Chemistry and decomposition of litter from Populus tremuloides Michaux grown at elevated atmospheric CO2 and varying N availability

It has been hypothesized that greater production of total nonstructural carbohydrates (TNC) in foliage grown under elevated atmospheric carbon dioxide (CO₂) will result in higher concentrations of defensive compounds in tree leaf litter, possibly leading to reduced rates of decomposition and nutrient cycling in forest ecosystems of the future. To evaluate the effects of elevated atmospheric CO₂ on litter chemistry and decomposition, we performed a 111 day laboratory incubation with leaf litter of trembling aspen (Populus tremuloides Michaux) produced at 36 Pa and 56 Pa CO₂ and two levels of soil nitrogen (N) availability. Decomposition was quantified as microbially respired CO₂ and dissolved organic carbon (DOC) in soil solution, and concentrations of nonstructural carbohydrates, N, carbon (C), and condensed tannins were monitored throughout the incubation. Growth under elevated atmospheric CO₂ did not significantly affect initial litter concentrations of TNC, N, or condensed tannins. Rates of decomposition, measured as both microbially respired CO₂ and DOC did not differ between litter produced under ambient and elevated CO₂. Total C lost from the samples was 38 mg g^?1 litter as respired CO₂ and 138 mg g^?1 litter as DOC, suggesting short‐term pulses of dissolved C in soil solution are important components of the terrestrial C cycle. We conclude that litter chemistry and decomposition in trembling aspen are minimally affected by growth under higher concentrations of CO₂.     

Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech

Beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) were grown from seed for two whole seasons at two CO₂ concentrations (ambient and ambient + 250 μmol mol^?1) with two levels of soil nutrient supply. Measurements of net leaf photosynthetic rate (A) and stomatal conductance (g_s) of well-watered plants were taken over both seasons; a drought treatment was applied in the middle of the second growing season to a separate sample of beech drawn from the same population. The net leaf photosynthetic rate of well-watered plants was stimulated in elevated CO₂ by an average of 75% in beech and 33% in oak; the effect continued through both growing seasons at both nutrient levels. There were no interactive effects of CO₂ concentration and nutrient level on A or g_s in beech or oak. Stomatal conductance was reduced in elevated CO₂ by an average of 34% in oak, but in beech there were no significant reductions in g_s except under cloudy conditions (–22% in elevated CO₂). During drought, there was no effect of CO₂ concentration on g_s in beech grown with high nutrients, but for beech grown with low nutrients, g_s was significantly higher in elevated CO₂, causing more rapid soil drying. With high nutrient supply, soil drying was more rapid at elevated CO₂ due to increased leaf area. It appears that beech may substantially increase whole-plant water consumption in elevated CO₂, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, thereby putting itself at risk during periods of drought.     

Interactive effects of growth‐limiting N supply and elevated atmospheric CO2 concentration on growth and carbon balance of Plantago major

To assess the interactions between concentration of atmospheric CO₂ and N supply, the response of Plantago major ssp. pleiosperma Pilger to a doubling of the ambient CO₂ concentration of 350 µl l^?1 was investigated in a range of exponential rates of N addition. The relative growth rate (RGR) as a function of the internal plant nitrogen concentration (N_i), was increased by elevated CO₂ at optimal and intermediate N_i. The rate of photosynthesis, expressed per unit leaf area and plotted versus N_i. was increased by 20-30% at elevated CO₂ for N_i above 30 mg N g^?1 dry weight. However, the rate of photosynthesis, expressed on a leaf dry matter basis and plotted versus N_i, was not affected by the CO₂ concentration. The allocation of dry matter between shoot and root was not affected by the CO₂ concentration at any of the N addition rates. This is in good agreement with theoretical models. based on a balance between the rate of photosynthesis of the shoot and the acquisition of N by the roots. The concentration of total nonstructural carbohydrates (TNC) was increased at elevated CO₂ and at N limitation, resulting in a shift in the partitioning of photosynthates from structural to nonstructural and, in terms of carbon balance, unproductive dry matter. The increase in concentration of TNC led to a decrease in both specific leaf area (SLA) and N_i at all levels of nutrient supply, and was the cause of the increased rate of photosynthesis per unit leaf area. Correction of the relationship between RGR and Ni for the accumulation of TNC made the effect of elevated CO₂ on the relationship between RGR and N_i disappear. We conclude that the shift in the relationship between RGR and N_i was due to the accumulation of TNC and not due to differences in physiological variables such as photosynthesis and shoot and root respiration, changes in leaf morphology or allocation of dry matter.     

