Sequestering Soil Carbon

There is worldwide interest in the sequestration of soil C to reduce the amount of C going into the atmosphere as carbon dioxide. An estimated 40 to 50 Pg of carbon (Pg C = 1 petagram carbon = 1 billion metric tons of carbon) have been released from the soil worldwide (Lal et al., 1997). Most of this loss has resulted from the conversion of forest and grassland ecosystems into agro-ecosystems for the production of food and fiber.

Crop production requires relatively large amounts of N, P, S, and other essential plant nutrients. Decomposition of SOM is the primary source of plant nutrients for the first few years after crop production is initiated. As SOM decomposition begins to slow significantly, manures or fertilizers are often required to supplement nutrient requirements. Legumes are sometimes used to supplement N needs, but the use of legumes is limited in semi-arid regions. Nitrogen is usually the first nutrient that becomes limiting as SOM decomposition slows, and P is often the second essential nutrient that limits crop production.

The many calls for sequestering soil C seldom discuss the amounts of other nutrients that must be available because C is only one of many SOM constituents. Himes (1997) stated that on a weight basis, the C/N ratio in soil organic matter was 12/1, C/P was 50/1, and C/S was 70/1. Therefore, to sequester 10,000 kg of C in humus, 833 kg of N, 200 kg of P, and 143 kg of S are required. The amounts of nutrients necessary to produce high crop yields, and at the same time sequester large amounts of C, are difficult to estimate but they are huge. As already stated, it is believed that 40 to 50 Pg of C have been released from the soil. Therefore, approximately 4 Pg of N and 1 Pg of P were likely mineralized, and most of the mineralized N and P have either been removed from the land with harvested crops or lost by runoff, erosion, leaching, and other processes.

Some rather optimistic estimates have been made about how much of the C lost from soils can be restored. Paustian et al. (1995) estimated that the 13.8 million ha in the U.S. Conservation Reserve Program (CRP) could sequester about 25 Tg of C over a 10-year period. This would require approximately 2.5 Tg of N, or 18 kg per year for each hectare. Nitrogen fertilizers are not generally used on these lands, and there are also few, if any, legumes present because most of the CRP lands are located in semi-arid regions. There is some

N deposited with precipitation, but far below the amounts needed. Lal et al. (1997) indicated that widespread adoption of conservation tillage could lead to global C sequestration of 1.48 to 4.90 Pg, and restoration of degraded soils could lead to an additional 3 Pg year-1. Again, the authors did not discuss the N requirements or the source. To gain some perspective, however, the restoration of 5 Pg of C in 1 year would require approximately 0.5 Pg of N, which is roughly six times the total worldwide usage of fertilizer N in 2000. About 0.1 Pg of P would also be required, and this is roughly seven times the total amount of fertilizer P used worldwide in 2000 (Fertilizer Institute, 2003). Consequently, sequestration of large amounts of C into SOM will require enormous amounts of N, P, and other nutrients, and these may be limiting in many cases, particularly in semi-arid regions where fertilizers are used sparingly and legumes are limited.

The potential for carbon sequestration in SOM will likely be higher in the first few years than in succeeding years. Decomposition of SOM is rapid the first few years after cultivation is initiated, and then it slows as it reaches a new equilibrium. The reverse is likely to occur with sequestration. If tillage is reduced or eliminated, there will be some C sequestered, but the amounts sequestered in succeeding years will likely slow as N, P, and possibly other nutrients become less available. For instance, C.A. Robinson (West Texas A&M University, unpublished data, 1996) sampled four sites on Pullman silty clay loam, a fine, mixed thermic Torrertic Paleustoll, located in a semi-arid region near Claude, TX (Figure 14.2). One site had been cropped to dryland wheat for more than 50 years with occasional fallow years. Tillage was primarily performed with a tandem disk plow. During the last few years before sampling in 1993, anhydrous ammonia at the rate of 22 kg ha-1 was applied in the fall prior to seeding wheat. The other three sites were grassland: one native grassland field, one previous cropland field returned to grass 37 years before sampling as part of the U.S. Department of Agriculture Soil Bank program of the 1950s, and a field that had been returned to grass 7 years before sampling as part of the CRP initiated in 1985 by the 1985 U.S.D.A. Food Security Act. Significant

Soil organic carbon, g kg 1

Figure 14.2 Soil organic carbon (top) and total nitrogen (bottom) by depth as affected by management systems. (Drawn from C.A. Robinson, West Texas A&M University, unpublished data, 1996.)

Soil organic carbon, g kg 1

Figure 14.2 Soil organic carbon (top) and total nitrogen (bottom) by depth as affected by management systems. (Drawn from C.A. Robinson, West Texas A&M University, unpublished data, 1996.)

amounts of C and N were sequestered in the SOM (Figure 14.2). The data clearly show that substantial amounts of C can be retained even under semi-arid conditions. Precipitation at the sites averages approximately 500 mm annually. The native grassland site had significantly more soil organic carbon (SOC) and N at each depth than the CRP site and the wheat site. The Soil Bank site was intermediate. The increase in SOC was mostly in the surface 15 cm, although some differences at lower depths were largely associated with differences in bulk density. The results were similar to those of Ibori et al. (1995) on abandoned fields in northeastern Colorado. Others, however, found that SOC differences due to tillage were limited to the top 7 cm (Havlin et al., 1990; Potter et al., 1997). Nitrogen accumulations showed similar trends, but there was less recovery of N than C, particularly at the lower depths. The CRP site that had been in grass for 7 years recovered 48% of the C, but only 25% of the N relative to the Soil Bank site that had been in grass for 37 years. This illustrates that the initial rate of C accumulation in cropland soils returned to grass is considerably greater than in future years, and this rate may be partially constrained by N, or possibly P, as discussed earlier. No legumes were found at any of the sites.

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