Indices of plant N availability in an alpine grassland under elevated atmospheric CO2

The objective of this study was to estimate whether elevated atmospheric [CO₂] alters plant N availability in a native high-elevation grassland in the Swiss Alps using two integrative, relatively non-disruptive methods. Estimates based on seasonal net plant N uptake, and those based on the amounts of NH ₄ ⁺ -N plus NO ₃ ^– -N captured by ion exchange resin (IER) bags, did not differ in plots treated with ambient (355 L L^–1) and elevated (680 L L^–1) [CO₂] in either the second (1993) or third (1994) growing season under treatment with elevated [CO₂]. The results of this study suggest that the effects of rising atmospheric [CO₂] on plant N availability may be negligible in this grassland. The results also contrast the relatively large effects of elevated atmospheric [CO₂] (increases and decreases) reported for highly disturbed artificial systems.     

Interactions of elevated CO2, NH3 and O3 on mycorrhizal infection,gas exchange and N metabolism in saplings of Scots pine

Four-year-old saplings of Scots pine (Pinus sylvestris) (L.) were exposed for 11 weeks in controlled-environment chambers to charcoad-filtered air, or to charcoal-filtered air supplemented with NH₃ (40 g m^–3), O₃ (110 g m^–3 during day/ 40 g m^–3 during night) or NH₃+O₃. All treatments were carried out at ambient (259 L L^–1) and at elevated CO₂ concentration (700 L L^–1). Total tree biomass, mycorrhizal infection, net CO₂ assimilation (P_n), stomatal conductance (g_s), transpiration of the shoots and NH₃ metabolization of the needles were measured. In ambient CO₂ (1) gaseous NH₃ decreased mycorrhizal infection, without significantly affecting tree biomass or N concentration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decreased mycorrhizal infection and the acitivity of GS in the needles, while it increased the activity og GDH; (3) exposure to NH₃+O₃ lessened the effects of single exposures to NH₃ and O₃ on reduction of mycorrhizal infection and on increase in GDH activity. Similar lessing effects on mycorrhizal infection as observed in trees exposed to NH₃+O₃ at ambient CO₂, were measured in trees exposed to NH₃+O₃ at elevated CO₂. Exposure to elevated CO₂ without pollutants did not significantly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO₂ _NH₃ strongly decreased root branching and mycorrhizal infection and temporarily stimulated P_n and g_s. The exposure to elevated CO₂+NH₃+O₃ also transiently stimulated P_n. The possible mechanisms underlying and integrating these effects are discussed. Elevated CO₂ clearly did not alleviate the negative effects of NH₃ and O₃ mycoorhiral infection. The significant reduction of mycorrhizal infection after exposure to NH₃ or O₃, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH₃ and O₃ induced stress.Abbreviations DW dry weight - FA filtered air - FA_a filtered air at ambient CO₂ - FA_e filtered air at elevated CO₂ - FW fresh weight - GDH glutamate dehydrogenase - GS glutamine synthetase - g_s stomatal conductance - P_n net CO₂ assimilation - RWR root weight ratio - SRL specific root length     

Effects of elevated CO2 and increased nitrogen deposition on photosynthesis and growth of understory plants in spruce model ecosystems

We studied the effects of atmospheric CO₂ enrichment (280, 420 and 560 l CO₂ l^–1) and increased N deposition (0,30 and 90 kg ha^–1 year^–1) on the spruce-forest understory species Oxalis acetosella, Homogyne alpina and Rubus hirtus. Clones of these species formed the ground cover in nine 0.7 m² model ecosystems with 5-year-old Picea abies trees (leaf area index of approx 2.2). Communities grew on natural forest soil in a simulated montane climate. Independently of N deposition, the rate of light-saturated net photosynthesis of leaves grown and measured at 420 l CO₂ l^–1 was higher in Oxalis and in Homogyne, but was not significantly different in Rubus compared to leaves grown and measured at the pre-industrial CO₂ concentration of 280 l l^–1. Remarkably, further CO₂ enrichment to 560 l l^–1 caused no additional increase of CO₂ uptake. With increasing CO₂ supply concentrations of non-structural carbohydrates in leaves increased and N concentrations decreased in all species, whereas N deposition had no significant effect on these traits. Above-ground biomass and leaf area production were not significantly affected by elevated CO₂ in the more vigorously growing species O. acetosella and R. hirtus, but the slow growing H. alpina produced almost twice as much biomass and 50% more leaf area per plant under 420 l CO₂ l^–1 compared to 280 l l^–1 (again no further stimulation at 560 l l^–1). In contrast, increased N addition stimulated growth in Oxalis and Rubus but had no effect on Homogyne. In Oxalis (only) biomass per plant was positively correlated with microhabitat quantum flux density at low CO₂, but not at high CO₂ indicating carbon saturation. On the other hand, the less shade-tolerant Homogyne profited from CO₂ enrichment at all understory light levels facilitating its spread into more shady micro-habitats under elevated CO₂. These species-specific responses to CO₂ and N deposition will affect community structure. The non-linear responses to elevated CO₂ of several of the traits studied here suggest that the largest responses to rising atmospheric CO₂ are under way now or have already occurred and possible future responses to further increases in CO₂ concentration are likely to be much smaller in these understory species.     

