Nitrogen and carbon partitioning in soybean under variable nitrogen supplies and acclimation to the prolonged action of elevated CO2

This study was conducted to determine reciprocal effects of low to high doses of nitrogenous fertilizer (N₃₀, N₄₀, N₅₀, N₆₀ and N₇₀ — 30, 40, 50, 60 and 70 kg ha⁻¹ respectively) and CO₂ enriched environment on C and N partitioning in soybean (Glycine max (L.) Merril cv JS-335). Plants were grown from seedling emergence to maturity inside open top chambers under ambient, AC (350±50 mol mol⁻¹) and elevated, EC (600±50 mol mol⁻¹) CO₂ and analyzed at seedling, vegetative, flowering, pod setting and maturity stages. Soybean responded to both CO₂ enrichment and N supply. Leaves, stem and root reserves at different growth stages were analyzed for total C and N contents. Consistent increase in the C contents of the leaf, stem and root was observed under EC than in AC. N contents in the different plant parts were found to be decreased under EC-grown plants specially at seedling and vegetative stage despite providing N doses to the soil. Significant increase observed for C to N dry mass ratio under EC in the root, stems and leaves at seedling and vegetative stage was decreased in the middle and later growth stages possibly due to combined impact of N doses to the soil and increased N₂ fixing activities due to EC conditions. Critical analysis of our findings reveals that the composition and partitioning of C and N of soybean under variable rates of N supply and CO₂ enrichment alter according to need under altered metabolic process. These changes eventually may lead to alteration in uptake of not only N but other essential nutrients also under changing atmosphere.     

Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply

With the ability to symbiotically fix atmospheric N₂, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO₂. The growth and photosynthetic responses of two perennial grassland species were compared to test the hypotheses that (1) the CO₂ response of wild species is limited at low N availability, (2) legumes respond to a greater extent than non-fixing forbs to elevated CO₂, and (3) elevated CO₂ stimulates symbiotic N₂ fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO₂ concentration (365 and 700 mol mol^–1) and N addition on whole plant growth and C and N acquisition in an N₂-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO₂ response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N m^–2 yr^–1). Whole plant growth, leaf net photosynthetic rates (A), and the proportion of N derived from N₂ fixation were determined in plants grown from seed over one growing season. Both species increased growth with CO₂enrichment, but this response was mediated by N supply only for the non-fixer, Achillea. Its response depended on mineral N supply as growth enhancements under elevated CO₂ increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO₂ regardless of N treatment. Although partial photosynthetic acclimation to CO₂ enrichment occurred, both species maintained comparably higher A in elevated compared to ambient CO₂ (+38%). N addition facilitated increased A in Achillea, however, in neither species did additional N availability affect the acclimation response of A to CO₂. Elevated CO₂ increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N₂ fixation, which resulted in a 47% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N₂-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO₂ that are independent of soil N supply. The enhanced amount of N derived from N₂ fixation under elevated CO₂ presumably helps meet the increased N demand in N₂-fixing species. This response may lead to modified roles of N₂-fixers and N₂-fixer/non-fixer species interactions in grassland communities, especially those that are inherently N-poor, under projected rising atmospheric CO₂.     

Optimal acclimation of the C3 photosynthetic system under enhanced CO2

A range of studies of C₃ plants have shown that there is a change in both the carbon flux and the pattern of nitrogen allocation when plants are grown under enhanced CO₂. This paper examines evidence that allocation of nitrogen both to and within the photosynthetic system is optimised with respect to the carbon flux. A model is developed which predicts the optimal relative allocation of nitrogen to key enzymes of the photosynthetic system as a function of CO₂ concentration. It is shown that evidence from flux control analysis is broadly consistent with this model, although at high nitrogen and under certain conditions at low nitrogen experimental data are not consistent with the model. Acclimation to enhanced CO₂ is also assessed in terms of resource allocation between photosynthate sources and sinks. A means of assessing the optimisation of this source-sink allocation is proposed, and several studies are examined within this framework. It is concluded that C₃ plants probably possess the genetic feedback mechanisms required to efficiently smooth out any imbalance within the photosynthetic system caused by a rise in atmospheric CO₂.Abbreviations A net rate of CO₂ assimilation - c _i intercellular CO₂ concentration - C_R ^A flux control coefficient for Rubisco with respect to flux A - FBPase fructose 1,6-bisphosphatase - k_app apparent catalytic rate constant - PCO photorespiratory carbon oxidation - PCR photosynthetic carbon reduction - PPFD photosynthetically active photon flux density - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate - Ru5P ribulose 5-phosphate - SBPase sedoheptulose 1,7-bisphosphatase     

