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Transcript
Nitrogen Cycle Atmosphere = 78% gaseous Nitrogen N2 tightly bonded together Most nitrogen in soil is associated with organic matter Most organic matter is about 5% N Inorganic N is only about 1 to 2% of N in soil 1. 2. 3. 4. 5. Necessary for chlorophyll synthesis Essential part of photosynthesis Essential part of amino acids Essential for carbohydrate use. Stimulates plant productivity Taken up by plants as NH4+ or nitrate (NO3) Deficiency Symptoms: chlorosis- pale yellow color occurring in older leaves first, stunted plants, or spindly plants that mature quickly. 1. Results in excessive vegetation 2. May cause lodging 3. May delay plant maturity 4. Plants may be more susceptible to disease. 5. Can cause nitrate poisoning in cattle grazing forages Occurs as microbes decompose organic matter for energy. 95-99% of soil N is in organic forms (proteins) Organic N Pool Inorganic N Pool (unavailable) (available) Occurs when crop residues high in carbon and low in Nitrogen are incorporated into the soil. Organic N Pool Inorganic N Pool (unavailable) (available) C:N ratio of decomposing tissue determines if mineralization will occur more than immobilization. Wide ratio (30:1) favors immobilization Narrow ratio (>20:1) favors mineralization 20 – 30:1 both occur equally Topsoil Alfalfa Rotted manure Cornstalks Grain straw Coal Oak Spruce 10:1 13:1 20:1 60:1 80:1 124:1 200:1 1000:1 When immobilization is greater than mineralization there is little N for plants. Applying N fertilizer can shift it to mineralization. Adding residue shifts it to immobilization. Decomposition of organic matter to ammonia (NH4) Several clay mixtures have the capacity to fix both ammonia and potassium ions (micas, vermiculite, and smectites). Volatilization – ammonia gas lost to the atmosphere. ▪ Greater as pH increases ▪ Ammonia gas is produced ▪ As soil dries ammonia volitalizes ▪ Plants can absorb ammonia from the air (cleaning air) The bacterial oxidation of ammonium to nitrite and then to nitrate. Step 1: ▪ NH4+ + 1½ O2 NO2 + 2H+ + H2O Step 2: ▪ NO2 + ½ O2 NO3 1. pH 4.5 – 10.0 2. Good drainage, adequate O2 3. Temperature 20 – 30o C Nitrates are lost to the atmosphere when they are reduced to Nitrous oxide or elemental nitrogen. ▪ 2NO3 2NO2 2NO(g) N2O(g) N2(g) 1. Nitrate must be available 2. Decomposable organic compounds must be available. 3. Soil air should contain less than 10% oxygen 4. Temperature should be 2-50oC 5. pH above 5.0 1. Taken up by plants 2. Leached through soil 3. Run-off 4. Lost to atmosphere through denitrification. 5. Lost to atmosphere through volatilization. Ammonium (NH4) is stable in the soil – held by the CEC sites. ▪ 1. Ammonium – N does not leach ▪ 2. NH4 is not subject to denitrification ▪ 3. Corn hybrids, wheat, cotton have higher yields when fertilized with a mixture of NH4 and NO3 1. Nitrification inhibitors – deactivate nitrifying bacteria ▪ Most used on sandy soils 2. Slow release forms of N ▪ Sulfur coated urea or formaldehyde coated urea. 1. Symbiotic – bacteria grows with the plant and fixes N benefiting both the plant and the bacteria. ▪ Fixes 100 – 500 kg N/ha/year ▪ Bradyrhizobium bacteria and legume roots ▪ Cyanobacteria – non-nodule forming 2. Non-symbiotic – carried out by free living bacteria in the soil. ▪ Fixes about 20 lbs N/a/year N2(g) + 6H+ + 6e- 2NH3 organic acids amino acids proteins Alfalfa group B. meliloti ▪ Alfalfa ▪ Sweetclover Clover Group B. trifolii ▪ Trifolium spp. Soybean B. japanicum