Belowground Biotic Responses to Atmosperic CO2 Enrichment

From ClimateWiki

Soil Fungi

Andrew and Lilleskov (2009) studied sporocarps (the reproductive structures of fungi), which can be significant carbon sinks for the ectomycorrhizal fungi that develop symbiotic relationships with plants by forming sheaths around their root tips, where they are the last sinks for carbon in the long and winding pathway that begins at the source of carbon assimilation in plant leaves. The researchers note “it is critical to understand how ectomycorrhizal fungal sporocarps are affected by elevated CO2 and ozone,” because, they continue, “sporocarps facilitate genetic recombination, permit long-distance dispersal and contribute to food webs,” and we need to know how these important processes will be affected by continued increases in the concentrations of these two trace constituents of the atmosphere.

Accordingly, the two researchers evaluated sporocarp biomass for a period of four years at the Aspen free-air CO2 enrichment (FACE) site near Rhinelander, Wisconsin, which provided, in their words, a “unique opportunity to examine the effects of both elevated CO2 and O3 on a forested ecosystem.” The examination was conducted during years four through seven of the aspen and aspen-birch forests’ exposures to ambient and enriched concentrations of the two gases: CO2 (350 and 550 ppm) and O3 (33–67 and 50-–00 ppb). The scientists found total mean sporocarp biomass “was generally lowest under elevated O3 with ambient CO2,” and it “was greatest under elevated CO2, regardless of O3 concentration.” They also found “a complete elimination of O3 effects on sporocarp production when [extra] CO2 was added.” And they state they “expect that the responses seen in the present study were conservative compared to those expected under regional to global changes in CO2 and O3.” Consequently, by itself or in combination with rising ozone concentrations, the ongoing rise in the atmosphere’s CO2 content appears destined to enhance the genetic recombination and long-distance dispersal of the ectomycorrhizal fungi that form symbiotic relationships with the roots of aspen and birch trees, thereby positively contributing to various food webs that will be found within aspen and aspen-birch forests of the future.

In another study dealing with soil fungi, Alberton et al. (2010) write, “roots of a very large number of plant species are regularly colonized by a group of ascomycete fungi with usually dark-pigmented (melanized) septate hyphae (Mandyam and Jumpponen, 2005; Sieber and Grunig, 2006)” that are referred to as “dark septate root endophytic (DSE) fungi,” with “most species belonging to the Leotiomycetes (Kernaghan et al., 2003; Wang et al., 2006).” To study these fungi, the three researchers grew Scots pine (Pinus sylvestris) plants from seed for 125 days in Petri dishes—both with and without inoculation with one of seven different species/strains of DSE fungi—within controlled environment chambers maintained at atmospheric CO2 concentrations of either 350 or 700 ppm, destructively harvesting some of the seedlings at the 98-day point of the experiment and the rest of them at the experiment’s conclusion. They found “across all plants (DSE-inoculated and control plants) under elevated CO2, shoot and root biomass increased significantly by 21% and 19%, respectively, relative to ambient,” with “higher values over the final four weeks (increases of 40% and 30% for shoots and roots, respectively).” In addition, they indicate “on average, shoot nitrogen concentration was 57% lower under elevated CO2,” and “elevated CO2 decreased root nitrogen concentration on average by 16%.”

Alberton et al. thus acknowledge their study “did not suggest a role for DSE fungi in increased nutrient uptake.” In fact, they emphasize that “under elevated CO2, DSE fungi even reduced nitrogen content of the pine seedlings.” But they also emphasize that “surprisingly, even under reduced nitrogen availability, elevated CO2 led to increases in both above-ground and below-ground plant biomass.”

To explain how that happened, the Brazilian and Dutch scientists write, “a potential mechanism for the increase of plant biomass even when plant nutrient uptake decreases is the production of phytohormones by DSE fungi.” They observe that “earlier authors noted that DSE fungi enhance plant growth by producing phytohormones or inducing host hormone production without any apparent facilitation of host nutrient uptake or stimulation of host nutrient metabolism (Addy et al., 2005; Schulz and Boyle, 2005),” further demonstrating that low levels of soil nitrogen availability need not be an insurmountable impediment to significant CO2-induced increases in plant growth and development.