Growth potential of Lemna gibba: Effect of CO2 enrichment at high photon flux rate

Lemna gibba L. was cultivated in continuous light (800–1200 μmol quanta m^?2s^?1, 320–400 W m^?2) and normal or CO₂-enriched air (1500 μl CO₂ l^?1), with a continuous nutrient supply. Increased CO₂ concentration increased the unit leaf rate (ULR) or net assimilation rate and decreased the leaf area ratio (LAR) (photosynthetic area per unit dry weight), but the relative growth rate was unchanged (0.43 g g^?1 day^?1). The changes in ULR and LAR indicate that organic matter production can be increased with CO₂ enrichment at high photon flux rate (PFR).     

Effect of CO2 enrichment on non-structural carbohydrates in leaves, stems and ears of spring wheat

Field-grown spring wheat (Triticum aestivum L. cv. Dragon) was exposed to ambient and elevated CO₂ concentrations (1.5 and 2 times ambient) in open-top chambers. Contents of non-structural carbohydrates were analysed enzymatically in leaves, stems and ears six times during the growing season. The impact of elevated CO₂ on wheat carbohydrates was non-significant in most harvests. However, differences in the carbohydrate contents due to elevated CO₂ were found in all plant compartments. Before anthesis, at growth stage (GS) 30 (the stem is 1 cm to the shoot apex), the plants grown in elevated CO₂ contained significantly more water soluble carbohydrates (WSC), fructans, starch and total non-structural carbohydrates (TNC) in the leaves in comparison with the plants grown in ambient CO₂. It is hypothesised that the plants from the treatments with elevated CO₂ were sink-limited at GS30. After anthesis, the leaf WSC and TNC contents of the plants from elevated CO₂ started to decline earlier than those of the plants from ambient CO₂. This may indicate that the leaves of plants grown in the chambers with elevated CO₂ senesced earlier. Elevated CO₂ accelerated grain development: 2 weeks after anthesis, the plants grown in elevated CO₂ contained significantly more starch and significantly less fructans in the ears compared to the plants grown in ambient CO₂. Elevated CO₂ had no effect on ear starch and TNC contents at the final harvest. Increasing the CO₂ concentration from 360 to 520 μmol mol^?1 had a larger effect on wheat non-structural carbohydrates than the further increase from 520 to 680 μmol mol^?1. The results are discussed in relation to the effects of elevated CO₂ on yield and yield components.     

Effect of daytime CO2 supplement on productivity and biochemical composition of Scenedesmus armatus under outdoor cultivation

The cultivation of Scenedesmus armatus was carried out under outdoor Thailand climate conditions. The highest actual growth rate occurred at around 9:00 a.m. to 3:00 p.m., with a wide pH range of 6.4 to 11. The supply of CO₂ had slight influence on growth characteristics but did exert some observable effects on nutritional accumulations. Adding CO₂ from 2 to 15% by volume in the aeration (0.2 vvm) caused an increase in lipid and protein from 19.8 to 25.6 and 37.8 to 48.2% w/w, respectively, whereas carbohydrate decreased from 42.5 to 26.2% w/w. Scenedesmus armatus cultivated with 2% CO₂-enriched air provided the highest the average of the average biomass productivity of 91.25 mg L^?1 d^?1, which corresponded to a CO₂ fixation of 165 mg CO₂ L^?1 d^?1 with the average lipid, protein, and carbohydrate productivities of 22.24, 38.34, and 30.67 mg L^?1 d^?1.     

