Elevated atmospheric CO2 in open top chambers increases net nitrification and potential denitrification

The control of soil nitrogen (N) availability under elevated atmospheric CO₂ is central to predicting changes in ecosystem carbon (C) storage and primary productivity. The effects of elevated CO₂ on belowground processes have so far attracted limited research and they are assumed to be controlled by indirect effects through changes in plant physiology and chemistry. In this study, we investigated the effects of a 4‐year exposure to elevated CO₂ (ambient + 400 µmol mol^?1) in open top chambers under Scots pine (Pinus sylvestris L) seedlings on soil microbial processes of nitrification and denitrification. Potential denitrification (DP) and potential N₂O emissions were significantly higher in soils from the elevated CO₂ treatment, probably regulated indirectly by the changes in soil conditions (increased pH, C availability and NO₃^– production). Net N mineralization was mainly accounted for by nitrate production. Nitrate production was significantly larger for soil from the elevated CO₂ treatment in the field when incubated in the laboratory under elevated CO₂ (increase of 100%), but there was no effect when incubated under ambient CO₂. Net nitrate production of the soil originating from the ambient CO₂ treatment in the field was not influenced by laboratory incubation conditions. These results indicate that a direct effect of elevated atmospheric CO₂ on soil microbial processes might take place. We hypothesize that physiological adaptation or selection of nitrifiers could occur under elevated CO₂ through higher soil CO₂ concentrations. Alternatively, lower microbial NH₄ assimilation under elevated CO₂ might explain the higher net nitrification. We conclude that elevated atmospheric CO₂ has a major direct effect on the soil microbial processes of nitrification and denitrification despite generally higher soil CO₂ concentrations compared to atmospheric concentrations.     

Effects of elevated O3 and CO2 concentrations on photosynthesis and stomatal conductance in Scots pine

Naturally regenerated Scots pines (Pinus sylvestris L.), aged 28–30 years old, were grown in open-top chambers and subjected in situ to three ozone (O₃) regimes, two concentrations of CO₂, and a combination of O₃ and CO₂ treatments From 15 April to 15 September for two growing seasons (1994 and 1995). The gas exchanges of current-year and 1-year-old shoots were measured, along with the nitrogen content of needles. In order to investigate the factors underlying modifications in photosynthesis, five parameters linked to photosynthetic performance and three to stomatal conductance were determined. Elevated O₃ concentrations led to a significant decline in the CO₂ compensation point (Г^*), maximum RuP₂-saturated rate of carboxylation (V_cmax), maximum rate of electron transport (J_max), maximum stomatal conductance (g_smax), and sensitivity of stomatal conductance to changes in leaf-to-air vapour pressure difference (?g_s/?D_v) in both shoot-age classes. However, the effect of elevated O₃ concentrations on the respiration rate in light (R_d) was dependent on shoot age. Elevated CO₂(700 μmol mol^?1) significantly decreased J_max and g_smax but increased R_d in 1-year-old shoots and the ?g_s/?D_v in both shoot-age classes. The interactive effects of O₃ and CO₂ on some key parameters (e.g. V_cmax and J_max) were significant. This may be closely related to regulation of the maximum stomatal conductance and stomatal sensitivity induced by elevated CO₂. As a consequence, the injury induced by O₃ was reduced through decreased ozone uptake in 1-year-old shoots, but not in the current-year shoots. Compared to ambient O₃ concentration, reduced O₃ concentrations (charcoal-filtered air) did not lead to significant changes in any of the measured parameters. Compared to the control treatment, calculations showed that elevated O₃ concentrations decreased the apparent quantum yield by 15% and by 18%, and the maximum rate of photosynthesis by 21% and by 29% in the current-year and 1-year-old shoots, respectively. Changes in the nitrogen content of needles resulting from the various treatments were associated with modifications in photosynthetic components.     

Effects of elevated atmospheric CO2 on fine root production and activity in an intact temperate forest ecosystem

We investigated the effects of elevated atmospheric CO₂ concentrations (ambient + 200 ppm) on fine root production and soil carbon dynamics in a loblolly pine (Pinus taeda) forest subject to free‐air CO₂ enrichment (FACE) near Durham, NC (USA). Live fine root mass (LFR) showed less seasonal variation than dead fine root mass (DFR), which was correlated with seasonal changes in soil moisture and soil temperature. LFR mass increased significantly (by 86%) in the elevated CO₂ treatment, with an increment of 37 g(dry weight) m^?2 above the control plots after two years of CO₂ fumigation. There was no long‐term increment in DFR associated with elevated CO₂, but significant seasonal accumulations of DFR mass occurred during the summer of the second year of fumigation. Overall, root net primary production (RNPP) was not significantly different, but annual carbon inputs were 21.7 gC m^?2 y^?1 (68%) higher in the elevated CO₂ treatment compared to controls. Specific root respiration was not altered by the CO₂ treatment during most of the year; however, it was significantly higher by 21% and 13% in September 1997 and May 1998, respectively, in elevated CO₂. We did not find statistically significant differences in the C/N ratio of the root tissue, root decomposition or phosphatase activity in soil and roots associated with the treatment. Our data show that the early response of a loblolly pine forest ecosystem subject to CO₂ enrichment is an increase in its fine root population and a trend towards higher total RNPP after two years of CO₂ fumigation.     

