Nutrient cycles

Ecosphere Photo

Earth Photo

Nutrient cycles

• Nutrient cycles, or “biogeochemical cycles,” involve natural processes that recycle nutrients in various chemical forms in a cyclic manner from the non-living environment to living organisms and back to the non-living environment again

• Types of nutrient cycles:– Hydrologic cycle– Atmospheric cycles– Sedimentary cycles

The water cycle

• No water, no life• determines ecosystem structure; water-living (aquatic) communities

important for supporting life on land• affects nutrient availability• Evaporation and transpiration lead to condensation, to precipitation, to

percolation and runoff, and all over again• Powered by energy from the sun and gravity• 84% of water vapor from the oceans (71% of the Earth’s surface)• 77% of precipitation falls back into the sea• Some precipitation locked in glaciers

• runoff, erosion, moves soil and weathered rock• primary sculptor of the earth’s landscape• dissolves many nutrient compounds, transporting nutrients• Percolation dissolves minerals and moves them into groundwater• Times for water to cycle through various pathways:

– Water table: 300-4600 years; Lakes: 13 years; Streams: 13 days; Atmosphere: 9 days; Ocean: 37,000 years; Glaciers: 16,000 years

• Evaporation = natural distillation; also purified by chemical and biological processes in the soil

• Hydrologic, atmospheric or sedimentary?

The carbon cycle

• Essential to life• Basic building block of carbohydrates, fats, proteins,

nucleic acids and all other organic compounds• CO2 is a heat-trapping greenhouse gas; regulates

heat, with major impacts on ecosystem function• Cycling times for CO2: Atmosphere, 3 years; Soil,

25-30 years; Oceans, 1,500 years• Hydrologic, atmospheric or sedimentary?

The phosphorous cycle

• essential nutrient of plants and animals, used in DNA, nucleic acids, fats, cell membranes, and bones, teeth and shells

• from phosphate deposits on land and shallow ocean sediments to living organisms and slowly back to the land and ocean

• Very little in the atmosphere, only as small particles of dust• much more rapidly through living components than through

geological formations; animals get by eating producers or animals that eat producers

• Animal wastes and decay return much of this phosphorous to the soil, streams, and eventually to ocean bottom and into rock cycle

• Hydrologic, atmospheric or sedimentary?

The nitrogen cycle

• Nitrogen is necessary for vital organic compounds such as amino acids, proteins, DNA and RNA

• In short supply in both terrestrial and aquatic ecosystems• N2 = 78% of the volume of the troposphere• Cannot be directly used by organisms• Must be converted to compounds that can enter food webs by

the process of “nitrogen fixation”

• Nitrogen fixation:– Specialized bacteria convert N2 to ammonia (NH3) by the

reaction

N2 + 3H2 = 2NH3

– Cyanobacteria in soils and water, and Rhizobium bacteria in small nodules in legume root systems

– Nitrification – NH3 converted by specialized aerobic nitrite (NO2-), toxic

– Converted to nitrate (NO3-) ions, which are easily taken up by plants as nutrients

• Nitrogen fixation:– Assimilation – NO3- taken up by plants and used to make

nitrogen-containing organic molecules

– Animals get nitrogen by eating plants or plant-eating animals

– Decomposers convert to NH3 and ammonium (NH4+); ammonification

– Denitirification - specialized bacteria convert NH3 and NH4+ back into NO2- and NO3, and then to N2 and N2O, released into the atmosphere

• Easily leached by water, limiting productivity potential.

• Hydrologic, atmospheric or sedimentary?

Back to the Ecosphere

How do humans affect nutrient cycles?

Water cycle:• Drain fresh water from streams, lakes, and underground

sources• Clear vegetation increasing runoff, reducing infiltration,

increasing erosion and risk of flooding• Modify water quality by adding nutrients (phosphates) and

changing ecological processes that naturally purify water

Carbon cycle:• Put more CO2 in the atmosphere than plants can remove• Deforestation reduces the amount of vegetation to remove CO2

• Burning fossil fuels and wood releases more CO2 than natural processes

• What happens when we have to much heat-trapping gas?

Phosphorous cycle:• Mine large phosphate rock for fertilizers and detergents• Cutting tropical forests; little phosphorous in soil, all bound up in

organic matter which usually rapidly recycles; but we remove the biomass or burn it, allowing it to be rapidly washed away by runoff, leaving the land unproductive

• Add excess phosphate to aquatic ecosystems in runoff from agricultural operations, causing explosive plant growth creating surface mats which block sunlight; dying plants feed bacteria which uses up most of the oxygen in the water.

Nitrogen cycle:• Emit nitric oxide (NO) when burning fuels; leads to acid rain• Emit heat-trapping nitrous oxide (NO2) into the atmosphere • Remove nitrogen from the earth’s crust for fertilizers, harvesting nitrogen-rich

biomass, and increase leaching through irrigation• Remove nitrogen from topsoil when burning grasslands and clearing forests; also

emits nitrous oxides• Add excess through runoff and sewage – promotes overgrowth of algae, which

dies, breaks down, and decomposition by bacteria depletes the water of oxygen; disrupts aquatic systems; reduces aquatic biodiversity

• Add excess nitrogen to atmosphere; allowing weedy plants to outcompete other plants, reducing biodiversity

Experimental impacts on

nitrogen cycling in a disturbed habitat

Nitrogen cycles in an experimental ecosystem