• Nutrient cycles, or “biogeochemical cycles,” involve natural processes that recycle nutrients in various chemical forms in a cyclic manner from the nonliving environment to living organisms and back to the non-living environment again
• Types of nutrient cycles:
– Hydrologic cycle
– Atmospheric cycles
– Sedimentary cycles

• 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?

• 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?

• 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?

• Nitrogen is necessary for vital organic compounds such as amino acids, proteins, DNA and RNA
• In short supply in both terrestrial and aquatic ecosystems
• N
2
= 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 N
2 reaction to ammonia (NH
3
) by the
N
2
+ 3H
2
= 2NH
3
– Cyanobacteria in soils and water, and
Rhizobium bacteria in small nodules in legume root systems
– Nitrification – NH
3
(NO
2
-), toxic converted by specialized aerobic nitrite
– Converted to nitrate (NO
3 by plants as nutrients
-) ions, which are easily taken up

– Assimilation – NO
3
- 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 NH
3 ammonification and ammonium (NH
4
+);
– Denitirification - specialized bacteria convert NH
3 back into NO
2
- and NO
3
, and then to N
2 into the atmosphere and N
2 and NH
O, released
4
+

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 CO
2 in the atmosphere than plants can remove
• Deforestation reduces the amount of vegetation to remove CO
2
• Burning fossil fuels and wood releases more CO
2 processes than natural
• 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 (NO
2
) 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