Photosynthetic acclimation in trees to rising atmospheric CO2: A broader perspective

Analysis of leaf-level photosynthetic responses of 39 tree species grown in elevated concentrations of atmospheric CO₂ indicated an average photosynthetic enhancement of 44% when measured at the growth [CO₂]. When photosynthesis was measured at a common ambient [CO₂], photosynthesis of plants grown at elevated [CO₂] was reduced, on average, 21% relative to ambient-grown trees, but variability was high. The evidence linking photosynthetic acclimation in trees with changes at the biochemical level is examined, along with anatomical and morphological changes in trees that impact leaf- and canopy-level photosynthetic response to CO₂ enrichment. Nutrient limitations and variations in sink strength appear to influence photosynthetic acclimation, but the evidence in trees for one predominant factor controlling acclimation is lacking. Regardless of the mechanisms that underlie photosynthetic acclimation, it is doubtful that this response will be complete. A new focus on adjustments to rising [CO₂] at canopy, stand, and forest scales is needed to predict ecosystem response to a changing environment.Abbreviations A/C_i photosynthesis as a function of internal [CO₂] - J_max maximum rate of electron transport - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - Vc_max maximum rate of carboxylation The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Acclimation of photosynthesis,respiration and ecosystem carbon flux of a wetland on Chesapeake Bay,Maryland to elevated atmospheric CO2 concentration

Acclimation of photosynthesis and respiration in shoots and ecosystem carbon dioxide fluxes to rising atmospheric carbon dioxide concentration (C _a ) was studied in a brackish wetland. Open top chambers were used to create test atmospheres of normal ambient and elevated C _a (=normal ambient + 34 Pa CO₂) over mono-specific stands of the C₃ sedge Scirpus olneyi, the dominant C₃ species in the wetland ecosystem, throughout each growing season since April of 1987. Acclimation of photosynthesis and respiration were evaluated by measurements of gas exchange in excised shoots. The impact of elevated C _a on the accumulation of carbon in the ecosystem was determined by ecosystem gas exchange measurements made using the open top chamber as a cuvette.Elevated C _a increased carbohydrate and reduced Rubisco and soluble protein concentrations as well as photosynthetic capacity(A) and dark respiration (R _d ; dry weight basis) in excised shoots and canopies (leaf area area basis) of Scirpus olneyi. Nevertheless, the rate of photosynthesis was stimulated 53% in shoots and 30% in canopies growing in elevated C _a compared to normal ambient concentration. Elevated C _a inhibited R _d measured in excised shoots (–19 to –40%) and in seasonally integrated ecosystem respiration (R _e ; –36 to –57%). Growth of shoots in elevated C _a was stimulated 14–21%, but this effect was not statistically significant at peak standing biomass in midseason. Although the effect of elevated C _a on growth of shoots was relatively small, the combined effect of increased number of shoots and stimulation of photosynthesis produced a 30% stimulation in seasonally integrated gross primary production (GPP). The stimulation of photosynthesis and inhibition of respiration by elevated C _a increased net ecosystem production (NEP=GPP–R _e ) 59% in 1993 and 50% in 1994. While this study consistently showed that elevated C _a produced a significant increase in NEP, we have not identified a correspondingly large pool of carbon below ground.     

Decomposition of litter produced under elevated CO2: Dependence on plant species and nutrient supply