Net grassland carbon flux over a subambient to superambient CO2 gradient

Increasing atmospheric CO₂ concentrations may have a profound effect on the structure and function of plant communities. A previously grazed, central Texas grassland was exposed to a 200‐µmol mol^?1 to 550 µmol mol^?1 CO₂ gradient from March to mid‐December in 1998 and 1999 using two, 60‐m long, polyethylene‐ covered chambers built directly onto the site. One chamber was operated at subambient CO₂ concentrations (200–360 µmol mol^?1 daytime) and the other was regulated at superambient concentrations (360–550 µmol mol^?1). Continuous CO₂ gradients were maintained in each chamber by photosynthesis during the day and respiration at night. Net ecosystem CO₂ flux and end‐of‐year biomass were measured in each of 10, 5‐m long sections in each chamber. Net CO₂ fluxes were maximal in late May (c. day 150) in 1998 and in late August in 1999 (c. day 240). In both years, fluxes were near zero and similar in both chambers at the beginning and end of the growing season. Average daily CO₂ flux in 1998 was 13 g CO₂ m^?2 day^?1 in the subambient chamber and 20 g CO₂ m^?2 day^?1 in the superambient chamber; comparable averages were 15 and 26 g CO₂ m^?2 day^?1 in 1999. Flux was positively and linearly correlated with end‐of‐year above‐ground biomass but flux was not linearly correlated with CO₂ concentration; a finding likely to be explained by inherent differences in vegetation. Because C₃ plants were the dominant functional group, we adjusted average daily flux in each section by dividing the flux by the average percentage C₃ cover. Adjusted fluxes were better correlated with CO₂ concentration, although scatter remained. Our results indicate that after accounting for vegetation differences, CO₂ flux increased linearly with CO₂ concentration. This trend was more evident at subambient than superambient CO₂ concentrations.     

Effects of Gamma Stress and Carbon Dioxide on Eight Bioactive Flavonoids and Photosynthetic Efficiency in <Emphasis Type="Italic">Centella asiatica</Emphasis>

Increased atmospheric CO₂ and gamma irradiation have a significant impact on the plant photosynthetic apparatus and organic compound production. In this study, we evaluated the effect of elevated CO₂ on the photosynthetic efficiency and production of defensive secondary metabolites (flavonoids) induced by gamma irradiation as a physical elicitor in Centella asiatica. Irradiated and non-irradiated 10-week-old plants of C. asiatica were exposed to 400 and 800 μmol mol^?1 of atmospheric CO₂ in growth chambers for 2 h every day until six weeks. A CO₂-enriched atmosphere initially improved the photosynthetic efficiency and ameliorated the detrimental impact of gamma irradiation on the photosynthetic apparatus, increasing carbon allocation into the flavonoid pathway. Elevated CO₂ combined with gamma irradiation resulted in the highest concentration of flavonoids in C. asiatica tissues compared with the other treatments. There was an enhancement in rutin (2.49 fold), naringin (2.15 fold), fisetin (4.07 fold), and morin (4.62 fold) with rising CO₂ concentrations from 400 to 800 μmol mol^?1 in the irradiated plants. With increasing CO₂ concentration, the compensation point and the respiration declined, whereas the apparent quantum yield and the maximum net photosynthesis (A _max) rate increased. The efficiency of photosystem II (PSII) was improved in the irradiated plants grown under high concentrations of CO₂. The total carbohydrate concentration reached the maximum value at the highest level of CO₂, followed by gamma irradiation combined with the highest level of CO₂. Irradiated plants of C. asiatica grown under elevated CO₂ could be superior to non-irradiated plants due to increased carbon availability both for the flavonoid biosynthesis and for the photosynthetic pathway.     

