Soil seems so simple—the stuff of childhood (mud pies taste a bit gritty), the smell after a rain, dark-crested fingernails after a hard day in the garden and stains on pants. Dirt. But it turns out that soil is in fact quite complicated, and a lot more interesting. And in this spongy matrix beneath our feet, made of mineral and of organic, of water and of space, an entire unseen world toils away.
Soil micro-organisms, in their work as decomposers, use carbon compounds in the organic matter in soil for energy—the same process as higher life forms like us—and they breathe out carbon dioxide. In fact, soil respiration can tell us a lot about the activity of microbes below ground: When conditions for microbial life improve (abundant nutrition, adequate moisture, warm temperatures), decomposition and therefore soil respiration increases.
So, you may be asking, “If all of these microbes are actively decomposing organic matter below ground, how does carbon even get stored in the first place?” It can build up in the soil precisely because the organic matter inputs exceed the outputs: the conditions below ground typically aren’t optimal for microbial decomposition.
I should point out that an impressive amount of carbon is stored in soil, nearly three times as much as that in the atmosphere and in living biomass combined. In light of the role of carbon gases in enhancing the greenhouse effect, we must pay particular attention to human activities that can result in the release of this massive store of carbon.
The impact of clear-cutting on forest soil has been debated for decades—largely inconclusively until now. Critical and timely information has been obtained by digging deeper than the typical soil surveys, down into the mineral soil to 50 centimetres. Over a decade of research carried out in the earth sciences department at St. Francis Xavier University has uncovered evidence that mineral soil organic matter, previously thought to be stable and resistant to microbial decomposition, is actually vulnerable to loss following disturbance.
One study in a set of sites in the Abraham’s Lake area in central Nova Scotia found that there was a 50 percent reduction in carbon storage following the clear-cutting of spruce forests, with the soil containing the lowest mineral carbon contents after 30-40 years. A second study in nearby Mooseland confirms this result, with a more modest 27 percent lower carbon storage at the 35-year-old site. Similar results have been obtained by research teams in New England and across Europe. In fact, there is now enough compelling global evidence to warrant a strong statement: intensive forest harvesting threatens soil carbon storage.
And it’s not just a carbon story. Soil erodes into streams following intensive forestry practices, and nitrogen and mineral nutrients are leached through the groundwater in disturbed watersheds. Research is also showing that these losses continue for years following clear-cutting and requires a long time (over 80 years) for soil to recover.
This evidence suggests that clear-cutting forests for energy production cannot be considered “green” nor “renewable.” This practice threatens not only our global climate (through enhanced greenhouse gas emissions from soils and waterways), but also affects the long-term productivity of our forests by degrading soil, which has serious implications for a sustainable forest industry in our province.
More and more of us now realize that healthy forests and sustainable livelihoods depend on healthy soil. This Earth Day, take a moment to reflect on its power and its vulnerability. As Franklin D. Roosevelt said, “A nation that destroys its soils destroys itself.”