Soil Fungus Soaks Up CO2
There have been arguments made for increased plant growth due to rising atmospheric CO2 levels, while others have argued against it. Now it seems that green plants and ocean algae are not the only forms of life involved. Opportunistic microorganisms are stepping in to sop up excess carbon. A new report in the Proceedings of the National Academy of Sciences (PNAS) has identified soil fungi as a major player in accelerating CO2 absorption. Arbuscular mycorrhizal fungi (AMF) have been identified as an intermediary between plants and other bacterial and fungal populations, acting as a buffer for other soil-borne communities. Existing organisms are not just working harder, new communities are developing to take advantage of increased CO2 levels, demonstrating that nature possesses self-regulation mechanisms science did not anticipate and has yet to discover.
Rising atmospheric CO2 levels are predicted to significantly impact the carbon cycle and, by extension, the functioning of terrestrial ecosystems. In a paper, appearing in the June 1, 2010, of PNAS, Barbara Drigoa et al. have revealed some previously unsuspected activity in our planet's soil. Their article, “Shifting carbon flow from roots into associated microbial communities in response to elevated atmospheric CO2,” provides a reassessment of how carbon flows through soil ecosystems. Their study is describe in the paper's abstract:
Rising atmospheric CO2 levels are predicted to have major consequences on carbon cycling and the functioning of terrestrial ecosystems. Increased photosynthetic activity is expected, especially for C-3 plants, thereby influencing vegetation dynamics; however, little is known about the path of ﬁxed carbon into soil-borne communities and resulting feedbacks on ecosystem function. Here, we examine how arbuscular mycorrhizal fungi (AMF) act as a major conduit in the transfer of carbon between plants and soil and how elevated atmospheric CO2 modulates the belowground trans-location pathway of plant-fixed carbon. Shifts in active AMF species under elevated atmospheric CO2 conditions are coupled to changes within active rhizosphere bacterial and fungal communities. Thus, as opposed to simply increasing the activity of soil-borne microbes through enhanced rhizodeposition, elevated atmospheric CO2 clearly evokes the emergence of distinct opportunistic plant-associated microbial communities.
Plants have a number of metabolic pathways they use to fix carbon from the atmosphere and it is accepted that carbon dioxide is essential to Earth's biosphere (see “Too Little CO2 To End Life On Earth”). But after the carbon is fixed, it makes its way through the plant's roots and into the complex ecosystem of the surrounding soil. Carbon fixed by plants can enter the soil through increased root turnover, the shedding of cells, plant tissue breakdown, or increased root exudation, the slow escape of liquids through pores or breaks in the cell membranes.
Arbuscular mycorrhizas (AM) are a mutually beneficial association between fungi from the phylum Glomeromycota and the roots of most plant species. They are a type of mycorrhiza in which the fungus penetrates the cortical cells of the roots of a vascular plant. The name mycorrhiza, which literally means fungus-root, was bestowed by A. B. Frank (1885) to describe non-pathogenic symbiotic associations between roots and fungi. Both symbionts receive important benefits: the plant supplies carbohydrates to the fungus, and the fungus improves plant nutrition by helping in the uptake of mineral nutrients such as phosphorus, nitrogen, zinc and water.
Plant and fungus in a mutually beneficial arrangement. Image Nature.
AM hyphae, branching threadlike filaments, enter into the plant cells, producing structures that are either balloon-like (vesicles) or branching root-like structures (arbuscules). It is believed that the development of AM symbiosis played a crucial role in the evolution of vascular plants and in the initial colonization of the land by plants. As far back as the Lower Devonian, around 400 million years ago, primitive plants were found to contain structures resembling vesicles. Arbuscular mycorrhizas are found in 85% of all plant families, and occur in many important food-crop species.
Once in the soil, a number of different microbial communities make use of the carbon containing material. Far from being a static system, the microbial composition of soil is changing in response to changes in atmospheric CO2. With more carbon being extracted from the atmosphere and transferred through plant roots, new fungal populations are arising that can rapidly absorb the carbon bounty.
“Based on our data, we present a conceptual model in which plant-assimilated carbon is rapidly transferred to AMF, followed by a slower release from AMF to the bacterial and fungal populations well-adapted to the prevailing (myco-)rhizosphere conditions,” the authors state. “This model provides a general framework for reappraising carbon-flow paths in soils, facilitating predictions of future interactions between rising atmospheric CO2 concentrations and terrestrial ecosystems.”
Conceptual model of carbon ﬂow in mycorrhizal plant–soil systems. Drigoa et al.
The authors' conceptual model of carbon ﬂow in mycorrhizal plant–soil systems, with the observed effects of elevated CO2 atmospheric concentrations on soil communities, is is summarized in the illustration above. Brown arrows indicate increases and decreases in the respective community sizes, as well as changes in community structure and carbon flow. Absence of an arrow indicates no significant change in the community size or structure. Red arrows indicate no effect of increased carbon availability because of elevated CO2 on the Actinomycetes spp. and Bacillus spp. communities. The mechanism and magnitude of the carbon flow along the soil–food web are indicated by the green arrows.
The pathway works like this: Plants and grasses absorb CO2 from the atmosphere and release part of the carbon (C) into the soil; This plant-derived C is taken up principally by the AMF, which rapidly receive it; AMF subsequently release the C gradually to other microbes. This highlights the key position of AMF in the release of plant-derived C to the soil microbial community. This is important because previous studies of the uptake of plant-derived carbon by soil microbes yielded much lower rates of absorption—primarily because the participation of AMF was limited. With larger amounts of C available, the fungi are stepping up and playing a bigger, unexpected role.
“Shifts in AMF populations in response to elevated CO2 conditions, therefore, result in marked changes in bacterial diversity and activity, stimulating the speciﬁc populations best capable of responding to the altered nutrient conditions of the (myco-)rhizosphere,” state Drigoa et al. in summary. “Together, these responses may not only impact community structure and biodiversity of the microbial communities in the rhizosphere, but they may also affect carbon-turnover processes in soil and as such, the direction and magnitude of terrestrial ecosystem feedbacks in response to enhanced atmospheric CO2 levels.” In other words, when atmospheric CO2 levels rise, nature changes in response.
Arbuscular mycorrhiza in clover root. Image J. Deacon.
This revelation comes as no suprise—at the end of 2009, I reported on a study in Geophysical Research Letters that showed ocean absorption of CO2 is not shrinking. That report contradicted alarmist predictions that the ocean would soon stop absorbing carbon dioxide, leading to accelerated global warming. Now, claims that terrestrial plant ecosystems would be damaged by rising CO2 levels, reducing their absorption capacity, have also been contradicted. Both of these discoveries serve to underscore the fact that climate science is just guessing at where CO2 really comes from, and where and how much CO2 nature absorbs.
Not only does this research indicate that CO2 absorption by soil microorganisms has increased, whole new communities of microbes have blossomed. This underscores a fundamental truth that scientists studying climate change do not seem to understand—nature is not static, it adjusts to changing conditions with changes of its own. How else could life have survived on Earth for billions of years? Even after devastating extinction events, nature bounces back with greater diversity than before. The effects of increased carbon dioxide levels predicted by climate alarmists are wrong because there is no way for them to know how nature will react. That, of course, does not stop them from predicting dire consequences ahead. Fortunately, nature pays no attention to climate scientists.
Be safe, enjoy the interglacial and stay skeptical.