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

fusion and by mass flow. Because they are so mobile, they can also be easily leached from the soil into groundwater and runoff, and thus are readily lost. (In some tropical soils, however, anions can be adsorbed onto positively charged aluminum or iron oxides and thus prevented from leaching.) Leaching of nitrate from fertilizer and animal waste can be an important source of water pollution.

Nitrate is also lost from ecosystems by the process of microbial denitrification, which is the reduction of NO2– and NO3– to various gaseous nitrogen compounds, including N2 and N2O. The latter is an important greenhouse gas that can contribute to global warming (see Chapter 22). The composition of the denitrifying bacterial community can differ strikingly between disturbed and undisturbed soils, and these differences can be reflected in very different responses to environmental conditions and in different rates of denitrification and production of N2O (Cavigelli and Robertson 2000).

Fire can be a major cause of nitrogen loss in some ecosystems. Most of the N released into the atmosphere during a fire comes from the vegetation itself, but substantial amounts can come from the burning of accumulated litter, and very hot fires can burn the soil organic matter in the uppermost part of the profile, thus volatilizing nitrogen from the soil.

Decomposition Rates and Nitrogen Immobilization

As soil organic matter is decomposed by microorganisms, much of the N and P they contain is incorporated into the living microbial biomass (see Figure 15.15) The ratio of C to N in plant tissues ranges from about 25:1 to 150:1, but the ratios are much lower in fungi (between 4:1 and 15:1) and decomposer bacteria (3:1–5:1). As decomposition occurs, soil C: N (as well as C: P) ratios decline as the biomass in decomposing plant material is replaced by biomass in the fungi and bacteria growing on it. When more of the soil N and other nutrients (such as P) become incorporated into living soil microorganisms as organic molecules, these nutrients become less available for uptake by plants. This phenomenon is called biological immobilization of the nutrients.

Eventually the microorganisms die, and some of the nutrients again become available for uptake by plants. However, nitrogen immobilization by microorganisms may be a very important short-term limitation on plant nutrient uptake and growth. This effect can be particularly strong when the plant tissues that are decomposing have relatively high C:N ratios or are rich in plant materials (such as lignin) that resist decomposition. High C: N ratios and the presence of lignin and similar materials result in slower decomposition rates and a longer period of N and P immobilization. For instance, evergreen leaves and leaves from plants native to low-nutri- ent environments typically decompose more slowly than

Ecosystem Processes 313

those from deciduous plants and plants from fertile sites. The leaves of many oak species, which are high in materials that physically and chemically resist decomposition, are incorporated into soil organic matter much more slowly than, for instance, maple leaves from the same site. Leaves of nitrogen-fixing species decompose more quickly than those of co-occurring nonfixers, with consequently less immobilization of soil nutrients. Decomposing animal tissues have much lower C:N ratios, and tend to decompose much more rapidly, than plant tissues.

Plant Uptake of Nitrogen

The balance between NO3– and NH4+ in soils differs among habitats, largely as a result of environmental effects on nitrification. While most plant species are better able to take up NO3–, plants adapted to environments where soil N is largely in the form of NH4+ (i.e., where nitrification is inhibited, as in cold, waterlogged soils) may show a preference for uptake of nitrogen in that form. In most plants, both forms of nitrogen are converted in the roots to amino groups (NH2–), which are then attached to various organic compounds (NH4+ is toxic if it accumulates in plant tissues).

The conversion of NO3– to amino groups is energetically expensive. The conversion of NH4+ to NH2– requires much less energy. However, NO3– is transported to plant roots readily by mass flow in the soil solution, but the lower mobility of NH4+ in the soil may require greater energy expenditure in the form of root proliferation (Schlesinger 1997).

Nitrogen uptake can be energetically costly for plants for a number of reasons. The uptake of nitrogen as either NO3– or NH4+ is aided by enzymes that expend energy to transport the ions across cell membranes in the roots. Active root growth and root proliferation into areas of the soil where N is more available also requires energy. In environments where N is less available, plants tend to allocate more biomass to roots, and root:shoot ratios are generally high, compromising the plant’s capacity to capture carbon.

