Major Cycles

The biota modifies chemical fluxes. In the absence of biota, the rate and direction of chemical fluxes would be controlled solely by the physical and chemical factors determining exchanges between abiotic pools. Chemicals would be retained at a site only to the extent that chelation or concentration gradients restricted leaching or diffusion. Exposed nutrients would continue to move with wind or water (erosion). Biotic uptake and storage of chemical resources creates a biotic pool that reduces chemical storage in abiotic pools, altering rates of exchange among abiotic pools and restricting movement of nutrients across chemical and topographic gradients. For example, the uptake and storage of atmospheric CO2 by plants (including long-term storage in fossil biomass, i.e., coal, oil and gas) and the uptake and storage of calcium carbonate by marine animals (and deposition in marine sediments) control concentration gradients of CO2 available for exchange between the atmosphere and ocean (Keeling et al. 1995, Sarmiento and Le Quéré 1996). Conversely, fossil fuel combustion, deforestation and desertification, and destruction of coral reefs are reducing CO2 uptake by biota and releasing CO2 from biotic storage, thereby increasing global CO2 available for exchange between the atmosphere and ocean. Biotic uptake of various sedimentary nutrients retards their transport from higher elevations back to marine sediments.

Consumers, including insects, affect the rate at which nutrients are acquired and stored (see Chapters 12-14). Consumption reduces the biomass of the lower trophic level, thereby affecting nutrient uptake and storage at that trophic level, and moves nutrients from consumed biomass into biomass at the higher trophic level (through secondary production) or into the detritus (through secretion and excretion) where nutrients become available to detritivores and soil microorganisms or are exported via water flow to aquatic food webs. Nutrients are recycled through decomposition of dead plant and animal biomass, which releases simple organic compounds or elements into solution for reacquisition by autotrophs.

Some nutrients are lost during trophic transfers. Carbon is lost (exported) from ecosystems as CO2 during respiration. Gaseous or dissolved CO2 remains available to organisms in the atmosphere and oceanic pools. Organic biomass can be blown or washed away. Soluble nutrients are exported as water percolates through the ecosystem and enters streams. The efficiency with which nutrients are retained within an ecosystem reflects their relative availability. Nutrients such as nitrogen and phosphorus often are limiting and tend to be cycled and retained in biomass more efficiently than are nutrients that are more consistently available, such as potassium and calcium. The following four examples exemplify the processes involved in biogeochemical cycling.

1. Hydric Cycle

Water availability, as discussed in Chapters 2 and 9, is one of the most important factors affecting the distribution of terrestrial organisms. Many organisms are modified to optimize their water balances in arid ecosystems (e.g., through their adaptations for acquiring and retaining water; Chapter 2). Water available to plants is a primary factor affecting photosynthesis and ecosystem energetics (see earlier in this chapter). Water absorbs solar energy, with little change in temperature, thereby buffering humid ecosystems against large changes in temperature. At the same time, water use by organisms significantly affects its passage through terrestrial ecosystems.

The primary source of water for terrestrial ecosystems is water vapor from evaporation over the oceans (Fig. 11.6). The availability of water to terrestrial ecosystems is controlled by a variety of factors, including the rate of evaporation from the ocean, the direction of prevailing winds, atmospheric and topographic factors that affect convection and precipitation, temperature, relative humidity, and soil texture. Water enters terrestrial ecosystems as precipitation and condensation and as subsurface flow and groundwater derived from precipitation or condensation at higher elevations. Condensation may be a major avenue for water input to arid ecosystems. Many plants in arid regions are adapted to acquire

Convection Current And Steps

Runoff

The hydric cycle. Net evaporation over the oceans is the source of water vapor carried inland by air currents. Water precipitated into terrestrial ecosystems eventually is returned to the ocean.

Runoff

The hydric cycle. Net evaporation over the oceans is the source of water vapor carried inland by air currents. Water precipitated into terrestrial ecosystems eventually is returned to the ocean.

water through condensation. Some desert insects also acquire water through condensation on specialized hairs or body parts (R. Chapman 1982). Vegetation intercepts up to 50% of precipitation, depending on crown structure and plant surface area (G. Parker 1983). Most intercepted water evaporates. The remainder penetrates the vegetation as throughfall (water dripping from foliage) and stemflow (water funneled to stems).

Vegetation takes up water primarily from the soil, using some in the synthesis of carbohydrates. Vascular plants conduct water upward and transpire much of it through the stomata. Evapotranspiration is the major mechanism for maintaining the upward capillary flow of water from the soil to the canopy. This active evaporative process greatly increases the amount of water moving back into the atmosphere, rather than flowing downslope, and may increase the availability of water for precipitation at a particular site, as discussed later in this chapter.

