Re: Does burning biomass or letting it decompose emit the same amount of CO2?

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!