Uptake of positively charged ions, such as NH4+ and many other nutrients, such as K+, from the soil could potentially lead to a charge imbalance in plant tissues. To counteract the positive charges, plant roots release hydrogen ions (H+) into the soil solution, acidifying the soil. When plants take up NO3–, they release negatively charged ions such as organic acids and HCO3– to maintain their charge balance. The release of these ions in turn affects soil chemistry and the availability of nutrients to plant roots and soil microorganisms. Some ions (e.g., Ca2+ and Na+) are actively excluded by the roots of some plants in environments where their availability is too high.

In some unusual cases, plants obtain nitrogen from sources other than NO3– and NH4+ in the soil solution.

“Carnivorous” plants native to extremely nitrogen-poor bogs may obtain nitrogen from decomposing insects (Figure 15.18). Other species may take up nitrogen in the form of amino acids directly from the soil.

Phosphorus in Terrestrial Ecosystems

Although phosphorus is needed for plant growth and function in smaller amounts than other macronutrients (see Table 4.2), it is fairly common for plant growth to be limited by P availability. We considered P availability in the soil and its uptake by plants in Chapter 4. Here we discuss P at the ecosystem and global levels.

Most of the phosphorus that cycles through ecosystems is found in microorganisms, in decomposing plant material, and in soil organic matter—that is, in organic material (Figure 15.19). This is curious, because most of the P in terrestrial systems occurs as minerals in rocks. Inorganic P is often complexed with other soil minerals and is relatively unavailable to plants directly (but see

Primary P minerals (from rock)

Figure 15.19

Phosphorus from minerals in rock (primary P) dissolves in the soil solution, from which it can be absorbed by plants and other living organisms. Some soluble soil phosphorus is transformed into various inorganic forms or is leached; however, the vast majority of the phosphorus taken up by organisms in an ecosystem has been recycled through other living organisms. Biological transformations are shown in green, geochemical transformations in white. (Modified from Schlesinger 1997, after Smeck 1985.)

Chapter 4 for a discussion of how mycorrhizal fungi and other organisms can access this phosphorus).

The original source of most of the phosphorus that enters the biosphere is weathering of rock containing apatite minerals. The minerals in the apatite group are the only minerals common in the lithosphere that con-

Living organisms

Soil organic P

Stable organic P

Secondary P minerals

(inorganic)

Leaching loss

to groundwater Occluded P (inorganic)

tain a substantial amount of phosphorus. As rock weathers and soils form, and as the soils develop and age, much of the phosphorus becomes bound in the interior of iron and aluminum oxide crystals, from which living organisms cannot remove it.

In old soils that have become dominated by iron and aluminum oxides (such as those that are common in Africa, Australia, and other environments, particularly in the Tropics), the lack of available phosphorus is one of the most important environmental factors affecting plants. Inorganic P may also be held in more available forms on the surface of other soil minerals by anion adsorption and other reactions. The complicated soil chemistry involved in these reactions depends on soil pH and other factors. It is therefore generally difficult for scientists to accurately measure the amount of phosphorus available to plants.

Unlike the global cycles of carbon, nitrogen, and water, the global cycle of phosphorus does not have a major atmospheric component (Figure 15.20). Most of the phosphorus that is used by organisms has been recycled in organic form through other living organisms, although its ultimate source is rock weathering. Organically bound phosphorus is released in the form of PO43– (orthophosphate) by plant roots, fungi (including those in mycorrhizae as well as free-living fungi), bacteria, and algae. All of these organisms mineralize phosphorus through the action of extracellular phosphatases (enzymes that cleave ester bonds). This process is expensive both energetically and in terms of the nitrogen invested in the phosphatase enzymes. Microorganisms as well as plants can take up the PO43– that is released, but if it is not immediately taken up, the orthophosphate does not remain available for long in the soil. Rather, it readily binds to organic particles or to soil minerals, becoming partially or completely unavailable to plants.

