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The second major agroforestry practice is the transformation of unproductive cropland into agroforestry-based crop/tree fallow rotations. Although various expressions of this practice are found throughout the tropics (Buresh and Cooper, 1999), it is illustrated in this Special Report with the recent movement to replenish soil fertility in subhumid areas of tropical Africa. Soil carbon stocks have dramatically decreased in smallholder farms of sub-Saharan Africa because of nutrient depletion, which is increasingly recognized as the fundamental biophysical cause for declining food security in this region (Sanchez et al., 1997a,b). Given the acute poverty and limited access to mineral fertilizers, an ecologically robust approach is being used by tens of thousands of farmers in eastern and southern Africa. This approach consists of bringing natural resources to farmer fields where crops can utilize them: nitrogen from the air by biological nitrogen fixation, phosphorus from indigenous phosphate rock deposits, and nutrient-rich shrub biomass from roadsides and farm hedges (Rao et al 1998; Kwesiga et al., 1999; Sanchez, 1999). Details of this practice are described in Fact Sheet 4.11. Replenishment of nitrogen and phosphorus has important effects on changes in carbon stocks.
Conventional cropping on clayey, oxidic Alfisols in Kabete, Kenya, shows a 28 percent decrease in topsoil carbon, from 36 to 26 t C ha-1 during an 18-year period (Kapkiyai et al., 1998). Another long-term trial in Muguga, Kenya, with similar soils shows a total loss of 91 t C ha-1 to a depth of 120 cm with 8 years of continuous cultivation without inputs. About half of this loss (48 t C ha-1) took place in the top 15 cm (Woomer et al, 1997). The loss of topsoil organic carbon associated with soil nutrient depletion has been estimated at an average rate of -0.22 t C ha-1 yr-1 (Sanchez et al., 1997b). When soil fertility is replenished, maize grain yields increase from 0.5 to 2 t C ha-1 and carbon sequestration rates become positive, averaging 1.5 t C ha-1 (Table 4-8). When more trees are planted on field boundaries and as orchards, carbon sequestration rates increase further to 3.5 t C ha-1. Overall carbon sequestration rates may range from 1.2 to 5.1 t C ha-1 yr-1, with a modal value of 3.1 t C ha-1 yr-1.
| Table 4-8: Estimates of carbon uptake rates and time-averaged system carbon stocks and differences in carbon stocks from land transformation from low-productivity cropland to sequential agroforestry in subhumid tropical Africa (Sanchez, 2000). | ||||||||
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Carbon Uptake Rates
(t C ha-1 yr-1) |
Duration
(yr) |
Carbon Stocks
(time-averaged) (t C ha-1) |
Differences in Modal
Carbon Stocks (time-averaged) (t C ha-1) |
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| Land-Use Practice |
Low
|
Modal
|
High
|
Low
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Modal
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High
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| Current nutrient-depleted small farmsa |
-0.22
|
?
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23
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-
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| Fertility-replenished farms with maize-tree fallow rotationsb |
1.0
|
1.5
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2.4
|
25
|
20
|
35
|
49
|
+12
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| Fertility-replenished farms (as above) + more trees on farmc |
3.5
|
25
|
70
|
+47
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a From Sanchez et al. (1997b), based on calculations from Smaling
(1993). b Calculations from agronomic data by Kwesiga and Coe (1994), Rao et al. (1998), Kwesiga et al. (1999), and Jama et al. (2000). Improved fallows of Sesbania sesban and other species planted for 1 year in eastern Africa and 2 years in southern Africa add 100-200 kg N ha-1 to the soil. Tree fallows are followed by one maize crop in eastern Africa and three consecutive maize crops in southern Africa. Where phosphorus is limited (primarily eastern Africa), replenishment also includes basal application of 125-250 kg P ha-1 as rock phosphate plus biomass transfer of 1.8 tons dry matter ha-1 of Tithonia diversifolia to every maize crop (Sanchez et al., 1997b). c Same as note b, plus adding trees as orchards and to farm boundaries (Woomer et al., 1997). |
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