|MadSci Network: Environment & Ecology|
Alan – Chemically speaking, combustion (the burning of biomass in the presence of oxygen) and respiration (the breakdown of organic material for energy) are very similar. The both take organic carbon and combine it with oxygen to generate energy and CO2. The difference is primarily the speed with which this process occurs. In the combustion of biomass, virtually all of the carbon (C) in the biomass is converted immediately to CO2 (plus other hydrocarbon gasses). The decomposition of wood and leaf litter, as you know, takes longer. Scientists use a number called the turnover time to represent how quickly organic matter breaks down in a given environment. It represents the time it takes, on average, for most of the organic matter to break down. In Germany, the turnover time for organic matter is probably about 4-10 years. In a tropical rainforest, where I work, it is about 1 year – leaf litter that falls in the dry season is mostly gone by the start of the next dry season. If you want to get slightly more complicated, the time to break down organic material varies based on the type of material. Cellulose, which makes up plant cell walls, breaks down fairly quickly while lignin, which makes leaves tough, takes much longer. But the important point is that microbes and fungi eventually break down all of the organic material and turn it into CO2. This means that most of the C in a log will end up in the atmosphere eventually. It will take minutes if you burn it or decades if you let it decay (although a small amount of the organic material may take hundreds to thousands of years to break down). Of course, our current problem with CO2 in the atmosphere doesn’t have to do so much with the amount of CO2 we emit but rather the rate at which we do so. Clearly, burning large quantities of biomass will increase the CO2 in the atmosphere much more quickly than the slow breakdown of wood in a forest (which is also balanced by the CO2 uptake of the living trees). As with most of our options for energy in the future, biomass will have to be balanced with other technologies like solar, wind and hydroelectric (plus energy conservation!). Great question!
Try the links in the MadSci Library for more information on Environment & Ecology.