Ecosystem Processes 315

Ecosystem Nutrient Cycling

and Plant Diversity

Is there any relationship between the characteristics of a plant community, such as diversity, and ecosystem processes such as nitrogen and phosphorus cycling? Recently considerable effort has been made to determine and understand the potential connections between these important ecosystem and community properties (Loreau et al. 2001; Tilman et al. 1997; see Chapter 14).

David Hooper and Peter Vitousek (1998) set out to answer this question experimentally in a California grassland. The grassland was located on serpentine soil (Box 15A). The plants were categorized into four functional groups that differed in phenology (seasonal growth pattern), root:shoot ratio, and other traits relevant to the use and cycling of N and P. The four functional groups were early-season annual forbs (herbaceous plants), late-season annual forbs, nitrogen fixers, and perennial bunchgrasses. Experimental plots contained plants of either a single functional group or combinations of functional groups. The researchers measured pools and fluxes of nitrogen and phosphorus, as well as other ecosystem variables, on the experimental plots.

Hooper and Vitousek found that plant functional group diversity increased the total use of nutrients, as they had hypothesized, because of seasonal differences in plant growth and resource use among the functional groups. They also found, however, that functional group richness did not decrease nitrogen leaching, although much more nitrogen was lost when plants were totally absent than when plants of any of the functional groups were present. Ecosystem nutrient retention was deter-

Figure 15.20

The global phosphorus cycle, showing major fluxes (in 1012 g P per year, shown in bold) and pools (in 1012 g P). Marine sediments, at approximately 4 × 1021, are by far the largest pool, and cycling by land plants, at 3000 × 1012 g P per year is the largest flux. (After Schlesinger 1997, with data from Jahnke 1992.)

BOX 15A

Serpentine Soils

Important ecological research has been conducted over the years on various aspects of the ecology of vegetation on serpentine soils (Kruckeberg 1954; Whittaker 1954; Hooper and Vitousek 1998). Serpentine soils are derived from serpentine rock, which is a magnesium silicate rock of metamorphic origin. These soils are poor in many nutrients—par- ticularly calcium, nitrogen, and phos- phorus—and they may have either very low or very high pH. Toxic elements, particularly heavy metals, are also characteristic of serpentine soils.

Areas with serpentine soils occur in many places in the world. Serpentine soils often occur in patches of limited extent, or “islands,” surrounded by more ordinary nonserpentine soils. These serpentine islands (sometimes called serpentine barrens) typically support distinctive plant communities that contrast strongly with the vegetation of the surrounding region. For instance, serpentine vegetation in California often contains many showy dicot

Serpentine grassland in the state of California. (Photograph courtesy of D. Hooper)

species, surrounded by grasslands. Serpentine vegetation is often very high in species diversity and may have many endemic species. The species from surrounding areas are unable to tolerate the toxic, low-nutrient soils, and this excludes them from serpentine patch-

es. In addition, Kruckeberg (1954) showed that the serpentine species appear to be excluded from the surrounding regions by competition (echoing Bradshaw’s findings for plants growing on mine tailings, discussed in Chapter 6).

mined at least as much by the indirect effects of plants on microbial activity and subsequent nutrient retention by microorganisms as by the indirect effects of plants on microbial activity. The researchers concluded that the identity of the particular plants present explained more about ecosystem nutrient cycling than could be explained by considering the number of functional groups present.

Ecosystem Processes

for Some Other Elements

We have examined the major ecosystem processes of primary importance for plants: those involving water, carbon, nitrogen, and phosphorus. Other nutrients, of course, are necessary for plant growth (see Table 4.2). Here we briefly discuss the cycling of two other important elements, sulfur and calcium, both as general examples and because of the influence of human activities on their cycles.

Sulfur

Sulfur is necessary primarily for building the amino acids cysteine and methionine, important constituents of various proteins. Like nitrogen, phosphorus, and carbon, sulfur is cycled biogeochemically in ecosystems. The weathering of rocks with minerals containing sulfur (such as pyrite and gypsum) is one major source of the sulfur that ultimately becomes available to plants. Atmospheric deposition is also an important source of sulfur. A large proportion of the sulfur pool in soils exists as a part of soil organic matter. Inorganic sulfate ions (SO42–) are adsorbed onto soil mineral particles, in equilibrium with SO42– dissolved in the soil solution. Bacterial mineralization and immobilization of sulfur are analogous to the same processes for nitrogen and phosphorus.

