Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland

In this study, the response of N₂ fixation to elevated CO₂ was measured in Scirpus olneyi, a C₃ sedge, and Spartina patens, a C₄ grass, using acetylene reduction assay and ¹⁵N₂ gas feeding. Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO₂ significantly (P < 0·05) caused a 35% increase in nitrogenase activity and 73% increase in ¹⁵N incorporated by Scirpus olneyi. In Spartina patens, elevated CO₂ (660 ± 1 μ mol mol ^{− 1}) increased nitrogenase activity and ¹⁵N incorporation by 13 and 23%, respectively. Estimates showed that the rate of N₂ fixation in Scirpus olneyi under elevated CO₂ was 611 ± 75 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} compared with 367 ± 46 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} in ambient CO₂ plants. In Spartina patens, however, the rate of N₂ fixation was 12·5 ± 1·1 versus 9·8 ± 1·3 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} for elevated and ambient CO₂, respectively. Heterotrophic non-symbiotic N₂ fixation in plant-free marsh sediment also increased significantly (P < 0·05) with elevated CO₂. The proportional increase in ¹⁵N₂ fixation correlated with the relative stimulation of photosynthesis, in that N₂ fixation was high in the C₃ plant in which photosynthesis was also high, and lower in the C₄ plant in which photosynthesis was relatively less stimulated by growth in elevated CO₂. These results are consistent with the hypothesis that carbon fixation in C₃ species, stimulated by rising CO₂, is likely to provide additional carbon to endophytic and below-ground microbial processes.     

Nitrogen‐15 budget in model ecosystems of white clover and perennial ryegrass exposed for four years at elevated atmospheric pCO2

Although there are many indications that N cycling in grassland ecosystems changes under elevated atmospheric CO₂ partial pressure (pCO₂), most information has been obtained in short‐term studies. Thus, N budgets were established for four years under ambient and 60 Pa pCO₂ at two levels of N fertilization in two contrasting model ecosystems: Trifolium repens L. (white clover) and Lolium perenne L. (perennial ryegrass) were planted in soil in boxes in the Swiss FACE experiment. While T. repens showed an 80% increase in harvested biomass with no change in biomass allocation under elevated atmospheric pCO₂ compared to ambient conditions, L. perenne showed an increase only in the biomass of the roots. During the four years of the experiment, the systems gained N both from N retained in the soil and from stubble/stolon and roots left after the final harvest; in total between 11 and 86 gN m⁻². Nitrogen retention in the soil was between 4 and 64 g m². The L. perenne system gained the most N and retained the most N in the soil at high N fertilization and elevated atmospheric pCO₂. The input of new C and N into the soil correlated well in the L. perenne systems but not in the T. repens systems. Elevated atmospheric pCO₂ led neither to an increase in N retention in the soil nor did it reduce the loss of N from the soil. In the L. perenne systems, N fertilization played the main role in both the retention of N and the sequestration of C, while in the T. repens systems symbiotic N₂ fixation may have controlled N retention in the soil.     

Oceanic sinks for atmospheric CO2

There is approximately 50 times more inorganic carbon in the global ocean than in the atmosphere. On time scales of decades to millions of years, the interaction between these two geophysical fluids determines atmospheric CO₂ levels. During glacial periods, for example, the ocean serves as the major sink for atmospheric CO₂, while during glacial–interglacial transitions, it is a source of CO₂ to the atmosphere. The mechanisms responsible for determining the sign of the net exchange of CO₂ between the ocean and the atmosphere remain unresolved. There is evidence that during glacial periods, phytoplankton primary productivity increased, leading to an enhanced sedimentation of particulate organic carbon into the ocean interior. The stimulation of primary production in glacial episodes can be correlated with increased inputs of nutrients limiting productivity, especially aeolian iron. Iron directly enhances primary production in high nutrient (nitrate and phosphate) regions of the ocean, of which the Southern Ocean is the most important. This trace element can also enhance nitrogen fixation, and thereby indirectly stimulate primary production throughout the low nutrient regions of the central ocean basins. While the export flux of organic carbon to the ocean interior was enhanced during glacial periods, this process does not fully account for the sequestration of atmospheric CO₂. Heterotrophic oxidation of the newly formed organic carbon, forming weak acids, would have hydrolyzed CaCO₃ in the sediments, increasing thereby oceanic alkalinity which, in turn, would have promoted the drawdown of atmospheric CO₂. This latter mechanism is consistent with the stable carbon isotope pattern derived from air trapped in ice cores. The oceans have also played a major role as a sink for up to 30% of the anthropogenic CO₂ produced during the industrial revolution. In large part this is due to CO₂ solution in the surface ocean; however, some, poorly quantified fraction is a result of increased new production due to anthropogenic inputs of combined N, P and Fe. Based on ‘circulation as usual’, models predict that future anthropogenic CO₂ inputs to the atmosphere will, in part, continue to be sequestered in the ocean. Human intervention (large-scale Fe fertilization; direct CO₂ burial in the deep ocean) could increase carbon sequestration in the oceans, but could also result in unpredicted environmental perturbations. Changes in the oceanic thermohaline circulation as a result of global climate change would greatly alter the predictions of C sequestration that are possible on a ‘circulation as usual’ basis.     

