Biogeochemical Cycles Chapter 2

Ecosystems provide vital services All life on Earth (including humans) depends on healthy, functioning ecosystems Ecosystem services: essential services provided by healthy, normally functioning ecosystems When human activities damage ecosystems, we must devote resources to supply these services ourselves Example: if we kill off insect predators, farmers must use synthetic pesticides that harm people and wildlife One of the most important ecosystem services: Nutrients cycle through the environment in intricate ways

Ecological processes provide services

Nutrients circulate through ecosystems Nutrients move through the environment in complex ways Matter is continually circulated in an ecosystem Nutrient (biogeochemical) cycle: the movement of nutrients through ecosystems Pool (reservoir): a location where nutrients remain for varying amounts of time (residence time) Source: a reservoir releases more materials than it accepts Sink: a reservoir that are accepts more than it releases Flux: the rate at which materials move between reservoirs Can change over time Copyright C Pearson Education, Inc., publishing as Benjamin Cummings 4

Humans affect nutrient cycling Human activities affect nutrient cycling Altering fluxes, residence times, and amounts of nutrients in reservoirs

The water cycle affects all other cycles Water is essential for biochemical reactions and is involved in nearly every environmental system and cycle Hydrologic cycle: the flow of liquid, gaseous, and solid water through the environment Less than 1% is available as fresh water Evaporation: conversion of liquid to gaseous water Transpiration: release of water vapor by plants Precipitation: rain or snow returns water to Earth’s surface Runoff: water flows into streams, lakes, rivers, oceans

Transpiration Animation: Transpiration Right-click / Select “Play”

Water is also stored underground Infiltration: water soaks down through rock and soil to recharge aquifers Aquifers: underground reservoirs of spongelike regions of rock and soil that hold … Groundwater: water found underground beneath layers of soil Water table: the uppermost level of groundwater held in an aquifer Water in aquifers may be ancient (thousands of years old)

The hydrologic cycle

Human impacts on the hydrologic cycle Humans have affected almost every flux, reservoir, and residence time in the water cycle Damming rivers slows water movement and increases evaporation Removal of vegetation increases runoff and erosion while decreasing infiltration and transpiration Overdrawing surface and groundwater for agriculture, industry, and domestic uses lowers water tables Emitting air pollutants that dissolve in water changes the nature of precipitation and decreases cleansing Copyright C Pearson Education, Inc., publishing as Benjamin Cummings 10

The Carbon Cycle Carbon cycle: describes carbon’s route in the environment Carbon forms essential biological molecules Through photosynthesis, producers move carbon from the air and water to organisms Respiration returns carbon to the air and water Oceans are the second largest reservoir of carbon Absorb carbon from the air, land, and organisms Decomposition returns carbon to the sediment, the largest reservoir of carbon Ultimately, it may be converted into fossil fuels Copyright C Pearson Education, Inc., publishing as Benjamin Cummings 11

The carbon cycle

Humans affect the carbon cycle Burning fossil fuels moves carbon from the ground to the air Since mid-1700s, people have added over 275 billion tons of carbon dioxide to the atmosphere Cutting forests and burning fields moves carbon from organisms to the air Less carbon dioxide is removed by photosynthesis Today’s atmospheric carbon dioxide reservoir is the largest in the past 800,000 years The driving force behind climate change

The Nitrogen Cycle Nitrogen makes up 78% of the atmosphere It is contained in proteins, DNA, and RNA It is also essential for plant growth Nitrogen cycle: describes the routes of nitrogen through the environment Nitrogen gas is inert and cannot be used by organisms It needs lightning, bacteria, or human intervention to become biologically active and available to organisms Then it is a potent fertilizer

Nitrogen must become biologically available Nitrogen fixation: nitrogen-fixing soil bacteria or lightning “fixes” nitrogen gas into ammonium Nitrogen-fixing bacteria live in legumes (e.g., soybeans) Nitrification: bacteria then convert ammonium ions first into nitrite ions then into nitrate ions Plants can take up these ions Nitrite and nitrate also come from the air or fertilizers Animals obtain nitrogen by eating plants or other animals Denitrifying bacteria: convert nitrates in soil or water to gaseous nitrogen, releasing it back into the atmosphere

The nitrogen cycle

Humans greatly affect the nitrogen cycle Historically, nitrogen fixation was a bottleneck: limited the flux of nitrogen from air into water-soluble forms Industrial fixation fixes nitrogen on a massive scale Overwhelming nature’s denitrification abilities Excess nitrogen leads to hypoxia in coastal areas Nitrogen-based fertilizers strip the soil of other nutrients Reducing soil fertility Burning forests and fossil fuels leads to acid precipitation, adds greenhouse gases, and creates photochemical smog

The phosphorus cycle Phosphorus cycle: describes the routes that phosphorus takes through the environment No significant atmospheric component Most phosphorus is in rocks With naturally low environmental concentrations, phosphorus is a limiting factor for plant growth Weathering releases phosphorus into water Allowing it to be taken up by plants Phosphorus is a key component of cell membranes, DNA, RNA, and other biochemical compounds

The phosphorus cycle

Humans affect the phosphorus cycle Fertilizer from lawns and farmlands Increases phosphorus in soil Its runoff into water increases phytoplankton blooms and hypoxia Wastewater containing detergents releases phosphorus to waterways

Controlling nutrient pollution in waterways Reduce fertilizer use in farms and lawns Change timing of fertilizer applications to minimize runoff Manage livestock manure applications to farmland Plant vegetation “buffers” around streams to trap runoff Restore wetlands and create artificial ones to filter runoff Improve sewage-treatment technologies Restore frequently flooded lands Reduce fossil fuel combustion

Conclusion Life interacts with its abiotic environment in ecosystems through which energy flows and materials are recycled Understanding biogeochemical cycles is crucial Humans are changing the ways those cycles function Understanding energy, energy flow, and chemistry increases our understanding of organisms How environmental systems function Thinking in terms of systems can teach us how to: Avoid disrupting Earth’s processes and to mitigate any disruptions we cause Copyright C Pearson Education, Inc., publishing as Benjamin Cummings 22