Influence of carbon dioxide enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition

The objective of this study was to test whether elevated [CO₂], [O₃] and nitrogen (N) fertility altered leaf mass per area (LMPA), non‐structural carbohydrate (TNC), N, lignin (LTGA) and proanthocyanidin (PA) concentrations in cotton (Gossypium hirsutum L.) leaves and roots. Cotton was grown in 14 dm³ pots with either sufficient (0·8 g N dm ^{? 3}) or deficient (0·4 and 0·2 g N dm ^{? 3}) N fertilization, and treated in open‐top chambers with either ambient or elevated ( + 175 and + 350 μ mol mol ^{? 1}) [CO₂] in combination with either charcoal‐filtered air (CF) or non‐filtered air plus 1·5 times ambient [O₃]. At about 50 d after planting, LMPA, starch and PA concentrations in canopy leaves were as much as 51–72% higher in plants treated with elevated [CO₂] compared with plants treated with ambient [CO₂], whereas leaf N concentration was 29% lower in elevated [CO₂]‐treated plants compared with controls. None of the treatments had a major effect on LTGA concentrations on a TNC‐free mass basis. LMPA and starch levels were up to 48% lower in plants treated with elevated [O₃] and ambient [CO₂] compared with CF controls, although the elevated [O₃] effect was diminished when plants were treated concurrently with elevated [CO₂]. On a total mass basis, leaf N and PA concentrations were higher in samples treated with elevated [O₃] in ambient [CO₂], but the difference was much reduced by elevated [CO₂]. On a TNC‐free basis, however, elevated [O₃] had little effect on tissue N and PA concentrations. Fertilization treatments resulted in higher PA and lower N concentrations in tissues from the deficient N fertility treatments. The experiment showed that suppression by elevated [O₃] of LMPA and starch was largely prevented by elevated [CO₂], and that interpretation of [CO₂] and [O₃] effects should include comparisons on a TNC‐free basis. Overall, the experiment indicated that allocation to starch and PA may be related to how environmental factors affect source–sink relationships in plants, although the effects of elevated [O₃] on secondary metabolites differed in this respect.     

GLYOXYLATE CYCLE ENZYME ACTIVITIES IN THE CYANOBACTERIUM ANACYSTIS NIDULANS1

The enzymes of the glyoxylate cycle, isocitrate lyase (EC.4.1.3.1) and malate synthase (EC.4.1.3.2), were measured in cell-free extracts from the cyanobacterium Anacystis nidulans Drouet during photoautotrophic growth in medium aerated with ordinary air (0.03% CO₂). Isocitrate lyase had an average specific activity of 112 nmoles·min^?1·mg protein^?1 whereas malate synthase had an average specific activity of 12.5 nmoles·min^?1·mg protein^?1. Unpurified isocitrate lyase showed classical Michaelis kinetics with a K_m of 8 mM. Isocitrate lyase activity was strongly inhibited by numerous cellular metabolites at 10 mM concentration. The previously reported low specific activity for isocitrate lyase may be due to metabolite inhibition caused by growth in high CO₂ concentrations. The activities reported for isocitrate lyase and malate synthase suggest the operation of the glyoxylate cycle in Anacystis nidulans under CO₂-limiting growth conditions.     

Overcoming drought-induced decreases in soybean leaf photosynthesis by measuring with CO2-enriched air

Soybean [Glycine max (L.) Merr. cv. Williams 82 and A3127] plants were grown in the field under long-term soil moisture deficit and irrigation to determine the effects of severe drought stress on the photosynthetic capacity of soybean leaves. Afternoon leaf water potentials, stomatal conductances, intercellular CO₂ concentrations and CO₂-assimilation rates for the two soil moisture treatments were compared during the pod elongation and seed enlargement stages of crop development. Leaf CO₂-assimilation rates were measured with either ambient (340 l CO₂ l^–1) or CO₂-enriched (1800 l CO₂ l^–1) air. Although seed yield and leaf area per plant were decreased an average of 48 and 31%, respectively, as a result of drought stress, leaf water potentials were reduced only an average of 0.27 MPa during the sampling period. Afternoon leaf CO₂-assimilation rates measured with ambient air were decreased an average of 56 and 49% by soil moisture deficit for Williams 82 and A3127, respectively. The reductions in leaf photosynthesis of both cultivars were associated with similar decreases in leaf stomatal conductance and with small increases in leaf intercellular CO₂ concentration. When the CO₂-enriched air was used, similar afternoon leaf CO₂-assimilation rates were found between the soil moisture treatments at each stage of crop development. These results suggest that photosynthetic capacity of soybean leaves is not reduced by severe soil moisture deficit when a stress develops gradually under field conditions.Abbreviations C_i intercellular CO₂ concentrations - A_a rates of CO₂ assimilation measured with ambient air - A_e rates of CO₂ assimilation measured with CO₂-enriched air - g_s stomatal conductances - RuBPCase ribulose-1,5-bisphosphate carboxylase     

