Is deep soil carbon the Missing Sink?

Deep soil carbon doesn't just add wads to what farmers could be paid. It could add precious gigatonnes to the global inventory and could explain the missing gigatonnes (i.e.. the hidden sink soaking up CO2 - scientists have proven it exists by maths, but no one has found it yet.)

Here's the mathematics: Each year we dump 8.5 billion tonnes of carbon into the atmosphere, 6.3 billion tonnes from fossil fuels and 2.2 bilion tonnes from deforestation. This is greater than the sum of the annual accumulation of carbon in the atmosphere (3.2 billion tonnes) plus the annual uptake by the oceans (2.4 billion tonnes) which is only 5.6 billion tonnes. The difference of 2.9 billion tonnes (i.e. 8.5-5.6=2.9) is the unknown carbon sink required to balance the carbon budget.

The Woods Hole Research Center calls it the Missing Carbon Sink: “Strangely, the difference between the net terrestrial sink and the emissions from land-use change suggests that there is a residual terrestrial sink, not well understood, that locked away as much as 3.0 billion tonnes C yr-1 during the last two decades. The exact magnitude, location and cause of this residual terrestrial sink are uncertain…” (1) (Biologist (2002) 49 (4))

Some scientists have argued that the mystery sink was peat bogs in Northern Europe. Others believe it to be Tropical Forests.  No one thought to look at soils: after all, science told us soils don’t have the capacity: “According to a recent global analysis based on changes in the concentrations of carbon dioxide and oxygen (Plattner et al., in review), the world’s terrestrial ecosystems (vegetation and soils) were a small net sink during the 1980s and 1990,” said Woods Hole. But here we have Harper and Tibbett recording carbon levels two to five times greater than would be reported with sampling to a depth of 0.5 m.(2)

 One bright spark in CSIRO was saying as much 20 years ago: “Atmospheric carbon budgets that ignore the possibility of terrestrial ecosystem responses to global atmospheric change do not balance; there is a 'missing sink' of about 0.4 - 4 Gt C yr^-1. … It is well within the bounds of possibility that increasing carbon storage in vegetation and soils in response to the globally increasing CO₂ concentration, temperature and nitrogen deposition can account for the missing C sink… Models of the global terrestrial C cycle indicate that an extra 0.5 - 4 Gt C yr^-1 could well be being stored in soils and vegetation today in response to the CO₂ fertilising effect... To obtain direct proof as to whether that this is happening or not is a major challenge.” (3)

Harper and Tibbett’s findings could just be the proof we need.

FOOTNOTES:

(1) R A Houghton, Terrestrialcarbon sinks: uncertain explanations, Biologist (2002)49 (4)

(2) R. J. Harper & M. Tibbett Thehidden organic carbon in deep mineral soils Plant & Soil (2013) 368:641–648

(3) RM Gifford, “TheGlobal Carbon Cycle: a Viewpoint on the Missing Sink” AustralianJournal of Plant Physiology (1994) 21(1) 1 - 15