A lot of folks wonder why I am not a big advocate of soil carbon sequestration to mitigate climate change. After all, it is widely recognized (and I thoroughly agree) that soils with lots of organic matter are good for plant growth, retention of water and nutrients, and soil health. Many farmers, who own their own land, practice no-till agriculture, plant cover crops, and apply manure to maintain soil organic matter at stable levels. Avoiding cultivation also saves money and fossil fuels used to power farmland equipment. In modern terms, these practices are win-win for farmers and the environment—maintain or restore soil carbon and reduce agriculture emissions to the atmosphere. We hardly need to subsidize them with government grants; no-till is now practiced on about 40% of agricultural land in the U.S.
Supplying food to the world’s population now accounts for about 1/3 of human emissions of greenhouse gases to the atmosphere, much of it from the loss of carbon content from soils converted to agriculture. Worldwide, soils have lost about 133 billion metric tons of carbon since first cultivation. The loss is often fairly rapid upon plowing virgin land. This historical loss is roughly equivalent to 12 years of today’s carbon dioxide emissions from fossil fuel combustion.
Some have suggested that we should restore soil organic matter by intensive management to increase soil carbon sequestration. Unfortunately, the rate of accumulation of soil organic matter is usually much slower than its loss, and substantial amounts of energy must be used to increase carbon sequestration in most soils. Thus, suggestions that we can mitigate the impact of fossil fuel combustion by increasing carbon storage in soils are bogus. We can strive to make agriculture a carbon-neutral activity, but not a net sink for atmospheric carbon dioxide from other sources.
One reason that soil carbon sequestration is slow stems from its nitrogen and phosphorus contents, which are also lost alongside soil organic carbon. To restore these nutrient stocks requires inputs of N and P from outside sources, at considerable energetic (and thus CO2) cost. While we can encourage the rapid cycling of P in soils, there is no good way to increase the overall content of P in soils beyond what is made available by rock weathering. Applications of nitrogen fertilizer result in ancillary emissions of nitrous oxide—arguably a greenhouse gas worse than CO2. Nitrous oxide emissions from soils also increase when N-fixing species are planted as an alternative to fertilizer. Nitrogen fertilizers substitute a host of environmental problems, and offer no net mitigation of greenhouse gas emissions from agriculture.
One recent paper suggested that with the large stock of carbon in global soils (about 2500 billion metric tons), we could mitigate fossil fuel emissions (10 billion metric tons) by increasing the soil carbon stock by only 0.4% per year. Sounds good, but the calculation assumes that all global soils could be involved, overlooking that vast areas of land are arid or unmanaged, and that large areas that are destined to warm in the coming years are likely to be sources, not sinks, of atmospheric carbon dioxide. Sequestration of carbon on the lands under active agriculture management would require rates exceeding 1000 gC/m2/yr to balance fossil fuel emissions—values never achieved in long-term field experiments.
So, when you hear of the Biden administration’s plans to encourage farmers to store carbon in soils as a climate change solution, be skeptical. We can and should try to reduce the emissions of carbon dioxide from agriculture, but a net uptake of carbon dioxide from other sources is just not in the cards.