The pH dependent substrate specificity of udp-glucose: isovitexin 2″-O-glucosyltransferase in Silene alba

In Silene alba plants the dominant allele of gene Fg controls an enzyme which catalyses the formation of isovitexin 2″-O-glucoside both in petals and green parts. Both isovitexin and isoorientin can act as substrate. K_mvalues for the isovitexin glucosylation are 0.09 mM for isovitexin and 0.3 mM for UDP-glucose, V_max 0.17 nmol min^?1 mg protein^?1. For the isoorientin glucosylation K_m values of 0.45 mM for isoorientin, of 0.75 mM for UDP-glucose and V_max of 0.27 nmol min^?1 mg protein^?1 are found. The pH optima for both substrates differ markedly. For the substrate with one hydroxyl in the B-ring, isovitexin, the pH optimum is pH 8.5. For isoorientin, which has two hydroxyls in the B-ring, a pH optimum of 7.5 is found. These results suggest that the B-ring hydroxylation pattern influences the pH at which the substrate has optimal affinity for the enzyme. The location of the carbon-carbon bound glucose on a the flavonoid skeleton is of importance for enzyme activity as well. Vitexin, which has glucose at the 8-position, was not a substrate. The glucosylation of vitexin could, however, be demonstrated in enzyme extracts of petals of plants, grown from seed collected in Armenia; in these petals apart from isovitexin glycosides, vitexin glycosides are found as well.     

CO2 responsiveness of plants: a possible link to phloem loading

Of the many responses of plants to elevated CO₂, accumulation of total non-structural carbohydrates (TNC in % dry weight) in leaves is one of the most consistent. Insufficient sink activity or transport capacity may explain this obvious disparity between CO₂ assimilation and carbohydrate dissipation and structural investment. If transport capacity contributes to the problem, phloem loading may be the crucial step. It has been hypothesized that symplastic phloem loading is less efficient than apoplastic phloem loading, and hence plant species using the symplastic pathway and growing under high light and good water supply should accumulate more TNC at any given CO₂ level, but particularly under elevated CO₂. We tested this hypothesis by carrying out CO₂ enrichment experiments with 28 plant species known to belong to groups of contrasting phloem-loading type. Under current ambient CO₂ symplastic loaders were found to accumulate 36% TNC compared with only 19% in apoplastic loaders (P=0.0016). CO₂ enrichment to 600 μmol mol^?1 increased TNC in both groups by the same absolute amount, bringing the mean TNC level to 41% in symplastic loaders (compared to 25% in apoplastic loaders), which may be close to TNC saturation (coupled with chlornplast malfunction). Eight tree species, ranked as symplastic loaders by their minor vein companion cell configuration, showed TNC responses more similar to those of apoplastic herbaceous loaders. Similar results are obtained when TNC is expressed on a unit leaf area basis, since mean specific leaf areas of groups were not significantly different. We conclude that phloem loading has a surprisingly strong effect on leaf tissue composition, and thus may translate into alterations of food webs and ecosystem functioning, particularly under high CO₂.     

