4.1 Biogeochemical cycles and the flow of energy in the earth

Learning objectives

After reading this module, students should be able to

• explain the concept of a biogeochemical cycle, incorporating the terms "pool" and "flux"
• describe the natural cycles of carbon, water, and nitrogen
• name some of the important ways human activity disrupts those cycles

Introduction

If people are to live sustainably, they will need to understand the processes that control the availability and stability of the ecosystem services on which their well-being depends. Chief among these processes are the biogeochemical cycles    that describe how chemical elements (e.g. nitrogen, carbon) or molecules (e.g. water) are transformed and stored by both physical and biological components of the Earth system. Storage occurs in pools    , which are amounts of material that share some common characteristic and are relatively uniform in nature, e.g. the pool of carbon found as carbon dioxide (CO ₂ ) in the atmosphere. Transformations or flows of materials from one pool to another in the cycle are described as fluxes    ; for example, the movement of water from the soil to the atmosphere resulting from evaporation is a flux. Physical components of the earth system are nonliving factors such as rocks, minerals, water, climate, air, and energy. Biological components of the earth system include all living organisms, e.g. plants, animals and microbes. Both the physical and biological components of the earth system have varied over geological time. Some landmark changes include the colonization of the land by plants (~400 million years ago), the evolution of mammals (~200 million years ago), the evolution of modern humans (~200 thousand years ago) and the end of the last ice age (~10 thousand years ago). The earth system and its biogeochemical cycles were relatively stable from the end of the last ice age until the Industrial Revolution of the eighteenth and nineteenth centuries initiated a significant and ongoing rise in human population and activity. Today, anthropogenic (human) activities are altering all major ecosystems and the biogeochemical cycles they drive. Many chemical elements and molecules are critical to life on earth, but the biogeochemical cycling of carbon, water, and nitrogen are most critical to human well-being and the natural world.

The natural carbon cycle

Most of the carbon on Earth is stored in sedimentary rocks and does not play a significant role in the carbon cycle on the timescale of decades to centuries. The atmospheric pool of CO ₂ is smaller [containing 800 GtC (gigatonnes of carbon) = 800,000,000,000 tonnes] but is very important because it is a greenhouse gas. The sun emits short-wave radiation that passes through the atmosphere, is absorbed by the Earth, and re-emitted as long-wave radiation. Greenhouse gases in the atmosphere absorb this long-wave radiation causing them, and the atmosphere, to warm. The retention of heat in the atmosphere increases and stabilizes the average temperature, making Earth habitable for life. More than a quarter of the atmospheric CO ₂ pool is absorbed each year through the process of photosynthesis by a combination of plants on land (120 GtC) and at sea (90 GtC). Photosynthesis is the process in which plants use energy from sunlight to combine CO ₂ from the atmosphere with water to make sugars, and in turn build biomass. Almost as much carbon is stored in terrestrial plant biomass (550 GtC) as in the atmospheric CO ₂ pool. On land, biomass that has been incorporated into soil forms a relatively large pool (2300 GtC). At sea, the phytoplankton that perform photosynthesis sink after they die, transporting organic carbon to deeper layers that then either are preserved in ocean sediments or decomposed into a very large dissolved inorganic carbon pool (37,000 GtC). Plants are called primary producers    because they are the primary entry point of carbon into the biosphere. In other words, almost all animals and microbes depend either directly or indirectly on plants as a source of carbon for energy and growth. All organisms, including plants, release CO ₂ to the atmosphere as a by-product of generating energy and synthesizing biomass through the process of respiration    . The natural carbon cycle is balanced on both land and at sea, with plant respiration and microbial respiration (much of it associated with decomposition, or rotting of dead organisms) releasing the same amount of CO ₂ as is removed from the atmosphere through photosynthesis.