Jessica Gurevitch, Samuel M. Scheiner, and Gordon A. Fox - The Ecology of Plants - 2002 / The Ecology of Plants Jessica Gurevitch, Samuel M. Scheiner, and Gordon A. Fox; 2002 / Ch 15

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Biogeochemical Cycles:

Quantifying Pools and Fluxes

A seed, with its embryo and other associated tissues, contains very limited quantities of energy and materials. Seeds are generally very small—orders of magnitude smaller than the plants into which they will grow. The spore that will develop into a mature fern is even smaller than most seeds. As the new individual grows into a mature plant, it transforms inorganic forms of carbon and other materials into complex organic molecules, and it stores the energy captured from photons of light in the form of chemical bonds in these molecules (see Part I). In terrestrial systems, most of the materials and energy used by heterotrophs—animals, fungi, bacteria, and pro- tists—are transformed from unusable inorganic forms into usable organic materials by plants. (Photosynthetic bacteria, diatoms, green and red algae, and other organisms play a similar role in aquatic systems and sometimes in soils.) The story is actually more complicated than that, however, because some of the materials needed by plants must first be transformed by microorganisms into forms the plants can use (see Chapter 4).

The quantity of material transformed into organic forms by an individual plant can range from fractions of grams to metric tons (1 metric ton = 1000 kg), depending on the size of the plant and the substance being transformed. At the ecosystem scale, this can add up to many metric tons of material. Terrestrial primary productivi- ty—the amount of carbon transformed from CO2 into organic carbon (C) by terrestrial plants per unit area per year—is typically on the order of 5 to 10 metric tons per hectare (far smaller amounts are also fixed by other organisms). On the global scale, fluxes of carbon, for example, are on the order of 1015 grams of carbon per year (gigatons).

A major focus of ecosystem ecology is understanding what regulates the pools (quantities stored) and fluxes (flows) of materials and energy in the various abiotic and biotic components of ecosystems. The pools and fluxes of some major mineral nutrients in a California chaparral system are shown as an example in Table 15.1.

A basic approach that is often used in accounting for the magnitude of pools and fluxes is the mass-balance approach to constructing nutrient budgets. Simply put, the mass-balance approach states that

inputs – outputs = ∆ storage

In other words, if we can measure everything that goes into a system and everything that comes out, the difference between the two quantities must be reflected by a change, ∆ , in the material stored in the system. The mass-balance approach allows ecosystem ecologists to account for difficult-to-measure quantities by subtrac-

tion from accurate measurements of the other terms (Vitousek and Reiners 1975).

Ecosystem ecologists also seek to make predictions about the future sizes of pools and the magnitude and direction of fluxes. The major fluxes and pools of a substance in a system are collectively called a cycle; for example, we might diagram the global water cycle, or at a smaller scale, the nitrogen cycle or phosphorus cycle of a forest ecosystem. Most of the nutrients that are important to plants are cycled biogeochemically—that is, both biological and chemical reactions are involved.

The material contained in a plant is composed mostly of carbon (C) and oxygen (O) (see Table 4.2). The oxygen available to aboveground parts of plants is almost never limiting to terrestrial plant growth (oxygen can, however, be limiting below ground, particularly in waterlogged soils). Carbon, in the form of CO2, is on average equally available everywhere on Earth (at sea level; it is considerably less available at high altitudes). Yet the production of plant biomass varies enormously over Earth’s surface (see Table 19.2). What are the causes of this tremendous variation?

Briefly, primary productivity is determined both by climatic factors (such as moisture, light, temperature, and length of the growing season) and by the supply of nutrients essential for plant growth. We have examined some of these factors in Part I and in Chapter 14. Here we look at the major factors controlling the ability of plants to obtain essential nutrients, as well as the roles of plants as conduits for the fluxes of those nutrients, as agents controlling those fluxes, and as reservoirs for their storage.

