Indirect Effects Of Elevated CO2 Research Articles

Developing a Redox Network for Coastal Saltmarsh Systems in the PFLOTRAN Reaction Model

AbstractCoastal ecosystems have been largely ignored in Earth system models but are important zones for carbon and nutrient processing. Interactions between water, microbes, soil, sediments, and vegetation are important for mechanistic representation of coastal processes and ecosystem function. To investigate the role of these feedbacks, we used a reactive transport model (PFLOTRAN) that has the capability to be connected to the Energy Exascale Earth System Model (E3SM). PFLOTRAN was used to incorporate redox reactions and track chemical species important for coastal ecosystems as well as define simple representations of vegetation dynamics. Our goal was to incorporate oxygen flux, salinity, pH, sulfur cycling, and methane production along with plant‐mediated transport of gases and tidal flux. Using porewater profile and incubation data for model calibration and evaluation, we were able to create depth‐resolved biogeochemical soil profiles for saltmarsh habitat and use this updated representation to simulate direct and indirect effects of elevated CO2 and temperature on subsurface biogeochemical cycling. We found that simply changing the partial pressure of CO2 or increasing temperature in the model did not fully reproduce observed changes in the porewater profile, but the inclusion of plant or microbial responses to CO2 and temperature manipulations was more accurate in representing porewater concentrations. This indicates the importance of characterizing tightly coupled vegetation‐subsurface processes for developing predictive understanding and the need for measurement of plant‐soil interactions on the same time scale to understand how hotspots or moments are generated.

Ocean acidification boosts reproduction in fish via indirect effects.

Ocean acidification affects species populations and biodiversity through direct negative effects on physiology and behaviour. The indirect effects of elevated CO2 are less well known and can sometimes be counterintuitive. Reproduction lies at the crux of species population replenishment, but we do not know how ocean acidification affects reproduction in the wild. Here, we use natural CO2 vents at a temperate rocky reef and show that even though ocean acidification acts as a direct stressor, it can indirectly increase energy budgets of fish to stimulate reproduction at no cost to physiological homeostasis. Female fish maintained energy levels by compensation: They reduced activity (foraging and aggression) to increase reproduction. In male fish, increased reproductive investment was linked to increased energy intake as mediated by intensified foraging on more abundant prey. Greater biomass of prey at the vents was linked to greater biomass of algae, as mediated by a fertilisation effect of elevated CO2 on primary production. Additionally, the abundance and aggression of paternal carers were elevated at the CO2 vents, which may further boost reproductive success. These positive indirect effects of elevated CO2 were only observed for the species of fish that was generalistic and competitively dominant, but not for 3 species of subordinate and more specialised fishes. Hence, species that capitalise on future resource enrichment can accelerate their reproduction and increase their populations, thereby altering species communities in a future ocean.

Direct and indirect effects of elevated CO2 and nitrogen addition on soil microbial communities in the rhizosphere of Bothriochloa ischaemum

Elevated CO2 and nitrogen (N) addition both affect soil microbial communities, which significantly influence soil processes and plant growth. Here, we evaluated the combined effects of elevated CO2 and N addition on the soil–microbe–plant system of the Chinese Loess Plateau. A pot cultivation experiment with two CO2 treatment levels (400 and 800 μmol mol−1) and three N addition levels (0, 2.5, and 5 g N m−2 year−1) was conducted in climate-controlled chambers to evaluate the effects of elevated CO2 and N addition on microbial community structure in the rhizosphere of Bothriochloa ischaemum using phospholipid fatty acid (PLFA) profiles and associated soil and plant properties. Structural equation modeling (SEM) was used to identify the direct and indirect effects of the experimental treatments on the structure of microbial communities. Elevated CO2 and N addition both increased total and fungal PLFAs. N addition alone increased bacterial, Gram-positive, and Gram-negative PLFAs. However, elevated CO2 interacting with N addition had no significant effects on the microbial community. The SEM indicated that N addition directly affected the soil microbial community structure. Elevated CO2 and N addition both indirectly affected the microbial communities by affecting plant and soil variables. N addition exerted a stronger total effect than elevated CO2. The results highlighted the importance of comprehensively studying soil–microbe–plant systems to deeply reveal how characteristics of terrestrial ecosystems may respond under global change.