Leaf and canopy responses to elevated CO2 in a pine forest under free-air CO2 enrichment

Physiological responses to elevated CO₂ at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO₂ using a free-air CO₂ enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO₂ over an 8-day exposure period was similar to that of trees exposed to current ambient CO₂ under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO₂-exposed trees showed minor (7%) but significant reductions in relative sap flux density compared to trees under ambient CO₂ conditions. Short-term (minutes) direct stomatal responses to elevated CO₂ were also relatively weak (5% reduction in stomatal aperture in response to high CO₂ concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO₂ for nearly 80 days compared to foliage grown under current ambient CO₂, so intrinsic leaf water-use efficiency at elevated CO₂ was enhanced primarily by direct responses of photosynthesis to CO₂. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO₂ (A _net-C _i curves) for Pinus taeda foliage grown under elevated CO₂ (550 mol mol^–1) for 50–80 days compared to those for foliage grown under current ambient CO₂ from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 mol mol^–1 CO₂ was enhanced by approximately 65% compared to the rate at ambient CO₂ (350 mol mol^–1). A similar level of enhancement under elevated CO₂ was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO₂ during the study period appears to be primarily attributable to direct photosynthetic responses to CO₂ in the pine forest, longer-term CO₂ responses and feedbacks remain to be evaluated.     

The effects of elevated atmospheric CO2 and soil P placement on cotton root deployment

Root proliferation into nutrient rich zones is an important mechanism in the exploitation of soil nutrients by plants. No studies have examined atmospheric CO₂ effects on cotton (Gossypium hirsutum L.) root distribution as affected by localized phosphorus (P). Cotton plants were grown in a Troup sand (loamy, thermic Grossarenic Kandiudults) using 17.2-l containers placed in open top field chambers (OTC) under ambient (360 mol mol^–1) or enriched (720 mol mol^–1) atmospheric CO₂ concentrations for 40 days. Equivalent amounts of P were added (150 mg P per kg of soil) to 100, 50, 25, 12.5, and 6.25% of the total soil volume; control containers with no added P were also included. Under extremely low P (controls), cotton was unresponsive to CO₂ enrichment. In treatments with both fertilized and unfertilized soil volumes, root proliferation was greater in the unfertilized soil under elevated CO₂ conditions. Stimulation of root growth occurred in the P-fertilized soil fraction; the pattern of stimulation was similar under both CO₂ levels. Under ambient CO₂, cotton plant response was positive (shoot mass, and total root mass and length) when soil P was confined to relatively small proportions of the total soil volume (6.25 and 12.5%). However, elevated CO₂ grown plants tended to respond to P regardless of its distribution.     

Elevated CO2 effects on carbon and nitrogen cycling in grass/clover turves of a Psammaquent soil

