Conversion of grassland to cropland

Conversion of pristine grassland to cropland in temperate regions mainly occurred between the mid-19th and the mid-20th centuries. Major conversions include the ploughing of prairie by settlers in the US (Figure 7.2) and later the large-scale land reclamation in the steppes of the Soviet Union during the 1950s.

Cultivation of grassland accelerates mineralization of soil organic matter and the release of mineral N, part of which is turned into N2O. Experimental ploughing up of a grass sward in southeast Scotland, for example, yielded 449kg N ha-1 by mineralization, and associated emissions of 2.0kg N2O-N ha-1 over 18 months. Even larger N2O emissions were observed after ploughing of a grass-clover sward, where 244kg N ha-1 were released by mineralization and 4.5kg as N2O-N ha-1 (Davies et al, 2001). The N2O EFs for these two sites were therefore 0.45 and 1.6 per cent, respectively. These values fall within the uncertainty range of the IPCC default value of 0.3 to 3 per cent (default value: 1 per cent) for the direct emission of N2O from agricultural soils. There are data available on the rates of decline of soil C and N after bringing land into cultivation (for example Tiessen et al, 1982; Bowman et al, 1990; Lobe et al, 2001) (Figure 7.3), but no associated measurements of N2O emissions.

Smith and Conen (2004) estimated the likely scale of emissions following conversion of grassland to cropland by analogy with the IPCC methodology at

Figure 7.2 Dramatic, swift and almost complete change of prairie grassland to cropland in Iowa between the (upper panel) 1850s and (lower panel) 1990s

Source: Adapted from Iowa Department of Natural Resources (2000)

Figure 7.2 Dramatic, swift and almost complete change of prairie grassland to cropland in Iowa between the (upper panel) 1850s and (lower panel) 1990s

Source: Adapted from Iowa Department of Natural Resources (2000)

that time. They assumed that mineral N liberated by mineralization of soil organic matter and plant remains, following land-use change, can be regarded as a comparable potential source of N2O to N being applied in the form of synthetic N fertilizer. The IPCC (2006) has subsequently adopted the same approach, in which the NH4+ and NO3~ resulting from the mineralization of soil organic matter following a change in land use is deemed to be of the same value as a substrate for microorganisms producing N2O by nitrification or denitrification as the NH4+ and NO3~ in an application of synthetic N fertilizer. By way of illustration, Smith and Conen (2004) applied this approach

Figure 7.3 Loss of N from soil after the cultivation of grassland in three ecosystems of the South African Highveld

Note: It can be expected (on the basis of IPCC methodology) that 1 per cent of the total N lost has been emitted as N2O.

Source: Data from Lobe et al (2001)

to the data from Lobe et al (2001) for the mineralization of N in savanna grassland soils of the Highveld of South Africa. The average rate of N loss from the soil organic matter in three soils was 7.8 per cent of the rate of C loss, corresponding to 90-170kg N ha-1 yr-1 over the first eight years after cultivation. The net change in emission from these soils over this period predicted by applying the then current IPCC default EF of 1.25 per cent was 1.1-2.1kg N2O-N ha-1 yr-1; applying the updated (IPCC, 2006) default factor of 1 per cent, the emission would be 0.9-1.7kg N2O-N ha-1 yr-1. This emission augments the global warming impact already caused by the loss of soil C in the form of CO2 through the ploughing of these grasslands by another 10 per cent. The historical loss of soil C and its impact on atmospheric CO2 enrichment may be mitigated by soil management conducive to enhancing soil organic matter contents, such as reversion to grassland or reduced tillage. However, once the N2O is released during the conversion of grasslands to cropland it cannot be similarly withdrawn from the atmosphere.

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