Carbon and nitrogen pools and mineralization in a grassland gley soil under elevated carbon dioxide at a natural CO2 spring

The growth and chemical composition of most plants are influenced by elevated CO₂, but accompanying effects on soil organic matter pools and mineralization are less clearly defined, partly because of the short‐term nature of most studies. Herein we describe soil properties from a naturally occurring cold CO₂ spring (Hakanoa) in Northland, New Zealand, at which the surrounding vegetation has been exposed to elevated CO₂ for at least several decades. The mean annual temperature at this site is ≈ 15.5 °C and rainfall ≈ 1550 mm. The site was unfertilized and ungrazed, with a vegetation of mainly C3 and C4 grasses, and had moderate levels of ‘available’ P. Two soils were present ? a gley soil and an organic soil – but only the gley soil is examined here. Average atmospheric CO₂ concentrations at 17 sampling locations in the gley soil area ranged from 372 to 670 ppmv. In samples at 0–5 cm depth, pH averaged 5.4; average values for organic C were 150 g, total N 11 g, microbial C 3.50 g, and microbial N 0.65 g kg^?1, respectively. Under standardized moisture conditions at 25 °C, average rates of CO₂‐C production (7–14 days) were 5.4 mg kg^?1 h^?1 and of net mineral‐N production (14 ?42 days) 0.40 mg kg^?1 h^?1. These properties were all correlated positively and significantly (P < 0.10) with atmospheric CO₂ concentrations, but not with soil moisture (except for CO₂‐C production) or with clay content; they were, however, correlated negatively and mainly significantly with soil pH. In spite of uncertainties associated with the uncontrolled environment of naturally occurring springs, we conclude that storage of C and N can increase under prolonged exposure to elevated CO₂, and may include an appreciable labile fraction in mineral soil with an adequate nutrient supply.     

Influences of PAR, temperature and water vapor concentration on gas exchange by ferns

The effects of photosynthetically active radiation (PAR), leaf temperature and the leaf-to-air water vapor concentration drop on net CO₂ uptake and water vapor conductance were surveyed for 14 species of ferns. Most previous studies indicated that ferns have extremely low maximal rates of net CO₂ uptake, below 2 umol m^?2 s^?1, whereas the average maximal rate observed here at 25⁰ C was 7 umol m^?2 s^?1. Net CO₂ uptake reached 90% of saturation at an average PAR (400 to 700 nm) of only 240 umol m^?2 s^?1, consistent with the typically shaded habitats of most ferns. Maximal CO₂ uptake rates were positively correlated with the PAR for 90% saturation (r²=0.59), the chlorophyII per unit leaf area (r²=0.30), the water vapor conductance (r²=0.65), and the CO₂ residual conductance (r²=0.69). A higher water vapor conductance (gwv) was correlated with a greater fractional change in gwv as the leaf-to-air water vapor concentration drop (Δc_wv) was raised from 5to20 g m^?3 (r²=0.90). Specifically, for species with low g_wv of about I mm s^?1 the ratio of g_wv at Δc_wv= 5 g m^?3 to that at Δc_wv= 20 g m^?3 was near 1, but it was near 2 for species with g_wv of about 4 mm s^?1. Such a relationship, which can prevent excessive transpiration, has apparently not previously been pointed out in surveys of other plant groups.     

Measurement of net ecosystem exchange,productivity and respiration in three spruce forests in Sweden shows unexpectedly large soil carbon losses

Measurement of net ecosystem exchange was made using the eddy covariance method above three forests along a north-south climatic gradient in Sweden: Flakaliden in the north, Knottåsen in central and Asa in south Sweden. Data were obtained for 2 years at Flakaliden and Knottåsen and for one year at Asa. The net fluxes (N_ep) were separated into their main components, total ecosystem respiration (R_t) and gross primary productivity (P_g). The maximum half-hourly net uptake during the heart of the growing season was highest in the southernmost site with ?0.787 mg CO₂ m^?2 s^?1 followed by Knottåsen with ?0.631 mg CO₂ m^?2 s^?1 and Flakaliden with ?0.429 mg CO₂ m^?2 s^?1. The maximum respiration rates during the summer were highest in Knottåsen with 0.245 mg CO₂ m^?2 s^?1 while it was similar at the two other sites with 0.183 mg CO₂ m^?2 s^?1. The annual N_ep ranged between uptake of ?304 g C m^?2 year^?1 (Asa) and emission of 84 g C m^?2 year^?1 (Knottåsen). The annual R_t and P_g ranged between 793 to 1253 g C m^?2 year^?1 and ?875 to ?1317 g C m^?2 year^?1, respectively. Biomass increment measurements in the footprint area of the towers in combination with the measured net ecosystem productivity were used to estimate the changes in soil carbon and it was found that the soils were losing on average 96–125 g C m^?2 year^?1. The most plausible explanation for these losses was that the studied years were much warmer than normal causing larger respiratory losses. The comparison of net primary productivity and P_g showed that ca 60% of P_g was utilized for autotrophic respiration.     