We investigated the effect of CO₂ concentration and soilnutrient availability during growth on the subsequent decomposition andnitrogen (N) release from litter of four annual grasses that differ inresource requirements and native habitat. Vulpia microstachys isa native grass found on California serpentine soils, whereas Avenafatua, Bromus hordaceus, and Lolium multiflorum areintroduced grasses restricted to more fertile sandstone soils (Hobbs & Mooney 1991). Growth in elevated CO₂ altered litter C:N ratio,decomposition, and N release, but the direction and magnitude of thechanges differed among plant species and nutrient treatments. ElevatedCO₂ had relatively modest effects on C:N ratio of litter,increasing this ratio in Lolium roots (and shoots at high nutrients),but decreasing C:N ratio in Avena shoots. Growth of plants underelevated CO₂ decreased the decomposition rate of Vulpialitter, but increased decomposition of Avena litter from the high-nutrient treatment. The impact of elevated CO₂ on N loss fromlitter also differed among species, with Vulpia litter from high-CO₂ plants releasing N more slowly than ambient-CO₂litter, whereas growth under elevated CO₂ caused increased Nloss from Avena litter. CO₂ effects on N release in Lolium and Bromus depended on the nutrient regime in whichplants were grown. There was no overall relationship between litter C:Nratio and decomposition rate or N release across species and treatments.Based on our study and the literature, we conclude that the effects ofelevated CO₂ on decomposition and N release from litter arehighly species-specific. These results do not support the hypothesis thatCO₂ effects on litter quality consistently lead to decreasednutrient availability in nutrient-limited ecosystems exposed to elevatedCO₂.     

Involvement of nitrogen and cytokinins in photosynthetic acclimation to elevated CO2 of spring wheat

Acclimation of photosynthetic capacity to elevated CO₂ involves a decrease of the leaf Rubisco content. In the present study, it was hypothesized that nitrogen uptake and partitioning within the leaf and among different aboveground organs affects the down-regulation of Rubisco. Given the interdependence of nitrogen and cytokinin signals at the whole plant level, it is also proposed that cytokinins affect the nitrogen economy of plants under elevated CO₂, and therefore the acclimatory responses. Spring wheat received varying levels of nitrogen and cytokinin in field chambers with ambient (370 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric CO₂. Gas exchange, Rubisco, soluble protein and nitrogen contents were determined in the top three leaves in the canopy, together with total nitrogen contents per shoot. Growth in elevated CO₂ induced decreases in photosynthetic capacity only when nitrogen supply was low. However, the leaf contents of Rubisco, soluble protein and total nitrogen on an area basis declined in elevated CO₂ regardless of nitrogen supply. Total nitrogen in the shoot was no lower in elevated than ambient CO₂, but the fraction of this nitrogen located in flag and penultimate leaves was lower in elevated CO₂. Decreased Rubisco: chlorophyll ratios accompanied losses of leaf Rubisco with CO₂ enrichment. Cytokinin applications increased nitrogen content in all leaves and nitrogen allocation to senescing leaves, but decreased Rubisco contents in flag leaves at anthesis and in all leaves 20 days later, together with the amount of Rubisco relative to soluble protein in all leaves at both growth stages. The results suggest that down regulation of Rubisco in leaves at elevated CO₂ is linked with decreased allocation of nitrogen to the younger leaves and that cytokinins cause a fractional decrease of Rubisco and therefore do not alleviate acclimation to elevated CO₂.     

Effects of elevated CO2 and nitrogen on nutrient uptake in ponderosa pine seedlings

This paper reports on the results of a controlled-environment study on the effects of CO₂ (370, 525, and 700 mol mol^-1) and N [0, 200, and 400 g N g soil^-1 as (NH₄)SO₄] on ponderosa pine (Pinus ponderosa) seedlings. Based upon a review of the literature, we hypothesized that N limitations would not prevent a growth response to elevated CO₂. The hypothesis was not supported under conditions of extreme N deficiency (no fertilizer added to a very poor soil), but was supported when N limitations were less severe but still suboptimal (lower rate of fertilization). The growth increases in N-fertilized seedlings occurred mainly between 36 and 58 weeks without any additional N uptake. Thus, it appeared that elevated CO₂ allowed more efficient use of internal N reserves in the previously-fertilized seedlings, whereas internal N reserves in the unfertilized seedlings were insufficient to allow this response. Uptake rates of other nutrients were generally proportional to growth. Nitrogen treatment caused reductions in soil exchangeable K⁺, Ca²⁺, and Mg²⁺ (presumably because of nitrification and NO₃ ^- leaching) but increases in extractable P (presumably due to stimulation of phosphatase activity).The results of this and other seedling studies show that elevated CO₂ causes a reduction in tissue N concentration, even under N-rich conditions. The unique response of N is consistent with the hypothesis that the efficiency of Rubisco increases with elevated CO₂. These results collectively have significant implications for the response of mature, N-deficient forests to evevated CO₂.     

