The PMOH Cycle

When you listen to journalists, sophists, and too many climate scientists, you’d think that cattle were the only source of atmospheric methane [CH4] and that their burps were solely to blame for climate change. While I’m being a tad bit sarcastic, such sarcasm is warranted given that some journalists even desire to rewild agricultural land into wetlands without seeming to realize that wetlands are one of the largest sources of biogenic methane (much larger than cattle). None of these journalists are demanding that wetlands (including beaver ponds) be drained. Journalists based in the US also seem to be equally unaware that GROSS enteric methane (cattle burps) only accounts for 2.17% of all GHG emissions in the US per data from the 2014 UN Climate Change Conference country level data as I noted in a recent blog: How much do cattle contribute to climate change?

Though as I noted in another recent blog, Methane persistence and hydroxyl radical availability, methane is emitted by a myriad of naturally occurring and anthropogenic biogenic sources in nature including shellfish, phytoplankton (Bizic 2021; Xu et al 2020), beaver ponds (Whitfield et al. 2015), rice paddies, wetlands, termites, arthropods, ruminants, dams, trees (Ezhumalai, 2021), or just about any place methanogenic archaea reside in anoxic environments (including soil and digestive tracts) or non-methanogenic bacteria occur in oxic environments (Ernst et al 2021). There’s also methane emitted by thermogenic (fossil fuels) and pyrogenic (fire) sources. So…no, methane isn’t only emitted by belching cows as some seem to believe (Lakhani et al. 2017).

Methane, like other carbon based gases particularly carbon dioxide [CO2], cycles.

Photosynthesis fixes atmospheric and soil respired CO2 into glucose in plants via the Calvin Cycle. That glucose is converted into a myriad of different carbon compounds including proteins, monoterpenes, phytonutrients, fats, exuded root exudates and the cellulose that ruminants- including cattle- consume. So some CO2 is converted to biomass, some CO2 ends up being stored, and some CO2 gets respired and used for photosynthesis again. Now the biomass, and more specifically the cellulose, consumed by the cattle, enters into the rumen where the bacteria, methanogenic archaea, protozoa, etc convert that cellulose into short chained fatty acids [SCFA’s], dihydrogen [H2], and methane [CH4]. It’s actually the archaea that are primarily responsible for the creation of methane via methanogenesis. The SCFA’s are used to build fats, proteins, etc. The methane is a by-product and most is “burped” out into the atmosphere. Though in the atmosphere, this CH4 collides with hydroxyl radicals [OH] and is eventually broken back down into CO2 and water vapor [H2O]. This is hydroxyl oxidation. The CO2 is then again fixed by photosynthesis into glucose and the cycle repeats itself. The H2O consolidates into rain or interacts with excited oxygen to form more OH.

This is the photosynthesis, methanogenesis and hydroxyl radical cycle or what I like to call the PMOH cycle. I explain hydroxyl oxidation in some more detail in my old blog: WTF happens to all that methane? One thing that should be made more apparent about this cycle is that a lot more CO2 is fixed via photosynthesis than is ultimately converted back to CO2 by hydroxyl oxidation. Why? Because in healthy soil ecosystems, 20 to 40% of the CO2 fixed in the cellulose is exuded from the roots of these plants consumed. These exudates include a wide array of carbon metabolites including sugars, fats, acids, etc. Additionally consumed cellulose is then converted mainly to SCFA’s rather than CH4. Thus each cycle reduces the amount of CO2 being cycled (and thus the top graphic isn’t correct but still illustrates the general mechanism).

This is what happens with enteric methane. Other biogenic methane undergoes a similar cycle except methanogenesis happens via methanogenic archaea in the guts or digestive tracts of other animals, mollusks, insects, etc or with methanogenic archaea in soil or other anoxic (without oxygen) environments or even oxic (with oxygen) environments with some types of very common bacteria. Though as more biogenic methane is being emitted, more methane is being oxidized so atmospheric levels of methane stay relatively balanced.

That is the atmospheric loads stay balanced until thermogenic and other trapped forms of methane are extracted and released into the atmosphere. That extraction can be for coal, natural gas, fracked gas, coal bed gas, etc.(hydrocarbons -see below image). The thermogenic methane and CO2 from fossil fuels hasn’t been in the atmosphere for over an hundred million years. So it’s very old carbon stored in the earth, but newly released carbon in the atmosphere. The released CH4 also eventually breaks down to CO2. Though since there’s now more CH4 and the same amount of OH, the CH4 lasts longer in the atmosphere. So, per modeling, the CH4 lasts 7.3 to 13 years rather than 5.9 to 9.2 years before all that thermogenic CH4 was emitted (Holmes 2018). The CO2 directly emitted and created via hydroxyl oxidation often exceeds the photosynthetic capacity to fix this CO2. That excess compounds and increases atmospheric CO2 levels. Excess CO2 that exceeds the carbon cycle can take up to a thousand years to break down.

