Soil carbon | Why it is such an important environmental issue

Soil carbon may make all the difference when we have so many mouths to feed. 

But before we get to the reasons for such a bold assertion we need some background.

Soil is complex material. Because it leaves dirt under our fingers and is gritty we think of it as mostly mineral and lifeless. In fact, soil is a biological wonderland full of organisms from amoeba to ants. And, of course, the matrix where plant roots grow in and around the smallest soil spaces to secure nutrients and water for the plant.

Even the damp, cold soil under a field of winter wheat contains biodiversity on par with a tropical rainforest.

But because we need electron microscopes to view most of the tiny critters and fancy chemistry to understand the variety of all the microbes, most of this life goes about its business unseen.

Soil carbon is both the fuel and the consequence of all this biology.

Soil carbon and organic carbon

Carbon in soil comes primarily from the biological activity of plants fixing carbon dioxide from the atmosphere into plant tissue through photosynthesis. In these days of attention to atmospheric carbon we sometimes call this pulling of CO2 into vegetation and soil via the action of plants biosequestration.

The carbon from biosequestration can be short lived in soil as plant tissue dies and is prone to decomposition aided by the activity of the many soil organisms. This is the normal, natural cycle of nutrients and carbon within soil that maintains plant productivity. Without it the plants would drown in their own dead leaves and fine roots.

Soil carbon also includes more stable carbon that can persist in the soil for millennia. This is the carbon from certain types of plant material, such as lignified wood in tree trunks, and the inorganic charcoal and ash from wild fires. This so-called ‘recalcitrant carbon’ is an important long-term pool of carbon in the global carbon cycle.

Organic carbon is the soil carbon that comes from biological activity. It includes

  • plant roots
  • root exudates
  • soil microbial biomass (bacteria and fungi)
  • soil animals
  • dead organic material from plants (roots, leaves and twigs)
  • dead animals (mostly among those living in the soil)

The non-living part is usually called humus and is what gives soil a dark colour.

When a soil is ‘healthy’ it will

  • absorb and hold moisture,
  • be biologically active so that nutrients are made available to plant roots,
  • have good structure to allow roots to penetrate and so provide a stable base for plants to grow toward the light
  • and together these properties sustain predictable plant growth.

Healthy soil has consistent proportions of inorganic and organic carbon because the inorganic carbon helps provide the physical structure and the moisture and nutrient exchange abilities, while the organic carbon fuels the biological activity.

Both are critical to maintain plant growth.

Soil and agriculture

It is trite to say that soil is the foundation of agriculture given that humans have cultivated soil to grow crops for 10,000 years.

What is less obvious is that this cultivation tends to decrease the carbon in soil. When natural vegetation is cleared and the soil ploughed for a crop the carbon in soil is exposed. The organic parts tend to dry out and oxidize and the loss of moisture means than many of the organisms die out too.

Soil scientists believe that over time there has been a net loss of carbon of at least 42 gigatons (154 gigatons CO2e) as a result of global agricultural activity. This is nearly 5 times the total greenhouse gas emissions from human activity [to 2008].

In most cases lost carbon also means loss of structure and biological activity that makes nutrients scarce. Farmers learnt to subsidize this loss by adding fertilizer. At first this involved manure from the newly domesticated livestock and more recently [in the last 200 years of the 10,000] as inorganic fertilizers manufactured on industrial scale.

Today many farms are input-output systems where in order to produce a crop the farmer must ensure that he adds sufficient fertilizer to the soil as an input.

This brings us to…

Important reason #1

Carbon in soil is vital to maintain farm productivity.

This reason will become acute as demand for food increases and as the cost of farm inputs rises. Ask a farmer what he thinks about oil at $200 a barrel and he may well swear at you because he knows that the price of the fertilizers he buys are tied to the price of oil.

soil carbon | important for food production

Carbon sequestered in soil

The good news is that although farming over 10,000 years has denuded many soils of their carbon it is possible to put some of it back. 

Sensible management of vegetation and tillage can replace up to 60% of lost carbon in many agricultural soils.

As an example soil scientists in Australia have estimated that in one state, New South Wales, prudent land management could sequester 4.9 Mt C/yr (18 Mt CO2e/yr) or roughly 10% of the annual greenhouse gas emissions from the state.

And, globally, through

  • woodland regeneration
  • no-till farming
  • cover crops
  • nutrient management
  • manuring and sludge applications
  • improved grazing
  • water conservation and harvesting
  • efficient irrigation
  • agroforestry
  • use of soil amendments (such as biochar)
  • there is the potential for soils to accumulate carbon sufficient to offset at least 5% of global fossil fuel emissions.

And this gives us…

Important reason #2

Soil carbon is an important offset for fossil fuel emissions

Although if we if we did manage to add carbon back into the soil and offset emissions the real benefit will be the 10-20% improvements in crop production for we can live with global warming but we cannot live without food.

Pragmatology | where is the environmental issue?

The environmental issue is that these two very good reasons to look after carbon in soil are not always heeded. 

Estimates are that in some 15.8 million km2 of agricultural land the soil is considered degraded with a measurable loss of productivity potential.

In a world where the current annual production of over 3.5 billion tons of food will need to double by 2050 just to keep everyone fed to be degrading land and soil seems careless at best.

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