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