温度增高、CO2浓度升高、施氮对蒙古栎幼苗非结构碳水化合物积累及其分配的综合影响

蒙古栎(Quercus mongolica)是中国东北地区天然次生林重要组成树种, 研究该树种幼苗有机碳积累及碳库容对未来气候变化的响应, 可为预测未来气候变暖情景下蒙古栎林的天然更新及幼苗的培育提供科学参考。该文旨在探讨CO₂浓度和温度升高综合作用对蒙古栎幼苗非结构性碳水化合物(NSC)积累及其分配的影响。实验环境条件用人工气候箱控制, 控制条件如下: 1) CO₂浓度倍增(700 μmol·mol^-1), 温度升高4 ℃处理(HCHT); 2) CO₂浓度正常(400 μmol·mol^-1), 温度升高4 ℃处理(HT); 3) CO₂浓度和温度均正常, 即对照组(CK); 每个气候箱幼苗分别在3种氮素水平下生长: N2 (15 mmol·L^-1, 高氮), N1 (7.5 mmol·L^-1, 正常供氮)和N0 (不施氮), 一共为9个处理。研究结果表明, 1) HCHT共同作用对NSC积累无促进作用, 但改变了植物各器官中NSC的分配比例, 叶片中可溶性糖和淀粉的积累明显增加, HCHT下N2水平有利于NSC的积累。2) HT明显影响了蒙古栎一年生幼苗NCS的积累和分配。在N2水平下, HT明显促进NSC的积累, 并增加了在主根中的分配比例。3)植株各器官可溶性糖含量的动态变化因处理不同而异。主根淀粉含量随时间逐渐增加, 而细根淀粉含量随时间逐渐减少。在未来气候变暖的情况下, 土壤中大量的氮供给, 可能将促进蒙古栎幼苗的生长、增加其碳库容和抵御不良环境的能力, 进而提高其天然更新潜力。     

Acclimation of nitrogen uptake capacity of rice to elevated atmospheric CO2 concentration

Background and Aims: Nitrogen (N) is a major factor affecting yield gain of cropsunder elevated atmospheric carbon dioxide concentrations [CO₂].It is well established that elevated [CO₂] increases root mass,but there are inconsistent reports on the effects on N uptakecapacity per root mass. In the present study, it was hypothesizedthat the responses of N uptake capacity would change with theduration of exposure to elevated [CO₂]. Methods: The hypothesis was tested by measuring N uptake capacity inrice plants exposed to long-term and short-term [CO₂] treatmentsat different growth stages in plants grown under non-limitingN conditions in hydroponic culture. Seasonal changes in photosynthesisrate and transpiration rate were also measured. Key Results: In the long-term [CO₂] study, leaf photosynthetic responsesto intercellular CO₂ concentration (Ci) were not affected byelevated [CO₂] before the heading stage, but the initial slopein this response was decreased by elevated [CO₂] at the grain-fillingstage. Nitrate and ammonium uptake capacities per root dry weightwere not affected by elevated [CO₂] at panicle initiation, butthereafter they were reduced by elevated [CO₂] by 31–41% at the full heading and mid-ripening growth stages. In theshort-term study (24 h exposures), elevated [CO₂] enhanced nitrateand ammonium uptake capacities at the early vegetative growthstage, but elevated [CO₂] decreased the uptake capacities atthe mid-reproductive stage. Conclusions: This study showed that N uptake capacity was downregulated underlong-term exposure to elevated [CO₂] and its response to elevated[CO₂] varied greatly with growth stage.     

氮素对高大气CO2浓度下小麦叶片光合作用的影响

通过测定小麦拔节期叶片的光合气体交换参数和光强-光合速率（P_n）响应曲线,研究了氮素对长期高大气CO₂浓度（760 μmol·mol^-1）下小麦叶片光合作用的影响.结果表明：在长期高大气CO₂浓度下,增施氮肥能提高小麦叶片P_n、蒸腾速率（T_r）和瞬时水分利用效率（WUE_i）;与正常大气CO₂浓度相比,高大气CO₂浓度下小麦叶片的P_n和WUE_i增加,气孔导度（G_s）和胞间CO₂浓度(C_i）降低.随光合有效辐射的增强,高大气CO₂浓度下小麦叶片的P_n和WUE_i均高于正常大气CO₂浓度处理,G_s则较低,而C_i和T_r无显著变化.高氮水平下小麦叶片G_s与P_n、T_r、WUE_i呈线性正相关,G_s与C_i在正常大气CO₂浓度下呈线性负相关,但高大气CO₂浓度下二者无相关性;低氮水平下小麦叶片的G_s与P_n、WUE_i无相关性,而与C_i和T_r呈线性正相关,表明高大气CO₂浓度下低氮水平的小麦叶片P_n由非气孔因素限制.     

