After completing this chapter, you will be able to
- Explain what nutrients are and give examples.
- Discuss the concept of nutrient cycling and describe important compartments and fluxes.
- Describe factors that affect the development of major soil types.
- Describe the cycles of carbon, nitrogen, phosphorus, and sulphur.
Nutrients are any chemicals that are needed for the proper functioning of organisms. We can distinguish two basic types of nutrients: (1) inorganic chemicals that autotrophic organisms require for photosynthesis and metabolism, and (2) organic compounds ingested as food by heterotrophic organisms. This chapter deals with the inorganic nutrients.
Plants absorb a wide range of inorganic nutrients from their environment, typically as simple compounds. For example, most plants obtain their carbon as gaseous carbon dioxide (CO2) from the atmosphere, their nitrogen as the ions (charged molecules) nitrate (NO3–) or ammonium (NH4+), their phosphorus as phosphate (PO43–), and their calcium and magnesium as simple ions (Ca2+ and Mg2+). The ions are obtained in dissolved form in soil water absorbed by plant roots. Plants utilize these various nutrients in photosynthesis and other metabolic processes to manufacture all of the biochemicals they need for growth and reproduction.
Some inorganic nutrients, referred to as macronutrients, are needed by plants in relatively large quantities. These are carbon, oxygen, hydrogen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulphur. Carbon and oxygen are required in the largest amounts because carbon typically comprises about 50% of the dry weight of plant biomass and oxygen somewhat less. Hydrogen accounts for about 6% of dry plant biomass, while nitrogen and potassium occur in concentrations of 1-2% and those of calcium, phosphorus, magnesium, and sulphur are 0.1-0.5%. Micronutrients are needed in much smaller amounts, and they include boron, chlorine, copper, iron, manganese, molybdenum, and zinc. Each of these accounts for less than 0.01% of plant biomass and as little as a few parts per million (ppm, or 10–6; 1 ppm is equivalent to 0.0001%; see Appendix A).
Image 5.1. The productivity of a natural ecosystem is often limited by the supply of nutrients. This can be investigated by experimentally adding fertilizer to the system. In this case, nitrogen fertilizer was added to a meadow in Arctic tundra on Ellesmere Island, resulting in increased productivity. The experimental plot is a slightly darker colour. Source: B. Freedman.
Heterotrophs obtain the nutrients they require from the food they eat, which may be plant biomass (in the case of a herbivore), other heterotrophs (carnivore), or both (omnivore). The ingested biomass contains nutrients in various organically bound forms. Animals digest the organic forms of nutrients in their gut and assimilate them as simple organic or inorganic compounds, which they use to synthesize their own necessary biochemicals through various metabolic processes.
Nutrient Flows and Cycles
Although Earth gains small amounts of material through meteorite impacts, these extraterrestrial inputs are insignificant in comparison with the mass of the planet. Essentially, at the global level, Earth is an isolated system in terms of matter. As a consequence of this fact, nutrients and other materials “cycle” within and between ecosystems. In contrast, energy always “flows through” ecosystems and the biosphere (Chapter 4). Nutrient cycling refers to the transfers, chemical transformations, and recycling of nutrients in ecosystems. A nutrient budget is a quantitative (numerical) estimate of the rates of nutrient input and output to and from an ecosystem, as well as the amounts present and transferred within the system.
The major elements of a nutrient cycle are shown in Figure 5.1. The outer boundary of the diagram defines the limits of an ecosystem. (It could even represent the entire biosphere, in which case there would be no inputs to or outputs from the system.) In ecological studies, the system is often defined as a particular landscape, lake, or watershed (a terrestrial basin from which water drains into a stream or lake). Each of these systems has inputs and outputs of nutrients, the rates of which can be measured.
The boxes within the boundary represent compartments, each of which stores a quantity of material. Compartment sizes are typically expressed in units of mass per unit of surface area. Examples of such units are kilograms per hectare (kg/ha) or tonnes per hectare (t/ha). In aquatic studies, compartment sizes may be expressed per unit of water volume (such as g/m3). The arrows in the diagram represent fluxes, or transfers of material between compartments. Fluxes are rate functions, and are measured in terms of mass per area per time (e.g., kg/ha-yr).
The system can be divided into four major compartments:
- The atmosphere consists of gases and small concentrations of suspended particulates and water vapour.
- Rocks and soil consist of insoluble minerals that are not directly available for uptake by organisms.
- Available nutrients are present in chemical forms that are water soluble to some degree, so they can be absorbed by organisms from their environment and contribute to their mineral nutrition.
- The organic compartment consists of nutrients present within living and dead organic matter. This compartment can be divided into three functional groups: (a) living biomass of autotrophs such as plants, algae, and autotrophic bacteria, (b) living heterotrophs including herbivores, carnivores, omnivores, and detritivores, and (c) and all forms of dead organic matter.