In another study of note, Compant et al. (2010) write, “virtually all land plant taxa investigated have well-established symbioses with a large variety of microorganisms (Nicolson, 1967; Brundrett, 2009),” some of which “are known to support plant growth and to increase plant tolerance to biotic and abiotic stresses (Bent, 2006).” Many of these microorganisms colonize the rhizosphere (Lugtenberg and Kamilova, 2009), while others “enter the root system of their hosts and enhance their beneficial effects with an endophytic lifestyle (Stone et al., 2000).” This is the case, as they put it, “for plant growth-promoting fungi such as arbuscular mycorrhizae, ectomycorrhizae and other endophytic fungi,” as well as for plant growth-promoting bacteria and the more specialized plant growth-promoting rhizobacteria. Many members of the first two categories, they note, “are applied as biocontrol agents, biofertilizers and/or phytostimulators in agriculture (Vessey, 2003; Welbaum et al., 2004) or as degrading microorganisms in phytoremediation applications (Denton, 2007).”

Consequently, and in order to determine how beneficial plant growth-promoting microorganisms might be affected by continued increases in the air’s CO2 content and by possible concomitant changes in climate, Compant et al. reviewed the results of 135 studies that investigated the effects of CO2 and changes in various climatic factors on “beneficial microorganisms and their interactions with host plants.” They found “the majority of studies showed that elevated CO2 had a positive influence on the abundance of arbuscular and ectomycorrhizal fungi,” which, in their words, “is generally in agreement with meta-analyses performed by Treseder (2004) and by Alberton et al. (2005).” But they also found “the effects on plant growth-promoting bacteria and endophytic fungi were more variable.” Nevertheless, they state, “in most cases, plant-associated microorganisms had a beneficial effect on plants under elevated CO2.” In addition, they report “numerous studies indicated that plant growth-promoting microorganisms (both bacteria and fungi) positively affected plants subjected to drought stress.” Temperature effects, on the other hand, were more of a wash, as Compant et al. state “the effects of increased temperature on beneficial plant-associated microorganisms were more variable, positive and neutral,” and “negative effects were equally common and varied considerably with the study system and the temperature range investigated.”

In concluding, Compant et al. note the stress of drought is disadvantageous for nearly all terrestrial vegetation, but plant growth-promoting microorganisms should help land plants overcome this potentially negative aspect of future climate change, as long as the air’s CO2 content continues to rise. Temperature effects, on the other hand, would appear to be no more negative than they are positive in a warming world, and when they might be negative, continued atmospheric CO2 enrichment should prove to be a huge benefit to plants by directly enhancing their growth rates and water use efficiencies. And under the best of climatic conditions, atmospheric CO2 enrichment should bring out the best of Earth’s plants, plus the best of the great majority of plant growth-promoting microorganisms that benefit them biochemically.

Soil amendment iuostqen?Yesterday I dug a 2 foot by 8 foot patch in my backyard that I plan to use for a small veggie/herb garden. The soil is in reasonably good condition and looks to be sandy and clayey. I plan to add some organic material from my compost heap when it produces some more hummus.Other than that, is there anything else I need to do with the soil to ready it for planting? I don't plan to plant anything fancy, just some basil, rosemary, mint, oregano, tomatoes, strawberries, blackberries and a few other things. I'm just concerned because I've never gardened before.The spot I've picked gets lots of sun 10-plus hours a day. I plan to start small and add on if my first fruits are successful. Any tips on knowing when the soil is ready to plant?Someone has said my garden isn't big enough. I can make the garden as large as I like. I just have to get the mattock out of the shed and start pounding away. Unfortunately, that doesn't answer my iuostqen as to what to do with the soil.

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Responses of Plants Under Stress to Atmospheric CO2 Enrichment