Effect of elevated CO2 concentration on seedling growth rate and photosynthesis inHevea brasiliensis

To study the effect of elevated CO₂ concentration on plant growth and photosynthesis, two clones ofHevea brasiliensis were grown in polybags and exposed to elevated concentration (700±25ppm) for 60 days. There was higher biomass accumulation, leaf area and better growth when compared to ambient air grown plantso From A/Ci curves it is clear that photosynthetic rates increases with increase in CO₂ concentrations. After 60 days of exposure to higher CO₂ concentration, a decrease in the carbon assimilation rate was noticed.     

氮素对高大气CO2浓度下小麦叶片光合作用的影响

通过测定小麦拔节期叶片的光合气体交换参数和光强-光合速率（P_n）响应曲线,研究了氮素对长期高大气CO₂浓度（760 μmol·mol^-1）下小麦叶片光合作用的影响.结果表明：在长期高大气CO₂浓度下,增施氮肥能提高小麦叶片P_n、蒸腾速率（T_r）和瞬时水分利用效率（WUE_i）;与正常大气CO₂浓度相比,高大气CO₂浓度下小麦叶片的P_n和WUE_i增加,气孔导度（G_s）和胞间CO₂浓度(C_i）降低.随光合有效辐射的增强,高大气CO₂浓度下小麦叶片的P_n和WUE_i均高于正常大气CO₂浓度处理,G_s则较低,而C_i和T_r无显著变化.高氮水平下小麦叶片G_s与P_n、T_r、WUE_i呈线性正相关,G_s与C_i在正常大气CO₂浓度下呈线性负相关,但高大气CO₂浓度下二者无相关性;低氮水平下小麦叶片的G_s与P_n、WUE_i无相关性,而与C_i和T_r呈线性正相关,表明高大气CO₂浓度下低氮水平的小麦叶片P_n由非气孔因素限制.     

Acclimation of photosynthesis to increasing atmospheric CO2: The gas exchange perspective

The nature of photosynthetic acclimation to elevated CO₂ is evaluated from the results of over 40 studies focusing on the effect of long-term CO₂ enrichment on the short-term response of photosynthesis to intercellular CO₂ (the A/C_i response). The effect of CO₂ enrichment on the A/C_i response was dependent on growth conditions, with plants grown in small pots (< 5 L) or low nutrients usually exhibiting a reduction of A at a given C_i, while plants grown without nutrient deficiency in large pots or in the field tended to exhibit either little reduction or an enhancement of A at a given C_i following a doubling or tripling of atmospheric CO₂ during growth. Using theoretical interpretations of A/C_i curves to assess acclimation, it was found that when pot size or nutrient deficiency was not a factor, changes in the shape of A/C_i curves which are indicative of a reallocation of resources within the photosynthetic apparatus typically were not observed. Long-term CO₂ enrichment usually had little effect or increased the value of A at all C_i. However, a minority of species grown at elevated CO₂ exhibited gas exchange responses indicative of a reduced amount of Rubisco and an enhanced capacity to metabolize photosynthetic products. This type of response was considered beneficial because it enhanced both photosynthetic capacity at high CO₂ and reduced resource investment in excessive Rubisco capacity. The ratio of intercellular to ambient CO₂ (the C_i/C_a ratio) was used to evaluate stomatal acclimation. Except under water and humidity stress, C_i/C_a exhibited no consistent change in a variety of C₃ species, indicating no stomatal acclimation. Under drought or humidity stress, C_i/C_a declined in high-CO₂ grown plants, indicating stomata will become more conservative during stress episodes in future high CO₂ environments.Abbreviations A net CO₂ assimilation rate - C_i (C_a) intercellular (ambient) partial pressure of CO₂ - operational C_i intercellular partial pressure of CO₂ at a given ambient partial pressure of CO₂ - g_s stomatal conductance - normal CO₂ current atmospheric mole fraction of CO₂ (330 to 355 mol mol^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

