In the past few years, many agronomists have promoted the sequestration of carbon in soils as a means of mitigating climate change by reducing the amount of carbon dioxide that accumulates in the atmosphere. For a long time, farmers have realized that soil organic matter improves the quality of soils, by improving water-hold capacity, friability, and nutrient content. But, however we do it—fertilization, irrigation, manuring, reduced tillage, and planting cover crops—increasing soil organic matter doesn’t look like such a good idea to mitigate climate change. The chemistry of humus substances includes a substantial amount of nitrogen and phosphorus that will limit our efforts.
One group has set an aspirational goal to increase the total amount of soil organic matter globally by 0.4% per year—what is often called the 4 per mille initiative. That amount would soak up the total carbon dioxide emitted from fossil fuel combustion each year.
But a couple of simple calculations show the futility of this approach, based on the nitrogen and phosphorus content in soil organic matter. Carbon sequestration in soils requires outside sources of nitrogen. With fossil fuel emissions of 9 x 1015 gC each year, and a 15-to-1.0 ratio of carbon to nitrogen in humus, the sequestration of carbon emitted from fossil fuel combustion would take 600 x 1012 g of nitrogen each year. For comparison, current production of nitrogen fertilizer is about 150 x 1012 gN/yr. Ignoring the current environmental problems associated with excessive use of nitrogen fertilizer, we’d need to use 4 times as much as today, just to sequester our fossil fuel carbon dioxide in soils. Even more N would be needed to support crop growth.
When nitrogen fertilizer is applied a small percentage—1 to 2%–is converted to nitrous oxide, which escapes to the atmosphere, where it acts as a powerful greenhouse gas. A rate of 1%/yr would add ~6 x 1012 g of nitrous oxide to the atmosphere, doubling the current emissions from agricultural soils globally and negating nearly all of the benefits of additional carbon storage in soil organic matter.
Similarly, for phosphorus, sequestration of 9 x 1015 g C each year would require 37 to 75 x 1012 gP/yr as fertilizer, compared to current global production of P fertilizer—about 34 x 1012 gP/yr. Phosphorus supplies are not unlimited, and we’d need to more than double our current use of P fertilizer to accommodate carbon sequestration in soils.
Moreover, the production of nitrogen and phosphorus fertilizer is not without its own fossil fuel emissions. For instance, the production of nitrogen fertilizer currently accounts for about 3% of fossil fuel emissions.
Soil organic matter is helpful to farmers, and it sequesters carbon. But, purposeful carbon sequestration in soil organic matter will require huge amounts of additional fertilizer production, with its own contributions to fossil fuel emissions and contamination of surface and groundwater.
There is a better way. Trees store carbon with much less nitrogen; the C/N ratio is wood is often 150 or more. Essentially you get 10 times the carbon storage per unit of nitrogen when the carbon is stored in wood versus soil organic matter. Increasing forest biomass provides a great storage capacity for atmospheric carbon dioxide. And, of course, wind, solar and tidal power emit no carbon dioxide to the atmosphere to start with.
What are we waiting for?
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Spohn, M. 2020. Increasing the organic carbon stocks in mineral soils sequesters large amounts of phosphorus. Global Change Biology 26: 4169-4177.