The charcoal residue created in such fires is eventually washed out of the soil of the forest floor and transported to the sea by rivers and thus enters Earth's carbon cycle.
Scientists at the Max Planck Institute for Marine Microbiology in Bremen, Germany, and Florida International University report their studies demonstrated soluble charcoal accounts for 10 percent of the total amount of dissolved organic carbon in the world's oceans.
"Most scientists thought charcoal was resistant. They thought, once it is incorporated into the soils, it would stay there," Florida researcher Rudolf Jaffe said. "But if that were the case, the soils would be black.
"From a chemical perspective, no one really thought it dissolves, but it does," Jaffe said. "It doesn't accumulate like we had for a long time believed. Rather, it is transported into wetlands and rivers, eventually making its way to the oceans."
The researchers said they estimate about 25 million tons of dissolved charcoal is transported from land to the sea each year.
The findings, they said, are important for better calculating the global carbon budget, a balancing act between sources that produce carbon and sinks that remove it. Such calculations are needed to assess global climatic effects and find ways to alleviate them, the researchers said.