Long-term response of Sitka spruce (Picea sitchensis (Bong.) Carr.) to CO2 enrichment and nitrogen supply. I. Growth,biomass allocation and physiology

ABSTRACT

After a 3-year exposure to elevated CO₂, young trees of Sitka spruce (Picea sitchensis (Bong.) Carr.) were planted in native, nutrient-deficient forest soil and grown for two more years with three CO₂ treatments in open-top chambers, and with two nutrient treatments (with and without supplied N). Elevated CO₂ resulted in larger fresh mass, dry mass, leaf area and leaf thickness in two-year old needles, but had no effect on one-year old and current needles. Tree height, basal diameter and biomass production also increased, regardless of N supply. In trees without added N, elevated CO₂ resulted in higher root-to-shoot and absorbing roots-to-stump ratios. Regardless of N supply, trees grown in elevated CO₂ had lower photosynthetic rates on a leaf area basis. Photosynthesis reduction was accompanied by a decline in Rubisco activity and leaf N concentration. Under elevated CO₂, added N elevated photosynthesis and Rubisco activity, suggesting a dependence on N availability of the photosynthetic response to elevated CO₂. Stomatal conductance of trees grown with added N decreased in response to elevated CO₂. This may account for the larger reduction in intercellular CO₂ concentration, and hence photosynthesis, in the trees supplied with N than in those without N supply.     

Root response to CO2 enrichment and nitrogen supply in loblolly pine

This paper examines how elevated CO₂ and nitrogen (N) supply affect plant characteristics of loblolly pine (Pinus taeda L.) with an emphasis on root morphology. Seedlings were grown in greenhouses from seeds during one growing season at two atmospheric CO₂ concentrations (375 and 710 μL L^-1) and two N levels (High and Low). Root morphological characteristics were determined using a scanner and an image analysis program on a Macintosh computer. In the high N treatment, elevated CO₂ increased total plant dry weight by 80% and did not modify root to shoot (R/S) dry weight ratio, and leaf and plant N concentration at the end of the growing season. In the low N treatment, elevated CO₂ increased total dry weight by 60%. Plant and leaf N concentration declined and R/S ratio tended to increase. Nitrogen uptake rate on both a root length and a root dry weight basis was greater at elevated CO₂ in the high N treatment and lower in the low N treatment. We argue that N stress resulting from short exposures to nutrients might help explain the lower N concentrations observed at high CO₂ in other experiments; Nitrogen and CO₂ levels modified root morphology. High N increased the number of secondary lateral roots per length of first order lateral root and high CO₂ increased the length of secondary lateral roots per length of first order lateral root. Number and length of first order lateral roots were not modified by either treatment. Specific root length of main axis, and to a lower degree, of first order laterals, declined at high CO₂, especially at high N. Basal stem diameter and first order root diameters increased at high CO₂, especially at high N. Elevated CO₂ increased the proportion of upper lateral roots within the root system.     

Effects of different levels of CO2 on photosynthesis and cell components of the red alga Porphyra leucosticta

Photosynthesis and cell composition of Porphyraleucosticta discs grown at low (< 0.0001% in air), current (control) and high (1% CO₂ in air)inorganic carbon (C_i) concentrations were analyzed. Carbohydrate content in discs grown at high C_i increased (15.1 mg g^-1 FW) with respect to the control (6.4 mg g FW^-1), whereas soluble protein content decreased to one-third (5.6 to2.1 mg g^-1 FW). Carbohydrate content was unaffected and soluble protein slightly increased in discs grown at low C_i. As a consequence of these changes, a lower C/N molar ratio (8.6) was found in the discs grown at low compared to high C_i(12.4). Nitrate reductase activity increased at high C_i from 0.3 ± 0.2 to 1.7 ± 0.4 μmolNO₂ ^- g^-1 FW h^-1indicating that reduction and assimilation of nitrate were uncoupled. The response of photosynthesis to increasing irradiance, estimated from O₂evolution vs. irradiance curves, was affected by the treatments. Maximum quantum yield (Φ _O2°) and effective quantum yield (Φ _O2) at 150 μmol photon m^-2s^-1 decreased by 20% and 50%, respectively, at low C_i. These differences could be due to changes in photosynthetic electron flow between PSII and PSI. Treatments also produced changes in maximal (F_v/F_m) and effective (ΔF/F_m′)quantum yield for photosystem II charge separation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of elevated atmospheric CO2 on soil enzyme activities at different nitrogen application treatments