Nitrogen rhizodeposition assessed by a NH3 shoot pulse-labelling of Lolium perenne L. grown on soil exposed to 9 years of CO2 enrichment

The effects of elevated CO₂ concentration upon rhizodeposition of nitrogen were investigated on field-grown Lolium perenne planted in soil cores set into the resident soil of a intensively managed ryegrass sward treated with elevated CO₂ for nine consecutive years, under two contrasted N fertilisation regimes (Swiss FACE Experiment). The planted cores were excavated from the ambiant (35 Pa pCO₂) and enriched (60 Pa pCO₂) rings at two dates during the growing season (spring and early autumn). The cores were brought back to the laboratory for a pulse-labelling of ryegrass shoots with ¹⁵NH₃, in order to quantify ¹⁵N-rhizodeposition.A recovery of 10–16% of the total ¹⁵N administred to the plant was recovered in the plant–soil system 48 h after the pulse-labelling; significant amounts of ¹⁵N were released into the soil adhering (adhering soil: AS) to the roots (0.44 μg ¹⁵N g AS⁻¹ and 0.60 μg g AS⁻¹ in the spring and the autumn samplings, respectively).In the spring sampling, there was no effect of atmospheric CO₂ concentration on N rhizodeposition. In the autumn sampling, elevated CO₂ stimulated N rhizodeposition that amounted to 7.2 and 5.2 mg ¹⁵N m⁻², under elevated and ambient CO₂, respectively. Nitrogen rhizodeposition was higher at high N (56 gN m⁻²) than at low N fertilisation (14 gN m⁻²), whatever the sampling date investigated.The mechanisms by which elevated atmospheric CO₂ leads to a stimulation of the net root-released N flux remains to be investigated: was it caused by a higher nitrogen immobilisation by the microbial biomass and a reduced re-assimilation of mineralized N and/or by a stimulation of N efflux from roots? Concomitant to the observed reduction of C rhizodeposition, the stimulation of net N efflux suggests that the quality of root released compounds was modified under elevated CO₂ concentration.     

Effects of long-term exposure to elevated CO2 and N fertilization on the development of photosynthetic capacity and biomass accumulation in Quercus suber L.

The effects of long‐term (4 year) CO₂ enrichment (70 Pa versus 35 Pa) and nitrogen nutrition (8 mm versus 1 mm NO₃^–) on biomass accumulation and the development of photosynthetic capacity in leaves of cork oak (Quercus suber L., a Mediterranean evergreen tree) were studied. The evolution of photosynthetic parameters with leaf development was estimated by fitting the biochemical model of Farquhar et al. (Planta 149, 78–90, 1980) with modifications by Sharkey (Botanical Review 78, 71–75, 1985) to A–C_i response curves. CO₂ enrichment had a small reduction effect on the development of the maximum CO₂ fixation capacity by Rubisco (V_Cmax), and no effect over maximum electron transport capacity (J_max), day‐time respiration (R_d) and Triose‐P utilization (TPU). However, there was a statistically significant effect of N fertilization and the interaction CO₂ × N over the evolution of V_Cmax, J_max and TPU. Relative stomatal limitation (estimated from A–C_i curves) was higher (+20%) for plants grown under ambient CO₂ than for plants grown under elevated CO₂. There was a significant effect of CO₂ and N fertilization over total biomass accumulation as well as leaf area. Plants grown at elevated CO₂ had 27% more biomass than plants grown at ambient CO₂ when given high N. However, for plants grown under low N there was no significant effect of CO₂ enrichment on biomass accumulation. Plants grown under low N also had significantly higher root : shoot ratios whereas there were no differences between CO₂ treatments. The larger biomass accumulation of Q. suber under elevated CO₂ is attributable to a higher availability of CO₂ coupled to a larger leaf area, with no significant decrease in photosynthetic capacity under CO₂ enrichment and elevated N fertilization. For low N fertilization, the effects of CO₂ enrichment over leaf area and biomass accumulation are lost, suggesting that in native ecosystems with low N availability, the effects of CO₂ enrichment may be insignificant.     

