Nitrogen and Phosphorus Cycles
Martha E. Rosemeyer
IES
January 29, 2003
One of three dynamic processes of ecosystems
Energy flowNutrient cyclesSuccession
Energy flows from the sun to entropy through a functioning food web
Two types of nutrient cycles
Volatile atmospheric reservior N, S
Non-volatile No atmospheric reservior P, K, Ca, Mg all micronutrients
Nitrogen cycle
2 3
Gliessman, 1998modified
N2ON2
Nitrification
N Pool Size (Stocks) are relatively big
Atmosphere g N 1021
Terrestrial biomass g N 1015
Soil organic matter g N 1015
Flows small Rate limiting steps with respect to getting N into terrestrial system:
Via lightning (very small)Via Biological N Fixation
natural legume crops
Via Industrial fixation of fertilizerVia Fossil fuel burning
What is biological nitrogen fixation (BNF)?
Second most important biochemical reaction of ecosystem after photosynthesis to life on earth
Involves the reduction of N from molecular state N2 to NH3 in which form it can be incorporated into an organic (C containing) molecular structure
Involves microbes
BNF
Regardless of organism uses nitrogenase enzyme for to fix N2 to ammonia NH3
NH3 + org. acids amino acids proteins
White clover, Trifolium repens“Trebol blanco”
seed
Temperate zone green manure crop
Human-caused NF = or greater Natural terrestrial BNF
BNF from crops 40 x 1012 g N fixed/yr
Fertilizer industry 80
Fossil fuel burning 20
Total human-caused 144
BNF terrestrial ecosystems 100
Microbes: The Ancient Ones
Responsible for all major processes on earth, including decomposition and photosynthesis and nutrient cycling
Major cycles of Earth could continue without plants and animals
Most microbes are beneficial!
Most bacteria 99% cannot be cultured
Therefore we know little about them!
A ton of microscopic bacteria maybe active in each acre of soil. Bacteria dot the surface of strands of fungal hyphae.
--Soil Biology Primer
Life on earth: starting with microbesFirst N fixed by lightning and meteor impactFormation of first amino acids and proteinN-fixation early because N-limited
environment in waterTerrestrial system: P limited environmentMycorrhizal fungi have allowed plants to
become terrestrial
Nitrogen cycle:mediated by microbes
2 3 Gliessman 1998modified
= Microbial mediation
P CycleP is an important plant nutrient, anionReleased from soil, taken up by plants and released
to soilGreen Rev ag is totally dependent on P rock P rock is basically a valuable, non-renewable
resource for agriculture some deposits contaminated with Cd, Pb, As
10% of the P which is applied to land is flushed way causing eutrophication of lakes
Dead zone in Gulf of Mexico due to nutrients from Mississippi
The phosphorus cycle
Gliessman 1998
= Microbial mediation
mycorrhizae
P cycle
Brady, 1999
Mycorrhizal fungusstained blue
Non mycorrhizal
Roots of Common Bean (Phaseolus vulgaris) from Costa Rica
Components of associationfungus + root = mycor + rhizaThey are the rule, not the exception:
Most of world’s vascular plants have association, except Brassicaceae (cabbage family) and a few other plant families
Fungus: takes up water and P, receives CHO from plantEnables plants to live in low P, low water environments
Mycorrhizae aid soil aggregation
Mycorrhizal fungi link root cells to soil particles. In thisphoto, sand grains are bound to a root by hyphae fromendophytes (fungi similar to mycorrhizae), and bypolysaccharides secreted by the plant and the fungi.Credit: Jerry Barrow, USDA-ARS Jornada Experimental Range, Las Cruces, NM.
Closing nutrient cycles
Making the cycle circularThe nutrient comes back to the “stock” or
“pool” that it started with-- none is lostIn a human time frameThis is an important component of
sustainability
Chinese have used “night soil” for centuries to close nutrient cycle