Vegetation stores large amounts of water intracellularly and extracellularly and controls the flux of water through the soil and into the atmosphere. Accumulation of organic material increases soil water storage capacity and further reduces downslope flow. Soil water storage mediates plant acquisition of other nutrients in dissolved form. Food passage through arthropods and earthworms, together with materials secreted by soil microflora, bind soil particles together, forming soil aggregates (Hendrix et al. 1990, Setala et al. 1996). These aggregates increase water and nutrient storage capacity and reduce erosibility. Burrowing organisms increase the porosity and water storage capacity of soil and decomposing wood (e.g., earthworms and wood borers) (e.g., Eldridge 1994). Macropore flow increases the rate and depth of water infiltration.

Some organisms also control water movement in streams. Swamp and marsh vegetation restricts water flow in low-gradient ecosystems. Trees falling into stream channels impede water flow. Similarly, beaver dams impede water flow and store water in ponds. However, water eventually evaporates or reaches the ocean, completing the cycle.

2. Carbon Cycle The carbon cycle (Fig. 11.7) is particularly important because of its intimate association with energy flow, via the transfer of chemical energy in carbohydrates, through ecosystems. Carbon is stored globally both as atmospheric carbon dioxide and as sedimentary and dissolved carbonates (principally calcium carbonate). The atmosphere and ocean mediate the global cycling of carbon among terrestrial and aquatic ecosystems. The exchange of carbon between atmosphere and dissolved or precipitated carbonates is controlled by temperature, carbonate concentration, salinity, and biological uptake that affects concentration gradients (Keeling et al. 1995, Sarmiento and Le Quéré 1996).

Carbon enters ecosystems primarily as a result of photosynthetic fixation of CO2 in carbohydrates. The chemical energy stored in carbohydrates is used to synthesize all the organic molecules used by plants and animals. Carbon enters many aquatic ecosystems, especially those with limited photosynthesis, primarily as allochthonus inputs of exported terrestrial materials (e.g., terrestrial organisms captured by aquatic animals, detritus, and dissolved organic material entering

Jpg Organic Matter Plant Cycle

The global carbon cycle. The atmosphere is the primary source of carbon for terrestrial ecosystems (left), whereas dissolved carbonates and bicarbonates are the primary source of carbon for marine ecosystems (right). Exchange of carbon between atmosphere, hydrosphere, and geosphere is regulated largely by biotic uptake and deposition.

The global carbon cycle. The atmosphere is the primary source of carbon for terrestrial ecosystems (left), whereas dissolved carbonates and bicarbonates are the primary source of carbon for marine ecosystems (right). Exchange of carbon between atmosphere, hydrosphere, and geosphere is regulated largely by biotic uptake and deposition.

with runoff or leachate). Carbon is transferred among trophic levels through consumption, converted into an astounding diversity of compounds for a variety of uses, and eventually is returned to the atmosphere as CO2 from respiration, especially during decomposition of dead organic material, completing the cycle. However, loss of carbon from an ecosystem is minimized by rapid acquisition and immobilization of soluble and fine particulate carbon by soil organisms and aquatic filter feeders, from which carbon becomes available for transfer within soil and aquatic food webs (de Ruiter et al., 1995, J. Wallace and Hutchens 2000).

However, some carbon compounds (especially complex polyphenols, e.g., lignin) decompose very slowly, if at all, and are stored for long periods as soil organic matter, peat, coal, or oil. Humic compounds are phenolic polymers that are resistant to chemical decomposition and constitute long-term carbon storage in terrestrial soils. These compounds contribute to soil water and nutrient-holding capacities because of their large surface area and numerous binding sites. Plants produce organic acids that are secreted into the soil through roots. These acids facilitate extraction of mineral nutrients from soil exchange sites, maintain ionic balance (with mineral cations), reduce soil pH, and often inhibit decomposition of organic matter. Similarly, peat accumulates in bogs where low pH inhibits decomposition and eventually may be buried, contributing to formation of coal or oil. Coal and oil represent long-term storage of accumulated organic matter that decomposed incompletely as a result of burial, anaerobic conditions, and high pressure. The carbon removed from the atmosphere by these fossil plants is now reentering the atmosphere rapidly, as a result of fossil fuel combustion, leading to increased atmospheric concentrations of CO2.

