Yan Li’s research team sampled water from a salty aquifer under the Tarim basin – a desert in north-west China – and from a glacier and river that supply it. Using carbon dating, they were able to produce a timeline showing the rate of carbon’s arrival in the groundwater over human history. They found that this rate rose dramatically, by more than a factor of 12 over the past 8000 years (Geophysical Research Letters, doi.org/6kf).
We already knew that crops suck carbon dioxide out of the air and release it into soil. Normally it would then escape back into the air. But the sheer amount of water needed in arid farming dissolves the CO2 and deposits it in aquifers, Li suggests.
If the process occurs elsewhere, as Li expects, there will be about a trillion tonnes of carbon stored in saline aquifers – 25 per cent more than in living plants.
For decades, global carbon budget accounting has identified a “missing” or “residual” terrestrial sink; i.e., carbon dioxide (CO2) released by anthropogenic activities does not match changes observed in the atmosphere and ocean. We discovered a potentially large carbon sink in the most unlikely place on earth, irrigated saline/alkaline arid land. When cultivating and irrigating arid/saline lands in arid zones, salts are leached downward. Simultaneously, dissolved inorganic carbon is washed down into the huge saline aquifers underneath vast deserts, forming a large carbon sink or pool. This finding points to a direct, rapid link between the biological and geochemical carbon cycles in arid lands which may alter the overall spatial pattern of the global carbon budget.