Some essential nutrients are needed by plants in relatively large quantities,* but their availability to plants is limited. Nitrogen (N) and phosphorus (P) are in this category; both are major constituents of essential organic molecules, but their supply is limited in most soils. Consequently, the availability of N and P controls primary productivity in many ecosystems. In contrast, in the case of some nutrients that are available in larger quantities, such as sulfur (S) and potassium (K), primary productivity can determine the rate at which they cycle in the ecosystem (Schlesinger 1997). In both cases, living organisms have a major effect on the geochemistry—the pools and fluxes—of these major components of living things.

In contrast, the cycling of elements that are not major constituents of living things, such as sodium (Na) or aluminum (Al), is relatively independent of the actions of living organisms (Schlesinger 1997). It is because plants have a major role in the local, regional, and global cycles of water and some chemical elements that we include

*You may wish to review Table 4.2, which summarizes the roles of the various mineral elements necessary for healthy plant growth and nutrition.

this topic in a plant ecology textbook. The large part that plants—and humans—play in global biogeochemistry also has major implications for global change, as we will see in Chapter 22.

Cycles of energy and materials are often illustrated by diagrams that show the fluxes and pools of a single element. While this makes for a convenient presentation, it is important to recognize that the cycles of various materials can be highly interconnected, and that they interact to affect plant growth. The carbon cycle, for example, both depends on and strongly affects the nitrogen and phosphorus cycles (Shaver et al. 1992). These cycles are interconnected by their mutual effects on plant

growth, tissue composition, leaf longevity, litter production, and litter decomposition rates. These interconnections determine the flux and pool sizes of C, N, and P. All three cycles are also simultaneously affected by climatic and local environmental factors such as temperature, precipitation, and light availability.

There are large differences in the magnitudes of pools and fluxes among different materials and ecosystems. Fluxes of water and carbon are vastly greater than fluxes of phosphorus, for example. Pools of many nutrients are much larger in living plants in tropical forests than they are in tundra plants. There are generally huge pools of stored carbon in bog soils, substantial (but far

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less) stored carbon in grassland soils, and very little stored carbon in desert soils.

The ultimate sources of the different essential nutrients also differ. The atmosphere is the source of N, O, and C, although plants obtain N from the air only indirectly. The atmosphere contains about 78% N and about 21% O. The concentration of CO2 in the atmosphere, while rising (see Figure 22.2), is surprisingly low—about one-third of one percent on average, or about 370 parts per million by volume (ppmv). Nevertheless, because the volume of the atmosphere is so large, the total pool of carbon in the atmosphere is very large. Large amounts of carbon are also stored as carbonate ions (CO32–) in rock and dissolved in seawater; additional carbon is contained in living biomass and in soils. Liquid water is the source of hydrogen (H), and is the ultimate source of the O released to the atmosphere by photosynthesis. Rock weathering is the major source for most of the other elements needed by plants (such as Ca, Mg, K, Fe, and P), while S comes both from atmospheric deposition and rock weathering (Schlesinger 1997). In addition to new inputs from the atmosphere and the weathering of rock, plants rely on ecosystem recycling of elements (Table 15.2).

Before proceeding with a discussion of individual cycles, we must confront the issue of scale (see Chapter 17), which we purposely blur in this chapter. Ecosystem ecologists treat local, regional, and global cycles separately. Our focus here is that point at which biogeochemistry most strongly interacts with plants. We consider aspects of the ecosystem ecology of several elements in other chapters where they are particularly relevant. Water moves over large scales on Earth, and we focus on the pools and fluxes of water at large scales here. Carbon, nitrogen, and phosphorus move through plants and affect plants at a range of scales, from the rhizosphere to the globe, and we consider these elements over a range of scales both in this chapter and elsewhere.

The Global Water Cycle

Terrestrial plants are the only living things (except humans) to have a substantial effect on the global water cycle (Figure 15.1). Most of the world’s available water (96.5%) is stored, not surprisingly, in the oceans, with all of the fresh (i.e., non-salty) water in the world’s rivers, lakes, ice, atmosphere, and groundwater constituting the remaining few percent. Living things store relatively minuscule amounts of water, but the amount of water that moves through plants by transpiration is large and globally important.

