1.5 Nitrogen Cycle - APES

Nitrogen Cycle Overview

Movement of N containing molecules between sources & sinks/reservoirs 

• Sources release N into the atmosphere; sinks take N out of the atmosphere in increasing amounts 

N reservoirs hold N for a relatively short period of time compared to C cycle

• EX: Soil, plants, atmosphere

Atmosphere = Main N reservoir 

• N in atm. exists mostly as N₂ gas, not usable by plants or animals 

N = Critical plant & animal nutrient

• All living things need N for DNA & amino acids to make proteins 

Nitrogen Fixation

Process of N₂ gas being converted into biologically available (useable by plants) NH₃ (ammonia) or NH₃^- (nitrate)

Bacterial Fixation

Certain bacteria that live in the soil, or in a symbiotic relationship with plant root nodules, convert N₂ into ammonia (NH₃)

Synthetic Fixation

Humans combust FFs to convert N₂ gas into nitrate (NH₃^-)

Assimilation

Plants and animals taking the N in and incorporating it into their body

Ammonification

Soil bacteria, microbes & decomposers converting waste and dead biomass back into NH₃ and returning it to soil

Nitrification

Conversion of NH₄ into nitrite (NO₂^-) & then nitrate (NO₃) by soil bacteria

Denitrification

Conversion of soil N (NO₃) into nitrous oxide (N₂O) gas which returns to atmosphere

Human Impacts on N Cycle

Climate: N₂O (nitrous oxide) = greenhouse gas which warms earth’s climate

• Produced by denitrification of nitrate in agricultural soils (especially when waterlogged/over watered)

Ammonia Volatilization: Excess fertilizer use can lead to NH₃ gas entering atm.

• NH₃ gas in atm. = Acid precipitation (rain) and respiratory irritation in humans and animals

• It also means less N stays in soil for crops to use for growth (lost profit)

Leaching & Eutrophication: Synthetic fertilizer use leads to nitrates (NO₃) leaching, or being carried out of soil by water

• Nitrates runoff into local waters, causing algae blooms that block sun → Kills aquatic plants