Farming practices that enhance production and the input of plant-derived residues to soil include crop rotations, reduced bare fallow, cover crops, high-yielding varieties, integrated pest management, adequate fertilization, organic amendments, irrigation, water table management, site-specific management, and other proper management practices. These practices are referred to collectively as agriculture intensification (Lal et al., 1999b; Bationo et al., 2000; Resck et al., 2000; Swarup et al., 2000). For more detail, see Section 126.96.36.199.
Use and Potential
Increasing global demand for food will drive continued agriculture intensification. Intensification can be applied to all cropping systems, with varying degrees of constraints because of economics and the availability of labor and technology. Rates of residue return to soil are also influenced by potential alternative uses as fodder and fuel. Intensification of systems with previously low use of purchased inputs (e.g., fertilizer, improved varieties, pesticides) may involve increased use of these inputs and/or intensive management using biological inputs (e.g., crop rotations, cover crops, manures). Where the use of purchased inputs is already high, intensification implies increased efficiency (and potentially reduced use) of fertilizer, pesticide, and other inputs. The principal means by which intensification influences soil carbon changes are through the amount and quality of carbon returned to soil (via roots, crop residues, and manures) and through water and nutrient influences on decomposition (Paustian et al., 2000a). Agricultural intensification can occur on all or nearly all of the world's existing cropland (1.6 Bha).
Current Knowledge and Scientific Uncertainties
The rates of SOC sequestration by agriculture intensification differ among soils and ecoregions. The influence of these practices on productivity and soil properties, including organic matter dynamics, has been studied for many decades; there is an extensive body of research results and many well-documented, long-term field studies around the world (Powlson et al., 1998). Uncertainties remain, however, regarding the interactions between different practices (e.g., crop rotations, water table management, fertilization) for different soil and climate conditions.
Rates of soil carbon sequestration can be established for predominant cropping/management systems on the basis of long-term benchmark experiments, on-site sampling, and modeling (e.g., Powlson et al., 1996; Eve et al., 2000). Annual statistics on cropland area and crop production are available globally at the country level (e.g., FAO, IGBP-DIS), and more detailed data on crop production and the extent and distribution of the practices described above exist to varying degrees for all Annex I countries and many non-Annex I countries. Several generalized models of carbon cycling in agricultural systems exist (see Chapter 2). Quantification of soil carbon changes can be estimated, using models, from the distribution of major cropping systems, data on production and residue returns, and associated soil and climate information, and/or with soil sampling designs. Scaling from local to regional to national levels can be done by using a combination of climate and soil maps, management and yield data, modeling, and geographic information systems (GIS).
These practices can increase soil carbon stocks for 25-50 years or until saturation is reached.
Monitoring, Verifiability, and Transparency
The amount of new carbon sequestration and its residence time (turnover rate) can be verified through ground truthing (on-site sampling) and well-calibrated models. Periodic monitoring can be done by using benchmark sites where SOC content and bulk density can be measured once every 5-10 years to a depth of 1 m. Because of the stratification of SOC, soil samples need to be taken in small depth increments in the surface layers. The practices to be used are well characterized.
Reversion to conventional agriculture practices (i.e., plowing, residue removal or burning, inappropriate irrigation, improper fertilizer use) can cause the loss of sequestered carbon.
Most carbon in agricultural systems is in the soil and has residence times of years to centuries (see Section 4.2).
Agriculture intensification has numerous ancillary benefits-the most important of which is the increase and maintenance of food production. Environmental benefits can include erosion control, water conservation, improved water quality, and reduced siltation of reservoirs and waterways. Soil and water quality is adversely affected by indiscriminate use of agriculture inputs and irrigation water. Where intensification involves increased use of nitrogen fertilizers, fossil energy use will increase, as may N2O emissions.
Relationship to IPCC Guidelines
These practices relate directly to the "Input Factors" used in the IPCC Guidelines for estimates of changes in soil carbon stocks. Default values for three levels of plant residue production and addition to soil are provided in the Workbook, with examples describing the types of management systems that each level would correspond to.
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