Conceptual Basis For Carbon Sequestration In Soils

The nonliving organic matter in soils is generally considered to be in three pools: (1) a labile pool that is free within the soil matrix, (2) a physically protected pool that is within larger aggregates, and (3) a chemically protected or mineral-associated pool that is within the smallest aggregates (e.g., Paustian et al., 1997; Jenkinson and Rayner, 1977). The specific aggregate sizes associated with these pools vary with soil type, but chemically protected organic matter can be considered to be within aggregates that are not disrupted by the normal forces that a soil is exposed to, namely, drying and wetting (and freeze-thaw) cycles and the mechanical forces of tillage. Consequently, the OC in these aggregates is very stable and is often referred to as the passive pool. Tillage leads to disruption of larger aggregates, partly by the mechanical forces of tillage, and partly because tillage is a mixing process that, over time, exposes all soil in the tilled layer to the harsh environment of the soil surface where wet-dry and/or freeze-thaw cycles are most frequent. The loss of OC associated with tillage-induced destruction of aggregates in agricultural soils can be considered as a conversion of OC from a protected pool to a labile pool that is then mineralized to CO2. The factors that regulate the size of the various soil OC pools are shown in Table 17.1.

Most of the OC in the labile pool is derived from recent inputs such as crop residues, and the size of this pool varies with the level of organic inputs into the soil and climatic factors (temperature and moisture) that affect the rate of biological decomposition processes. The size of the labile OC pool is little affected by soil properties. It is the most important contributor to total soil OC in cold climates where both plant growth and biological decomposition processes are slow, but where the decomposition is slower than production, allowing an accumulation of substantial amounts of OC over long

Table 17.1

Size

Organic Carbon Pools and Factors Controlling Pool

Organic Carbon Pool Labile (free)a

Physically protected (particulate, slow) Chemically protected (passive, mineral associated)

Location in Soil Factors Controlling Pool Size Free in soil 1. Amount and frequency of matrix organic inputs

2. Temperature and moisture regimes Within large 1. Soil texture aggregates 2. Tillage Within small 1. Soil texture aggregates 2. Soil mineralogy

Parentheses give alternative names for pools.

a periods of time. Thus, the average per ha carbon stock in soils of boreal forests (343 metric tons/ha) is almost three times that in tropical forests (123 metric tons/ha), whereas the vegetation stock in tropical forests (120 metric tons/ha) is about twice that in boreal forests (64 metric tons/ha) (Intergovernmental Panel on Climate Change [IPCC], 2001).

The bulk of the organic matter in noncultivated temperate and tropical region soils is associated with aggregates that provide varying degrees of protection against biological decomposition processes. The potential for formation of aggregates in soils depends on soil texture and mineralogy, and increases with the fineness (surface area) of soil particles. The stability of aggregates, and hence their associated OC, varies greatly with soil texture and mineralogy. Except for oxisols, which have very stable large aggregates, the dividing point between large and small aggregates, and between the physically protected and passive organic matter pools, is most likely in the 50- to 250-^m size range.

The total OC content of soils increases with the ability to form aggregates, and hence with increasing clay content. Figure 17.1 shows how tillage and soil texture relate to soil aggregation and carbon sequestration. Sandy soils with little capacity to form aggregates have low OC contents that vary little with cultivation. Finer-textured soils have higher OC contents that are reduced by tillage-induced destruction of aggregates. The upper and lower lines represent the range in the maximum and minimum OC contents, respectively, as a function of soil texture and tillage. The difference between the two lines represents the OC protected within aggregates. The minimum OC content is associated with the passive or stable OC pool. For a clay soil with an initial OC content at point A, the sequestration potential is the difference between the maximum value and the initial value, that is, B - A. In most situations, it is unlikely that the OC content of soils under NT agriculture can quite reach that of their original natural ecosystems, due to lower levels of residue return, and hence a smaller labile OC pool.

The equilibrium OC content that can be achieved by a given carbon sequestration practice will vary with the extent

No-till soils

Maxim aggre (fores grassl

No-till soils

Maxim aggre (fores grassl

Sand

Silt

Clay

Soil texture (% clay)

Figure 17.1 Influence of texture and aggregation on carbon sequestration in soils

Sand

Silt

Clay

Soil texture (% clay)

Figure 17.1 Influence of texture and aggregation on carbon sequestration in soils of tillage. Carbon sequestration management regime M1, which represents the maximum OC level that can be reached under NT agriculture, gives a different pattern of carbon accumulation and a higher final or equilibrium OC level than regime M2, which represents a practice that has some tillage and hence is less effective.

Estimates of OC accumulation rates following a switch from CT to NT are fairly consistent. The IPCC (2000) suggests that average OC accumulation rates will range between 0.1 to 0.8 metric tons C/ha/year, with mean values of 0.4 to 0.5 metric tons C/ha/year for moist temperate and tropical environments. West and Marland (2002) estimate an average rate of 0.34 ± 0.1 metric tons C/ha/year for cropland in the United States, and West and Post (2002) provide a global average of 0.57 ± 0.14 metric tons C/ha/year by considering 67 long-term experiments around the world. However, the rate of OC accumulation depends on the rate at which macroaggregates are generated, which will vary as a function of soil texture and fertility, cropping system, and residue return levels, all affecting soil biological activity as well as physical-chemical processes that together affect soil structure. The rate at which C accumulates in soils when tillage is stopped or reduced will vary with management. However, the equilibrium OC level associated with a particular reduced tillage practice is independent of the rate of OC accumulation.

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