The influence of CO2 enrichment, phosphorus deficiency and water stress on the growth, conductance and water use of Pinus radiata D. Don

Abstract. Seedlings of Pinus radiata D. Don were grown in growth chambers for 22 weeks with two levels of phosphorus, under either well-watered or water-stressed conditions at CO₂ concentrations of either 330 or 660mm³ dm^?3. Plant growth, water use efficiency and conductance were measured and the relationship between these and needle photosynthetic capacity, water use efficiency and conductance was determined by gas exchange at week 22. Phosphorus deficiency decreased growth and foliar surface area at both CO₂concentrations; however, it only reduced the maximum photosynthetic rates of the needles at 660 mm³ CO₂ dm^?3 (plants grown and measured at the same CO₂ concentration). Water stress reduced growth and foliar surface area at both CO₂ concentrations. Increases in needle photosynthetic rates appeared to be partly responsible for the increased growth at high CO₂ where phosphorus was adequate. This effect was amplified by accompanying increases in needle production. Phosphorus deficiency inhibited these responses because it severely impaired needle photosynthetic function. The relative increase in growth in response to high CO₂ was higher in the periodically water-stressed plants. This was not due to the maintenance of cell volume during drought. Plant water use efficiency was increased by CO₂ enrichment due to an increase in dry weight rather than a decrease in shoot conductance and, therefore, transpirational water loss. Changes in needle conductance and water use efficiency in response to high CO₂ were generally in the same direction as those at the whole plant level. If the atmospheric CO₂ level reaches the predicted concentration of 660 mm³ dm^?3 by the end of next Century, then the growth of P. radiata will only be increased in areas where phosphorus nutrition is adequate. Growth will be increased in drought-affected regions but total water use is unlikely to be reduced.     

Initial cultivation of a temperate-region soil immediately accelerates aggregate turnover and CO2 and N2O fluxes

The immediate effects of tillage on protected soil C and N pools and on trace gas emissions from soils at precultivation levels of native C remain largely unknown. We measured the response to cultivation of CO₂ and N₂O emissions and associated environmental factors in a previously uncultivated U.S. Midwest Alfisol with C concentrations that were indistinguishable from those in adjacent late successional forests on the same soil type (3.2%). Within 2 days of initial cultivation in 2002, tillage significantly (P=0.001, n=4) increased CO₂ fluxes from 91 to 196 mg CO₂‐C m^?2 h^?1 and within the first 30 days higher fluxes because of cultivation were responsible for losses of 85 g CO₂‐C m^?2. Additional daily C losses were sustained during a second and third year of cultivation of the same plots at rates of 1.9 and 1.0 g C m^?2 day^?1, respectively. Associated with the CO₂ responses were increased soil temperature, substantially reduced soil aggregate size (mean weight diameter decreased 35% within 60 days), and a reduction in the proportion of intraaggregate, physically protected light fraction organic matter. Nitrous oxide fluxes in cultivated plots increased 7.7‐fold in 2002, 3.1‐fold in 2003, and 6.7‐fold in 2004 and were associated with increased soil NO₃^? concentrations, which approached 15 μg N g^?1. Decreased plant N uptake immediately after tillage, plus increased mineralization rates and fivefold greater nitrifier enzyme activity, likely contributed to increased NO₃^? concentrations. Our results demonstrate that initial cultivation of a soil at precultivation levels of native soil C immediately destabilizes physical and microbial processes related to C and N retention in soils and accelerates trace gas fluxes. Policies designed to promote long‐term C sequestration may thus need to protect soils from even occasional cultivation in order to preserve sequestered C.     