It has been predicted that elevated atmospheric CO₂ will increase enzyme activity as a result of CO₂-induced carbon entering the soil. The objective of this study was to investigate the effects of elevated atmospheric CO₂ on soil enzyme activities under a rice/wheat rotation. This experiment was conducted in Wuxi, Jiangsu, China as part of the China FACE (Free Air Carbon Dioxide Enrichment) Project. Two atmospheric CO₂ concentrations (580±60) and (380±40) μmol·mol^-1) and three N application treatments (low-150, normal-250 and high-350 kg N·hm^-2) were included. Soil samples (0-10 cm) were collected for analysis of β-glucosidase, invertase, urease, acid phosphates and β-glucosaminidase activities. The results revealed that with elevated atmospheric CO₂ β-glucosidase activity significantly decreased (P < 0.05) at low N application rates; had no significant effect with a normal N application rate; and significantly increased (P < 0.05) with a high N application rate. For urease activity, at low and normal N application rates (but not high N application rate), elevated atmospheric CO₂ significantly increased (P < 0.05) it. With acid phosphatase elevated atmospheric CO₂ only had significant higher effects (P < 0.05) at high N application rates. Under different CO₂ concentration, effects of N fertilization are also different. Soil β-glucosidase activity at ambient CO₂ concentration decreased with N fertilization, while it increased at elevated CO₂ concentration. In addition, invertase and acid phosphatase activities at elevated CO₂ concentration, significantly increased (P < 0.05) with N treatments, but there was no effect with the ambient CO₂ concentration. For urease activity, at ambient CO₂ concentration, N fertilization increased it significantly (P < 0.05), whereas at elevated CO₂ concentration it was not significant. Additionally, with β-glucosaminidase activity, there were no significant effects from N application. In general, then, elevated atmospheric CO₂ increased soil enzyme activity, which may be attributed to the following two factors: (1) elevated atmospheric CO₂ led to more plant biomass in the soil, which in turn stimulated soil microbial biomass and activity; and (2) elevated atmospheric CO₂ increased plant photosynthesis, thereby increasing plant-derived soil enzymes.     

Effects of elevated atmospheric CO2 on soil enzyme activities at different nitrogen application treatments

It has been predicted that elevated atmospheric CO₂ will increase enzyme activity as a result of CO₂-induced carbon entering the soil. The objective of this study was to investigate the effects of elevated atmospheric CO₂ on soil enzyme activities under a rice/wheat rotation. This experiment was conducted in Wuxi, Jiangsu, China as part of the China FACE (Free Air Carbon Dioxide Enrichment) Project. Two atmospheric CO₂ concentrations (580±60) and (380±40) μmol·mol^-1) and three N application treatments (low-150, normal-250 and high-350 kg N·hm^-2) were included. Soil samples (0-10 cm) were collected for analysis of β-glucosidase, invertase, urease, acid phosphates and β-glucosaminidase activities. The results revealed that with elevated atmospheric CO₂ β-glucosidase activity significantly decreased (P < 0.05) at low N application rates; had no significant effect with a normal N application rate; and significantly increased (P < 0.05) with a high N application rate. For urease activity, at low and normal N application rates (but not high N application rate), elevated atmospheric CO₂ significantly increased (P < 0.05) it. With acid phosphatase elevated atmospheric CO₂ only had significant higher effects (P < 0.05) at high N application rates. Under different CO₂ concentration, effects of N fertilization are also different. Soil β-glucosidase activity at ambient CO₂ concentration decreased with N fertilization, while it increased at elevated CO₂ concentration. In addition, invertase and acid phosphatase activities at elevated CO₂ concentration, significantly increased (P < 0.05) with N treatments, but there was no effect with the ambient CO₂ concentration. For urease activity, at ambient CO₂ concentration, N fertilization increased it significantly (P < 0.05), whereas at elevated CO₂ concentration it was not significant. Additionally, with β-glucosaminidase activity, there were no significant effects from N application. In general, then, elevated atmospheric CO₂ increased soil enzyme activity, which may be attributed to the following two factors: (1) elevated atmospheric CO₂ led to more plant biomass in the soil, which in turn stimulated soil microbial biomass and activity; and (2) elevated atmospheric CO₂ increased plant photosynthesis, thereby increasing plant-derived soil enzymes.