Soil 13C–15N dynamics in an N2-fixing clover system under long-term exposure to elevated atmospheric CO2

Reduced soil N availability under elevated CO₂ may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N₂‐fixing system. We studied the effects of Trifolium repens (an N₂‐fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO₂ under FACE conditions. ¹⁵N‐labeled fertilizer was applied at a rate of 140 and 560 kg N ha^?1 yr^?1 and the CO₂ concentration was increased to 60 Pa pCO₂ using ¹³C‐depleted CO₂. The total soil C content was unaffected by elevated CO₂, species and rate of ¹⁵N fertilization. However, under elevated CO₂, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer‐N (f_N) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO₂, had a significant effect on f_N values of the total soil N pool. The fractions of newly sequestered C (f_C) differed strongly among intra‐aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO₂, the ratio of fertilizer‐N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more ¹⁵N fertilizer than T. repens: 179 vs. 101 kg N ha^?1 for the low rate of N fertilization and 393 vs. 319 kg N ha^?1 for the high N‐fertilization rate. As the loss of fertilizer‐¹⁵N contributed to the ¹⁵N‐isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N₂ fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non‐N₂‐fixing L. perenne system. This suggests that N₂ fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.     

不同包装条件对党参皂甙含量的影响

本文通过不同水分梯度下的党参进行充N2 、CO2 气体或真空处理 ,在室温条件下贮藏 3年后 ,测定其皂甙含量变化。研究表明 :在党参含水量不大于 17%时 ,采用真空材料 ,进行充CO2 气体的党参经较长时间的密封储藏 ,其皂甙含量较高 ,是一种较好的保质贮藏方式     

Soil CO2 efflux of two silver birch clones exposed to elevated CO2 and O3 levels during three growing seasons

In the present open‐top chamber experiment, two silver birch clones (Betula pendula Roth, clone 4 and clone 80) were exposed to elevated levels of carbon dioxide (CO₂) and ozone (O₃), singly and in combination, and soil CO₂ efflux was measured 14 times during three consecutive growing seasons (1999–2001). In the beginning of the experiment, all experimental trees were 7 years old and during the experiment the trees were growing in sandy field soil and fertilized regularly. In general, elevated O₃ caused soil CO₂ efflux stimulation during most measurement days and this stimulation enhanced towards the end of the experiment. The overall soil respiration response to CO₂ was dependent on the genotype, as the soil CO₂ efflux below clone 80 trees was enhanced and below clone 4 trees was decreased under elevated CO₂ treatments. Like the O₃ impact, this clonal difference in soil respiration response to CO₂ increased as the experiment progressed. Although the O₃ impact did not differ significantly between clones, a significant time × clone × CO₂× O₃ interaction revealed that the O₃‐induced stimulation of soil respiration was counteracted by elevated CO₂ in clone 4 on most measurement days, whereas in clone 80, the effect of elevated CO₂ and O₃ in combination was almost constantly additive during the 3‐year experiment. Altogether, the root or above‐ground biomass results were only partly parallel with the observed soil CO₂ efflux responses. In conclusion, our data show that O₃ impacts may appear first in the below‐ground processes and that relatively long‐term O₃ exposure had a cumulative effect on soil CO₂ efflux. Although the soil respiration response to elevated CO₂ depended on the tree genotype as a result of which the O₃ stress response might vary considerably within a single tree species under elevated CO₂, the present experiment nonetheless indicates that O₃ stress is a significant factor affecting the carbon cycling in northern forest ecosystems.