Ecosystem CO2 starvation and Earth’s minimum CO2 concentration: an experimental assessment

Joe Quirk (Researcher), David Johnson (Research technician), Jonathan Leake (Co-I),  David Beerling (PI), 

Funded by The Leverhulme Trust 
 
Polar ice-core records indicate that Earth’s atmospheric CO2 concentration ([CO2]a) varied between ~180 and ~300 ppm during the last eight glacial cycles, while isotopic signatures of foraminifera indicate it varied from ~220 – 280 ppm over the past 2.1 million years (Ma). Other proxies suggest [CO2]a did not drop below 200 – 250 ppm for at least the last 24 Ma of the Cenozoic. This lack of variability in the lower boundary of [CO2]a concentrations is surprising because major tectonic episodes and coincident environmental conditions should have enhanced the global weathering of calcium-bearing silicate minerals, increased the export of calcium to the oceans and contributed to a major long-term CO2 sink via precipitation of oceanic calcium carbonate sediments.

Cenozoic Earth system hypothesis.

To explain these observations, a mechanistic “Earth system” hypothesis has recently been advanced. It postulates that the lower limit of [CO2]a is buffered by a strong negative feedback in which the capacity of land plants to weather calcium-bearing silicates diminishes in a non-linear manner as [CO2]a falls to critically low ‘starvation’ levels limiting plant productivity. The hypothesis builds on foundational studies in contemporary forested ecosystems establishing the role of vegetation in the weathering of minerals in modern and ancient systems. Field studies of vegetation from multiple ecosystems indicate that rates of silicate mineral weathering are increased by the actions of plants by several times. In a geological context, the rise of the first deep-rooted forested ecosystems from around 380 Ma is linked to a 90% drop in[CO2]a through the Palaeozoic, affirming the critical role of plants in driving atmosphere-geosphere interactions via mineral weathering.

Biological weathering mechanisms.

The direct ability of plants, and their root symbiotic partnerships with mycorrhizal fungi, to enhance weathering is driven by three main mechanisms: (1) the capacity of roots and associated microorganisms to physically and chemically increase mineral surface areas, (2) lowering of soil solution pH by mycorrhizosphere respired CO2, organic acids and proton exchange during nutrient uptake, and (3) chelation by organic ligands secreted into the mycorrhizosphere enhancing mineral dissolution and nutrient supply for plant growth. All of these weathering activities contribute to acquisition of mineral nutrients by plants from rocks using photosynthate carbon energy. Of the two major types of mycorrhiza, the ectomycorrhizal (EM) associates of trees are considered to be especially effective at mineral weathering, in comparison to arbuscular mycorrhizal (AM) which associate with both herbaceous and woody plants, including grasses and most tree species. EM fungi increase the rate of dissolution of calcium by secreting ligands (e.g., oxalic acid) at the scale of individual grains of calcium-rich minerals and rocks, such as apatite and basalt, but there is no evidence of such exudation by AM fungi.

Atmospheric CO2 dependency of biological processes.

Critically, as [CO2]a approaches low levels (180-200 ppm), experimental evidence indicates that the primary productivity and nutrient demand of forest trees and grasslands declines, together with proportional and total carbon allocation to roots (and therefore mycorrhizal partners). Such responses reveal the potential for non-linear effects of CO2 starvation in diminishing weathering activities by terrestrial vegetation and its mycorrhizal partnerships.

We are testing the unifying hypothesis that [CO2]a starvation will have the dual effect of reducing plant demand for phosphorus, and carbon-energy supply to mycorrhizas. These fundamental effects of low [CO2]a, in concert with linked shifts from EM to AM plant dominance, are likely to strongly repress plant and mycorrhiza-driven weathering of primary calcium silicate/phosphate minerals. We aim to investigate the potential role of these biosphere-geosphere interactions in the Cenozoic carbon cycle.

Our experiments address fundamental questions regarding the role of interactions between changes in [CO2]a, vegetation shifts from forest to grassland and biological weathering, and how this feeds back into the climate system to maintain CO2 above Earth system minimum values of 180 – 200 ppm. The work is funded by The Leverhulme Trust and carried out using state of the art growth chambers in the Sir David Read Controlled Environment Facility, that maintain the atmospheric CO2 in matched growth chambers at low (200 ppm), ambient (~450 ppm) and high (1200 ppm) levels.
  


Arbuscular mycorrhizal fungi spore indentation in muscovite. 3D Image acquired using vertical scanning interferometry.                                                                        



Mineral chips embedded in silicone and mounted on a glass slide prior to VSI imaging.
 

Basalt
 

Betula pendula saplings used for weathering studies to establish the influence of ectomycorrhizal fungi on weathering of basalt grains.

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