The mounting pressures of global warming have brought mitigation strategies to the forefront of atmospheric research, but a growing number of experts are searching below the Earth’s surface. Some suggest that the world’s soils can trap and remove enough carbon dioxide from the atmosphere to offset a significant portion of fossil fuel emissions thanks to the entirely natural, inexpensive process of sequestration. But many scientists have expressed mixed opinions on this optimistic approach.
All soils contain varying quantities of organic matter, or any material derived from a living organism. This could be lawn clippings and branches or sawdust and earthworms, including everything in between. Microbes are an essential element of organic matter, decomposing dead material and providing nutrients back to the soil. Resistance to decomposition is used to designate three basic categories: living organic matter (e.g., plant roots and soil fauna), dead organic matter (e.g., manure and residues), and long-dead humus. Humus is organic matter held within soil aggregates, inaccessible to microbes, whereas plant debris is readily decomposed.
Because it consists primarily of carbon, organic matter is commonly referred to as soil organic carbon. It collectively forms a huge pool in the carbon biogeochemical cycle, larger than both the atmosphere and all biomass on Earth.
Soils store, or “sequester,” atmospheric carbon dioxide by way of organic matter. As plants perform photosynthesis, they assimilate carbon into their roots, stems, and leaves, in turn supporting the entire food web of other carbon-storing organisms within the soil. Sequestration is balanced by the carbon dioxide respired and released by microbial communities as they decompose dead organic matter. As a result, soil organic carbon increases when photosynthetic inputs exceed the outputs of respiration.
Soils are estimated to counteract nearly a quarter of fossil fuel emissions each year, yet millennia of cultivated agriculture have caused a massive reduction to this stock. Soils around the world have lost upwards of 70%, or 116 gigatons, of the carbon they once held due to the destruction of natural ecosystems for homogenized croplands. Deforestation, coupled with poor farm management practices, has accelerated the microbial release of carbon dioxide at the expense of plants that could have structurally assimilated carbon.
Despite their historic losses, modern croplands have great potential as a carbon sink. Soil carbon sequestration is cheaper and requires less energy than other climate change mitigation strategies, offering ecological benefits as well.
With proactive management, croplands alone can store an additional 1.85 gigatons of carbon annually. Crop rotation or cover crops, for example, add biomass of diverse decompositional resistance to empty fields. The resulting year-round photosynthesis and staggered microbial respiration allows soils to store additional carbon with each passing harvest. Other practices like reduced tilling and erosion control minimize the chance of breaking soil aggregates, which, when exposed, are otherwise susceptible to microbes. Further tactics apply manure, compost, or biochar, a natural charcoal substance, to enrich soils with important plant nutrients while prioritizing sustainability.
More intensive sequestration undertakings focus on ecosystem restoration. Planting efforts reintroduce native vegetation, which is better adapted to take advantage of photosynthesis and grow larger than their non-native counterparts. The creation of wetlands, whose waterlogged conditions reduce decomposition rates, also yields net soil carbon sequestration.
Biogeochemical cycles are highly complex, posing challenges for scientists who seek to predict detailed carbon fluxes with certainty. Consequently, the prospects of soil carbon sequestration are not yet fully understood. Outside of farm management practices, soil organic carbon is impacted by factors like climate, topography, parent rock material, and the organisms which inhabit it. Although some scientists suggest that soil carbon can be returned to croplands, other researchers believe this idea is a flawed, optimistic dream.
It is inherently difficult to encourage billions of agricultural workers worldwide to adopt new farming practices. Many are hesitant to heed the advice of scientists, or are downright distrustful of incentives and regulations from government authorities. Soil carbon sequestration has reached a difficult cultural crossroads between scientific research and local farmers, in which neither show particular interest in walking the path of the other.
Even still, economic barriers impede agricultural modernization. Farmers predominantly lack the resources required to implement climate-smart practices and purchase the necessary equipment. In the United States, the majority of croplands are rented instead of bought, lending even less motivation to farmers seeking to invest in long-term soil carbon solutions. Few farms are supported under subsidized soil programs, while the larger, influential farms are excluded due to their size. Seeing as such problems exist in one of the wealthiest countries in the world, harsher challenges face soil carbon sequestration initiatives in developing nations.
Regardless of whether every farmer agrees and is financially willing to pursue soil carbon sequestration, its physical possibility remains questionable. Emerging discourse in the scientific community implies that potential soil carbon sequestration rates are overestimated. Rather, studies show that widespread farm management changes must be accelerated if they are to contribute to climate change mitigation, as they may offset just 5% of annual carbon dioxide emissions at present.
More troublesome, however, is a dangerous and self-reinforcing feedback loop between global warming and soil microbes. Between rising temperatures and deforestation, microbial decomposers break down soil organic matter faster than plants can assimilate carbon into the soil, thereby increasing their respiration. When carbon dioxide is released, its properties as a greenhouse gas hold additional heat in the atmosphere, starting the cycle anew. The study of this process, albeit recent, highlights the flaws of considering soils as central to combating climate change.
The practicality of sequestration as a global mitigation strategy is limited, and prioritizing it may distract from the underlying issue of fossil fuel emissions. Comparably, other mitigation strategies have more tangible success; investment in renewable energy and land conservation, for instance, may be of greater importance in tackling the climate crisis.
Better farm management practices have a multitude of benefits outside of carbon sequestration, like increased agricultural productivity, groundwater storage, or climate resilience, and emphasizing soil health overall could have the best societal impact. Although crucial to all aspects of life, soils may not be the anticipated catchall solution to climate change that scientists once thought they were.
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