Drainage and cultivation of organic soils

Moving even further away from the tropics, we find another form of land-use change that had a major impact on N2O emissions. Boreal and subarctic peatlands have accumulated around 455Pg C during the postglacial period

(Gorham, 1991). A mean C:N ratio of 25.8 (Batjes, 1996) indicates a pool of organic N around 17.6Pg in these soils. While vegetation adapted to flooded conditions may thrive in a wetland, a lack of oxygen in flooded soil severely limits the mineralization of shed litter by microorganisms. Consequently, organic matter, and the N that is a part of it, has accumulated. Emissions of N2O from undisturbed wetlands are small (Martikainen et al, 1993; Brumme et al, 1999) for at least four reasons. First, small rates of mineralization mean little availability of NH4+. Second, oxygen limitation restricts nitrification, one of the pathways by which N2O can be produced. Consequently, third, very little NO3- is generated that may be turned into N2O by denitrification. Fourth, under oxygen limitation, N2O is itself a welcome electron acceptor to the microbial community and isturned efficiently into N2 (Vieten et al, 2009). If one desires to grow agricultural crops, grass or commercial forest on organic soils, then these soils have to be drained. Once the water level has been lowered, the layers of organic matter above that level become aerated; supplied with atmospheric oxygen, microbial decomposers proliferate and rapid mineralization sets in. Rogiers et al (2008) have estimated drainage-induced mineralization equivalent to 5-9Mg C ha-1 yr-1 in a subalpine sedge peat in Switzerland. Similar rates have been observed in other parts of the world. Mineralization produces NH4+, and the availability of oxygen permits nitrification, which produces NO3-and some N2O. Denitrification rates increase and so does total N2O production. Because of a better aeration compared to the situation previously, denitrification is now less complete and a larger proportion of the reduced NO3- is released as N2O rather than as N2.

Drainage of organic soils and associated effects on N2O emissions have been well documented in Scandinavia. Weslien et al (2009) describe the historical development in Sweden, where in the 1930s around 0.65-0.70 x 106ha of organic soil were farmed. Economic and political reasons led to the abandonment of 0.40-0.45 x 106ha by the 1990s. Nowadays, most of the abandoned area is probably under forest. Alm et al (2007) describe an even more dramatic situation in Finland. Of the original 10.4 x 106ha of pristine ecosystems, 5.4-5.7 x 106ha have been drained for forestry and 0.7-1.0 x 106ha for agriculture. Further major areas of drained organic soils are found in the Baltic states and in Russia (Rydin and Jeglum, 2006). Emissions of N2O from drained organic soils are, per unit area, much larger than those discussed for tropical forests in the first section. The IPCC (2006) default value for temperate organic crop and grassland soils is 8kg ha-1 yr-1 with an uncertainty envelope of 2-24kg ha-1 yr-1. Maljanen et al (2007) have summarized field measurements on cultivated organic soils in Finland. They range, in kg N2O-N ha-1 yr-1, for barley fields from 5.4 to 24.1, for grass from 1.7 to 11.0 and for fallow from 3.8 to 37.0. Although it is common practice on highly fertile drained organic soils to add no N fertilizer at all to cereal production (Kasimir Klemedtsson et al, 2009), part of the reported emissions may be associated with the use of mineral N fertilizer as part of the cultivation. However, such fertilization seems to make little difference to the overall emission total. Comparing cultivated with abandoned fields, Maljanen et al (2007) found emissions to be similar, even 30 years after abandonment. A lack of correlation between emissions and time since abandonment suggests that enhanced emissions will still continue for a much longer time.

Afforestation of abandoned organic cropland does not ameliorate the situation because availability of mineral N still remains high (Makiranta et al, 2007). Mineralization of organic matter and subsequent nitrification continues to provide a source of NO3- for denitrification (Weslien et al, 2009), probably the dominant process of N2O production in cultivated organic soils (Maljanen et al, 2003). Nevertheless, greater assimilation and storage of CO2 by forest, compared to crops, may improve the greenhouse gas balance, even if N2O emissions remain unchanged (Weslien et al, 2009).

In the previous section we discussed evidence for about 1 per cent of mineralized N being emitted in the form of N2O. This assumption is endorsed by a large number of studies (summarized in IPCC, 2006) on mineral soils. However, a larger proportion than 1 per cent of mineral N may be emitted as N2O from organic soils. Because of generally more acidic conditions in organic soils, compared to most mineral soils, reduction of N2O to N2 may be severely inhibited (Stevens and Laughlin, 1998; Simek et al, 2002). Data in Maljanen et al (2007) and Weslien et al (2009) support this hypothesis.

Maljanen et al (2007) report CO2 and N2O emissions from different land uses on cultivated organic soil. One type of land use is bare soil without any plants. Here, CO2 emissions are a good proxy for mineralization of soil organic matter. Where plants are present, part of the CO2 comes from root respiration, which is not easily separated from CO2 resulting from the mineralization of soil organic matter. The mean CO2 emission from the four fallow sites was 5910kg CO2-C ha-1 yr-1. Annual mineralization of N estimated from the average C:N ratio of these sites (21.6) was 274kg N ha-1 yr-1. Measured mean N2O emission was 16.7kg N2O-N ha-1 yr-1, or 6 per cent of the mineralized N. While N2O emissions from these disturbed ecosystems may not decline in the foreseeable future, their share of the greenhouse gas budget declines because emissions from other sources increase. Still, they can constitute a surprisingly large fraction of a country's greenhouse gas inventory. For Finland in 2004, for example, N2O emissions from the cultivation of organic agricultural soils constituted 1.7 per cent of the total net greenhouse gas emissions in terms of global warming potential (Lapvetelainen et al, 2007).

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