Modeling of Environmental Systems

Transcription

1 Modeling of Environmental Systems The next portion of this course will examine the balance / flows / cycling of three quantities that are present in ecosystems: Energy Water Nutrients We will look at each of these at two scales: Global Ecosystem Before we can build models of these phenomena, we need to have some background on the functioning of these systems with respect to these quantities

2 Global Cycling of Nutrients In addition to the movement of water, there are other forms of matter that are critical to sustaining ecosystem function that move through the environment in a cyclical manner A few elements of critical importance, which we refer to as nutrients, include: Nitrogen Phosphorus (Potassium, Sulfur, Magnesium, and Calcium) Additionally, the movement of carbon is also quite important, but because of carbon s relative abundance, we usually do not refer to it as a nutrient, but rather as substrate or some similar term

3 The Global Nitrogen Cycle Organisms require nitrogen to live: While carbon is a much more common element in tissues, nitrogen is required as well Nitrogen is required by the enzymes which mediate the biochemical reactions in which carbon is fixed in photosynthesis and oxidized in respiration In plant tissues, the ratio of carbon:nitrogen is around 50:1, and if nitrogen is not available in this relative abundance to carbon, it can be the limiting factor in ecosystem productivity On the global scale, nitrogen limits the rate of net primary production on land and in the sea

4 Forms of Nitrogen in the Environment In the gross sense, nitrogen is very common on Earth: About 78% of the atmosphere is nitrogen in its inorganic molecular form as N 2 However, most organisms are incapable of making use of nitrogen in this form, so instead most organisms need to gain access to nitrogen that are part of several other chemical species, such as NH 4+, NO, N 2 O, NO 2 -, NO 3 - An important part of the global cycling of nitrogen is its transformation between these various forms Certain organisms, particular microbes, subsist on the energy gained through various transformations of nitrogen

5 Microbial Transformations of Nitrogen Wollast, R Interactions between major biogeochemical cycles in marine Ecosystems, pp in G.E. Likens (Ed.), Some Perspectives of the Major Biogeochemical Cycles. Wiley, New York.

6 Nitrogen Transformations There are two sorts of transformations that are of particular importance: Nitrogen fixation converts nitrogen from its inorganic molecular form (an even form) to one of the organic ( odd ) forms, e.g. A certain type of bacteria might fix molecular nitrogen to ammonia: N 2 NH 3 The opposite sort of process also occurs, where organic ( odd ) forms of nitrogen are converted back to inorganic ( even ) molecular nitrogen, and this is called denitrification, e.g. Another sort of bacteria might denitrify nitrate or nitrite: NO 2- or NO 3- N 2

7 The Global Nitrogen Cycle Atmosphere (3.9x10 9 ) Human Activity Land 100 Biological Fixation 140 Lightning Fixation <= (3.5x10 3 in terrestrial biomass) (95 to140x10 3 in soil organic matter) Denitrification <= 200 River Flow Denitrification Ocean (5.7x10 5 ) Biological Fixation 15 Flow units are g N/year, stores are (10 12 g N) Transformation processes: Fixation Denitrification

8 The Global Nitrogen Cycle Plants can only make use of nitrogen as NH 4+ &NO 3 - Because of the strength of the triple bond between the pair of nitrogen atoms in inorganic molecular nitrogen (N 2 ), this chemical species is practical inert All of the nitrogen that is available to biota was made available through nitrogen fixation -- either by lightning fixation ( <= 5 Tg/yr) or by microbes (140 Tg/yr, of which asymbiotic microbes accounts for 44 Tg/yr and symbiotic fixation in higher plants accounts for the remaining amount)

9 The Global Nitrogen Cycle If we assume that terrestrial net primary production is about 60 Pg/yr (1Pg = 1000Tg = g), and we assume the average C/N ratio is 50:1, then the nitrogen requirement of land plants is about 1200 Tg Thus, lightning and microbial fixation of nitrogen only accounts for 12% of the demand. The remaining of the nitrogen is derived from recycling and decomposition of dead materials in the soil Rivers carry 36 Tg/yr into the ocean, half of which is now from human additions of fixed nitrogen. This has a great impact on the coastal sea and estuaries, and is of both environmental and economic concern

