CHAPTER 21 The Earth System and the Cycling of Carbon and Nutrients

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1 CHAPTER 21 The Earth System and the Cycling of Carbon and Nutrients Introduction The realization that ecosystems function like other natural systems, with throughputs of energy and the cycling of nutrients, was one of the major intellectual revolutions to affect the study of ecology and biogeography in the 2oth century. No longer is it possible to understand how ecosystems work and where ecosystems are distributed solely in a descriptive manner. Whilst the behaviour of plant and animal species is a vital area of study, processes involving energy transformations and the movement of necessary nutrients occur at global, regional and local scales. These processes link organisms and therefore emphasize interconnections both within and between ecosystems. Studies of energy flow and nutrient cycling were stimulated by the International Biological Programme which started in the 1960s. There are now many data for particular ecosystems, whether local or on the biome scale. This enables detailed balance sheets to be drawn up and relationships between organisms depicted as diagrams or equations. Much of the stimulus to studying ecosystems in this trophic dynamic way came from the work of Hutchinson, Lindeman and E.P. Odum in the mid-twentieth century. This chapter covers the theory and principles of energy flow and nutrient cycling, together with detailed case studies of temperate deciduous woodland, temperate prairie and the - tropical rain forest. A major concern in the 21 st century has become the carbon cycle. Terms such as carbon footprint have now entered everyday usage. Biogeographers have a key role to play here. Topics emerging in biogeography are: the carbon cycle; carbon in soils and its sequestration there; dissolved organic carbon in freshwaters; and, carbon, vegetation and the importance of climate change. Biogeographers are also contributing to the ongoing debates within society at large on: carbon and energy production; carbon and food production; carbon and waste management; and, last but not least, the planning and management of carbon neutrality for institutions and governments. Chapter summary General principles of energy flow The structure and function of ecosystems are governed by energy flow and nutrient cycling. Gross primary productivity is the sum total of the sun s radiant energy which is fixed by photosynthesis. Net primary productivity is the energy converted into potential energy after the respiration of the autotrophs.

2 Energy and nutrients are passed through the various trophic levels of ecosystems, from producers, to herbivores, to various levels of carnivores, with dead material and waste products passing through the decomposer chain of detrivores. Pyramids of numbers and biomass were introduced by Sir Charles Elton to explain ecosystem structure, though it was left to Lindeman to propose the - fundamental pyramid of energy. Biomass and productivity Biomass or standing crop is the weight of live organic material per unit area. The energy equivalent of organic material can be determined in the laboratory, using a calorimeter. The energy content of organic materials, both in ecosystems and also in food, is measured as calories per unit weight or Joules per unit weight. Net ecosystem production is the increase in biomass per unit area per unit time, and thus is the increase after losses by grazing and decomposition have taken place. In contrast, net primary productivity includes changes in biomass plus both grazing and decomposition losses. Energy flows in a deciduous woodland The study of the energy budget of a real-world ecosystem is a detailed, long-term investigation over many years, using careful sampling techniques. The oak woodland of Wytham Wood, England, has been studied by field, laboratory and computer techniques for many years by scientists from Oxford University. By means of an energy flow diagram, it is possible to represent the stores of energy in the different compartments (biomass) and the rates of energy flow in primary and secondary production, and in consumption. The ecological efficiency of an ecosystem is an important property and reflects the energy produced as a ratio of the energy received. The trophic level of an organism is the level or levels at which an organism transfers energy (i.e. feeds). Ecosystem production The global biomass of the tropical rain forest exceeds the combined biomasses of all other biomes, whether terrestrial or aquatic. The highest net primary productivity figures per annum are reached by coral reefs in tropical oceans. Forests account for 90 per cent of the world s biomass, grasslands and deserts 7 per cent, with the remaining 3 per cent being in aquatic ecosystems.