Soil CO2 efflux in a boreal pine forest under atmospheric CO2 enrichment and air warming

The response of forest soil CO₂ efflux to the elevation of two climatic factors, the atmospheric concentration of CO₂ (↑CO₂ of 700 μmol mol⁻¹) and air temperature (↑ T with average annual increase of 5°C), and their combination (↑CO₂+↑ T ) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines ( Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO₂ efflux in May–October increased with elevated CO₂ by 23–37%, with elevated temperature by 27–43%, and with the combined treatment by 35–59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO₂ was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO₂ alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO₂ efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO₂ emissions. The decrease in the temperature sensitivity of soil CO₂ efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO₂ and air temperature consistently increased forest soil CO₂ efflux over the 4-year period, their combined effect being additive, with no apparent interaction.     

The influence of elevated CO2 and O3 on fine roots and mycorrhizas of naturally growing young Scots pine trees during three exposure years

Young Scots pine trees naturally established at a pine heath were exposed to two concentrations of CO₂ (ambient and doubled ambient) and two O₃ regimes (ambient and doubled ambient) and their combination in open-top field chambers during growing seasons 1994, 1995 and 1996 (late May to 15 September). Filtered ozone treatment and chamberless control trees were also included in the treatment comparisons. Root ingrowth cores were inserted to the undisturbed soil below the branch projection of each tree at the beginning of the fumigation period in 1994 and were harvested at the end of the fumigation periods in 1995 and 1996. Root biomasses were determined from different soil layers in the ingrowth cores, and the infection levels of different mycorrhizal types were calculated. Elevated O₃ and CO₂ did not have significant effects on the biomass production of Scots pine coarse (Ø > 2 mm) or fine roots (Ø < 2 mm) and roots of grasses and dwarf shrubs. Elevated O₃ caused a transient stimulation, observable in 1995, in the proportion of tuber-like mycorrhizas, total mycorrhizas and total short roots but this stimulation disappeared during the last study year. Elevated CO₂ did not enhance carbon allocation to root growth or mycorrhiza formation, although a diminishing trend in the mycorrhiza formation was observed. In the combination treatment increased CO₂ inhibited the transient stimulating effect of ozone, and a significant increase of old mycorrhizas was observed. Our conclusion is that doubled CO₂ is not able to increase carbon allocation to growth of fine roots or mycorrhizas in nutrient poor forest sites and realistically elevated ozone does not cause a measurable limitation to roots within a period of three exposure years.     

Acclimation of photosynthesis and respiration to elevated atmospheric CO2 in two Scrub Oaks

Abstract For two species of oak, we determined whether increasing atmospheric CO₂ concentration (C_a) would decrease leaf mitochondrial respiration (R) directly, or indirectly owing to their growth in elevated C_a, or both. In particular, we tested whether acclimatory decreases in leaf‐Rubisco content in elevated C_a would decrease R associated with its maintenance. This hypothesis was tested in summer 2000 on sun and shade leaves of Quercus myrtifolia Willd. and Quercus geminata Small. We also measured R on five occasions between summer 1999 and 2000 on leaves of Q. myrtifolia. The oaks were grown in the field for 4 years, in either current ambient or elevated (current ambient + 350 µmol mol^?1) C_a, in open‐top chambers (OTCs). For Q. myrtifolia, an increase in C_a from 360 to 710 µmol mol^?1 had no direct effect on R at any time during the year. In April 1999, R in young Q. myrtifolia leaves was significantly higher in elevated C_a—the only evidence for an indirect effect of growth in elevated C_a. Leaf R was significantly correlated with leaf nitrogen (N) concentration for the sun and shade leaves of both the species of oak. Acclimation of photosynthesis in elevated C_a significantly reduced maximum RuBP‐saturated carboxylation capacity (V_{c max}) for both the sun and shade leaves of only Q. geminata. However, we estimated that only 11–12% of total leaf N was invested in Rubisco; consequently, acclimation in this plant resulted in a small effect on N and an insignificant effect on R. In this study measurements of respiration and photosynthesis were made on material removed from the field; this procedure had no effect on gas exchange properties. The findings of this study were applicable to R expressed either per unit leaf area or unit dry weight, and did not support the hypothesis that elevated C_a decreases R directly, or indirectly owing to acclimatory decreases in Rubisco content.     