The major transfers of material between compartments, or fluxes, are also shown in Figure 5.1. These are important transfer pathways within nutrient cycles. For instance, insoluble forms of nutrients in rocks and soil become available for uptake by organisms through various chemical transformations, such as weathering, that render the nutrients soluble in water. This is reversed by reactions that produce insoluble compounds from soluble ones. These latter reactions form secondary minerals such as carbonates (e.g. limestone, CaCO3, and dolomite, MgCO3), oxides of iron and aluminum (Fe2O3 and Al(OH)3), sulphides (e.g., iron sulphide, FeS2), and other compounds that are not directly available for biological uptake.
Figure 5.1. Conceptual Diagram of a Nutrient Cycle. This diagram shows the major elements of a nutrient cycle for a particular ecosystem, such as a watershed. Each box represents a compartment (atmosphere, soil and rocks, organic material, and available nutrients) that contains a quantity of material. The arrows represent fluxes, or transfers of material between compartments. Source: Modified from Likens et al. (1977).
Other fluxes in nutrient cycles include the biological uptake of nutrients from the atmosphere or from the available pool in soil. For example, plant foliage assimilates carbon dioxide (CO2) from air, and roots absorb nitrate (NO3–) and ammonium (NH4+) ions dissolved in soil water. Plants then metabolically fix these nutrients into their growing biomass. The organic nutrients may then enter the food web and are eventually deposited as dead biomass. Organic nutrients in dead biomass are recycled through decay and mineralization, which regenerate the supply of available nutrients.
These concepts are examined in more detail in the following sections. Initially, we examine the soil ecosystem, which is where most nutrient cycling occurs within terrestrial habitats. We will then examine key aspects of the cycling of carbon, nitrogen, phosphorus, and sulphur.
The Soil Ecosystem
Soil is a complex and variable mixture of fragmented rock, organic matter, moisture, gases, and living organisms that covers almost all terrestrial landscapes. Soil provides mechanical support for growing, even for trees as tall as 100 m. Soil also stores water and nutrients for use by plants and provides habitat for the many organisms that are active in the decomposition of dead biomass and recycling of its nutrient content. Soil is a component of all terrestrial ecosystems, but it is also in itself a dynamic ecosystem.
Soil develops over long periods of time toward a mature condition. Fundamentally, soil is derived from a so-called parent material, which consists of rocks and minerals that occur within a metre or so of the surface. Parent materials in most of Canada were deposited through glacial processes, often as a complex mixture known as till, which contains rock fragments of various sizes and mineralogy. In some areas, however, the parent materials were deposited beneath immense inland lakes, usually in post-glacial times. Such places are typically flat and have uniform, fine-grained soils ranging in texture from clay to sand. (Clay particles have a diameter less than 0.002 mm, while silt ranges from 0.002 to 0.05 mm, sand from 0.05 to 2 mm, gravel from 2 to 20 mm, and coarse gravel and rubble are larger than 20 mm.) Figure 5.2 presents a textural classification of soil based on the percentage of clay-, silt-, and sand-sized particles.
Figure 5.2. A Textural Classification of Soils. The percentage composition of clay-, silt-, and sand-sized particles is used to classify soils into the 12 major types that are shown. Source: Modified from Foth (1990).
In other regions, parent materials known as loess are derived from silt that was transported by wind from other places. Because of their very small particle size, soil rich in clay has an enormous surface area, giving it important chemical properties such as the ability to bind many nutrient ions.
The characteristics of the parent material have an important influence on the type of soil that eventually develops. However, soil development is also profoundly affected by biological processes and climatic factors such as precipitation and temperature.
For example, water from precipitation dissolves certain minerals and carries the resulting ions downward. This process, known as leaching, modifies the chemistry and mineralogy of both the surface and deeper parts of the soil. In addition, inputs of litter (dead biomass) from plants increase the content of organic matter in soil. Fresh litter is a food substrate for many decomposer species of soil-dwelling animals, fungi, and bacteria. These organisms eventually oxidize the organic debris into carbon dioxide, water, and inorganic nutrients such as ammonium, although some material remaining as complex organic matter, known as humus. As soils develop, they assume a vertical stratification known as a soil profile, which has recognizable layers known as horizons. From the surface downward, the major horizons of a well-developed soil profile are as follows:
Soil that has been modified by human influences may be stratified differently. In cultivated land, for example, a homogeneous plough layer (Ap) of 15-20 cm develops at the surface. The plough layer is uniform in structure because it has been repeatedly mixed up for many years. In addition, the soil of agricultural land is often deficient in organic matter, compacted by the repeated passage of heavy machinery, and degraded in structure, nutrient concentration, and other qualities important to its ability to support crop productivity. These subjects are examined in more detail in Chapters 14 and 24.
