Bert G. Drake

Altered soil microbial community at elevated CO2 leads to loss of soil carbon

Increased carbon storage in ecosystems due to elevated CO2 may help stabilize atmospheric CO2 concentrations and slow global warming. Many field studies have found that elevated CO2 leads to higher carbon assimilation by plants, and others suggest that this can lead to higher carbon storage in soils, the largest and most stable terrestrial carbon pool. Here we show that 6 years of experimental CO2 doubling reduced soil carbon in a scrub-oak ecosystem despite higher plant growth, offsetting ≈52% of the additional carbon that had accumulated at elevated CO2 in aboveground and coarse root biomass. The decline in soil carbon was driven by changes in soil microbial composition and activity. Soils exposed to elevated CO2 had higher relative abundances of fungi and higher activities of a soil carbon-degrading enzyme, which led to more rapid rates of soil organic matter degradation than soils exposed to ambient CO2. The isotopic composition of microbial fatty acids confirmed that elevated CO2 increased microbial utilization of soil organic matter. These results show how elevated CO2, by altering soil microbial communities, can cause a potential carbon sink to become a carbon source.

Predicting ecosystem responses to elevated CO2 concentrations

One of the many changes occurring in the biosphere due to human activities is the increase in the carbon dioxide concentration in the atmosphere. This change is due both to the burning of fossil fuels and to deforestation. We do not know how these changes are affecting terrestrial ecosystems. This ignorance is partly because we have relatively poor records of the functional and structural response of any ecosystem through time. More studies are required to be able to accurately assess the effects of carbon dioxide.

Observed increase in local cooling effect of deforestation at higher latitudes

Deforestation in mid- to high latitudes is hypothesized to have the potential to cool the Earth’s surface by altering biophysical processes. In climate models of continental-scale land clearing, the cooling is triggered by increases in surface albedo and is reinforced by a land albedo–sea ice feedback. This feedback is crucial in the model predictions; without it other biophysical processes may overwhelm the albedo effect to generate warming instead. Ongoing land-use activities, such as land management for climate mitigation, are occurring at local scales (hectares) presumably too small to generate the feedback, and it is not known whether the intrinsic biophysical mechanism on its own can change the surface temperature in a consistent manner. Nor has the effect of deforestation on climate been demonstrated over large areas from direct observations. Here we show that surface air temperature is lower in open land than in nearby forested land. The effect is 0.85 ± 0.44 K (mean ± one standard deviation) northwards of 45° N and 0.21 ± 0.53 K southwards. Below 35° N there is weak evidence that deforestation leads to warming. Results are based on comparisons of temperature at forested eddy covariance towers in the USA and Canada and, as a proxy for small areas of cleared land, nearby surface weather stations. Night-time temperature changes unrelated to changes in surface albedo are an important contributor to the overall cooling effect. The observed latitudinal dependence is consistent with theoretical expectation of changes in energy loss from convection and radiation across latitudes in both the daytime and night-time phase of the diurnal cycle, the latter of which remains uncertain in climate models.

Growth and senescence in plant communities exposed to elevated CO2 concentrations on an estuarine marsh

SummaryThree high marsh communities on the Chesapeake Bay were exposed to a doubling in ambient CO2 concentration for one growing season. Open-top chambers were used to raise CO2 concentrations ca. 340 ppm above ambient over monospecific communities of Scirpus olneyi (C3) and Spartina patens (C4), and a mixed community of S. olneyi, S. patens, and Distichlis spicata (C4). Plant growth and senescence were monitored by serial, nondestructive censuses. Elevated CO2 resulted in increased shoot densities and delayed sensecence in the C3 species. This resulted in an increase in primary productivity in S. olneyi growing in both the pure and mixed communities. There was no effect of CO2 on growth in the C4 species. These results demonstrate that elevated atmospheric CO2 can cause increased aboveground production in a mature, unmanaged ecosystem.

Growth and photosynthetic response of nine tropical species with long-term exposure to elevated carbon dioxide

SummarySeedlings of nine tropical species varying in growth and carbon metabolism were exposed to twice the current atmospheric level of CO2 for a 3 month period on Barro Colorado Island, Panama. A doubling of the CO2 concentration resulted in increases in photosynthesis and greater water use efficiency (WUE) for all species possessing C3 metabolism, when compared to the ambient condition. No desensitization of photosynthesis to increased CO2 was observed during the 3 month period. Significant increases in total plant dry weight were also noted for 4 out of the 5 C3 species tested and in one CAM species, Aechmea magdalenae at high CO2. In contrast, no significant increases in either photosynthesis or total plant dry weight were noted for the C4 grass, Paspallum conjugatum. Increases in the apparent quantum efficiency (AQE) for all C3 species suggest that elevated CO2 may increase photosynthetic rate relative to ambient CO2 over a wide range of light conditions. The response of CO2 assimilation to internal Ci suggested a reduction in either the RuBP and/or Pi regeneration limitation with long term exposure to elevated CO2. This experiment suggests that: (1) a global rise in CO2 may have significant effects on photosynthesis and productivity in a wide variety of tropical species, and (2) increases in productivity and photosynthesis may be related to physiological adaptation(s) to increased CO2.