Effects of elevated CO₂ (525 and 700 L L^–1), and a control (350 L L^–1 CO₂), on biochemical properties of a Mollic Psammaquent soil in a well-established pasture of C3 and C4 grasses and clover were investigated with continuously moist turves in growth chambers over four consecutive seasonal temperature regimes from spring to winter inclusive. After a further spring period, half of the turves under 350 and 700 L L^–1 were subjected to summer drying and were then re-wetted before a further autumn period; the remaining turves were kept continuously moist throughout these additional three consecutive seasons. The continuously moist turves were then pulse-labelled with ¹⁴C-CO₂ to follow C pathways in the plant/soil system during 35 days.Growth rates of herbage during the first four seasons averaged 4.6 g m^–2 day^–1 under 700 L L^–1 CO₂ and were about 10% higher than under the other two treatments. Below-ground net productivity at the end of these seasons averaged 465, 800 and 824 g m^–2 in the control, 525 and 700 L L^–1 treatments, respectively.in continuously moist soil, elevated CO₂ had no overall effects on total, extractable or microbial C and N, or invertase activity, but resulted in increased CO₂-C production from soil, and from added herbage during the initial stages of decomposition over 21 days; rates of root decomposition were unaffected. CO₂ produced h^–1 mg^–1 microbial C was about 10% higher in the 700 L L^–1 CO₂ treatment than in the other two treatments. Elevated CO₂ had no clearly defined effects on N availability, or on the net N mineralization of added herbage.In the labelling experiment, relatively more ¹⁴C in the plant/soil system occurred below ground under elevated CO₂, with enhanced turnover of ¹⁴C also being suggested.Drying increased levels of extractable C and organic-N, but decreased mineral-N concentrations; it had no effect on microbial C, but resulted in lowered microbial N in the control only. In soil that had been previously summer-dried, CO₂ production was again higher, but net N mineralization was lower, under elevated CO₂ than in the control after autumn pasture growth.Over the trial period of 422 days, elevated CO₂ generally appears to have had a greater effect on soil C turnover than on soil C pools in this pasture ecosystem.     

Long-term effects of CO2 enrichment and temperature increase on a temperate grass sward

Perennial ryegrass swards were grown in large containers on a soil and were exposed during two years to elevated (700 L L^-1) or ambient atmospheric CO₂ concentration at outdoor temperature and to a 3 °C increase in air temperature in elevated CO₂. The nitrogen nutrition of the grass sward was studied at two sub-optimal (160 and 530 kg N ha^-1 y^-1) and one non-limiting (1000 kg N ha^-1 y^-1) N fertilizer supplies. At cutting date, elevated CO₂ reduced by 25 to 33%, on average, the leaf N concentration per unit mass. Due to an increase in the leaf blade weight per unit area in elevated CO₂, this decline did not translate for all cuts in a lower N concentration per unit leaf blade area. With the non-limiting N fertilizer supply, the leaf N concentration (% N) declined with the shoot dry-matter (DM) according to highly significant power models in ambient (% N=4.9 DM^-0.38) and in elevated (%N=5.3 DM^-0.52) CO₂. The difference between both regressions was significant and indicated a lower critical leaf N concentration in elevated than in ambient CO₂ for high, but not for low values of shoot biomass. With the sub-optimal N fertilizer supplies, the nitrogen nutrition index of the grass sward, calculated as the ratio of the actual to the critical leaf N concentration, was significantly lowered in elevated CO₂. This indicated a lower inorganic N availability for the grass plants in elevated CO₂, which was also apparent from the significant declines in the annual nitrogen yield of the grass sward and in the nitrate leaching during winter. For most cuts, the harvested fraction of the plant dry-matter decreased in elevated CO₂ due, on average, to a 45–52% increase in the root phytomass. In the same way, a smaller share of the plant total nitrogen was harvested by cutting, due, on average, to a 25–41% increase in the N content of roots. The annual means of the DM and N harvest indices were highly correlated to the annual means of the nitrogen nutrition index. Changes in the harvest index and in the nitrogen nutrition index between ambient and elevated CO₂ were also positively correlated. The possible implication of changes in the soil introgen cycle and of a limitation in the shoot growth potential of the grass in elevated. CO₂ is discussed.Abbreviations 350 outdoor climate - 700 outdoor climate +350 L L^-1[CO₂] - 700+ outdoor climate +350 L L^-1 (CO₂) and +3 °C - N-- low N fertilizer supply - N+ high N fertilizer supply - N++ non-limiting N fertilizer supply - DM dry-matter     

Effect of changes in water content on photosynthesis,transpiration and discrimination against 13CO2 and C18O16O in Pleurozium and Sphagnum

