Soil Organic Matter Soil Carbon and Carbon Sequestration in Zambia

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Soils under almost all farming systems in Zambia are low in N, P, and SOM (Stromgaard, 1984). It has been easy to detect and demonstrate evidence of N deficiency in cropped soils, but declines in SOM have been rather difficult to identify and quantify (Lungu, 1987). On the other hand, when crop residues are returned to the soil and legume cover crops are grown, they lead to increased organic C content and improved water retention.

The contributions of decomposed legume litter to SOC were assessed in a study of soil chemical and physical properties in a 7-year-old alley cropping trial containing Leucaena leucoceph-ala and Flemingia congesta in northern Zambia. Trees had a

Diagram Tieridging

Benefits

Increased N in soil

\ \

Increased yields

\

Improved nutrition

Reduced N fertilizer requirement

Reduced pollution risk

Increased biomass for livestock

/

Greater use of indigenous

V /

Microbes

Benefits

Improved soil physical

/ \

Properties

\

Porosity I

\

Aeration

Drainage

Infiltration 1

1

Water Retention

/

Soil Stabilization and \ /

i Conservation V

Figure 25.3 Relationship between integrated nutrient management, soil carbon sequestration, and improved farming systems.

Table 25.2 Litter Production and Maize Yields from 3-Year Improved Agroforestry Fallows in Eastern Zambia (metric tons/ha)

Treatment

Fallow Surface litter Maize (12% moisture) Grain Stover

Sesbania sesban Fully fertilized Gliricidia sepium Leucaena leucocephala Grass fallow Senna siamea Maize without fertilizer

9.30

8.20

8.20

8.33

Source: From 1996 report by International Centre for Research in Agroforestry for Zambia, cited in Lungu, O.I.M., and V.R.N. Chinene. 1999. Carbon sequestration in agricultural soils of Zambia. In Zambia Status Paper, presented at Workshop on Carbon Sequestration in Soils and Carbon Credits: Review and Development of Options for Semi-Arid and Sub-Humid Africa, Earth Resources Observation Systems Data Center, Sioux Falls, SD. Available at: www.kafuku.org/wwwboard/messages/ 5.html. With permission.

beneficial effect on soil chemical properties under hedge rows, particularly under Leucaena, and high levels of organic C, Mg, K, ECCE, and pH values (Dalland et al., 1993). Higher levels of organic C in the alley crop treatments were responsible for improvements observed in soil physical properties.

In Eastern province, maize grain yields following 3-year fallows were consistently greater than under conventional grass fallow of similar age (Table 25.2) in improved agroforestry fallows experiments (Lungu and Chinene, 1999). Improved fallows generated substantial surface leaf litter and rhizosphere biomass that contributed to C and N, and resulted in improved crop yields. The rhizosphere biomass contributed by Sesbania sesban root mass was 1.7 metric tons ha-1 for a 2-year fallow with 90% concentrated in the top 50 cm of soil.

A study of the effect of burning and ash fertilization in chitemene demonstrated that surface soils of protected woodlands had the highest values of C and lowest values of bulk density, compared to burned or cleared land (Araki, 1993). The relationship between bulk density and C content of both infield (burned) and outfield (cleared) soils of the miombo woodland were negatively correlated. Infield soils had less and higher values of bulk density than outfield soils. The range of sequential changes in the chitemene practice fluctuated between 0.5% and 2.5% for C, and between 0.9% and 1.4% for bulk density. The C content of infield soils decreased to 1% after cultivation and gradually increased with time. The C content of the outfield decreased to 1.1% a few years after clearing and gradually increased to 1.7% in 50 years. This decrease in SOM might be due to a reduction in the litter fall, and accelerated decomposition rate from increased exposure to solar radiation. Levels of 1.1% to 1.3% may be a minimum requirement for sustainable agriculture capable of utilizing weak plateau soils of the miombo woodland (Araki, 1993).

The impact of topsoil depth (TSD) and management on soil properties has been assessed using farmyard manure (FYM), N and P fertilizer, tie ridging, and ordinary farmer practices in Tanzania. SOC and P generally declined with a decrease in TSD, while SOC, N, P, K, and Mg were significantly increased by FYM application. The application of N and P fertilizers also had significant effects on SOC and P. Applying FYM increased SOC by about 0.55% in comparison with normal farmer practices, improved available water capacity (AWC), and root growth in soils with unstable structure and low SOC content (Gajri et al., 1994). The SOC pool was increased by adopting conservation tillage, using crop residue (mulch), incorporating cover crops in the rotation cycle, and using improved agricultural practices. The total potential for SOC sequestration in arable land of the tropics was estimated to be about 4 to 8 Pg (1015 g, or 1 gigaton) over the next 50 years (Lal, 1995a, 1995b).

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