Feeding behavior, life history, and virus transmission ability of Bemisia tabaci Mediterranean species (Hemiptera: Aleyrodidae) under elevated CO2

The continuous rise of CO2 concentrations in the atmosphere is reducing plant nutritional quality for herbivores and indirectly affects their performance. The whitefly (Bemisia tabaci, Gennadius) is a major worldwide pest of agricultural crops causing significant yield losses. This study investigated the plant-mediated indirect effects of elevated CO2 on the feeding behavior and life history of B. tabaci Mediterranean species. Eggplants were grown under elevated and ambient CO2 concentrations for 3 weeks after which plants were either used to monitor the feeding behavior of whiteflies using the Electrical Penetration Graph technique or to examine fecundity and fertility of whiteflies. Plant leaf carbon, nitrogen, phenols and protein contents were also analyzed for each treatment. Bemisia tabaci feeding on plants exposed to elevated CO2 showed a longer phloem ingestion and greater fertility compared to those exposed to ambient CO2 suggesting that B. tabaci is capable of compensating for the plant nutritional deficit. Additionally, this study looked at the transmission of the virus Tomato yellow leaf curl virus (Begomovirus) by B. tabaci exposing source and receptor tomato plants to ambient or elevated CO2 levels before or after virus transmission tests. Results indicate that B. tabaci transmitted the virus at the same rate independent of the CO2 levels and plant treatment. Therefore, we conclude that B. tabaci Mediterranean species prevails over the difficulties that changes in CO2 concentrations may cause and it is predicted that under future climate change conditions, B. tabaci would continue to be considered a serious threat for agriculture worldwide.

Direct and indirect effects of elevated CO2 are revealed through shifts in phytoplankton, copepod development, and fatty acid accumulation.

Change in the nutritional quality of phytoplankton is a key mechanism through which ocean acidification can affect the function of marine ecosystems. Copepods play an important role transferring energy from phytoplankton to higher trophic levels, including fatty acids (FA)—essential macronutrients synthesized by primary producers that can limit zooplankton and fisheries production. We investigated the direct effects of pCO2 on phytoplankton and copepods in the laboratory, as well as the trophic transfer of effects of pCO2 on food quality. The marine cryptophyte Rhodomonas salina was cultured at 400, 800, and 1200 μatm pCO2 and fed to adult Acartia hudsonica acclimated to the same pCO2 levels. We examined changes in phytoplankton growth rate, cell size, carbon content, and FA content, and copepod FA content, grazing, respiration, egg production, hatching, and naupliar development. This single-factor experiment was repeated at 12°C and at 17°C. At 17°C, the FA content of R. salina responded non-linearly to elevated pCO2 with the greatest FA content at intermediate levels, which was mirrored in A. hudsonica; however, differences in ingestion rate indicate that copepods accumulated FA less efficiently at elevated pCO2. A. hudsonica nauplii developed faster at elevated pCO2 at 12°C in the absence of strong food quality effects, but not at 17°C when food quality varied among treatments. Our results demonstrate that changes to the nutritional quality of phytoplankton are not directly translated to their grazers, and that studies that include trophic links are key to unraveling how ocean acidification will drive changes in marine food webs.

Amplification of heat extremes by plant CO2 physiological forcing

Plants influence extreme heat events by regulating land-atmosphere water and energy exchanges. The contribution of plants to changes in future heat extremes will depend on the responses of vegetation growth and physiology to the direct and indirect effects of elevated CO2. Here we use a suite of earth system models to disentangle the radiative versus vegetation effects of elevated CO2 on heat wave characteristics. Vegetation responses to a quadrupling of CO2 increase summer heat wave occurrence by 20 days or more—30–50% of the radiative response alone—across tropical and mid-to-high latitude forests. These increases are caused by CO2 physiological forcing, which diminishes transpiration and its associated cooling effect, and reduces clouds and precipitation. In contrast to recent suggestions, our results indicate CO2-driven vegetation changes enhance future heat wave frequency and intensity in most vegetated regions despite transpiration-driven soil moisture savings and increases in aboveground biomass from CO2 fertilization.

Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO2

Increasing concentrations of atmospheric carbon dioxide are expected to affect carbon assimilation and evapotranspiration (ET), ultimately driving changes in plant growth, hydrology, and the global carbon balance. Direct leaf biochemical effects have been widely investigated, whereas indirect effects, although documented, elude explicit quantification in experiments. Here, we used a mechanistic model to investigate the relative contributions of direct (through carbon assimilation) and indirect (via soil moisture savings due to stomatal closure, and changes in leaf area index) effects of elevated CO2 across a variety of ecosystems. We specifically determined which ecosystems and climatic conditions maximize the indirect effects of elevated CO2 The simulations suggest that the indirect effects of elevated CO2 on net primary productivity are large and variable, ranging from less than 10% to more than 100% of the size of direct effects. For ET, indirect effects were, on average, 65% of the size of direct effects. Indirect effects tended to be considerably larger in water-limited ecosystems. As a consequence, the total CO2 effect had a significant, inverse relationship with the wetness index and was directly related to vapor pressure deficit. These results have major implications for our understanding of the CO2 response of ecosystems and for global projections of CO2 fertilization, because, although direct effects are typically understood and easily reproducible in models, simulations of indirect effects are far more challenging and difficult to constrain. Our findings also provide an explanation for the discrepancies between experiments in the total CO2 effect on net primary productivity.

