Factors Influencing Cycling Processes

A number of factors alter the rates and pathways of biogeochemical fluxes. Variation in fluxes reflects the chemical properties and source of the nutrient;

Solutions The Phosphorus Cycle

Sedimentary cycle. Phosphorus and other nongaseous nutrients precipitate from solution and are stored largely in sediments of marine origin. These nutrients become available to terrestrial ecosystems primarily through chemical weathering of uplifted sediments.

Sedimentary cycle. Phosphorus and other nongaseous nutrients precipitate from solution and are stored largely in sediments of marine origin. These nutrients become available to terrestrial ecosystems primarily through chemical weathering of uplifted sediments.

interactions with other cycles; and the composition of the community, especially the presence of specialized organisms that control particular fluxes. Hence, changes in community composition resulting from disturbance and recovery alter the rates and pathways of chemical fluxes.

The chemical properties of various elements and compounds, especially their solubility and susceptibility to pH changes, and biological uses affect cycling behavior. Some elements, such as Na and K, form compounds that are readily soluble over normal ranges of pH. These elements generally have high rates of input to ecosystems via precipitation but also high rates of export via runoff and leaching. Other elements, such as Ca and Mg, form compounds that are not as soluble over usual ranges of pH and have lower rates of input and export. Elements such as nitrogen and phosphorus are necessary for all organisms, relatively limiting, and generally conserved within organisms. For example, deciduous trees usually resorb nitrogen from senescing foliage prior to leaf fall (Marschner 1995). Sodium has no known function in plants and is not retained in plant tissues, but it is required by animals for osmotic balance and for muscle and nerve function. Consequently, it is conserved tightly by these organisms. In fact, animals often seek mineral sources of sodium (e.g., Seastedt and Crossley 1981b). Many decay fungi accumulate sodium (Cromack et al. 1975, Schowalter et al. 1998), despite absence of apparent use in fungal metabolism, perhaps to attract animal vectors of fungal spores.

Biogeochemical cycles interact with each other in complex ways (Daufresne and Loreau 2001, Elser and Urabe 1999, Rastetter et al. 1997, Sterner and Elser 2002). For example, precipitation affects decomposition and carbon storage in soils (Schuur et al. 2001). Some plants respond to increased atmospheric CO2 by reducing stomatal opening, thereby acquiring sufficient CO2 while reducing water loss. Hence, increased size of the atmospheric pool of CO2 may alter transpiration, permitting some plant species to colonize more arid habitats. Similarly, the calcium cycle interacts with cycles of several other elements. Calcium carbonate generally accumulates in arid soils as soil water evaporates. Acidic precipitation, such as resulting from industrial emission of nitrous oxides and sulfur dioxide into the atmosphere, dissolves and leaches calcium carbonate from soils and sediments. Soils with high content of calcium carbonate are relatively buffered against pH change, whereas soils depleted of calcium carbonate become acidic, increasing export (through leaching) of other cations as well.

Some biogeochemical fluxes are controlled by particular organisms. The nitrogen cycle depends on several groups of microorganisms that control the transformation of nitrogen among various forms that are available or unavailable to other organisms (see earlier in this chapter). Soil biota secrete substances that bind soil particles into aggregates that facilitate retention of soil water and nutrients. Some plants (e.g., western redcedar, Thuja plicata, and dogwoods, Cornus spp.) accumulate calcium in their tissues (Kiilsgaard et al. 1987) and generally increase pH and buffering capacity of surrounding soils. Their presence or absence thereby affects retention of other nutrients, as well. Oaks, Quercus spp., and spruces, Picea spp., emit large amounts of carbon as volatile isoprene that affects the oxidation potential of the atmosphere (Lerdau et al. 1997). Changes in community composition following disturbance or during succession affect rates and pathways of biogeochemical fluxes. Early successional communities frequently are inefficient because of limited competition for resources by the small biomass, and early successional species have little selective pressure to retain nutrients. For example, the early successional tropical tree, Cecropia spp., has large, thin leaves that transpire water more rapidly than the smaller, more scle-rotized leaves of later successional species. Although later successional communities are not always efficient, declining resource supply relative to growing biomass promotes efficiency of nutrient retention within the ecosystem (E. Odum 1969, Schowalter 1981).

Agricultural and silvicultural systems are inefficient largely because communities composed of a single, or few, plant species cannot acquire or retain all available forms of matter effectively. Furthermore, the diversity of organisms in natural systems may increase per capita resource acquisition or provide overall resistance to herbivores and pathogens (Cardinale et al. 2002, A. Hunter and Arssen 1988). Nitrogen fixation often is controlled by noncommercial species, such as symbiotic nitrogen-fixing lichens, herbs and shrubs, or structures such as large decomposing woody litter, that are suppressed or eliminated by management activities. Necessary nitrogen then must be supplied anthropogenically, often in excess amounts that leach into groundwater and streams. Exotic species also can alter nutrient cycling processes. Liu and Zou (2002) reported that invasion of tropical pastures and wet forest in Puerto Rico by exotic earthworms significantly increased decomposition rates.

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