Distribution of surface soil mercury of Wuda old mining area,Inner Mongolia,China

In this study, mercury (Hg) concentrations in dustfall and topsoil were investigated. Three hundred forty-four samples were collected, including dustfall and topsoil samples, across an area of ～180 km² in Wuda, China. Dustfall Hg concentration in Wuda ranged from 10 to 6453 ng·g^?1_, with an average of 305 ng·g^?1, and topsoil Hg concentration ranged from 3 to 1537 ng·g^?1, with an average value of 135 ng·g^?1. The average dustfall Hg concentrations in the coalfield, industrial park, and urban areas were 289 ng·g^?1, 809 ng·g^?1, and 160 ng·g^?1, respectively, and the corresponding average topsoil Hg concentrations were 216 ng·g^?1, 242 ng·g^?1, and 91 ng·g^?1. Hg concentrations were significantly higher in the coalfield, industrial park, and urban areas compared with background values for Wuda and China tide soil. The coal Hg concentrations ranged from 273 to 346 ng·g^?1, with an average value of 317 ng·g^?1. Comparison of the Hg concentrations of Wuda coal with other regions and countries, indicated that Hg concentrations were significantly enriched in coal, highlighting that coal is the primary Hg source for Wuda District. While coal fires provided the primary source of Hg, some higher Hg values were caused by factors such as gangue hills and coalfield topography, the use of activated carbon with HgCl₂ as a catalyst in the industrial park, and several coal washeries in wasteland areas. In addition to atmospheric Hg, underground coal seam emissions served as another potential Hg source in the coalfield. The surface soil Hg in the coalfield and industrial park areas should be given closer attention in the future.     

A rhizolysimeter facility for studying the dynamics of crop and soil processes: description and evaluation

Increased atmospheric carbon dioxide (CO₂) concentration will likely cause changes in plant productivity and composition that might affect soil decomposition processes. The objective of this study was to test to what extent elevated CO₂ and N fertility-induced changes in residue quality controlled decomposition rates. Cotton (Gossypium hirsutum L.) was grown in 8-l pots and exposed to two concentrations of CO₂ (390 or 722 μmol mol^-1) and two levels of N fertilization (1.0 or 0.25 g l^-1 soil) within greenhouse chambers for 8 wks. Plants were then chemically defoliated and air-dried. Leaf, stem and root residues were assayed for total non-structural carbohydrates (TNC), lignin (LTGA), proanthocyanidins (PA), C and N. Respiration rates of an unsterilized sandy soil (Lakeland Sand) mixed with residues from the various treatments were determined using a soda lime trap to measure CO₂ release. At harvest, TNC and PA concentrations were 17 to 45% higher in residues previously treated with elevated CO₂ compared with controls. Leaf and stem residue LTGA concentrations were not significantly affected by either the elevated CO₂ or N fertilization treatments, although root residue LTGA concentration was 30% greater in plants treated with elevated CO₂. The concentration of TNC in leaf residues from the low N fertilization treatment was 2.3 times greater than that in the high N fertilization treatment, although TNC concentration in root and stem residues was suppressed 13 to 23% by the low soil N treatment. PA and LTGA concentrations in leaf, root and stem residues were affected by less than 10% by the low N fertilization treatment. N concentration was 14 to 44% lower in residues obtained from the elevated CO₂ and low N fertilization treatments. In the soil microbial respiration assay, cumulative CO₂ release was 10 to 14% lower in soils amended with residues from the elevated CO₂ and low N fertility treatments, although treatment differences diminished as the experiment progressed. Treatment effects on residue N concentration and C:N ratios appeared to be the most important factors affecting soil microbial respiration. The results of our study strongly suggest that, although elevated CO₂ and N fertility may have significant impact on post-harvest plant residue quality of cotton, neither factor is likely to substantially affect decomposition. Thus, C cycling might not be affected in this way, but via simple increases in plant biomass production. This revised version was published online in June 2006 with corrections to the Cover Date.