Land-atmosphere cycling

The agricultural emissions of NHj, NjO and NO must be considered in context the processes which lead to net loss from the soil and vegetation are natural and form a part of the land-atmosphere cycling of this vital nutrient. The current agricultural processes, however, create conditions in which the small natural background fluxes, in the range of a few ngNm s are dwarfed by losses from fertilized land. 

Land/atmospheric interfacial processes which impact climate and biological activity on earth are illustrated in Figure 3. Emissions of carbon dioxide, methane, nitrogen dioxide, and chlorofluorocarbons (CFCs) have been linked to the transmission of solar radiation to the surface of the earth as well as to the transmission of terrestrial radiation to space. Should solar radiation be an internal process or an external driver of the hydrologic cycle, weather, and air surface temperatures Compounds of sulfur and nitrogen are associated with acidic precipitation and damage to vegetation, aquatic life, and physical structures. 

As shown in Figure 2.1, the free water on Earth s surface is now transported between the land, atmosphere, ocean, and mantle through a global hydrological cycle. From... 

Cyclic salts Salt ions that undergo rapid cycling between the ocean, atmosphere and land. This cycle involves ejection into the atmosphere via bursting bubbles and return via either dry deposition onto the sea surface or onto land followed by runoff back into the ocean. 

Land-atmosphere exchange processes include the evaporation of soil moisture, from the leaf surface, stems, and trunks of plants, as well as transpiration, precipitation, and evaporation olf the surface of unstable water accumulations low in the ground (Figure 4.11). The water flow from the soil through the plant is the least studied link in this chain. The importance of the process of transpiration in the global water cycle cam be judged from available estimates, according to which the process of... 

Human civilization interferes more and more with the cycles that cormect land, water, and atmosphere, and pollution seriously affects water quahty. In order to assess the stresses caused to aquatic ecosystems by chemical perturbation, the distribution of pollutants and their fate in the environment must be investigated (see Air pollution). 

Water is omnipresent on the earth. Constant circulation of water from the ocean to the atmosphere (evaporation) and from the atmosphere to land and the oceans (precipitation, mnoff, etc) is generally known as the hydrologic cycle (see Fig. 1) (1 2). Within the hydrologic cycHc, there are several minor and local subcycles where water is used and returned to the environment. 

The oceans hold about 97% of the earth s water. More than 2% of the total water and over 75% of the freshwater of the world is locked up as ice ia the polar caps. Of the remaining 1% of total water that is both Hquid and fresh, some is groundwater at depths of > 300 m and therefore impractical to obtain, and only the very small difference, possibly 0.06% of the total water of this planet, is available for human use as it cycles from sea to atmosphere to land to sea. Only recently have humans been able to regulate that cycle to their advantage, and even now (ca 1997), only infinitesimally, ia some few isolated places. 

The continuous interchange of nitrogen between the atmosphere and the biosphere is called the nitrogen cycle. Global estimates are difficult to obtain and there are frequently regional and local impacts which vary greatly from the mean. However, some indication of the size of the various reservoirs of nitrogen in the atmosphere, on land, and in the seas is... 

Figure 1. Changes in global climate due to increased atmospheric CO2 will alter carbon cycle processes in land, continent margins, and oceans, which will in turn effect the atmospheric C02concentration. Processes that may have effects large enough to Eilter future projections of atmospheric CO2 are listed under their geographic region.

Several studies, based on models, examined the effects of land-use change on the global carbon cycle and conclude that there is a net release of carbon due to land clearing. However, the results and conclusions of these studies are based on assumed sizes of vegetation carbon pools which are inputs to the models. For example, Melillo et al. 24) concluded that boreal and temperate deciduous forests of the northern hemisphere are net sources of atmospheric carbon. Their analysis used values for carbon density derived by Whittaker and Likens 19) from work by Rodin and Bazilevich (27). Rodin and Bazilevich extrapolated results of small, unrelated studies in Europe and the USSR to estimate total biomass of Eurasian boreal and temperate deciduous forests. Their estimates have since been extrapolated to forests worldwide and are used often today. 

Bjorkstrom, A. 1979. A model of CO2 interaction between atmosphere,oceans, and land biota. In The Global Carbon Cycle, Bolin, B. Degens, E. T. Kempe, S. Ketner, P., Eds. SCOPE 13 J Wiley Sons New York, NY, 1979 pp 403-457. 

The cycles of carbon and the other main plant nutrients are coupled in a fundamental way by the involvement of these elements in photosynthetic assimilation and plant growth. Redfield (1934) and several others have shown that there are approximately constant proportions of C, N, S, and P in marine plankton and land plants ("Redfield ratios") see Chapter 10. This implies that the exchange flux of one of these elements between the biota reservoir and the atmosphere - or ocean - must be strongly influenced by the flux of the others. 

Five components of the hydrosphere play major roles in climate feedbacks - atmospheric moisture, clouds, snow and ice, land surface, and oceans. Changes to the hydrologic cycle, among other things, as a result of altered climate conditions are then referred to as responses. Interactions with climate can best be explored by examirung potential response to a climate perturbation, in this case, predicted global warming. 

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

Unlike other biogeochemical elements, phosphorus does not have a significant atmospheric reservoir. Thus, while some amount of phosphorus is occasionally dissolved in rain, this does not represent an important link in the phosphorus cycle. River runoff is the primary means of transport between the land surface and oceans, and unlike the other elements discussed. 

Large amounts of carbon are found in the terrestrial ecosystems and there is a rapid exchange of carbon between the atmosphere, terrestrial biota, and soils. The complexity of the terrestrial ecosystems makes any description of their role in the carbon cycle a crude simplification and we shall only review some of the most important aspects of organic carbon on land. Inventories of the total biomass of terrestrial ecosystems have been made by several researchers, a survey of these is given by Ajtay etal.(1979). 

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