Soil organic carbon impact

Soil contributes to a greater extent to total carbon storage than do above-ground vegetation in most forests (Johnson and Curtis 2001). The total amount of soil organic carbon (SOC) in the upper meter of soil is about 1500 x 1015 g C (Eswaran et al. 1993 Batjes 1996), and the global atmospheric pool of CO2 is about 750 x 1015 g C (Harden et al. 1992). The CO2 emission from soil into atmosphere is about 68.0-76.5 1015 g C per year, and this is more than 10 times the CO2 released from fossil fuel combustion (Raich and Potter 1995). Variations in SOC pools and SOM turnover rates, therefore, exert substantial impacts on the carbon cycles of terrestrial ecosystems in terms of carbon sequestration in soil and CO2 emission from soil. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

Grayston SJ, Vaughan D, Jones D (1996) Rhizosphere carbon flow in trees, in comparison with annual plants the importance of root exudation and its impact on microbial activity and nutrient availability. Appl Soil Ecol 5 29-56 Gregorich EG, Ellert BH, Monreal CM (1995) Turnover of soil organic matter and storage of com residue carbon estimated from natural 13C abundance. Can J Soil Sci 75 161-167... 

Carbohydrate partitioning from shoot to roots can comprise a substantial proportion of plant-photosynthetic C02 fixation (20-70%), and 4-70% of this fraction can be released into the soil as organic rhizodeposition (Lynch and Whipps, 1990 Grayston et al., 1996). This is not only a significant loss of reduced carbon but can also contribute by 30-40% to the total input of soil-organic matter with considerable impact as a source of carbon and nitrogen for soil microorganisms. 

Figure 15.18. Comparison of global reservoirs and their residence times (t in years) (Example 15.3). The reservoirs of the atmosphere, of surface fresh waters, and of living biomass are significantly smaller than the reservoirs of sediment and marine waters and are thus more susceptible to distuibance. For example, the combustion of fossil fuel (from the reservoir of organic carbon in sediments) will have an impact on the smaller reservoirs CO2 in the atmosphere will be markedly enlarged. This combustion also fixes some N2 to NO and NO2 sulfur, associated with the organic carbon, introduces CO2 into the atmosphere. These nitrogen and sulfur compounds are washed out relatively rapidly into soil and aquatic ecosystems. The total groundwater reservoir may be twice that of surface fresh water but, however, is less accessible. (From Stumm, 1986.)...

The Century biogeochemical model was developed to study the impact of climate and atmospheric perturbations on soil organic matter and ecosystem dynamics. Century is a general model of plant-soil interactions that incorporates simplified representations of key processes relating to carbon assimilation and turnover (Figure 16). 

In terms of key interactions at the ecosystem level, it is expected that plant production and its associated effects on the amounts and transformations of organic carbon in the soil-plant system will have the greatest overall impact on the dynamics of organic phosphorus. The amounts, forms, distribution and associated dynamics of organic carbon and phosphorus in soil are influ-... 

FIGURE 17.23 Vertical distribution of dissolved organic carbon in soil pore water and water column at impacted and unimpacted sites in the WCA-2A of the Everglades. (Reddy, K. R., Unpublished Results, University of Florida.)... 

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