A smaller proportion of black carbon created during combustion will remain in soil than have been estimated before. Contrary to previous understanding, burying black carbon in the ground in order to restrain climate change will not create a permanent carbon reserve.

Instead, a part of black carbon will dissolve from soil to rivers. The flux of dissolved black carbon from the rivers to the ocean was estimated in a research article published in Science.

The burning of organic matter creates 40 million tons of black carbon every year. Black carbon is formed through the incomplete combustion of organic matter, e.g. in forest fires, slash-and-burn and controlled burning of fields. The general assumption has been that black carbon would remain in soil even for millions of years.

However, recently published research indicates that a remarkable proportion of black carbon in soil will dissolve to the water system. In the light of new research results, much discussed "bio-carbon" may not be that beneficial in terms of mitigating climate change. Carbon is given the prefix "bio" when it is used both for energy production and soil enrichment. In any case, the stability of carbon in soil has been a central factor of bio-carbon applications.

By sampling rivers all around the world, the researchers estimated that the annual amount of black carbon flowing via rivers to the ocean is 27 million tons per year.

"Each sample included a significant amount of black carbon," says a research participant Anssi Vahatalo, Senior Lecturer from the University of Jyvaskyla.

"On average, the amount of black carbon was ten per cent of the amount of dissolved organic carbon. The results prove that the proportion of water soluble black carbon may be as much as 40 per cent of black carbon created annually.

Water samples from the largest rivers in the world
The basis of the research was the 'Big river'-project started by Senior Lecturer Anssi Vahatalo while he was working as an Academy Research Fellow at the University of Helsinki before moving to the University of Jyvaskyla. For this project, water samples were collected from the ten largest rivers in the world.

"These rivers carry one third of fresh water running to oceans, and their catchment area covers 28% of the whole land area in the world. Water samples were taken, e.g. from Amazon, the largest river in the world," says Vahatalo.

In addition to the samples used in the river project, the research published in Science was supplemented with samples from many other rivers all over the world. The total number of researched samples was 174.

Global Charcoal Mobilization from Soils via Dissolution and Riverine Transport to the Oceans; Science 19 April 2013, Vol. 340 no. 6130 pp. 345-347, DOI: 10.1126/science.1231476; Rudolf Jaffe, Yan Ding, Jutta Niggemann, Anssi V. Vahatalo, Aron Stubbins, Robert G. M. Spencer, John Campbell, Thorsten, Dittmar