Photosynthetic gas exchange characteristics of two common boreal forest mosses, Sphagnum (section acutifolia) and Pleurozium schreberi, were measured continuously during the time required for the moss to dry out from full hydration. Similar patterns of change in CO₂ assimilation with variation in water content occurred for both species. The maximum rates of CO₂ assimilation for Sphagnum (approx. 7 mol m^–2 s^–1) occurred at a water content of approximately 7 (fresh weight/dry weight) while for Pleurozium the maximum rate (approx. 2 mol m^–2 s^–1) occurred at a water content of approximately 6 (fresh weight/dry weight). Above and below these water contents CO₂ assimilation declined. In both species total conductance to water vapour (expressed as a percentage of the maximum rates) remained nearly constant at a water content above 9 (fresh weight/dry weight), but below this level declined in a strong linear manner. Short-term, on-line ¹³CO₂ and C¹⁸O¹⁶O discrimination varied substantially with changes in moss water content and associated changes in the ratio of chloroplast CO₂ to ambient CO₂ partial pressure. At full hydration (maximum water content) both Sphagnum and Pleurozium had similar values of ¹³CO₂ discrimination (approx. 15). Discrimination against ¹³CO₂ increased continuously with reductions in water content to a maximum of 27 in Sphagnum and 22 in Pleurozium. In a similar manner C¹⁸C¹⁶O discrimination increased from approximately 30 at full hydration in both species to a maximum of 150 in Sphagnum and 90 in Pleurozium, at low water content. The observed changes in C¹⁸O¹⁶O were strongly correlated to predictions of a mechanistic model of discrimination processes. Field measurements of moss water content suggested that photosynthetic gas exchange by moss in the understory of a black spruce forest was regularly limited by low water content.     

Independent and contrasting effects of elevated CO2 and N-fertilization on root architecture in Pinus ponderosa

The effects of elevated CO₂ and N-fertilization on the architecture of Pinus ponderosa Dougl. ex P. Laws & C. Laws fine roots and their associated mycorrhizal symbionts were measured over a 4-year period using minirhizotron tubes. The study was conducted in open-top field-exposure chambers located near Placerville, Calif. A replicated (3 replicates), 3×3 factorial experimental design with three CO₂ concentrations [ambient air (354 mol mol^–1), 525 mol mol^–1, and 700 mol mol^–1] and three rates of N-fertilization (0, 100 and 200 kg ha^–1 year^–1) was used. Elevated CO₂ and N treatment had contrasting effects on the architecture of fine roots and their associated mycorrhizae. Elevated CO₂ increased both fine root extensity (degree of soil exploration) and intensity (extent that roots use explored areas) but had no effect on mycorrhizae. In contrast, N-fertilization had no effect on fine root extensity or intensity but increased mycorrhizal extensity and intensity. To better understand and model the responses of systems to increasing CO₂ concentrations and N deposition/fertilization it is necessary to consider these contrasting root architectural responses.     

Elevated CO2 alters anatomy,physiology, growth,and reproduction of red mangrove (Rhizophora mangle L.)

Mangroves, woody halophytes restricted to protected tropical coasts, form some of the most productive ecosystems in the world, but their capacity to act as a carbon source or sink under climate change is unknown. Their ability to adjust growth or to function as potential carbon sinks under conditions of rising atmospheric CO₂ during global change may affect global carbon cycling, but as yet has not been investigated experimentally. Halophyte responses to CO₂ doubling may be constrained by the need to use carbon conservatively under water-limited conditions, but data are lacking to issue general predictions. We describe the growth, architecture, biomass allocation, anatomy, and photosynthetic physiology of the predominant neotropical mangrove tree, Rhizophora mangle L., grown solitarily in ambient (350 ll^–1) and double-ambient (700 ll^–1) CO₂ concentrations for over 1 year. Mangrove seedlings exhibited significantly increased biomass, total stem length, branching activity, and total leaf area in elevated CO₂. Enhanced total plant biomass under high CO₂ was associated with higher root:shoot ratios, relative growth rates, and net assimilation rates, but few allometric shifts were attributable to CO₂ treatment independent of plant size. Maximal photosynthetic rates were enhanced among high-CO₂ plants while stomatal conductances were lower, but the magnitude of the treatment difference declined over time, and high-CO₂ seedlings showed a lower P_max at 700 ll^–1 CO₂ than low-CO₂ plants transferred to 700 ll^–1 CO₂: possible evidence of downregulation. The relative thicknesses of leaf cell layers were not affected by treatment. Stomatal density decreased as epidermal cells enlarged in elevated CO₂. Foliar chlorophyll, nitrogen, and sodium concentrations were lower in high CO₂. Mangroves grown in high CO₂ were reproductive after only 1 year of growth (fully 2 years before they typically reproduce in the field), produced aerial roots, and showed extensive lignification of the main stem; hence, elevated CO₂ appeared to accelerate maturation as well as growth. Data from this long-term study suggest that certain mangrove growth characters will change flexibly as atmospheric CO₂ increases, and accord with responses previously shown in Rhizophora apiculata. Such results must be integrated with data from sea-level rise studies to yield predictions of mangrove performance under changing climate.