10 Human Impacts on the Nitrogen Cycle Humans have been modifying the nitrogen cycle when we began planting crops that result in heightened nitrogen fixation We now produce fertilizer (80 Tg N/yr) through the burning of natural gas and molecular nitrogen at high temperatures and pressures, a form of fixation that produces ammonia Fossil fuel combustion releases about 20 Tg N/yr: This is not technically fixation since much of this N was already in organic form, but it takes N that was stored and releases it to the atmosphere (as NO x ), which eventually precipitates on land, having considerable impact on forested ecosystems downwind (non-point source pollution)

11 The Global Phosphorus Cycle Phosphorus is also necessary for organic life: It is an essential component in DNA, cell membranes, and the two related organic compounds which provide a key mechanism for the storage and release of energy Adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are made up of adenosine bonded to either two or three phosphate groups When a phosphate group is removed from ATP to produce ATP, energy is released: ATP ADP + energy + phosphate When a phosphate group is added to ADP to produce ATP, energy must be added: ADP + energy + phosphate ATP Rechargeable Batteries

12 The Global Phosphorus Cycle The ATP/ADP cycle provides energy for cellular activity and is a key part of plant productivity Photosynthesis, respiration and ATP/ADP are related: Photosynthesis stores energy, respiration releases it, and ATP is the central molecule in this process Thus, plants require phosphorus to live, and much like nitrogen, if it is not available in sufficient quantity, it can be the limiting factor in productivity However, the global phosphorus cycle differs from that from nitrogen in several ways: In particular, the global phosphorus cycle has no significant gaseous component

13 The Global Phosphorus Cycle A small amount of P in dust Significant flow from land to ocean via rivers (21 Tg/yr) Terrestrial P derived from weathering of Calcium phosphate minerals (apatite) Schlesinger, W.H Biogeochemistry: An Analysis of Global Change. Harcourt, Brace and Co., USA: p.397. Most phosphorus compounds are not very water soluble, thus few chemical transformations

14 The Global Phosphorus Cycle The largest flow of phosphorus in the global cycle is from rivers to the oceans (21 Tg/yr), and about 10% of this is in reactive form which can be used by marine organisms The remainder is strongly bound to soil particles that deposit on the continental shelf. On a time scale of hundreds of millions of years, these sediments mineralize and become rock, and are uplifted and subject to rock weathering on land So while there are significant stores of P on land and in the sea, very little is accessible to organisms. Thus, there is significant internal cycling where the available P is reused quite efficiently in ecosystems

15 The Global Carbon Cycle Living tissue is primarily composed of carbon, so estimates of the disposition of carbon globally (like NPP) give us a good sense of the extent to which ecosystems are thriving or struggling Carbon is abundantly available in the atmosphere as two gaseous species, CO 2 and CH 4 Carbon is withdrawn from the atmosphere and added to organic biomass through photosynthesis, and vice-versa occurs through the process of respiration Over the past billions of years, the concentration of atmospheric CO 2 has diminished as its removal from the atmosphere has exceeded its addition, demonstrating organisms ability to change the planet

16 The Global Carbon Cycle Units are Pg (10 15 ) rather than Tg (10 12 ) Fossil fuel burning Compare Net increase LULC change as Veg. is cleared ~15% of the atmospheric pool is taken by terrestrial organisms Another large pool in soils, increasingly available as permafrost melts? Schlesinger, W.H Biogeochemistry: An Analysis of Global Change. Harcourt, Brace and Co., USA: p.359.

17 The Global Carbon Cycle Units are Pg (10 15 ) rather than Tg (10 12 ) Human Activity 6.9 Change in the Atm. 5.2 Potential feedbacks: As [CO 2 ] increases, plant uptake increases = 1.7 Where are the other 1.7 Petagrams of C? Uncertainties in figures? Boreal (and temperate) forest growth? As global climate changes, vegetation distribution changes, and [CO 2 ] changes? Schlesinger, W.H Biogeochemistry: An Analysis of Global Change. Harcourt, Brace and Co., USA: p.359.

18 Global Flux of Carbon Botkin, DB, Keller EA Environmental Science: Earth as a Living Planet. Wiley, USA: p.66.

19 NDVI from AVHRR Feb 27-Mar 12 Apr 24-May 7 Jun 19-Jul 2 Jul 17-Jul 30 Aug 14-Aug 27 Nov 6- Nov19

20 Remote Sensing of Vegetation in the News