Effects of elevated atmospheric CO2 on net ecosystem CO2 exchange of a scrub–oak ecosystem

We report the results of a 2‐year study of effects of the elevated (current ambient plus 350 μmol CO₂ mol^?1) atmospheric CO₂ concentration (C_a) on net ecosystem CO₂ exchange (NEE) of a scrub–oak ecosystem. The measurements were made in open‐top chambers (OTCs) modified to function as open gas‐exchange systems. The OTCs enclosed samples of the ecosystem (ca. 10 m² surface area) that had regenerated after a fire, 5 years before, in either current ambient or elevated C_a. Throughout the study, elevated C_a increased maximum NEE (NEE_max) and the apparent quantum yield of the NEE (φ_NEE) during the photoperiod. The magnitude of the stimulation of NEE_max, expressed per unit ground area, was seasonal, rising from 50% in the winter to 180% in the summer. The key to this stimulation was effects of elevated C_a, and their interaction with the seasonal changes in the environment, on ecosystem leaf area index, photosynthesis and respiration. The separation of these factors was difficult. When expressed per unit leaf area the stimulation of the NEE_max ranged from 7% to 60%, with the increase being dependent on increasing soil water content (W_soil). At night, the CO₂ effluxes from the ecosystem (NEE_night) were on an average 39% higher in elevated C_a. However, the increase varied between 6% and 64%, and had no clear seasonality. The partitioning of NEE_night into its belowground (R_below) and aboveground (R_above) components was carried out in the winter only. A 35% and 27% stimulation of NEE_night in December 1999 and 2000, respectively, was largely due to a 26% and 28% stimulation of R_below in the respective periods, because R_below constituted ca. 87% of NEE_night. The 37% and 42% stimulation of R_above in December 1999 and 2000, respectively, was less than the 65% and 80% stimulation of the aboveground biomass by elevated C_a at these times. An increase in the relative amount of the aboveground biomass in woody tissue, combined with a decrease in the specific rate of stem respiration of the dominant species Quercus myrtifolia in elevated C_a, was responsible for this effect. Throughout this study, elevated C_a had a greater effect on carbon uptake than on carbon loss, in terms of both the absolute flux and relative stimulation. Consequently, for this scrub–oak ecosystem carbon sequestration was greater in the elevated C_a during this 2‐year study period.     

Elevated atmospheric [CO2] promotes frost damage in evergreen tree seedlings

Growth under elevated [CO₂] promoted spring frost damage in field grown seedlings of snow gum ( Eucalyptus pauciflora Sieb. ex Spreng.), one of the most frost tolerant of eucalypts. Freezing began in the leaf midvein, consistent with it being a major site of frost damage under field conditions. The average ice nucleation temperature was higher in leaves grown under elevated [CO₂] (– 5·7 °C versus – 4·3 °C), consistent with the greater incidence of frost damage in these leaves (34% versus 68% of leaves damaged). These results have major implications for agriculture, forestry and vegetation dynamics, as an increase in frost susceptibility may reduce potential gains in productivity from CO₂ fertilization and may affect predictions of vegetation change based on increasing temperature.     

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.     

Elevated atmospheric CO2 lowers herbivore abundance, but increases leaf abscission rates

Increased levels of atmospheric carbon dioxide (CO₂) are likely to affect the trophic relationships that exist between plants, their herbivores and the herbivores' natural enemies. This study takes advantage of an open‐top CO₂ fertilization experiment in a Florida scrub oak community at Kennedy Space Center, Florida, consisting of eight chambers supplied with ambient CO₂ (360 ppm) and eight chambers supplied with elevated CO₂ (710 ppm). We examined the effects of elevated CO₂ on herbivore densities and levels of leaf consumption, rates of herbivore attack by natural enemies and effects on leaf abscission. Cumulative levels of herbivores and herbivore damage were significantly lower in elevated CO₂ than in ambient CO₂. This may be because leaf nitrogen levels are lower in elevated CO₂. More herbivores die of host plant‐induced death in elevated CO₂ than in ambient CO₂. Attack rates of herbivores by parasitoids are also higher in elevated CO₂, possibly because herbivores need to feed for a longer time in order to accrue sufficient nitrogen (N), thus exposing themselves longer to natural enemies. Insect herbivores cause an increase in abscission rates of leaves throughout the year. Because of the lower insect density in elevated CO₂, we thought, abscission rates would be lower in these chambers. However, abscission rates were significantly higher in elevated CO₂. Thus, the direct effects of elevated CO₂ on abscission are greater than the indirect effects on abscission mediated via lower insect densities. A consequence of increased leaf abscission in elevated CO₂ is that nutrient deposition rates to the soil surface are accelerated.     

Root responses along a subambient to elevated CO2 gradient in a C3–C4 grassland

Atmospheric CO₂ (C_a) concentration has increased significantly during the last 20 000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community‐level and single species root responses to rising C_a are not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C₃–C₄ grassland exposed to a gradient of C_a from preglacial to future levels (230–550 μmol mol^?1). Root windows and minirhizotron tubes were installed below naturally occurring stands of the C₄ perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over 2 years. Community root biomass increased significantly (P<0.05) with C_a over initial conditions, with linear or curvilinear responses depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated C_a in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high C_a, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to C_a under moist conditions in June 2000, with the lowest rates at C_a<300 μmol mol^?1 and peak activity at 450 μmol mol^?1 in a quadratic model. B. ischaemum roots at subambient C_a had higher SRLs and slightly higher carbohydrate concentrations than those at higher C_a, which may be related to drier soils at low C_a. Our data emphasize that belowground responses of plant communities to C_a can be quite different from those of the individual species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing C_a.