Have you ever wondered about the invisible cycle that sustains life on Earth? It's a delicate dance between the atmosphere, living beings, and the soil beneath our feet. Every breath we take, every tree that sways in the wind, every creature that burrows underground – they're all part of a complex choreography, with carbon as the leading player.
Carbon is sequestered in the soil through biological processes. The sequestration of carbon is a continuous process: plants photosynthesize and capture carbon from the atmosphere, but they also respire – a process almost every living being undergoes, in which carbon dioxide is a byproduct. But what does all this mean? Is carbon sequestering in the soil a good thing or a bad thing?
Carbon is present everywhere on our planet. According to the Deep Carbon Observatory (DCO), the Earth contains about 1.85 billion billion tonnes of carbon, with only 0.02% present in its crust. It's not an infinite resource, and carbon needs to be recycled and stored, much like the water cycle – there is also a carbon cycle. For decades, research has emphasized the importance of soil in agriculture and organic farming, stressing the need to naturally replenish minerals like carbon, nitrogen, and other essential micronutrients. However, this article focuses on the carbon cycle's significance in the soil ecosystem.
Carbon sinks
We all know carbon is essential to life – a major building block in our bodies, forming compounds like proteins, fats, carbohydrates, and energy stores. But how does it move through the environment? We cannot directly extract carbon from the air, so we need sources to trap it – carbon sinks.
A carbon sink absorbs more carbon from the atmosphere than it releases. By this definition, plants, soil, and the ocean are all carbon sinks. The opposite, a carbon source (like cars, planes, and boats burning carbon), releases more carbon into the atmosphere than it absorbs. This isn't necessarily harmful – carbon sources are essential for completing the cycle. However, human activities' scale could lead to drastic changes like global warming and disruption of pre-existing cycles.
When plants and animals die, organisms like fungi break them down and decompose them into their constituent minerals. These varied organisms use processes to break down cells and digest them externally, leaving carbon ready in organic forms to dissolve into the soil (Fig.1).
Fig.1 Carbon CycleCarbon cycle
The carbon cycle keeps carbon cycling through the atmosphere. Any living organisms have only "borrowed" from this cycle and will eventually become part of other organisms. Life leads to more life. In the soil context, fossil fuels are essentially carbon (hydrocarbon) reserves that can be utilized as energy stores. They represent carbon stored from organisms that died hundreds of millions of years ago, preserved by being buried in soil or underwater.
More readily available carbon sources for living organisms include wood, grasses and most plants. Since plants are sinks, they store carbon, which gets released into the soil as SOCs (Soil Organic Carbon). Humus – dead and decaying matter – is crucial because it stores carbon and other essential minerals. When these minerals are passed on to cows, humans, and other plants, they break them down and respire, a process that produces carbon dioxide as a byproduct. This atmospheric carbon dioxide is then exhaled into the atmosphere, and the cycle repeats.
But what does it have to do with soil?
According to the Global Forest Resources Assessment 2020, about 45% of carbon in forests is stored mostly in the SOM (Soil Organic Matter) and in living biomass (Fig.2). Only a small fraction is stored in dead wood and litter, meaning it's constantly being used by plants and soil organisms. Plants take in this carbon as SOCs. In agricultural areas, SOCs have been found to be lower than in natural ecosystems, as agriculture leads to the release of 50 to 100 Gigatons of carbon into the atmosphere. This is largely due to decreased plant roots and increased soil erosion. However, studies show that plants growing in higher CO2 concentrations will fix more carbon through photosynthesis, thus producing greater biomass.
Increased atmospheric CO2 also leads to global warming, reducing soil water availability and limiting photosynthesis. The increased temperature also accelerates SOM decomposition, releasing more carbon from the soil into the atmosphere, creating a feedback loop where higher temperatures lead to more SOM decomposition.
Without soil's carbon sequestration, we would have much higher atmospheric CO2 levels, making it vital to protect soil and forests – not just the number of plants and animals, but also their diversity, which plays a role in how much carbon is stored in the soil. When we plant crops, we create a debt.
Soil is vital to our survival. Research shows we can even try using agricultural land as a carbon sink by planting perennial crops with deeper roots that help the soil store more carbon.
Soil can't hold on to carbon indefinitely, but it can keep it for some time in rotation – forests can grow more, develop more niches for plants, and develop whole ecosystems by increasing the amount of trapped biomass.
Fig.2 Proportion of carbon stock in forest carbon pools, 2020Heading 1
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