Review of forests, wood products and wood biotechnology ... - GWDG

Conen, 2004). N2O is produced via two main processes, nitrification and
denitrification. Nitrification is the oxidation of ammonia to nitrate via the
intermediate nitrite. The oxidation of ammonia into nitrite is performed by two
groups of organisms, ammonia oxidizing bacteria and ammonia oxidizing archaea.
Denitrification is the reduction process to nitrogen through a series of
intermediate gaseous nitrogen oxide product. Nitrous oxide is an intermediate in
the reaction sequence of denitrification and a by-product of nitrification (IPCC,
2001, Duxbury, 1982). Therefore these microbial processes are mainly regulated
by temperature, pH, water content, C availability and nitrate content.
Microbial processes depend on water. Denitrification rate increases as soil water
content rises. Nitrification also increases with water content up to a level where
oxygen is restricted (Granli & Bockman, 1994). Then the diffusivity of soil gas is
affected by soil water content (Firestone and Davidson, 1989).
In organic agriculture the application of organic fertilizer like manure or slurry is
an important mean to increase the humus content. No synthetic fertilizer is used
and existing resources can be applied. Production rests from animal husbandry
can close the gap in the agricultural cycle and can be recycled. The use of fertilizer
can be regarded as sustainable.
It is supposed that organic fertilized fields emit more N2O due to the higher C
availability and nitrogen stocks (Groffman & Tiedje, 1991). IPCC (2001)
published an emission factor for nitrous oxide from agricultural fields that
depends on the nitrogen input which is mainly fertilizer.
N2O -N emission = 1.25% * N-input
So far, the calculated emission only depends on the nitrogen input and there is no
differentiation between soil texture, fertilizer type etc. A more detailed calculation
would permit a better differentiation between sites. Therefore a better basis for
extrapolations to countrywide or worldwide emission would be possible.
Methane
Soil mostly takes methane up, except for the cultivation of rice. Rice fields release
methane in large amounts of around 90 Tg CH4 yr -1. Worldwide the uptake of
soils is ~ 30 Tg CH4 yr -1 (IPCC, 2007). CH4 is produced when organic materials
decompose in oxygen-deprived conditions, notably from fermentative digestion
by ruminant livestock, from stored manures, and from rice grown under flooded
conditions (Mosier et al. 1998). Rice cultivation and animal husbandry are the main
sources for methane in agriculture. Soils with a low soil moisture content (not
water-logged, no wetlands) are usually sinks for methane. This is a very important
fact in the discussion of climate change as soil is able to transform methane. The
methane uptake depends on the oxygen availability and therefore on the soil
moisture content. The higher the moisture content is the less pores are air-filled.
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