Oorsig / Review
Each bite primes the carbon pump
Through the perfection of evolution , each time a cow takes a bite of grass it triggers the plant to release exudates in order to receive in return the nutrients needed for rapid regrowth . The grass ’ s rate of photosynthesis goes up , more grass ( biomass ) is grown and more carbon is removed from the air and pushed down into the soil . The action of grazing repeatedly pushes the grass back into it ’ s growth stage and as long as grass is actively growing , not senescing , carbon is being removed from the atmosphere and sequestered into the ground . In a positive feedback loop , this liquid carbon pathway supports a healthy underground ecosystem , building soil organic matter , creating humus and soils that are deep , well-functioning and fertile ( 3 ). When cows graze , the carbon cycle is boosted and each bite primes the carbon pump .
Roaming , grazing cows sequester carbon
Once you understand this process , it becomes clear that although the cow is breathing or belching as she grazes , for all the carbon she cycles through her metabolism , there is carbon also being removed from the air and sequestered into the soil . This means that , when she is exclusively fed her natural diet of grass , there is a net reduction in atmospheric carbon . Roaming , grazing cows sequester carbon .
There is much debate , though limited scientific evidence , about the size of the vast herds of herbivores that roamed the grasslands of the world up to pre-industrial times ( 6 ). But one thing is for sure : the hundreds of millions , or billions , of them were not causing any climatic problems . In fact , quite the reverse . By grazing and stimulating rapid photosynthesis , they , along with the herbivorous megafauna before them , played a pivotal role in the reduction of atmospheric carbon that brought CO2 down to pre-industrial levels ( 2 ). And when grazed appropriately , cattle will continue to do this .
Cows produce nutritious food powered by the sun
The beauty of working with natural systems in this way is that not only does a cow ’ s grazing stimulate and speed up carbon sequestration in the soil , but she also provides us with a nutritious by-product to her helpful environmental services in the form of meat or milk . And all achieved with no inputs other than the free , perennial energy source of sunlight .
But we can ’ t talk about ruminants without a bit more on methane , so let ’ s continue the journey into how carbon mooooves ….
Carbon cycling through methane
Another natural cycle occurs when carbon is used to produce methane in the metabolism of microbes known as methanogens . The main natural sources of methane are wetlands and forest fires as well as the guts of ruminants and termites . It is the same microbial action and associated release of methane that finalises plant decomposition , be it on the ground in the rotting of uneaten plant biomass or in the gut of an animal that is digesting plant matter , called enteric fermentation .
As discussed , what is important about methane being produced by microbes in a cow ’ s rumen rather than from microbes on the ground , is that the cow is also providing us with food by turning grass , something we can ’ t eat , into something highly nutritious that we can .
As with other natural cycles , the movement of carbon through methane and back to CO2 can take different lengths of time . The short route involves methanotrophs , methane oxidising bacteria , which have been shown in studies in Australia ( 7 ) to oxidise more methane than cattle produce . The action of methanotrophs also causes methane to diffuse from the atmosphere into soils to be oxidised ( 4 ). The slightly longer route is when methane in the atmosphere is oxidised by hydroxyl radicals back into water and carbon dioxide ( 8 ), which can take up to 12 years . The hydroxyl radical is nature ’ s main methane sink , oxidising 90 % of methane emissions .
It is critical to understand that methane , produced by methanogens either in boggy soils or the guts of termites or ruminants and then oxidised back to CO2 either by methanotrophs or by the hydroxyl radical , is part of the cycling and recycling of carbon . No new carbon is added to the system . While the number of cattle on the planet remains constant , there is no increase in atmospheric methane as a result of their enteric fermentation because nature ’ s methane sinks balance production .
Furthermore , cows kept on pasture convert into methane just 1 % of the carbon taken in by plants during photosynthesis ( 12 ). And whilst this may have a global warming potential 25 times greater than CO2 , it is still considerably less than the carbon sequestration benefits of well-managed grasslands previously noted ( 13 ).
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