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Lecture 21 - Nutrient Supply and Cycling.docx

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Western University
Biology 2483A
Hugh Henry

LECTURE 21: NUTRIENT SUPPLY AND CYCLING  Biochemistry – the study of the physical, chemical and biological factors that influence the movement and transformation of elements  Understanding biogeochemistry is important in determining the availability of nutrients—chemical elements required for metabolism and growth  Nutrients must be in certain forms for uptake by organisms. The rate of physical and chemical transformations determines the supply of nutrients Nutrient Requirements and Sources  Nutrients enter ecosystems through the chemical breakdown of minerals in rocks or through fixation of atmospheric gases  All organisms have similar nutrient requirements but amounts and specific nutrients needed vary  Carbon (C) is the main component of structural compounds in plants; nitrogen (N) is largely tied up in enzymes  C:N ratios reflect biochemistry: Animals have lower C:N ratios (6 for humans); plants have C:N ratios of 10 – 40  Herbivores must consume more food than carnivores to get enough nutrients such as N  All plants require a set of essential nutrients  Some species have specific requirements. Some C a4d CAM plants require sodium. (All animals require it.)  Some plants that host N-fixing bacteria require cobalt  Some plants in selenium-rich soil require it, but it is toxic to most plants  Plants and microorganisms take up nutrients in simple, soluble forms from the environment  Animals mostly get nutrients in food as large, complex molecules. Some of these are broken down; others are absorbed intact, such as some amino acids  All nutrients are ultimately derived from abiotic sources: Minerals in rocks and gases in the atmosphere  Nutrients may be cycled within an ecosystem, repeatedly passing through organisms and the soil or water  Minerals – solid substances with characteristic chemical properties  Rocks – collections of different minerals  Elements are released from rock minerals by weathering  Mechanical weathering—the physical breakdown of rocks  Expansion and contraction from freeze–thaw and drying–rewetting cycles break rocks into smaller pieces  Plant roots and gravity (e.g., landslides) also contribute  Mechanical weathering exposes minerals to the processes of chemical weathering—chemical reactions release soluble forms of the mineral elements  Weathering is one of the processes that result in soil formation  Soil – mix of mineral particles, organic matter (mostly decomposing plant matter), water and organisms  The water contains dissolved organic matter, minerals, and gases (the soil solution)  Soil properties influence availability of nutrients to plants: o Texture – determined by particle size, the coarsest particles are sand o Clays are the smallest particles (< 2 μm). A semicrystalline structure and negative charges on the surface can hold onto cations and exchange them with the soil solution  Clay particles 2+n +e a reser2+ir for some nutrient ions such as Ca , K , and Mg  Cation exchange capacity—the ability of a soil to hold and exchange cations; it is related to the amount and types of clay particles present  Texture also influences soil water-holding capacity o Soils with a high proportion of sand have large spaces between the particles, and do not hold water well. Water drains through quickly  Parent material – the rock or mineral material that was broken down by weathering to form a soil o Parent material may be bedrock, or sediment deposited by glaciers (till), or by wind (loess), or by water  Chemistry and structure of the parent material determines rate of weathering, and amount and type of minerals released, thus it influences soil characteristics such as fertility 2+ + 2+ o Example: Soils derived from limestone have high levels of Ca , K , and Mg  The parent material exerts an influence on abundance, growth, and diversity of plants in an ecosystem. Gough et al. (2000) showed that variation in parent material pH was correlated with plant species richness in Arctic ecosystems  Climate influences rate of soil development. Soil development is fastest in warm, wet conditions  Tropical forest soils have experienced high rates of weathering and leaching for a long time, and are nutrient-poor  Most of the nutrients in these ecosystems are in the living tree biomass  In other terrestrial ecosystems, the proportion of nutrients in the soil is greater  When tropical forests are cleared and burned, the nutrients are lost in smoke and ash and soil erosion  These ecosystems can take centuries to return to their previous state  Organisms, especially plants, bacteria and fungi, contribute organic matter to soils  Organic matter is a reservoir of nutrients such as nitrogen and phosphorus  Organisms can also affect weathering rates through the release of CO2and organic acids Nutrient Transformations  Chemical and biological transformations in ecosystems alter the chemical form and supply of nutrients  Foremost among nutrient transformations is the decomposition of organic matter, which releases nutrients back into the ecosystem  Detritus includes dead plants, animals, and microorganisms, and egested waste products  Nutrients in detritus are made available by decomposition—the process by which detritivores break down detritus to obtain energy and nutrients  Decomposition releases nutrients as simple, soluble organic and inorganic compounds that can be taken up by other organisms  Fresh, undecomposed organic matter on the soil surface is known as litter  Animals such as earthworms, termites, and nematodes consume the litter, breaking it up into progressively finer particles  This fragmentation increases surface area, which facilitates chemical breakdown  Mineralization – chemical conversion of organic matter into inorganic nutrients  Heterotrophic microorganisms release enzymes into the soil that break down organic macromolecules  Abiotic and biotic controls on decomposition and mineralization determine nutrient availability to autotrophs  Decomposition and mineralization rates are faster in warm, moist conditions  Soil moisture influences the availability of water and2O to microorganisms  Wet soils have low O2concentrations, which inhibits detritivores  Some of the nutrients released are used by decomposers themselves  The C:N ratio of detritus represents energy content to nutrient content ratio  Organic matter with high C:N will have a low net release of nutrients because the heterotrophic microbes are limited more by nitrogen than by energy  The properties of carbon compounds influence decomposition rates: o Lignin strengthens plant cell walls, and is difficult for soil microbes to degrade. It decomposes very slowly o The amount of lignin in cell walls varies with plant species  Plant litter may contain secondary compounds o High concentrations can inhibit the microorganisms and decrease nutrient release o Or, some compounds stimulate microbial growth, leading to
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