Mechanisms driving Antarctic microbial community responses to ocean acidification: a network modelling approach

Rising atmospheric CO2 concentrations and the subsequent changes to ocean chemistry may have pronounced effects on marine microbial communities, particularly for the cold Southern Ocean. Changes to the microbial community in this region could affect the way nutrients are cycled, impact the efficiency of carbon drawdown, and cause shifts in food supply to higher trophic levels. Increased CO2 could affect the bioavailability of iron to phytoplankton. Fertilisation experiments show that iron can influence phytoplankton community composition, favouring large phytoplankton species in iron-replete conditions. The potential interactive effects of CO2 and iron bioavailability are currently poorly understood but are likely to be important in determining CO2-induced changes to the microbial community. We employ a qualitative network modelling approach to evaluate alternative hypotheses regarding the effects of elevated CO2 on Antarctic microbial communities in incubation experiments. We used a sequential approach to model development and testing, where we first formulated a base model for microbial community interactions, and then sequentially added direct and indirect effects of elevated CO2 on particular groups. We found that model simulations were most consistent with observations from incubation experiments when we assumed an indirect effect of CO2 on phytoplankton. In particular, when we assumed a negative effect of elevated CO2 on the uptake of iron by large phytoplankton, as a result of a decrease in iron bioavailability. Our findings show that qualitative network models can be used to test hypotheses relating to results from experimental studies, and help identify key processes to target in future studies.

The use of branch enclosures to assess direct and indirect effects of elevated CO2 on photosynthesis, respiration and isoprene emission of Populus alba leaves

We used a novel system of branch enclosures to study the impact of elevated CO2 (900 ppm) on the gas-exchange characteristics of developed and developing leaves of white poplar (Populus alba L.), as well as of leaves subsequently developing at ambient CO2, outside the enclosures in which the CO2 concentration was raised. We found no significant effect of elevated CO2 on photosynthesis, respiration and isoprene emission, as the rates of developed and developing leaves inside the enclosures, and of leaves developing outside the enclosures, were similar to those recorded using enclosures maintained at ambient CO2. The enclosure system, however, largely influenced the rates of gas-exchange. In fact, leaves already developed inside the enclosures showed rates of photosynthesis, stomatal conductance, and isoprene emission higher than leaves developing inside the enclosures, and also higher than leaves developing outside the enclosure. These differences were caused by a higher efficiency in the light use and by a higher Ribulose 1.5 bisphosphate carboxylase (Rubisco) activity in leaves fully developed inside enclosures than in the other leaf classes. The experiment overall suggests that branch enclosures may alter the physiology of the plants, reducing or counteracting the impact of elevated CO2, which we predicted to stimulate photosynthesis and uncouple isoprene emission from photosynthesis. This may be an important bias against the use of enclosure systems for studies of the impact of environmental constraints and global change factors on physiological features.

Below‐ground process responses to elevated CO2 and temperature: a discussion of observations, measurement methods, and models

SummaryRising atmospheric CO2 and temperatures are probably altering ecosystem carbon cycling, causing both positive and negative feedbacks to climate. Below‐ground processes play a key role in the global carbon (C) cycle because they regulate storage of large quantities of C, and are potentially very sensitive to direct and indirect effects of elevated CO2 and temperature. Soil organic matter pools, roots and associated rhizosphere organisms all have distinct responses to environmental change drivers, although availability of C substrates will regulate all the responses. Elevated CO2 increases C supply below‐ground, whereas warming is likely to increase respiration and decomposition rates, leading to speculation that these effects will moderate one another. However, indirect effects on soil moisture availability and nutrient supply may alter processes in unexpected directions. Detailed, mechanistic understanding and modelling of below‐ground flux components, pool sizes and turnover rates is needed to adequately predict long‐term, net C storage in ecosystems. In this synthesis, we discuss the current status of below‐ground responses to elevated CO2 and temperature and potential feedback effects, methodological challenges, and approaches to integrating models and measurements.

Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem

AbstractWe measured the short‐term direct and long‐term indirect effects of elevated CO2 on leaf dark respiration of loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) in an intact forest ecosystem. Trees were exposed to ambient or ambient + 200 µmol mol−1 atmospheric CO2 using free‐air carbon dioxide enrichment (FACE) technology. After correcting for measurement artefacts, a short‐term 200 µmol mol−1 increase in CO2 reduced leaf respiration by 7–14% for sweetgum and had essentially no effect on loblolly pine. This direct suppression of respiration was independent of the CO2 concentration under which the trees were grown. Growth under elevated CO2 did not appear to have any long‐term indirect effects on leaf maintenance respiration rates or the response of respiration to changes in temperature (Q10, R0). Also, we found no relationship between mass‐based respiration rates and leaf total nitrogen concentrations. Leaf construction costs were unaffected by growth CO2 concentration, although leaf construction respiration decreased at elevated CO2 in both species for leaves at the top of the canopy. We conclude that elevated CO2 has little effect on leaf tissue respiration, and that the influence of elevated CO2 on plant respiratory carbon flux is primarily through increased biomass.

Stimulated N2O flux from intact grassland monoliths after two growing seasons under elevated atmospheric CO2.

Long-term exposure of native vegetation to elevated atmospheric CO2 concentrations is expected to increase C inputs to the soil and, in ecosystems with seasonally dry periods, to increase soil moisture. We tested the hypothesis that these indirect effects of elevated CO2 (600 μl l-1 vs 350 μl l-1) would improve conditions for microbial activity and stimulate emissions of nitrous oxide (N2O), a very potent and long-lived greenhouse gas. After two growing seasons, the mean N2O efflux from monoliths of calcareous grassland maintained at elevated CO2 was twice as high as that measured from monoliths maintained at current ambient CO2 (70 ± 9 vs 37 ± 4 μg N2O m-2 h-1 in October, 27 ± 5 vs 13 ± 3 μg N2O m-2 h-1 in November after aboveground harvest). The higher N2O emission rates at elevated CO2 were associated with increases in soil moisture, soil heterotrophic respiration, and plant biomass production, but appear to be mainly attributable to higher soil moisture. Our results suggest that rising atmospheric CO2 may contribute more to the total greenhouse effect than is currently estimated because of its plant-mediated effects on soil processes which may ultimately lead to increased N2O emissions from native grasslands.

Direct and indirect effects of elevated CO(2) on whole-shoot respiration in ponderosa pine seedlings.

We determined the short-term direct and long-term indirect effects of CO(2) on apparent dark respiration (CO(2) efflux in the dark) in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings grown in 35 or 70 Pa CO(2) partial pressure for 163 days in naturally lit, controlled-environment chambers. Two soil N treatments (7 and 107 ppm total N, low-N and high-N treatments, respectively) were imposed by watering half the plants every 2 weeks with 15/15/18 fertilizer (N,P,K) and the other half with demineralized water. Direct effects of ambient CO(2) partial pressure on apparent dark respiration were measured during short-term manipulations (from minutes to hours) of the CO(2) environment surrounding the aboveground portion of individual seedlings. Short-term increases in the ambient CO(2) partial pressure consistently resulted in significant decreases in CO(2) efflux of seedling in all treatments. Efflux of CO(2) decreased by 3 to 13% when measurement CO(2) partial pressure was increased from 35 to 70 Pa, and by 8 to 46% over the entire measurement range from 0 to 100 Pa. No significant interactions between the indirect effects of growth CO(2) partial pressure and the direct effects of the measurement CO(2) partial pressure were found. Seedlings grown in the high-N treatment were significantly less sensitive to short-term changes in CO(2) partial pressures than seedlings grown in the low-N treatment. Apparent respiration tended to decrease in seedlings grown in elevated CO(2), but the decrease was not significant. Nitrogen had a large effect on CO(2) efflux, increasing apparent respiration more than twofold on both a leaf area and a leaf or shoot mass basis. Both the direct and indirect effects of elevated CO(2) were correlated with changes in the C/N ratio. A model of cumulative CO(2) efflux for a 160-day period demonstrated that, despite a 49% increase in total plant biomass, seedlings grown in the high-N + high-CO(2) treatment lost only 2% more carbon than seedlings grown in the high-N + low-CO(2) treatment, suggesting increased carbon use efficiency in plants grown in elevated CO(2). We conclude that small changes in instantaneous CO(2) efflux, such as those observed in ponderosa pine seedlings, could scale to large changes in carbon sequestration.