Veggie Is The Most Low-Carbon Diet, Right? Well, It Depends Where You Live

It is often claimed that a vegetarian diet is better for the environment, because grazing animals such as cattle and sheep produce a lot of methane, a far more potent greenhouse gas than carbon dioxide.

The areas needed for livestock grazing can also be much larger than those used for harvests to make an equivalent quantity of food, so more land is cleared for meat than harvests, which causes more carbon to be lost from the landscape.

But wait. As is often the case with complex environmental cycles, particularly those altered by human activities, this is only part of the story. While it is true that ruminants emit a lot of methane, and this is currently the greatest slice of the agricultural emissions pie, it is also true that these are not the only emissions associated with human agriculture.

Cropland generally use more inorganic fertiliser than grassland, which means that the more plants you eat, the more of your greenhouse footprint comes from nitrous oxide another potent greenhouse gas linked to use of industrially created fertiliser.

Unfortunately, this means that sticking to a climate-friendly diet isnt always only a matter of giving up steak and lamb chops. You also have to consider the clay forms and farming practises in the places where your food is created. And the bad news for Europeans is that eating meat is much harder to justify than it is in Australia, for example, where livestock tends to be less intensively farmed.

Emissions And Soils

Nitrous oxide emissions come from the turnover of nitrogen compounds in the clay, which in turn come from both organic materials( manure, soil organic materials) and synthetic fertilisers( primarily inorganic nitrogen ).

This means that the biggest greenhouse impact would come from feeing livestock animals that are disconnected from the clay, kept in barns and fed on crops( for instance, beef cattle fed on corn snack) rather than extensively grazing on pastures. This represents a climate double whammy because the harvests lead to nitrous oxide emissions and the animals then create methane.

The other greenhouse gas to consider is, unsurprisingly, carbon dioxide. Healthy soils contain lots of organic matter, which helps to reduce eroding, boosts water storage capability( and therefore drought resilience ), and acts as a storehouse for nutrients( thereby reducing the need for fertiliser ).

When land is cleared for agriculture, the amount of soil organic matter can decline dramatically. And because carbon constructs up around 50-55% of soil organic matter, this land clearing is not merely depletes soil health but releases greenhouse gas, as the soil organic carbon is converted to carbon dioxide and released.

Soil organic matter can be restored by plants, which take up atmospheric carbon dioxide as they grow. When they die, their biomass is then( partly) incorporated into the clay and converted into clay organic matter.

So Does Farming Help Soils ?

The soil organic carbon pond is the largest land-based carbon store and the most dynamic globally of the non-living carbon ponds. There is at least twice as much carbon stored in the worlds soils as there is in the atmosphere.

So planting crops to store more carbon sounds like an attractive idea. Regrettably, however, cultivated clays contain up to 70% less soil organic materials than natural clays, so croplands are actually a net greenhouse emitter.

Conversely, soils used for grazing animals have much higher soil organic matter content than in cropped systems, and approximately the same sum as natural soils. This is probably because many grazed systems are permanent grasslands, where plants constantly grow and add to the clay carbon pond( even after the animals have eaten their fill ).

But this distinction is not captured by official figures from the Intergovernmental committee of experts on Climate Change( IPCC ), which only reports non-CO emissions from agriculture, and assumes the CO emissions from agriculture to be net zero( CO emissions due to clay carbon loss appear in the Forestry and other land use category ).

This means that the greenhouse emissions due to harvests, and carbon storage in grassland lands, may both be underestimated. This matter is highlighted by our research, which shows that carbon losses from cropped clay extend far deeper than previously believed.

Previous calculates assumed that only the topsoil( generally the top 30 cm) was affected, but we have shown that, in Australia at least, this is not the case the lower carbon content of cropped soil is detectable all the route down the soil profile. We also found that, at these deeper depths, natural and grazing soils contained very similar amounts of carbon.

As if that were not all complicated enough, there is yet another factor: when livestock manure is returned to the clay, that is something that boosts soil carbon, making for healthier soils and partially offsetting the animals’ greenhouse emissions. Declining utilize of animal manure on European crops has been associated with a decrease in soil carbon storage.

Food For Thought

So what does this all mean? Well, 90% of our energy intake comes directly from the soil, so agricultural practices obviously got a big consequence on soil health. If you care about conserving clays as well as minimising your greenhouse emissions, its not as simple as just going vegetarian.

Grazing animals can be good for soils, even though their methane emissions are bad for the atmosphere. Working out where the balance sits is a fiendishly tricky question. This is because agricultural emissions are related to individual site factors( such as climate or soil type) as well as agricultural practices( such as fertiliser regime or grazing intensity ).

Perhaps the best approach is try to source your food from local suppliers( to reduce your food miles) who do not use intensive agricultural practices( such as frequent tillage or indoor mass-rearing of animals ).

If you feed meat, choose free-range, grass-fed animals instead of those fed in barns use food from harvests. Get to know how your food is produced, and prefer the most sustainable alternatives, whether meaty or not. Small selections can help to save our soils.

Eleanor Hobley, Postdoctoral Fellow, Technical University of Munich and Martin Wiesmeier, Researcher, Technical University of Munich

Read more: www.iflscience.com

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