The flux of water through plants can also be important regionally. Plants can contribute a major share of the water in the atmosphere and in regional precipitation, but the effects of plants on local precipitation patterns vary greatly. Precipitation in California, for instance, comes largely from water evaporated over the Pacific Ocean. In contrast, between one-quarter and one-half of the rainfall over the Amazon basin comes from evapotranspiration (evaporation and transpiration) from the Amazon forest itself, with the rest coming from outside the region (Salati and Vose 1984; Eltahir and Bras 1994). Cutting large areas of the Amazon forest can therefore result in decreased evapotranspiration, decreased rainfall, and increased temperatures at the ground surface (Lean and Warrilow 1989; Shukla et al. 1990).

Plants can also indirectly affect water fluxes by intercepting precipitation, reducing the impact of rain hitting the ground, and decreasing runoff from the soil. If vegetation is removed—for example, by clear-cut- ting forests—runoff and soil erosion can be greatly increased, particularly in areas where the terrain is hilly or mountainous. This can sometimes result in flooding and mudslides, further damaging vegetation and threatening human lives and homes. Even in semiarid regions, removal of vegetation can result in reduced precipitation, increased soil warming, and the onset of desertifi-

Atmospheric water (vapor)

Aquifer

Groundwater 23,000

Figure 15.1

The global water cycle. The numbers show the pools (in units of 1000 km3 of water, shown in regular type) held in various major components of the global ecosystem, and the fluxes among those components (1000 km3/year, shown in boldfaced type). Notice that the amount of water leaving the land to return to the oceans (40,000 km3/year) is equal to the amount of water that returns from the oceans to the land as water vapor evaporated from the oceans. The numbers given are approximate, as estimates vary considerably between different authors. (After Schlesinger 1997 and Gleick 1996; data from Lvovitch 1973 and Chahine 1992.)

cation (Schlesinger et al. 1990; Dirmeyer and Shukla 1996).

In intact vegetation, the balance between precipitation, evapotranspiration, and runoff varies greatly among biomes (Table 15.3). In deserts and grasslands, all or most of the water that enters the system as precipitation is lost by evaporation or transpiration, leaving almost nothing for runoff and recharge of groundwater. Both temperate

and tropical forests generally lose less water by evapotranspiration than they receive as precipitation, so considerable amounts often run off into rivers and streams and percolate to the groundwater. Over the Earth as a whole, rivers carry about a third of the precipitation that falls over land back to the oceans. While the amount of rainfall over the oceans is greater than that over land (see Figure 15.1), the evaporation of water from the oceans

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is also much greater than the evapotranspiration of water over land. The net movement of water vapor from the oceans to the land through the atmosphere is balanced by the runoff of water from the land to the oceans in the form of groundwater and river flow. While only a minuscule amount of water is stored as water vapor in the atmosphere, the flux of water vapor through the atmosphere is tremendous.

Potential evapotranspiration (PET) is the maximum amount of water that would be lost to evapotranspiration in a particular place if water were freely available in the soil and plant cover were 100%. PET depends on the energy available to evaporate water, which depends on the ambient temperature. You may be surprised to know that PET can be much greater per unit area than the amount of water that would be lost by evaporation from an open body of water. This is because the surface area of the roots taking up water is much greater than the surface area of the ground in which the plants are rooted, and because the surface area of the leaves from which water is evaporating is often greater than the surface area of the ground—that is, the leaf area index (LAI) is often greater than 1.0.

Actual evapotranspiration (AET) is equal to the amount of water that enters the system in precipitation minus the amount that is lost in runoff and percolation to groundwater (the amount of water stored in living systems is minimal relative to the other terms). In areas with long dry periods, PET is much greater than actual evapotranspiration, but in tropical rainforests PET and AET are approximately equal. Actual evapotranspiration is closely linked to other major ecosystem processes, particularly productivity, decomposition, and other

soil processes (Schlesinger 1997). This is not surprising, given the dependence of plant and microbial activity on soil water content and the close functional linkage between carbon assimilation and water loss in plants (see Chapters 2 and 3).

