Plants found to produce methane
After
carbon dioxide, methane
is the second most important anthropogenic greenhouse gas. In the past two centuries the concentration
of atmospheric methane has more than doubled. Rising concentrations are causing changes in climate and
contributing to global warming.
Total
annual emissions
of methane are about
600 million tonnes, split almost
equally between anthropogenic and natural sources. Anthropogenic methane contributors include mining
and burning of fossil fuels, digestive processes in ruminant animals such as cattle, rice paddies and
the burying of waste in landfills. To date known natural sources of methane include wetlands, termites
and oceans.
Biological production of methane up to now
was considered only
to occur
under strictly anaerobic conditions. However, AFBI scientists, in conjunction with researchers at the
Max Planck Institute in Heidelberg, Germany and The Institute for Marine and Atmospheric Research Utrecht
in The Netherlands, have discovered that plants produce methane under oxic conditions. These findings
were reported in the prestigious journal Nature in January 2006. This paper was the third most downloaded
paper from the Nature website during January 2006.
Based
on laboratory and some field experiments it is estimated that living plants release between 60 and 240
million tonnes of methane per year. Thus plants could account for 10-30% of methane found
in the atmosphere. Methane release by plants was found to increase with rising temperatures and solar
radiation. It was estimated that the largest emissions come from tropical regions because that is where
the most biomass is located.
Laboratory and field investigations
with dried
leaf tissue,
fresh detached leaves and intact plants were conducted. Laboratory experiments involved the incubation
of plant material in an atmosphere of methane-free air, in order to eliminate the high background of
methane normal to atmospheric air. Stable carbon isotope analysis of methane was employed to show
beyond doubt that this was an undiscovered process of methane production. Field experiments involved
incubation of intact plants in Plexiglass incubation chambers. Tissue from thirty species of plants,
from deciduous trees to grasses, were evaluated for methane release and the effects of temperature and
solar radiation on release rates were investigated.
Methane
release was observed
from
all plant material regardless of whether the leaf tissue was dry or fresh or if the plant was intact
or not. Emission rates were directly related to both temperature of incubation and solar radiation.
Living plants were found to produce the largest quantities of methane, with a considerable range of
emissions between the different species.
Methane emissions
measured in the
laboratory
and field experiments were scaled on a global basis relative to annual nett primary production, distinguishing
between various types of biomes, length of vegetation period and average daily sunshine hours. On an
annual basis living vegetation was calculated to release 62-236 million tonnes with the main contribution,
46-169 million tonnes, assigned to tropical forest and grasslands. Plant litter was considerably lower
in the range 0.5-6.6 million tonnes.
The implications
of this research
are significant
and may shed some light on a number of unexplained phenomena. These include the large plumes of methane
observed above tropical forests, the fact that rice fields emit less methane when there is less sunlight,
and the high levels of methane found in ice formed 2,000 years ago, when plants covered more of the
Earth's surface. Plants as a natural source of methane provides closure for the carbon isotope
mass balance of methane in the pre-industrial atmosphere, a finding of considerable importance to atmospheric
chemists when modelling past and future climates of the Earth.
The
research
has created
considerable controversy worldwide in both scientific and political arenas. It is generally accepted
that, as atmospheric levels of carbon dioxide rise, plant growth will increase - a phenomenon known
as 'carbon dioxide fertilisation' - and thus methane emissions will also increase. This has led to the
debate that new forests might increase greenhouse warming through methane emissions rather than decrease
it by storing carbon dioxide. However, calculations of the climatic benefits gained through carbon sequestration
by reforestation far exceed the relatively small negative effect of methane emissions, which may reduce
carbon uptake by 4 percent.
The
discovery of methane emissions by plants will require rewriting of certain sections in textbooks that
describe natural sources and mechanisms of formation of methane. However, more basic information is
necessary before the full impact of this discovery is finally known.