Nitrogen and carbon dynamics in C3 and C4 estuarine marsh plants grown under elevated CO2 in situ

SummaryCarbon dioxide concentrations were elevated in three estuarine communities for an entire growing season. Open top chambers were used to raise CO2 concentrations ca. 336 ppm above ambient in monospecific communities of Scirpus olneyi (C3) and Spartina patens (C4), and a mixed community of S. olneyi, S. patens and Distichlis spicata (C4). Nitrogen and carbon concentration (% wt) of aboveground tissue was followed throughout growth and senescence. Green shoot %N was reduced and %C was unchanged under elevated CO2 in S. olneyi. This resulted in a 20%–40% increase in tissue C/N ratio. There was no effect of CO2 on either C4 species. Maximum aboveground N (g/m2) was unchanged in S. olneyi, indicating that increased productivity under elevated CO2 was dependent on reallocation of stored N. There was no change in the N recovery efficiency of S. olneyi in pure stand and a decrease in the mixed community. Litter C/N ratio was not affected by elevated CO2 suggesting that decomposition and N mineralization rates will also remain unchanged. Continued growth responses to elevated CO2 could, however, be limited by the ability of S. olneyi to increase the total aboveground N pool.

An open top chamber for field studies of elevated atmospheric CO2 concentration on saltmarsh vegetation

Small open top chambers (0-8m X 1Om) were developed to maintain elevated CO2 concentrations in three plant communities in a brackish marsh ecosystem. Mean annual CO2 concentrations were 350 ? 22 VU 11 in chambers which received no added CO2 and 686 ? 30 1 1-1 in chambers with elevated CO2 concentrations. Light quality was not affected in the photosynthetically active wavelengths but the chamber reduced light quantity by 10%. Night-time air temperatures inside the chamber (Ti) averaged 20C above air temperature outside the chamber (To) due to heating from the air blowers. Air temperature profiles through the plant canopy and boundary layer showed that daytime temperature differences (T, To) were greater than night-time differences and this day/night difference also depended on the plant community. Effects of the chamber on the micro-environment of the plant communities resulted in a significant growth enhancement in the plant community dominated by the C3 sedge Scirpus olneyi Grey but not in the other two communities. Key-words: Elevated C02, open top chamber, microclimate

Elevated Atmospheric CO2 Effects on Belowground Processes in C3 and C4 Estuarine Marsh Communities

Belowgound carbon allocation is a major component of a plants carbon budget, yet relatively little is known about the response of roots to elevated atmospheric CO{sub 2}. We have exposed three brackish marsh communities dominated by perennial macrophytes to twice ambient CO{sub 2} concentrations for two full growing seasons using open top chambers. One community was dominated by the C{sub 3} sedge Scirpus olneyi, one was dominated by the C{sub 4} grass Spartina patens, and one was a mixture of S. olneyi, S. patens, and Distichlis spicata, a C{sub 4} grass. Root and rhizome growth were studied in the 2nd yr of exposure by measuring growth into peat cores previously excavated and refilled with sphagnum peat devoid of roots. Growth under elevated CO{sub 2} resulted in an 83% increase in root dry mass per core in the Scirpus community. Those roots were also significantly lower in percentage of nitrogen than roots from ambient-grown plants. There was no effect of elevated CO{sub 2} on root growth or nitrogen content in the Spartina community or in the C{sub 4} component of the Mixed community.

Direct Inhibition of Plant Mitochondrial Respiration by Elevated CO2

Doubling the concentration of atmospheric CO2 often inhibits plant respiration, but the mechanistic basis of this effect is unknown. We investigated the direct effects of increasing the concentration of CO2 by 360 [mu]L L-1 above ambient on O2 uptake in isolated mitochondria from soybean (Glycine max L. cv Ransom) cotyledons. Increasing the CO2 concentration inhibited the oxidation of succinate, external NADH, and succinate and external NADH combined. The inhibition was greater when mitochondria were preincubated for 10 min in the presence of the elevated CO2 concentration prior to the measurement of O2 uptake. Elevated CO2 concentration inhibited the salicylhydroxamic acid-resistant cytochrome pathway, but had no direct effect on the cyanide-resistant alternative pathway. We also investigated the direct effects of elevated CO2 concentration on the activities of cytochrome c oxidase and succinate dehydrogenase (SDH) and found that the activity of both enzymes was inhibited. The kinetics of inhibition of cytochrome c oxidase were time-dependent. The level of SDH inhibition depended on the concentration of succinate in the reaction mixture. Direct inhibition of respiration by elevated CO2 in plants and intact tissues may be due at least in part to the inhibition of cytochrome c oxidase and SDH.

Interactions between C3 and C4 salt marsh plant species during four years of exposure to elevated atmospheric CO2

Elevated atmospheric CO2 is known to stimulate photosynthesis and growth of plants with the C3 pathway but less of plants with the C4 pathway. An increase in the CO2 concentration can therefore be expected to change the competitive interactions between C3 and C4 species. The effect of long term exposure to elevated CO2 (ambient CO2 concentration +340 µmol CO2 mol-1) on a salt marsh vegetation with both C3 and C4 species was investigated. Elevated CO2 increased the biomass of the C3 sedgeScirpus olneyi growing in a pure stand, while the biomass of the C4 grassSpartina patens in a monospecific community was not affected. In the mixed C3/C4 community the C3 sedge showed a very large relative increase in biomass in elevated CO2 while the biomass of the C4 species declined. The C4 grassSpartina patens dominated the higher areas of the salt marsh, while the C3 sedgeScirpus olneyi was most abundant at the lower elevations, and the mixed community occupied intermediate elevations.Scirpus growth may have been restricted by drought and salt stress at the higher elevations, whileSpartina growth at the lower elevations may be affected by the higher frequency of flooding. Elevated CO2 may affect the species distribution in the salt marsh if it allowsScirpus to grow at higher elevations where it in turn may affect the growth ofSpartina.