wheat, indicating a release of organic S through mineralisation. All methods tested except 0.5 M NaHCO₃-ICP-AES produced satisfactory results in the regression with plant dry matter response and S uptake in the pot experiment. In general, 0.016 M KH₂PO₄ appeared to be the best extractant and this extraction followed by ICP-AES determination was considered to be a good method to standardise on.