Image 5.2. Soil in natural ecosystems often develops a vertical stratification. Typically, there are organic-rich horizons on the surface and mineral-rich ones below. This soil “pit” was dug in a spruce-dominated stand of boreal forest in Labrador. Beneath the darker organic surface layer is a light-coloured mineral horizon from which iron and aluminum ions have been leached downward by percolating water. The next reddish layer is part of the B horizon, where iron and aluminum are deposited. The lightish bottom layer is the parent material, which in this case is sand deposited by the Churchill River thousands of years ago. Source: B. Freedman
Broadly speaking, soil within a particular kind of ecosystem, such as tundra, conifer forest, hardwood forest, or prairie, tends to develop in a distinctive way. Soils are classified by the ecological conditions under which they developed. The highest level of classification arranges soils into groups called orders, which can themselves be divided into more detailed assemblies. The most important soil orders in Canada are:
The Importance of Soil
The soil ecosystem is extremely important. Terrestrial plants obtain their water and much of the nutrients they need from the soil, absorbing them through their roots. Soil also provides habitat for a great diversity of animals and microorganisms that play a crucial role in litter decomposition and nutrient cycling.
Soil is economically important because it critically influences the kinds of agricultural crops that can be grown (this topic is examined in Chapter 14). Some of the most productive agricultural soils are alluvial deposits found along rivers and their deltas, where periodic flooding and silt deposition bring in abundant supplies of nutrients. As long as they are not too stony, chernozem and brunisol are also fertile and useful for agriculture. Much prairie agriculture is developed on chernozem soils, while much of the fertile agricultural land of southern Quebec and Ontario has brunisol types.
The Carbon Cycle
Carbon is one of the basic building blocks of life and the most abundant element in organisms, accounting for about half of typical dry biomass. Key aspects of the global carbon cycle are presented in Figure 5.3 (see also Chapter 17 and Figure 17.1). Gaseous carbon dioxide (CO2) is the most abundant form of carbon in the atmosphere, where it occurs in a concentration of about 400 ppm (0.04%), although methane (CH4, 1.8 ppm) is also significant.
Figure 5.3. Model of the Global Carbon Cycle. Carbon is stored in the various compartments (atmosphere, organic material, oceans, and soil/rock) and moves from one box to another. The amounts of carbon in compartments are expressed in units of billions of tonnes of carbon (109 t or gigatonnes, Gt), while fluxes between them are in 109 t/y. Based on data from Blasing (1985), Solomon et al. (1985), and Freedman (1995).
Atmospheric CO2 is a critical nutrient for photosynthetic organisms, such as plants and algae. Plants absorb this gas through tiny pores (called stomata) in their foliage, fix it into simple sugars, and then use the fixed energy to support their respiration and to achieve growth and reproduction. The biomass of autotrophs is available to be consumed by heterotrophs and passed through food webs. All organisms release CO2 to the atmosphere as a waste product of their respiratory metabolism.
CO2 is also the most common emission associated with the decomposition of dead organic matter. However, if this process occurs under anaerobic conditions (in which oxygen, O2, is not present), then both CO2 and CH4 are emitted. Because anaerobic decomposition is relatively inefficient, dead organic matter often accumulates in wetlands such as swamps and bogs, eventually forming peat. Under suitable geological conditions of deep burial, high pressure and temperature, and a lack of oxygen, peat and other organic materials may be slowly transformed into carbon-rich fossil fuels such as coal, petroleum, and natural gas (see Chapter 13).
Atmospheric CO2 also dissolves into oceanic water, forming the bicarbonate ion (HCO3–), which can be taken up and fixed by photosynthetic algae and bacteria, which are the base of the marine food web. Various marine organisms also use oceanic CO2 and HCO3– to manufacture their shells of calcium carbonate (CaCO3), an insoluble mineral that slowly accumulates in sediment and may eventually lithify into limestone (also CaCO3).
Over almost all of geological time, the amount of CO2 absorbed by the global biota from the atmosphere was similar to that released through respiration and decomposition. Consequently, the cycling of this nutrient can be viewed as a steady-state system. In modern times, however, anthropogenic emissions have changed the atmospheric carbon balance. Global emissions of CO2 and CH4 are now larger than the uptake of these gases, an imbalance that has resulted in increasing concentrations in the atmosphere. This phenomenon appears to be intensifying the greenhouse effect of Earth and resulting in global warming (see Chapter 17).
The Nitrogen Cycle
Nitrogen is another important nutrient for organisms, being an integral component of many biochemicals, including amino acids, proteins, and nucleic acids. Like the carbon cycle, that of nitrogen has an important atmospheric phase. However, unlike carbon, nitrogen is not a significant constituent of rocks and minerals. Consequently, the atmospheric reservoir plays a paramount role in the cycling of nitrogen (Figure 5.4).
Figure 5.4. Model of the Global Nitrogen Cycle. Nitrogen occurs in three main compartments: the atmosphere, terrestrial organic material, and oceanic organic material. The amounts of nitrogen stored in compartments are expressed in units of millions of tonnes of nitrogen (106 t or megatonnes, Mt), while fluxes are in 106 t/y. Based on data from Hutzinger (1982) and Freedman (1995).