Biophysical Potential

Past, current, and future soil and biomass carbon (C) dynamics under various management practices for the Old Peanut Basin are discussed in Tschakert (2004a) and Tschakert et al. (2004). Current soil C values were measured in the field and amounted to 11.3 metric tons ha-1 on average for the upper 0 to 20 cm soil layer. The average C stock in the 20 to 40 cm layer was 6.9 metric tons ha-1, with a mean soil bulk density of 1.6 g cm-3. Nowadays, such a low range is common for the area (Rabot, 1984; Badiane et al., 2000) and consistent with mean values for both the Sudano-Sahelian zone (Manu et al., 1991) and global averages for drylands, which range from 0.2% to 0.8% (Lal, 2002a, 2002b). However, measurements showed large variation between individual fields, with values ranging from 3.9 metric tons ha-1 to 29.6 metric tons ha-1 for upper soil C. This variation reflects different clay content and management practices. Observed tree C was 6.3 metric tons C ha-1, while litter, root, and herbaceous C together accounted for 5 metric tons ha-1.

The biophysical potential to increase current carbon stocks can be approximated by the amount lost over time. Results from C dynamics simulations with CENTURY suggest that total system C in the area, including soil to a 20-cm depth, has declined from 60 metric tons ha-1 under native savanna and before the introduction of agriculture to the present level of 17 metric tons ha-1. This constitutes a total loss of 43 metric tons C ha-1 or 71% of the precultivation stocks. As shown in Figure 22.4, forest C decreased more rapidly than soil C. The results suggest a decline from 35 metric tons ha-1 in 1850 to 4.2 metric tons ha-1 in 2001 (0.2 metric tons C ha-1 year-1) compared to 20 metric tons ha-1 and 11.9 metric tons ha-1 for soil in 1850 and 2001, respectively (0.05 metric tons C ha-1 year-1).

How much C can in fact be sequestered through improved land use and management practices in order to boost today's low soil organic carbon values and approach historical values depends to a large extent on soil texture, precipitation, temperature, evapotranspiration, and the availability of nitrogen

1800 1850 1900 1950 2000 2050

Years

Figure 22.4 Changes in soil and tree carbon for the Old Peanut Basin, simulated with CENTURY. (From Tschakert, P. 2004a. Carbon for farmers: assessing the potential for soil carbon sequestration in the Old Peanut Basin of Senegal. Climatic Change, 67(2-3): 273-290. With permission.)

1800 1850 1900 1950 2000 2050

Years

Figure 22.4 Changes in soil and tree carbon for the Old Peanut Basin, simulated with CENTURY. (From Tschakert, P. 2004a. Carbon for farmers: assessing the potential for soil carbon sequestration in the Old Peanut Basin of Senegal. Climatic Change, 67(2-3): 273-290. With permission.)

in the soil. Results from CENTURY simulations indicate that, over a 50-year period, total system C could be increased by 244% from a mean present value of 17.3 metric tons ha-1 to a maximum of 40.8 metric tons ha-1. Simulated changes in soil and forest C for the 2001-2050 period are depicted in Figure 22.4. The largest gains in soil C can be expected under an optimal intensification scenario. (The optimum agricultural intensification scenario includes 3-year rotation cycle with groundnuts-millet-fallow, 150 kg of fertilizer [10-10-20] on groundnuts, 5 metric tons of sheep manure, and 2 metric tons of Leucaena leucocephala prunings on millet, 4 tons of manure on fallow, and improved millet seeds, as well as reducing tree pruning.) At the other extreme, soil C is likely to drop to 8.7 metric tons ha-1 under a millet-sorghum rotation with no external inputs. Significant increases in tree C can only be achieved under scenarios involving agroforestry (such as 250 to 300 nitrogen-fixing Faidherbia albida per hectare) or a conversion of croplands to tree plantations. Other management and land use practices are also likely to result in more soil C such as, for example, 3- to 10-year improved fallows in rotation with cropping cycles, the application of 4 to 10 metric tons of manure with and without mineral fertilizer, and the conversion of cropland to grassland with and without tree protection. Under these scenarios, soil C is expected to increase by 2 to 5.3 metric tons ha-1 over 25 years. Losses in soil C are projected for crop rotation without fallow and stubble grazing with no other inputs, reaching 1.7 to 3.9 metric tons ha-1 over 25 years.

Simulated changes in crop yields resulting from improved management practices are depicted in Figure 22.5. They varied from -62% to +200% for millet and -45% to +133% for groundnuts compared to 1980 and 2000 values (653 kg ha-1 and 707 kg ha-1, respectively). This correlates well with the simulated changes in soil C. Under the worst-case scenario (millet-sorghum rotation with no external inputs), CENTURY suggested a drop from 653 kg ha-1 to less than 300 kg ha-1. Clearly, this option would not be in the interest of farmers. Millet yields as high as 2 metric tons ha-1 and groundnut yields of 1.6 metric tons ha-1 could be reached under the n a se ld m el0

r0 cs c cn

200 150-

ng to

□ Groundnuts

Management scenarios

1 Fallow - crop rotation(3:4)

2 Compost 2t

3 Fallow + grazing - crop rotation (3:4)

4 Cow manure 4t

5 Horse manure 1.5t

6 Sheep manure 5t

7 Sheep manure 10t

8 Cow manure 4t + fertilizer

9 Fallow + leucaena prunings -crop rotation (10:6)

10 Fallow + manure - crop rotation(10:6)

11 Fallow + manure - crop rotation(3:4)

12 Fallow + leuceana prunings -crop rotation (3:4)

13 Agricultural intensification

Figure 22.5 Simulated changes via CENTURY in crop yields (percent) as a result of improved management practices.

optimal agricultural intensification scenario. All gains in millet under fallow scenarios (scenarios 9 through 12) are based on the assumption that food crops are planted the first year after fallowing, a typical practice. Increases in groundnuts exceed those in millet (scenarios 6 to 8) when the first benefit from high manure and/or mineral fertilizer inputs made on the latter during the previous year. This reflects the typical 1-year lag effect of manure on millet (Badiane and Lesage, 1996).

When climate change is taken into account, however, such projected gains in both soil C and crop yields become more uncertain. Climate change simulations were also performed with CENTURY (Tschakert et al., 2004). They suggest that larger threats to C stocks and agricultural production loom in the future, and effective actions taken today may at best result in no additional losses in the future. Under extreme conditions, they might even be futile. For instance, optimal agricultural intensification under a 'high increase' climate change scenario would result in a decrease of soil C by 5% by the year 2025 and by 40% by the year 2100. (Climate change assumptions under the 'high increase' scenario [2000-2100] are: 2020 — temperature +2°C, precipitation -30%; 2050 — temperature

+3.8°C, precipitation -48%; and 2080 — temperature +5.8°C, precipitation -48%.) Crop yields are even more at risk. The model predicts a decline in both millet and groundnuts yields of 5.5% by the year 2025. By the year 2100, yields would drop by as much as 93%. In other words, the steep increase in soil C simulated for the first 20 years of intensification (+85%) would counteract the projected impact of climate change during the same period of time. However, after this period, climatic conditions would be too unfavorable and losses inevitable.

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