Amanita fulva from long-term experiment at Harvard Forest.Photo: John Rousk

When a plant invests carbon in its roots to gain nutrients, its growth is restricted by, for example, nitrogen and phosphorus, and carbon leaking as dissolved organic matter in the soil. When a plant dies, leaves, branches and other plant material fall to the ground. To keep the mineral that limits plant production in circulation, there is thus a constant need for reacting organic material.

The decomposition of organic matter in the soil is carried out mainly by fungi and bacteria, which convert dead plant material to recycled mineral nutrients and carbon dioxide. During the past century, scientists have concluded that organic quality materials, such as plant leaves that are high in soluble sugars, are mainly broken down by bacteria. Materials with lower quality, such as cellulose and lignin in wood is broken down mainly by the fungus. Furthermore, research has previously shown that the organic material broken down by fungi leads to less leaching of nutrients and carbon dioxide than if bacteria had broken down the material. If there is more carbon dioxide and nitrogen produced by the same quantity of decomposed material, this means a direct effect on eutrophication, and the soil's contribution to the greenhouse effect due to decomposition of organisms. 

In a long-term experiment that has been going on for 23 years, researchers from Lund University and the US examined the significance of fungal and bacterial-dominated degradation. 

• In sharp contrast to expectations that there was no connection between the organic material of high quality and bacterial contribution to degradation. Furthermore, there was a strong suggestion to the contrary, says John Rousk, researcher in Microbial Ecology at Lund University. 
• There was also no evidence that the organic material  broken down by fungi reduced leaching of soil carbon to the atmosphere as carbon dioxide, or the leaching of nutrients to eutrophication. Again, there was rather a tendency to the contrary, Rousk said. 

The researchers' results show that the basic understanding of how microorganisms control the turnover of carbon and nutrients in the soil needs to be revised. The discovery has significant implications for the development of climate models that would integrate the microorganisms to be able to estimate how climate and environmental changes affect nutrient cycles. The development of climate models is currently steeped in the notion that bacteria and fungi break down the material in totally different ways and with different power. 

The study has been published in the scientific journal Ecological Monographs.