25.1 Characteristics of Agro-Ecological Regions in Zambia 609
25.1.1 Region 1 611
25.1.2 Region 2 611
25.1.3 Region 3 612
25.2 Anthropogenic Practices and SOC Sequestration in Zambia 612
25.2.1 Farming Systems 612
25.2.2 Soil Carbon Dynamics in Smallholder Agriculture 614
25.2.3 Impact of Farming Systems and Agroforestry on Soil Organic Carbon 615
25.2.4 Integrated Soil Nutrient Management 617
25.2.5 Soil Organic Carbon Sequestration
25.2.6 Carbon Stocks in Natural Ecosystems and
C Loss with Land Conversion in Africa 620
25.3 Potential for Sequestering Soil Organic Carbon in Zambia Soils 621
25.3.1 Soil Organic Matter, Soil Carbon, and Carbon Sequestration in Zambia 623
25.3.2 Conservation Tillage, Cover Crops, and Residue Management 627
188.8.131.52 Conservation Tillage 627
184.108.40.206 Management of Residual Organic Matter and Soil Organic Matter Sequestration 627
25.3.3 Increasing Carbon Sequestration through Nutrient Recapitalization and Agroforestry 628
25.3.4 Mitigation Options in Zambian
Forest Sector 628
220.127.116.11 Maintaining Existing Stocks 629
18.104.22.168 Expanding Carbon Sinks 629
25.3.5 Characteristics of Mitigation Options 630
25.5 Summary and Conclusion 630
Carbon (C) is one of the most important and abundant elements on Earth, occurring in five general pools, that is, soil organic C (SOC) and soil inorganic C (SIC), oceanic, geologic, atmospheric, and biotic components. Carbon pools are of immense significance to the growing human population, which depends on soil quality for agricultural sustainability, poverty alleviation, and improved nutritional and health status. Carbon is a constituent part of humus, which improves soil quality by binding soil particles into aggregates. It enhances the chemical and physical properties of the soil and crop productivity. Carbon also combines with oxygen to form carbon dioxide (CO2), a raw material for photosynthesis and, hence, an important component of dry matter for the production of food and fiber. Carbon dioxide absorbs heat from sunlight, thus helping to keep the Earth warm. Increased concentrations of CO2 and other greenhouse gases will raise mean annual temperatures and cause excessive global heating, and melting of ice, glaciers, and permafrost. In turn, higher temperatures are expected to result in increased flooding in coastal areas from rising sea levels and droughts in low-rainfall areas, adversely affecting climate and agricultural production, especially in tropical and subtropical zones (Lal, 2001).
The SOC pool occurs as a complex mixture of nonhumic substances, such as carbohydrates, proteins, and amino acids, and humic products of secondary synthesis. The SIC pool consists of carbonates and bicarbonates. Together, the SOC and SIC pools form terrestrial C, and have a great impact on soil quality, and thus, on plant and animal life.
SOC is important for various reasons:
• It is a major sink/source of essential plant nutrients because of its strong impact on the effective cation exchange capacity in soils with low-activity clays.
• The release of plant nutrients through mineralization of soil organic matter (SOM) is essential for agronomic productivity.
• Mineralization of organic matter leads to emission of CO2 under aerobic conditions and methane (CH4) under anaerobic environments.
• Depletion of SOC causes soil degradation that results in crusting, compaction, reduced water retention and transmission properties, accelerated soil erosion, and decline of soil fertility (Baldock and Nelson, 2000; Rochette et al., 2000).
When accentuated by plow tillage, clean cultivation, and residual biomass removal and burning, SOC also significantly reduces agronomic productivity through soil crusting, compaction, accelerated erosion, water and nutrient imbalance, leaching, and acidification (Sanchez et al., 1982; Cassel and Lal, 1992).
SOM has numerous functional roles within ecosystems that range from the molecular to global, such as complexation of toxic cations in the soil solution (Hue et al., 1986), where the soil serves as an important source, sink, and buffer of greenhouse gasses (Hall, 1989; Post et al., 1990). Generally, SOM is closely associated with soil fertility as a source and sink of mineralizable nutrients (Duxbury et al., 1989), with the retention of nutrients and water (Russell, 1973; Lal, 1986), as an agent of soil structural stability (Oades, 1984), and with detoxification of naturally occurring and human-manufactured substances. Invariably, SOM declines once land is converted to agriculture in tropical and subtropical regions of the world.
SOC has both on-site and off-site beneficial effects. On-site beneficial effects can be derived through:
1. Increased aggregation and structural properties
2. Increased available water holding capacity
3. Improved macro porosity
4. Increased infiltration capacity
5. Decreased crusting, compaction, and soil erosion
6. Improved cation exchange capacity (CEC) and nutrient retention capacity
7. Decreased nutrient losses by leaching
8. Increased soil's trafficability and tilth
9. Increased number and diversity of soil biota
10. Increased soil capacity to biodegrade chemicals
Off-site benefits of SOC include:
1. Lower rates of sediment transport in natural waters
2. Lower siltation of waterways and reservoirs
3. Lower rates of transport of pollutants of natural waters
4. Lower emissions of CO2, CH4, and N2O, from C and N displaced by soil erosion
Thus, increased SOC content and retention help enhance soil properties, and ultimately, contribute to increased agronomic productivity and sustainability.
Recognizing the significant role that C plays in agriculture, this chapter examines some of the factors that affect terrestrial C sequestration in Zambia. The agroecological zones, anthropogenic factors that affect biomass production, causes of C emissions, and strategies to sequester C in soils in Zambia are also discussed.
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