GES175, Science of Soils Lecture 8 NITROGEN

Slide 8.2

Slide 8.3

Oxidation States of Soil N Slide 8.4 Oxidation States of Soil N N Form Name Oxidation state organic-N -3 NH4+ ammonium -3 â á N2 dinitrogen gas 0 (oxidation) (reduction) NO2- nitrite +3 â á NO3- nitrate +5 â á

Nitrogen Redox Processes Oxidation: loss of e- Reduction: gain of e- -3 +5 NH4+ ¾¾¾¾¾® NO3- 8 e- transfer

N-cycle 5 5 4 2 3 1 3 Slide 8.6 plant & animal residues N2 organic-N NH4+

Mineralization vs. Immobilization Fate of N if added to soil???

Low C:N (high N content) Slide 8.8 Low C:N (high N content) Alfalfa, peas, grass

High C:N (low N) Slide 8.9 straw, bark, sawdust

Ammonia Volatilization - gaseous loss of N

Ammonia Volatilization Urea: CO(NH2)2 ¾¾¾¾¾¾® NH3 +CO2 + H2O urea soil enzymes & H2O - Most volatilization when: ü coarse or sandy-textured soils ü low clay and low organic matter (which adsorb NH4+) ü dry alkaline surface

Nitrification * Autotrophic bacteria NH4+ ¾¾¾® NO2- ¾¾¾® NO3- ammonium nitrite nitrate - oxidation of N * Autotrophic bacteria • obtain energy from N oxidation • Nitrosomonas NH4+ ¾® NO2- + energy • Nitrobacter NO2- ¾® NO3- + energy

Nitrification (cont’d) * Rapid in well-aerated, warm, moist soils • aerobic organisms (O2 is required) • little NO2- accumulation * Acid-forming process NH4+ +3/2O2¾® NO2- + 2H+ + H2O

Nitrogen (nitrate?) Leaching  Eutrification

Denitrification

Denitrification Gaseous loss of N upon N reduction + e- + e- + e- + e- NO3- ¾¾® NO2- ¾¾® NO ¾¾® N2O ¾¾® N2 nitric nitrous oxide oxide

Denitrification (cont’d) * Microorganisms responsible: • facultative anaerobes - prefer O2 but will use N for a terminal e- acceptor • mostly heterotrophic - use organic-C for energy source (reductions require energy)

Denitrification (cont’d) * Denitrification enhanced by: • low O2 (flooding) • high O.M. (energy source) • high NO3-

Denitrification (cont’d) * Metabolic reduction is not denitrification (no N gas formation) NO3- organisms ® NH4+ ® organic-N - N is reduced for use in protein formation

Nitrogen Fixation N2 ¾(organisms)® NH4+ * Symbiotic relation between bacteria and plants: - legumes + - rhizobium

Nitrogen Fixation Bacteria: Rhizobium genus (species specific) R. meliloti - alfalfa R. trifolii - clover R. phaseoli - beans - bacteria require plant to function - inoculation of seed (coat seed with proper bacteria)

Process: nodule Rhizobium

(b) Process: C from plant photosynthesis î N from fixation of N2 ì organic-C organic-N N2 Rhizobium C from plant photosynthesis î N from fixation of N2 ì Þ symbiosis

Quantity of N Fixed Alfalfa and clover provide Beans and peas » 100 - 250 kg N/ha/yr (mature stand, good fertility & pH) Beans and peas » less fixation but high protein food with minimum N input added N fertilizer  lowered N fixation

Symbiotic Nodules - Nonlegumes * Organisms l actinomycetes - Frankia * Plants l Alders and other trees

Symbiotic - without nodules * Azolla/Anabaena complex é é é blue-green algae (N-fixer) é in leaves floating fern in rice paddies * Rhizosphere organisms l use root exudates (C) l large areas

Nonsymbiotic N-fixation: Free-living Organisms * Bacteria and blue-green algae l aerobic and anaerobic l small amounts: 5 - 50 kg/ha/yr l inhibited by available soil N