3 The proportion of NPP consumed by herbivores declines as one moves from the tropics to the poles in forests and also in grasslands. The leaf area index, LAI, is an important plant characteristic which appears to correlate closely with ecosystem production. Global carbon budgets Carbon is a crucial nutrient and chemical on a world scale, being fixed in photosynthesis and released in respiration in very large amounts. Much research effort is currently being devoted to measuring the carbon budgets of the world s biomes; using eddy covariance techniques; this involves measurements of: microclimatic elements, the CO 2 flux, the energy flux, respiration by plants and soil, and methane production. Eddy covariance techniques enable biogeographers to determine whether biomes are net carbon sources or net carbon sinks. Global carbon budgets are also much influenced by human activities; for example, land-use activities such as land clearance and firing release carbon to the atmosphere. The release of carbon dioxide by these means, and in many industrial processes, leads to global warming, which in turn will increase the production and fixation of carbon. From studies of the boreal coniferous forest biome it appears that vegetation can act as both a source and a sink of carbon. The boreal forest acted as a source of carbon before about 1890, as a sink between 1890 and 1970, and as a source again in recent decades. Plant nutrients Plant growth requires eighteen essential nutrient elements, which are supplied by the atmosphere, the soil and water. The major nutrients are carbon, hydrogen, oxygen, nitrogen, phosphorus and potassium, with the first four coming from the atmosphere and hydrosphere, the latter two from the soil. Trace elements or micro-nutrients are required but in very small quantities, e.g. iron, manganese, copper, zinc, boron, chlorine, cobalt, molybdenum and selenium. Intermediate or minor nutrients are calcium, magnesium and sulphur. Nutrients are absorbed by plants in the ionic form, with the nature of the ion (whether cation or anion) determining many aspects of nutrient cycles. The nitrogen cycle Unlike energy, nutrients can cycle and individual atoms can be reused continuously.

4 The nitrogen cycle is a major, complex cycle which must work efficiently so that plants can receive large amounts of this major nutrient. Key pathways in the nitrogen cycle are formed by micro-organisms, viz. ammonification, nitrification, denitrification, nitrate reduction and nitrogen - fixation. Micro-organisms which promote nitrogen use generally require fertile soils which are well aerated, moist, warm and of neutral reaction. Soils where nitrogen is in short supply are cold, waterlogged or arid, poorly aerated, and either acid or alkaline. The phosphorus cycle Phosphorus is absorbed as the anion, like sulphur, but has several problems associated with it. Phosphorus does not have a gaseous store in the atmosphere, but is found only in rock minerals and natural waters. Being cycled in the anionic form, phosphorus is easily leached from soils, owing to the lack of colloidal adsorption. Phosphorus easily combines with iron and manganese at low ph levels and with calcium and magnesium at high ph levels to form compounds unavailable to plants. Because of leaching and fixation problems, the store of phosphorus in soil organic matter is a vital reservoir of the nutrient. Biogeochemical cycling of base cations Base cations include potassium, calcium, magnesium, sodium and the metallic trace elements. The entire group cycles largely in the cationic chemical form, and this is the form in which it is absorbed by plants. Soil humic and clay colloids act as an important store of these nutrients, with the cationic exchange sites protecting the majority of ions against leaching losses. The Hubbard Brook catchment in the United States has been studied intensively over time, and quantities can now be provided for the stores and flows of these nutrients. Human clearance of trees has a disastrous effect on these cycles, removing the biomass store and leading to the erosion of the soil colloid store. Nutrient cycling in tropical rain forests The bulk of nutrients in tropical rain forests are stored in the living vegetation, though fertile soils in special areas such as volcanic regions and alluvial plains can have large soil stores.

5 The humic material in tropical soils is low in content, and is thus not able to provide much of a nutrient store. Mineral colloids consist of kaolinite and oxides of iron and aluminium, all of which have low cation exchange capacities. An important pathway of nutrient absorption by tropical rain forest trees is via the root fungi, mycorrhizae, which can absorb nutrients from the soil and pass them to the plants. Traditional peasant farming systems of shifting agriculture are sustainable if the fallow period is long enough, but plantation systems lead to many losses of nutrients. Figure 1 Eddy covariance tower measuring carbon fluxes in the Siberian tundra. An eddy covariance experiment in the tundra of Siberian Russia. The 4-metre high tower has an anemometer on top, and the horizontal tube carries a net radiometer. Readings are made of temperature, humidity, CO 2 fluxes and methane (CH 4 ). Changes in the carbon and hydrological cycles are inevitable with global warming, and scientists are working to determine the direction of such changes. Source: Jon Lloyd (NB: Slide 2 has been used as plate 21.3 in the book.)

