The scenarios show that CS in tropical dryland soils can be achieved at the various sites (Table 21.2). The land management practices were chosen to be in accordance with the current farming systems and provide specific real cases for CS strategies in different dryland systems. For example, application rates of organic matter are commensurate with quantities that should be available to local farmers. However, at the field level, important tradeoffs may occur, preventing adoption of the best strategies for CS. Crop residues may be required for livestock feed or fuel, or during difficult times may be sold. Animal manures may be burned for fuel. Many socioeconomic factors will interact to determine which scenario or combination of scenarios is implemented in each growing season, all of which must be analyzed for each case.
Some of the results predict that soil carbon can be restored to precultivation levels or in certain circumstances to above these levels. The true "native soil carbon level" is often difficult to establish in those systems where agricultural activity has been present for at least several centuries or millennia, such as in the Nigeria and Kenya cases. To achieve quantities of soil carbon in excess of the "natural level" implies that the agricultural system has greater productivity than the native system, assuming that carbon is not being imported. The scenarios that predict the highest CS rates are often associated with the introduction of trees to the system. The inputs of carbon from trees are more resistant to
Table 21.2 Summary of Findings on C Stocks (metric tons ha-1) and Rates (metric tons ha-1 year-1) of Carbon Sequestration or Loss in Four Dryland Ecosystems Compared with Rates of CS in Drylands Reported by Lal (1999)
C stock before cultivation Current C stocks Conventional tillage Green and farmyard manures, plant residues, and fallows in rotations Effect of trees
Inorganic fertilizers as sole source of nutrients Zero tillage Zero tillage + green or farmyard manure Zero tillage + inorganic fertilizers
Plus additional 0.05 to 0.15 -0.12 to 0.08
Plus additional 0.05 to 0.15 -0.01
-0.05 to -0.01 -0.07 to 0.06 -0.3 to -0.1 -0.17 to 0.19 —
Gj decomposition than those from herbaceous crops, and consequently can cause marked increases in the level of soil carbon (Falloon and Smith, 2002). The highest rates of sequestration (0.1 to 0.25 metric tons ha-1 year-1) occur when zero-tillage systems also include cultivation of green manures and additions of farmyard manure. Inorganic fertilizers were generally inefficient in providing the necessary nutrients, and therefore increase CS. CS is greatly enhanced by including cover crops in the rotation cycle. Cover crops enhance soil biodiversity which is known to increase CS. The modeling data from the case studies are similar with rates of soil carbon sequestration under various land management regimes in drylands reported by Lal (1999), and reproduced in the last column of Table 21.2.
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