What goes around comes around

The carbon cycle and the water cycle – some surprises.


When rain falls to Earth, some water is absorbed by plants, some is stored in the soil and some runs to the sea. Water then returns to the atmosphere via transpiration from plants and evaporation from surfaces. So far, so normal; this is the soil-plant-atmosphere continuum that most of us last saw in junior school.

Humanity developed at an atmospheric-carbon-dioxide level over 100 parts-per-million (ppm) less than today (Da 2019, Ebi and Loladze 2019). This is also true for the plants and animals that we depend on for food. Plant health in turn depends on systems their genetics can respond to, especially soil moisture and a specific air mixture. All life that we depend on is optimally healthy at a CO2 level of ~280ppm (today’s level is 415ppm).

Carbon dioxide is essential to plants up to about 300ppm but above that it is not a helpful part of healthy living systems.

All life systems adapt very slowly to fundamental change


It’s a dangerous myth that carbon dioxide is ‘fertiliser’ as the plant life we depend on is changing so fast that we won’t be able to keep up. For example weeds (unlike cultivated plants) have unmodified genomes capable of responding positively to higher levels of carbon – an uphill struggle for us.

High levels of lignin (in plant leaves) make plants less flammable. CO2 levels below 350ppm maintain this preferred state. Unfortunately several studies show lignin is falling in many plants, especially grasses, leading to rapid expansion of fires (Blank et al. 2006).

Another way excess CO2 does not help is in making plants more water-efficient; it may also be leading to the breaking of plant / soil-fungi relationships. We’ve bred crop plants to be more water-efficient but this has been directed to optimize the plants for our use. Additionally soil fungi (mycorrhizae) have always added a lot of strength to soil, although this is poorly understood. With a rapidly changing atmosphere plants are seeking new ways to survive and their solutions will likely not benefit us.

Some field soils appear to be getting wetter (from my own field observations, asking growers and reading widely). A high CO2 level makes plants more water-efficient (Newton et al., 2006) resulting in plant roots concentrating near the surface instead of growing deeply. In a wet-climate region, roots taking less water from the soil result in heavier soils with no reinforcing roots – this leads to an increase of shallow landslides or slips.

As roots shift upward so mycorrhizae are expected to move nearer the surface (roots and mycorrhizae are a symbiosis; association is obligatory). In a healthy soil mycorrhizae spread from just below the surface to metres down, with a myriad of species, doing a lot of things we know almost nothing about. What we do know is they are vital in storing carbon, retaining water and helping plants obtain water and nutrients, their potential loss is immeasurable (Johnson et al, 2013 and Wright, 1996 are good places to start).


Elevated carbon also affects freshwater chemistry (Gies, 2018 is a great place to start) and may reduce stream-bottom stability and in-stream nutrient cycling where most work is done by periphyton, microscopic organisms living as mats or so-called biofilms. With freshwater little is known, but with marine and tidal biofilms there are significant detrimental effects (Ahrendt, 2014).


Ahrendt, S.R., Jennifer J.M., Visscher, P.T., Koss, L.L. and Foster, J.S. 2014. Effects of Elevated Carbon Dioxide and Salinity on the Microbial Diversity in Lithifying Microbial Mats. Minerals, 4, 145-16.

Blank, R.R., White R.H. and Ziska L.H. 2006. Combustion properties of Bromus tectorum L.: influence of ecotype and growth under four CO2 concentrations. International Journal of Wildland Fire, 15, 227-236. https://www.fs.usda.gov/treesearch/pubs/24450

Da, J., Zhang Y., Li G., Meng X. and Ji J. 2019 Sept. Low CO 2 levels of the entire Pleistocene epoch. Nature Communications. https://doi.org/10.1038/s41467-019-12357-5

Ebi, K. and Loladze, I. 2019. Elevated atmospheric CO2 concentrations and climate change will affect our food’s quality and quantity. Lancet Planetary Health, 3, 7, PE283-E284, JULY 01

Erica Gies, E. 2018. Like Oceans, Freshwater Is Also Acidifying: Rising CO2 in lakes and reservoirs may harm animals that live in those ecosystems

Johnson, N.C., Angelard, C., Sanders, I.R. Kiers, T.E. 2013. Predicting community and ecosystem outcomes of mycorrhizal responses to global change. Ecology Letters, 16: 140-153. https://onlinelibrary.wiley.com/doi/full/10.1111/ele.12085

Newton P.C., Allard V., Carran R.A., Lieffering M. 2006. Impacts of elevated CO2 on a grassland grazed by sheep: the NZ FACE experiment. in: Nosberger 2006. Managed Ecosystems and CO2 : Case Studies, Processes and Perspectives.

Wright, S. 2002. Glomalin: Hiding Place for a third of the World’s Stored Soil Carbon
USDA, Agricultural Research, 50: 9, September. https://agresearchmag.ars.usda.gov/2002/sep/soil

3 thoughts on “What goes around comes around

Add yours

  1. It is very interesting that carbon levels have such a complicated domino effect. I always hear about “high carbon” being bad, but this is the first time I have heard someone explain many layers of the repercussions.

    1. Thank you for your comment. Yes, carbon sure is complex, I’ll write more on it further down the track, including how we may respond to high carbon.

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