6 Essay questions 1. Explain how photosynthesis and respiration are controlled by environment and plant physiology. 2. Discuss the magnitudes and differences between Gross Primary Productivity (GPP), Net Primary Productivity (NPP), Net Ecosystem Productivity (NEP) and Net Biome Productivity (NBP). 3. Describe the different types of biogeochemical cycles of nutrients that operate on land. Explain how feedbacks link the cycles together and with the climate system. Discussion topics 1. 1.Choose a particular biome (e.g. tundra, boreal forest, savanna, tropical rainforest) and discuss how the carbon cycle operates within it, and how this cycle might respond to future climate change. 2. Examine the ways in which changes in land-use influence the global carbon cycle. 3. Discuss how carbon in the oceans interacts with carbon in the atmosphere. 4. How do nutrient cycles interact with each other (e.g. C with N; N with P; C with P)? Give examples at the global scale, the biome scale, and the local ecosystem scale Discuss the extent to which variations in climate should be ascribed to the functioning of the terrestrial biosphere. Further reading Borman, F.H. and Likens, G.E. (1979) Pattern and Process in a Forested Ecosystem, New York: Springer-Verlag. The detailed account of the Hubbard Brook experimental catchment. Combines useful illustrative data with a clear enunciation of principles. Colinvaux, P. (1973) Introduction to Ecology, New York: Wiley. A readable, yet detailed, advanced text, covering all aspects of ecosystems. Chapters 9 to 15 cover energy and nutrient aspects. Dickinson, G., and Murphy, K. (1998) Ecosystems, London: Routledge. An extremely useful summary of the properties of ecosystems. Odum, E.P. (1959) Fundamentals of Ecology, Philadelphia: W.B. Saunders. The classic and innovative text which was a landmark on the functioning of ecosystems. A book written by an ecologist for ecologists, but plenty of good illustration for the biogeographer. Royal Society (2001) The role of carbon sinks in mitigating global change, London: The Royal Society, Policy Documents 10/01. Saugier, B., Roy, J. and Mooney, H.A. (2001) Global Terrestrial Productivity, San

7 Diego, Academic Press. As the title suggests, this is a good source of information on the productivities, and hence carbon budgets, of the world s biomes. Schlesinger, W.H. (1997) Biogeochemistry: an analysis of global change, 2 nd edition, London: Academic Press. This is an advanced textbook with chapters on all the majoe chemical elements of climatic and biological interest. The chapter on the carbon cycle is particularly relevant. Tivy, J. (1993) Biogeography, 3rd edition, London: Longman. This book has for many years been a standard text on biogeography. Many examples from around the world. Trudgill, S.T. (1988) Soil and Vegetation Systems, second edition, Oxford: Clarendon Press. A detailed treatment of productivity and nutrients in ecosystems, using a systems approach in the text and diagrams. Web resources The famous Broadbalk Winter Wheat Experiment at Rothamsted Experimental Station (see below) has been in operation since It allows nutrient cycles to be assessed in cereal farming, and also monitors leaching losses from the experimental plots. The Natural Environment Research Council s (NERC) CLASSIC (Climate and Land- Surface Systems Interaction Centre) Earth Observation Centre at the University of Exeter uses satellite data to detect changes in land-cover, and to model how these changes feedback on climate and the carbon cycle. QUEST-QUERCC (Quantifying and Understanding the Earth System-Quantifying Ecosystem Roles in the Carbon Cycle) This is the National Oceanographic and Atmospheric Administration (NOAA) website on the global carbon cycle, and the distribution of carbon dioxide and greenhouse gases. Research into nutrient cycles, fertiliser use and farming has been carried out at the world famous Rothamsted Experimental Station, Harpenden, for the past 160 years. The activities of the Agriculture and Environment Division, Rothamsted Research UK, allow you to see both the traditional and the new experimental programmes being carried out.