The Global Water and Nitrogen Cycles
(R. Robarts & R. Wetzel, SIL News V. 29, Jan 2000)
Global Water CycleThe water cycle contains the largest chemical flux on earth. Water distributes heat around the globe and thus creates climate, and water is the single most important factor regulating land-plant productivity worldwide. Without water life would not exist (perhaps on ANY planet), and despite the fact that 70% of the earth's surface is covered with water, that water is salty and can't be used for drinking, agriculture, or industry. Only about 0.014% of the water at earth's surface is useable by plants, humans, and other animals.
In attempting to understand element cycles as part of the major functioning of ecosystems, it is useful to follow a specific "approach". This general approach was followed in the previous lecture on the carbon cycle, and it can be used to help understand any element cycle. It consists of 3 parts and is formally outlined below:
Figure 1. The Laurentian Great Lakes.
Accounting for Water
|Rocks (not usable)||25,000|
|Oceans (97.4% of usable water)||1,350|
|Lakes and Rivers||0.025|
Figure 2. The distribution of water at the earth's surface.
As you can see from the Table and from Figure 2, most of the water on earth is tied-up in rocks and unavailable. Of the water that is at the surface of the earth and available for cycling, only a very small percentage is fresh water. Of that fresh water, about 20% is contained solely in the Laurentian Great Lakes in North America (Figure 1), and another 20% is contained in a single lake in Siberia, Lake Baikal.
There are 4 major pathways of cycling in the global water cycle (Figure 3): precipitation, evaporation, vapor transfer from ocean to land, and return flow in rivers and groundwaters from land to oceans. The following gives the flux of these different pathways:
= 3,176 years
Figure 3. The Global Water Cycle - Pathways and Fluxes. (Values in 103 km3/yr).
ControlsThere are several major controls on the water cycle, including human consumption, temperature increases, and land use changes.
(A). Human consumption. The consumption of water by humans has increased dramatically since the industrial revolution, and today water is a critically lacking resource in certain areas such as deserts and semi-deserts. In addition to this local vulnerability, it is quite likely that water shortages due to human consumption will occur at the regional scale in the near future. For example, the southwestern United States (in all seriousness) has proposed to "buy" water from the Great Lakes states and build a pipeline from Lake Michigan -- so far Michigan, Wisconsin, and other nearby states and Canadian provinces have declined such offers.
Figure 4. Rates of water withdrawn from surface and groundwater sources, and consumption per individual for representative countries.
(B). Temperature. The second major control on the cycling of water on earth is temperature. Increasing temperature increases the rates of evaporation and ice melting, and causes sea level to rise. Severe droughts, like in the Sahel in Africa, are caused by small changes in the geographical distribution of water that are in turn caused by changes in temperature. In Figure 5 below there are some examples of the effect that increased global temperatures have had on glaciers in recent years.
2. Sea level rise. Sea level has been rising in the world in recent years. Figures 6 and 7 below show first how large these changes have been in various parts of the world, and second how much of this increase is due simply to the thermal expansion of water as temperature Increases. Figure 8 shows the effect of a rise of 4.7 m in sea level on Florida. Note that while 4.7 m may seem like a large increase, during the last glaciation sea level was a full 100 m lower than it is today; with that time perspective, a change of 4.7 m does not seem so large.
Figure 8 & 9 (below). Impact of sea level rise on low-lying areas of Florida (left). The conveyor belt circulation (right) of the ocean may be altered by increasing freshwater inputs to the Arctic ocean.
(C). Land use changes. Currently most of the land use changes on earth, such as deforestation, are at a local scale. However, it may soon become important at regional scales and for the entire globe in the future. For example, in a study done at Hubbard Brook in New Hampshire, run-off increased by up to 400% after deforestation. Nutrient cycles are strongly linked to hydrologic cycle, and so nutrient export was also increased. These increases are only temporary, however, and the likely end result of such land use changes is that precipitation will be decreased (this will be discussed in more detail in the upcoming lecture on the Tropical Rainforest), and soils will become less fertile. This illustrates one of the key points about element cycles, which is that they are most often linked and it is difficult to study them in isolation. In this example, we found that the water cycle strongly controls the nutrient cycling due to the transport of nutrients in runoff. In the next section we will examine the nitrogen cycle specifically as an example of a global cycle of an important nutrient.
The cycling of nitrogen is different from the cycling of water in at least one important area, which is that the "forms" of nitrogen are more varied than the form of water, which is always H2O and in either a liquid, gas, or solid form. The nitrogen cycle is complex then in part because of the many chemical forms of N such as: Organic-N; NO3; NH4; and the gases N2, N2O, NO + NO2 (=NOx). Figure 10 below gives an overview of the global nitrogen cycle.
Figure 10. The Global Nitrogen Cycle
Just as we did for the water cycle, or first step in understanding the nitrogen cycle is to examine the distribution of N on earth. The Table below gives the distribution of N in x1015 grams. Notice that the largest pool of available N is in the atmosphere.
The pathways and the reactions involved in the nitrogen cycle are also more complicated than in the water cycle due, again, to the fact that there are different chemical forms. The major pathways are shown in Figure 10, and these pathways can be linked to specific chemical reactions that are listed below and shown in Figure 11.
Figure 11. The processes controlling the conversion of one form of nitrogen into another form. There are aerobic processes (with oxygen) at the top of the figure, and anaerobic processes (w/o oxygen) at the bottom of the figure. Notice that the chemical conversions begin with the fixation of N2 gas from the atmosphere by N-fixation, and end with the return of N2 gas to the atmosphere by denitrification.
Nitrogen Fluxes (with respect to the atmosphere). Given the information above, we can calculate some of the fluxes of various nitrogen chemical species and their residence times in the atmosphere.
* RT of N2 = 24.68 million years
* RT of NOx = 0.01 yr = 3.6 days
There are very many "controls" on the overall nitrogen cycle, but in this lecture we will focus on the controls that are related directly to a major environmental problem on earth, which is acid rain. The general way in which acid rain is formed is given in Figure 12 below.
Figure 12. Diagram of the pathways and reactions leading to the formation of acid rain in our atmosphere.
Effects of acid rain
It is important to learn and understand that most biogeochemical questions must be solved by combining information about several element cycles. This is because most element cycles interact strongly with surrounding elements, and so for example to help solve the problems of acid rain we must first understand the controls on the elements that interact. The two Figures below (13 and 14), illustrate some of these interactions.
Figures 13 and 14 (left and below). Chemical reactions of acid rain components in soils.
The important aspects of these figures can be summarized as follows:
Figure 15. Diagram
of the situation in a typical forest before acid rain (~1880) and after
By measuring the buffering capacity of soils, you can determine regions of sensitivity to acid rain, as is shown below in the map of the United States.
Figure 16. Regions of sensitivity to acid rain in the United States. Also shown are the isopleths of the pH of precipitation; for example, all of the eastern U.S. currently has an average pH of rainfall between 4-5, where "neutral" rainwater has a pH of ~6.
"ELEMENT CYCLES INTERACT" and they cannot be studied in isolation.
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