3. Nitrogen Cycle

Nitrogen is a critical element for synthesis of proteins and nucleic acids and is available in limited amounts in most ecosystems. The atmosphere is the reservoir of elemental nitrogen, making nitrogen an example of a nutrient with an atmospheric cycle (Fig. 11.8). Most organisms cannot use gaseous nitrogen and many other nitrogen compounds. In fact, some common nitrogen compounds are toxic in small amounts to most organisms (e.g., ammonia). Nitrogen cycling is mediated by several groups of microorganisms that transform toxic or unavailable forms of nitrogen into biologically useful compounds.

Gaseous N2 from the atmosphere becomes available to organisms through fixation in ammonia, primarily by nitrogen-fixing bacteria and cyanobacteria. These organisms are key components of most ecosystems but are particularly important in ecosystems subject to periodic massive losses of nitrogen, such as through fire. Many early successional plants, especially in fire-dominated ecosystems, have symbiotic association with nitrogen-fixing bacteria in root nodules. These plants can use the ammonia produced by the associated bacteria, but most plants require nitrate (NO3) as their source of nitrogen.

Nitrogen Cycle Nitrifying Bacteria

The nitrogen cycle. Bacteria are the primary organisms responsible for transforming elemental nitrogen into forms available for assimilation by plants. Note that the return of nitrogen to the atmospheric pool occurs almost exclusively under anaerobic conditions.

The nitrogen cycle. Bacteria are the primary organisms responsible for transforming elemental nitrogen into forms available for assimilation by plants. Note that the return of nitrogen to the atmospheric pool occurs almost exclusively under anaerobic conditions.

Ammonium compounds also are produced by lightning and volcanic eruptions. Nitrifying bacteria oxidize ammonia to nitrite (NO2) and nitrate, which then is available to plants for synthesis of amino acids and nucleic acids and transferred to higher trophic levels through consumption. The nitrogen compounds in dead organic matter are decomposed to ammonium by ammonifying bacteria. Organic nitrogen enters aquatic ecosystems as exported terrestrial organisms, detritus, or runoff and leachate solutions. Nitrogen in freshwater ecosystems similarly is transferred among trophic levels through consumption, eventually reaching marine ecosystems. Under anaerobic conditions, which occur naturally and as a result of anthropogenic eutrophication or soil compaction, the biotic cycle can be disrupted by anaerobic denitrifying bacteria that convert nitrate to gaseous nitrogen, which is lost to the atmosphere, thereby completing the cycle. However, nitrogen loss is minimized by soil organisms that aerate the soil through excavation and by the rapid acquisition and immobilization of soluble nitrogen by soil microorganisms and aquatic filter feeders, from which nitrogen becomes available to plants and to soil and aquatic food webs.

4. Sedimentary Cycles Many nutrients, including phosphorus and mineral cations, are available only from sedimentary sources. These nutrients are cycled in similar ways, as exemplified by phosphorus (Fig. 11.9). Phosphorus is biologically important in molecules that mediate energy exchange during metabolic processes (adenosine triphosphate [ATP] and adenosine diphosphate [ADP]) and in phospholipids. Like nitrogen, it is available to organisms only in certain forms and is in limiting supply in most ecosystems. Phosphorus and mineral cations become available to terrestrial ecosystems as a result of chemical weathering or erosion of geologically uplifted, phosphate-bearing sediments.

Phosphate enters an ecosystem from weathered bedrock and moves among terrestrial ecosystems through materials washed downslope or filtered from the air. Phosphorus is highly reactive but available to plants only as phosphate, which often is bound to soil particles. Plants extract phosphorus (and mineral cations) from cation exchange and sorption sites on soil particles and from soil solution. Phosphorus then is synthesized into biological molecules and transferred to higher trophic levels through consumption; it eventually is returned to the soil as dead organic matter and is decomposed. Phosphorus enters aquatic ecosystems largely in particulate forms exported from terrestrial ecosystems. It is transferred between aquatic trophic levels through consumption, eventually being deposited in deep ocean sediments, completing the cycle. Phosphorus loss is minimized by soil organisms and aquatic filter feeders, which rapidly acquire and immobilize soluble phosphorus and make it available for plant uptake and exchange among soil and aquatic organisms.

Oplan Termites

Oplan Termites

You Might Start Missing Your Termites After Kickin'em Out. After All, They Have Been Your Roommates For Quite A While. Enraged With How The Termites Have Eaten Up Your Antique Furniture? Can't Wait To Have Them Exterminated Completely From The Face Of The Earth? Fret Not. We Will Tell You How To Get Rid Of Them From Your House At Least. If Not From The Face The Earth.

Get My Free Ebook


Post a comment