N Pool Size (Stocks) are relatively big Atmosphereg N10 21 Terrestrial biomass g N 10 15 Soil organic matter g N 10 15
Flows small Rate limiting steps with respect to getting N into terrestrial system: §Via lightning (very small) §Via Biological N Fixation l natural l legume crops §Via Industrial fixation of fertilizer §Via 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 N 2 to NH 3 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 N 2 to ammonia NH 3 §NH 3 + org. acids amino acids proteins
Human-caused NF = or greater Natural terrestrial BNF BNF from crops 40 x 10 12 g N fixed/yr Fertilizer industry 80 Fossil fuel burning20 Total human-caused 144 BNF terrestrial ecosystems100
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 may be active in each acre of soil. Bacteria dot the surface of strands of fungal hyphae. --Soil Biology Primer
Life on earth: starting with microbes §First N fixed by lightning and meteor impact §Formation of first amino acids and protein §N-fixation early because N-limited environment in water §Terrestrial system: P limited environment §Mycorrhizal fungi have allowed plants to become terrestrial
P Cycle §P is an important plant nutrient, anion §Released from soil, taken up by plants and released to soil §Green Rev ag is totally dependent on P rock § P rock is basically a valuable, non- renewable resource for agriculture l 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
Mycorrhizal fungus stained blue Non mycorrhizal Roots of Common Bean (Phaseolus vulgaris) from Costa Rica
Components of association §fungus + root = mycor + rhiza §They 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 plant §Enables plants to live in low P, low water environments
Mycorrhizae aid soil aggregation Mycorrhizal fungi link root cells to soil particles. In this photo, sand grains are bound to a root by hyphae from endophytes (fungi similar to mycorrhizae), and by polysaccharides secreted by the plant and the fungi. Credit: Jerry Barrow, USDA-ARS Jornada Experimental Range, Las Cruces, NM.
Closing nutrient cycles §Making the cycle circular §The nutrient comes back to the “stock” or “pool” that it started with-- none is lost §In a human time frame §This is an important component of sustainability
Chinese have used “night soil” for centuries to close nutrient cycle