Fræðaþing landbúnaðarins - 08.02.2008, Page 539

VEGGSPJÖLD | 537
The effects of different land use and different temperatures on the
emission of the greenhouse gases nitrous oxide (N20), carbon dioxide
(C02) and methane (CH4) from organic soil cores in Iceland
Elisabeth Jansen ab, Jón Guðmundsson b, Hlynur Óskarsson b
“ HÍ, Háskóli íslands
b LBHÍ, Landbúnaðarháshóli Islands
Introduction
Climate change
Nitrous oxide (N20) is one of the most important of the greenhouse gases (GHG’s)
contributing to global warming additionally to carbon dioxide (C02) and methane (CH4)
(excluding water vapour). Per unit of weight, it has approximately 300 times the effect of
a C02 molecule for producing global warming in an approximately 120 year time
horizon, and methane has 23 times the effect of C02 (Minami, 2002: Houghton, 2001).
The N20 molecule is long-lived and accumulates in the atmosphere causing the
concentration to increase with time. The increase in N20, C02j CH4 and other green house
gases during the industrial era is signifícant. The atmospheric N20 abundance has
increased from 270 ppb pre-industrial level (in 1750) to 314 ppb in 1998 and to 319 ppb
in 2005. Ice core data on the N20, C02, and CH4 2000 years back in time show little
change in mixing ratios. The last 200 years exhibit a relative rapid increase of these gases
in the atmosphere (Forster et al., 2007). N20 is also the major source of ozone depleting
nitric oxide (NO) and nitrogen dioxide (N02) in the stratosphere. An estimate has
predicted that a doubling of the N20 concentration in the atmosphere could result in a
10% decrease of the ozone layer which would increase the ultraviolet radiation reaching
the earth by 20% (Crutzen and Ehhalt, 1977). Intemational agreements such as the Kyoto
Protocol to the United Nations Framework Convention on Climate Change (UNFCCC)
have the reduction of human-induced greenhouse gases as their main target. To be able to
reduce and alter the emission trend of the human induced GHG’s, all participants of the
agreement need to work collectively and collect and report accurate knowledge of the
GHG emission in their region (IPCC, 1998).
Soil microbial activity
Nitrous oxide (N20), carbon dioxide (C02) and methane (CH4) are natural gases emitted
by soil microbial organisms under different circumstances (Prather et al., 2001). C02 is a
result of aerobic respiration from a vast majority of soil organisms in the process of
decomposition of organic material. CH4 is a result of an integral part of the metabolism of
a large number of Euryarchaeota bacteria. These organisms are called methanogens and
they are physiologically strictly anaerobic (Madigan, Martinko and Parker, 2000).
Microorganism does not produce N20 as a main product, rather as a by product in the
natural circle of nitrogen referred to as nitrification (aerobe process) and denitrijication
(anaerobe process). Organically bound nitrogen goes first through the process of
mineralization (decay) which converts the organically bound N into inorganic mineral
forms (organic N NH4+ and N03'). The nitrogen bound in amino groups (as proteins
or in humic compounds) is attacked by microbes and simple amino compounds are
formed. They are hydrolyzed and the nitrogen is released as ammonium ion (NH4 )