Sulfur is lost from ecosystems in a variety of forms. In addition to the loss of sulfate ions from ecosystems by leaching, many plants release volatile organic sulfur

(particularly H2S), and waterlogged, anaerobic soils can also produce large quantities of sulfur gases by bacterial reduction.

On a global scale, large pools of sulfur are dissolved in seawater. In the atmosphere, sulfur-containing gases are short-lived and are found at low concentrations. However, dust storms, sea spray, and volcanic eruptions can contribute large amounts of particulate sulfur (as well as some sulfur in the form of gases and aerosols) to the atmosphere. Human activities are the largest current source of atmospheric sulfur gas, although recent air pollution controls have decreased the yearly input of sulfur from industrial sources considerably. The SO2 that enters the atmosphere reacts with water to form sulfuric acid, which, along with nitric acid (from anthropogenic NO and NO2), is the chief cause of acid precipitation (see Chapter 22).

Calcium

Calcium is abundant in the Earth’s crust. It is an important component of limestone and other common rocks such as gypsum (CaSO4 •H2O) and calcium carbonate (CaCO3). Weathering of these rocks releases calcium ions (Ca2+), which are carried by rivers to the oceans; large pools of calcium ions are found in seawater. Marine invertebrates use this calcium to build calcium carbonate shells; when these organisms die, their remains form thick sediments. Through burial, metamorphosis, and crustal movement, these marine sediments may ultimately be transformed into calcium-bearing terrestrial rock.

Calcium ions readily dissolve in the soil solution and are taken up by plants passively in the transpiration stream. Calcium concentrations can vary dramatically among soils, from being apparently limiting to plant growth to being present in excessive amounts. In humid regions with low-pH soils, calcium ions can be heavily leached from the soil. Huntington (2000) studied fluxes of calcium in southeastern U.S. forests, and found that losses of calcium caused by logging and soil leaching often were greater than inputs from atmospheric deposition and rock weathering, leading to severe calcium depletion in the forest soils. His results suggest that calcium depletion could result in widespread problems for southeastern forest trees over the next few decades.

In contrast, in many deserts, calcium can accumulate in the soil. Rainwater dissolves the calcium ions and then evaporates, precipitating and thus concentrating the calcium into either a soft or a cement-hard layer called a calcic horizon (also called a “pan” or “caliche”). The excessive amounts of calcium in these calcareous, high-pH soils can be actively excluded by the roots of some plant species.

Within an ecosystem, the calcium contained in plant biomass is returned to the soil through litterfall and sub-

Ecosystem Processes 317

sequent litter decomposition, as well as throughflow (rainwater) that passes through the canopy and travels down the surface of tree trunks, carrying dissolved ions leached from the plant surfaces. Some calcium is carried in particulate form in the atmosphere and deposited in rainfall or by dry deposition (Figure 15.21).

The requirement of plants for calcium is generally very high, second only to that for nitrogen (see Table 4.2). Calcium is unusual in that it plays both chemical and structural roles in plant tissues, including regulation of growth and response to stress, stomatal function, cell division and cell wall synthesis, and structural support in both leaves and wood (McLaughlin and Wimmer 1999). Decreasing calcium availability may lead to decreased growth and higher mortality in forest trees, making them more susceptible to pathogens, for example.

Some recent evidence suggests that calcium depletion is occurring in many forests, with negative consequences for forest vitality (Likens et al. 1998), although other researchers have disputed these findings (Yanai et al. 1999). The potential for calcium depletion is a result

34

Figure 15.21

Local calcium cycle within a forest ecosystem in the United Kingdom. Pool amounts are given in kg/ha, and annual fluxes (arrows) are shown in kg/ha/year. (After Schlesinger 1997 and Whittaker 1970.)