Carbon in Ecosystems

Decomposition and Soil Food Webs

From the shortest-lived microorganism to the longestlived tree, everything that lives eventually dies. If the organic detritus—the dead bodies and waste products of living and once-living organisms—were not decomposed, most ecosystem processes would come to a screeching halt, and needless to say, the piled-up dead bodies and wastes would not be a pretty sight. Sooner or later, much of the carbon and other material and some of the energy released by the process of decomposition are taken up by living organisms and used again. How are materials that were once part of living organisms recycled?

Decomposition transforms dead plants and animals, their shed or removed parts (e.g., leaves, bark, branches, and roots), and animal feces into soil organic matter and ultimately into inorganic nutrients, CO2, water, and energy. In terrestrial ecosystems, decomposition involves both physical and chemical alteration of the original materials by a complex food web, which exists mostly (but not entirely) on or just under the surface of the soil (Figure 15.2).

Decomposition is largely an aerobic process (one requiring oxygen), and it occurs slowly, if at all, in waterlogged soils or other anaerobic environments. Warm,

Organic

matter

Flagellates

Amoebae

Bacteria

Ciliates

Dead material (from all boxes)

Figure 15.2

Diagram of a soil food web. Autotrophs at the soil surface and upper layers of the soil include plants, with aboveground photosynthetic parts and roots below ground, photosynthetic single-celled protists (including diatoms and green algae), and cyanobacteria (photosynthetic bacteria). Heterotrophs include many different kinds of bacteria, sin- gle-celled animals (amoebae, ciliates, flagellates), mycorrhizal and saprophytic (decomposing) fungi, nematodes, soil microarthropods and macroarthropods, including mites, and a variety of vertebrates as well. (After Ingham, unpublished, at http:// www.soilfoodweb.com)

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are spectacularly abundant and critical in soil food webs as herbivores that eat plant roots, carnivores that eat soil animals, and consumers of soil-dwelling bacteria and fungi. Unicellular protists, including ciliates and amoebas, live in the films of soil water between and surrounding soil particles and prey on soil bacteria. By their voracious consumption of soil bacteria and fungi, soil microarthropods, nematodes, and protists are responsible for converting large amounts of microbial nitrogen and phosphorus into forms available to plants, as well as releasing microbial carbon as respired CO2.

Saprophytic fungi rely on non-living organic material for their carbon and energy. They are the major decomposers of dead leaves and other plant litter. They account for approximately half of the microbial biomass in grassland soils and about 90% in temperate forest soils. Fungi are heterotrophs that secrete powerful digestive enzymes externally, decomposing their substrate.

The fine hyphae of fungi can penetrate plant organs and cells, overcoming the defensive cuticle to gain access to the interior and speeding decomposition. However, both plant structural defenses, such as lignin, and various aspects of plant defensive chemistry, including toxins and pH levels, can inhibit their effectiveness. Such differences in plant chemistry and physical structure contribute to large differences in decomposition rates

Figure 15.5

Global map of estimated NPP (total net primary productivity, above and below ground) for terrestrial plants in metric tons/ha/year of total dry organic matter. (Unpublished data from N. I. Bazilevich, Global Primary Productivity Database, 1993, http://ceos.cnes.fr:8100/cdrom-006/ceos1/casestud/ ecoreg/datasets/602/baz.htm)

among different species (see Figure 15.3) and different plant parts (leaves versus wood, for instance). The extensive multicellular mycelia that make up a fungus can obtain carbon from one source and nitrogen from a different source at some distance, further enhancing the ability of fungi to use dead plant materials as a source of nutrients and energy.

Bacteria play an essential role in soil food webs and nutrient cycling, although they are generally not very important in the initial stages of decomposition in most ecosystems. Bacteria can lyse (rupture) and break down living and dead cells of plants, animals, fungi, and other bacteria. Their presence in the soil is highly variable in space and time; they are found in greatest abundance at warm temperatures and when water and nutrients are available. They are found largely in soil macropores (larger soil pores), in the rhizosphere, and coating aggregates of soil particles (see Chapter 4). They move through the soil passively, carried by water or animals. We will return to the roles of various functional groups of bacteria below.

Productivity

Through photosynthesis, plants and other photosynthetic organisms capture carbon and energy, transform-