Understanding that different greenhouse gases have different lifespans and that different sources of greenhouse gases (biogenic versus thermogenic) impact atmospheric loads of both CH4 and CO2 differently was something just recently recognized by the IPCC in their latest 6th Assessment as follows in Chapter 7 on p. 122 :

And as follows from p. 123:

And as follows from p. 124:

One of the metrics recognized was GWP* developed by Dr. Myles Allen and his team at Oxford University’s Martin School. Allen served on the IPCC’s prior 3rd, 4th and 5th Assessments. All prior Assessments used GWP20 or GWP100 as the recognized metrics. These earlier metrics don’t account for the different lifespans or different sources of different green house gases. So as noted biogenic sources, especially enteric methane, have been grossly over accounted by a factor of 3-4 over a 20 year time horizon while methane emissions from “new” sources (old in the ground , but new in the atmosphere thermogenic emissions) have been under accounted for by a 4-5 factor during the same time frame using earlier metrics (e.g. GWP100) for carbon equivalencies in life cycle assessments [LCA’s].

GWP* is relatively new and doesn’t make for convenient memes or sound bites. So most discussions of methane don’t recognize that biogenic sources of methane are over-accounted and that thermogenic sources of methane are under counted. So most analysis is usually ass-backwards including the dismissal of thermogenic emissions from transporting food all over the world. Such emissions from transportation, though small in gross amount, like other thermogenic emissions, are old fossil sources of trapped methane that are “newly” added to the atmosphere.

Whereas (again) enteric methane from ruminants are a biogenic source. This CH4 may seem like a larger gross amount, but any methane ruminants burp replaces old methane ruminants burped 7 to 10 years previously. So ruminants are cycling and recycling the same carbon over and over again. And with soil carbon sequestration, each cycle can reduce or slow down the amount of carbon being cycled by fixing carbon into a whole myriad of other carbon compounds including mycorrhizal necromass, chitin, proteins, fats, lignin, etc. Each cycle can be further reduced by better feed conversion of cellulose to SCFA’s. Better feed conversion can be accomplished through feed additives (e.g. seaweed) or certain forages (Vazquez-Carrillo et al. 2020; Verma et al. 2022) that reduce the amount of methanogenesis occurring in a head of cattle’s rumen.

Note too that whether or not cellulose, other carbohydrates, fats or proteins goes through the digestive tracts and fore or rear guts of animals, all forms of carbon biomass breakdown at different rates back to CO2 directly or to CO2 from CH4 via hydroxyl oxidation. Plants in particular decompose, oxidize or burn. All three of these processes release either methane, carbon monoxide [CO] or carbon dioxide. Aerobic compost piles emit CO2. Anaerobic compost emits CH4. Burning biomass releases CO and CH4. Oxidized CO become CO2. Hydroxyl oxidized CH4 becomes CO2. When cellulose goes through a rumen, the rate of carbon (and nutrient) cycling increases. In semi-arid and arid environments, decomposition of plant material through a rumen is essential to cycle carbon and nutrients. Journalists residing in humid places like the UK, Cape Cod and Miami never seem to understand how brittle ecosystems with seasonal (rather than year round) rainfall work.

Production methods and context also matter quite a bit. Cattle finished in an AMP managed pasture eating grasses without any inputs will emit a lot less total greenhouse gases than cattle finished in a feedlot fed feeds grown using a lot of synthetic nitrogen [Syn N] and other inputs (Stanley et al. 2018). Syn N takes a lot of energy to make via the Haber Bosch process (to split atmospheric dinitrogen N2) that releases a lot of CO2 and uses thermogenic methane (natural or fracked gas) as the source for hydrogen (H2 for NH3). Extraction and transport of the thermogenic CH4 results in a lot of emissions and fugitive emissions of this thermogenic CH4 into the atmosphere. Syn N when applied to fields, like other forms of inorganic nitrogen is NOT well utilized by plants. Plus Syn N compacts soils. So a lot of Syn N volatilizes as nitrous oxide [N2O] into the atmosphere or leaches/run-offs as nitrates [NO3] into waterways where it causes algae blooms and hypoxia (dead zones in rivers, lakes and oceans).

But problems with synthesis and application of synthetic nitrogen are subject matter for another future blog.

References:

Bizic, M 2021. Phytoplankton photosynthesis: an unexplored source of biogenic methane emission from oxic environments

Xu, H et al 2020. Underestimated methane production triggered by phytoplankton succession in river-reservoir systems- evidence from a microcosm study

Whitfield, C.J. et al. 2015. Beaver-mediated methane emission: The effects of population growth in Eurasia and the Americas

Ezhumalai, R. 2021 Emission of Methane from Dead Trees/snags of tropical and subtropical forest Ecoregions

Ernst, L. et al 2021. Methane formation driven by reactive oxygen species across all living organisms

Lakhani, N et al. 2017. Methanogenesis: Are ruminants only responsible: A review.

Holmes, C. D. 2018. Methane Feedback on Atmospheric Chemistry: Methods, Models, and Mechanisms

Vazquez-Carrillo, M. F. et al. 2020 Effects of Three Herbs on Methane Emissions from Beef Cattle

Verma, S. et al. 2022. Linking metabolites in eight bioactive forage species to their in vitro methane reduction potential across several cultivars and harvests

Stanley, P. et al 2018. Impacts of soil carbon sequestration on life cycle greenhouse gas emissions in Midwestern USA beef finishing systems

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