318 Chapter 15

of several factors, including decreasing amounts of calcium in rainfall and removal of calcium from the ecosystem in biomass (by logging and other human activities). Most important are the direct negative effects of anthropogenic nitrogen deposition (particularly in the form of HNO3), which results in displacement of calcium ions in the soil, causing them to be leached into stream water and lost.

Summary

Ecosystem ecologists study the flow of energy and materials through ecosystems. In terrestrial ecosystems, plants play the essential role of primary producers: nearly all production of biomass depends on plant photosynthesis. Fluxes of water and carbon, as well as nitrogen and most other nutrients, also depend critically on the plants in an ecosystem, since plants constitute a large proportion of the living biomass, and other organisms acquire carbon and nutrients from food webs that are based mainly on plants. These materials cycle through the biosphere at very different rates. Thus, changes in the amount of plant cover or in the efficiency of uptake by plants have differing effects on each biogeochemical cycle. Similarly, changes in the different biogeochemical cycles have very different effects on plants. Plants and the water cycle affect one another quickly, as changes in regional plant cover can affect humidity and rainfall within years to decades, and changes in moisture can affect plants within days to weeks. Changes in the carbon cycle occur on a larger spatial and longer time scale, because CO2 is (on average) evenly distributed in the atmosphere, and much carbon is stored as biomass and dissolved in the oceans.

Additional Readings

Classic References

Tansley, A. G. 1935. The use and abuse of vegetational concepts and terms. Ecology 16: 284–307.

Lindeman, R. L. 1942. The trophic-dynamic aspect of ecology. Ecology 23: 399–418.

Odum, E. P. 1960. Organic production and turnover in old field succession. Ecology 41: 34–49.

Contemporary Research

Gohlz, H. L., D. A. Wedin, S. M. Smitherman, M. E. Harmon and W. J. Parton. 2000. Long-term dynamics of pine and hardwood litter in contrasting environments: toward a global model of decomposition. Global Change Biol. 6:751-765.

Net primary productivity (NPP) the total energy captured minus respiratory losses by primary producers, is a critical component of many ecosystem processes. Recent technological developments have made it possible to obtain far better measurements of productivity than was possible in the past. Nevertheless, we are still not able to model the carbon cycle with full confidence, although doing so is important for predicting the consequences of deforestation and CO2 emissions from fossil fuels. The carbon that is captured in NPP accumulates in an ecosystem primarily in living biomass, plant litter, and soil organic matter, and is lost from the ecosystem primarily through soil microbial respiration.

The nitrogen cycle depends strongly on microbial activity. Biologically available nitrogen originates in soils mainly by microbial fixation, and the decomposition of biomass recycles available nitrogen in the soil. Nitrogen is often limiting to plants; increases in the amount of available nitrogen can have major effects on plant growth rates. Use of synthetic fertilizer by humans has roughly doubled the amount of biologically available nitrogen in the world, with serious consequences for both terrestrial and aquatic ecosystems.

The global phosphorus cycle does not have a major atmospheric component. Phosphorus is limiting for plant growth in many ecosystems, particularly those on old soils. Sulfur and calcium are necessary for plant growth; their availability varies widely among different soils. Human activities may be depleting calcium in some ecosystems. Improving our understanding of biogeochemical cycles is critical both for understanding how plants and plant communities change over time and for understanding the environmental challenges presented by anthropogenic change.

Shaver, G. R. and F. S. Chapin III. 1991. Production-biomass relationships and element cycling in contrasting Arctic vegetation types. Ecol. Monogr. 61: 1–31.

Vogt, K. A., C. C. Grier, C. E. Meier and R. L. Edmonds. 1982. Mycorrhizal role in net primary production and nutrient cycling in Abies amabilis ecosystems in western Washington. Ecology 63: 370–380.

Additional Resources

Chapin, F. S., P. Matson and H. A. Mooney. 2002. Principles of Terrestrial Ecosystem Ecology. Springer-Verlag, New York.

Schlesinger, W. H. 1997. Biogeochemistry: An Analysis of Global Change. 2nd ed. Academic Press, New York.

Waring, R. H. and S. W. Running. 1998. Forest Ecosystems: Analysis at Multiple Scales. 2nd ed. Academic Press, New York.