Agriculture Reference
In-Depth Information
pre-existing soil carbon. However, the mechanisms leading to the priming effect remain
poorly understood (Kuzyakov et al., 2000). It is commonly believed that the low quality of
SOM limits the amount of available energy for soil microorganisms, and in turn the rate of
SOM mineralization. Thus, the priming effect is thought to result from an increase in overall
microbial activity due to the higher availability of energy and nutrients released from fresh
organic matter (Löhnis, 1926; Bingeman et al., 1953; Sørensen, 1974). However, mineral
nutrients appeared that induce no or little effect on SOM mineralization (Wu et al., 1993;
Shen & Bartha, 1997).
A possible role of added N is a chemically stabilizing C in the soil since N compounds
may react with lignin in the process of humus formation, as a mechanism of C stabilization
(Paustian et al., 1992). In addition, most SOM stabilizes with a C:N ratio of approximately
10:1 again indicating that if soil C storage is to increase, N is needed (Schulten & Schnitzer,
1997).
At any rate, the positive role of N fertilization in pooling C, may be offset by changing
the CH 4 soil-atmosphere exchange and soil N 2 O emissions (see later in this chapter).
E FFECT OF N F ERTILIZATION ON
S OIL -A TMOSPHERE CH 4 E XCHANGE
Methane (CH 4 ) is considered the second greenhouse gas after CO 2 as atmospheric
concentration, despite a short residence time in the atmosphere (about 10 years), since its
ability to absorb infrared radiation makes is 20 to 30 times more efficient than CO 2 (Rodhe,
1990). Methane is also involved in changes in the chemical composition of the atmosphere
since it is chemically reactive. In particular, it reacts with hydroxyl radicals in the
troposphere, thereby reducing its oxidative power and its ability to eliminate pollutants such
as chloro-fluoro carbons (CFCs), and it also leads to the production of other greenhouse gases
(O 3 , CO, CO 2 ) (Cicerone & Oremland, 1988).
Soil-atmosphere methane exchange is the result of two antagonistic microbial processes,
strictly connected and linked by a syntrophic relationship with other transformation processes
in the soil. Methane is produced in anaerobic soils or in anaerobic micro-sites of drained soils
by methanogens. In aerobic soils, methane produced in anaerobic microsites and methane
from air is oxidized into CO 2 by methanotrophs. A soil is considered CH 4 source when the
balance between production and consumption is positive; when the balance is negative, the
soil is considered a CH 4 sink. Globally, soils most efficient as CH 4 sources are generally
those which are anaerobic because they are often submerged or water-saturated. In these cases
a significant methanogenic activity develops at intervals: ricefields, landfills, peat soils,
(Whalen et al., 1990; Nesbit & Breitenbeck, 1992; Sundh et al., 1995). However, 60 to more
than 90 % of CH 4 produced in anaerobic soils is re-oxidized in their aerobic zones, so the
balance between CH 4 production and oxidation is usually positive (Le Mer & Roger, 2001).
Methane oxidation by aerobic upland soils is rarely higher than 0.1 mg CH 4 m -2 h -1 . Forest
soils are the most active, followed by grasslands and cultivated soils (Le Mer & Roger, 2001).
Methanogenic bacteria are responsible of the complete mineralization of organic matter
in anaerobic environments, where sulfate and nitrate concentrations are low, through a
fermentation process which produces CH 4 and CO 2 according to the reaction:
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