1 Nitrogen cycle Forms of inorganic N  plants need inorganic forms of N to grow - cannot use organic forms  ammonium, NH 4 + held on clay  other forms not held by clay particles - nitrate NO 3 -, and nitrite NO 2 - quickly....  leach or  transform into nitrogen gases (N 2, NO, or N 2 O) and ammonia gas (NH 3 ) & evaporate

2 Nitrogen fixation  mostly (90%) by bacteria : - Azotobacter spp., Rhizobium spp. Azospirillum spp.  and lightning, ultraviolet radiation [can also be produced by electrical equipment, and artificially by Haber-Bosch process (N2 + H2 + catalysts + T + P)]  ammonia (NH 3 ) and nitrates (NO 3 - ) formed  bacteria become encased in nodules that grow on the roots of plants  cyanobacteria (formerly blue-green algae) also fix N for liverworts, hornworts, cycads, and at least one genus of flowering plants (Gunnera); also symbiotic relationship with fungi – combination called lichen

3 Azotobacter vinelandii Rhizobium leguminosarum

4  non-symbiotic forms may fix N for cereals, e.g:  Azospirillum spp.  Klepsiella spp.  Euterobacter spp  Acetobacter spp.

5 Legume Root Nodules Over 15,000 species in Leguminosae family, ranging from forage legumes to grain legumes to trees. Many of these have organs on their roots called nodules which are packed full of bacteria called "rhizobia". The rhizobia can live either within the nodule or in the soil, but they can only fix N 2 while they are inside the nodule. Inside the nodule, the rhizobia are provided with carbon and energy from the plant.

6 Nodules on root

7 Innoculation fixing of N often can be improved by adding the bacteria to the soil at the time of planting

8 Nitrification  plants can assimilate NH 3 as well as NO 3 but plants can incorporate the nitrates more easily into their tissues  but most NH 3 converted to NO 2 - and then to NO 3 by aerobic bacteria  process is called “nitrification”  bacteria that convert ammonia to nitrites : (Nitrosomonas, Nitrosospira, Nitrosococcus, & Nitrosolobus) - family Nitrobacteraceae - use inorganic chemicals as an energy source  bacteria that convert nitrites (toxic to plants) to nitrates : Nitrobacter, Nitrospina, and Nitrococcus  applying dilute solutions of ammonia results in an increase in soil nitrates through the action of nitrifying bacteria  inorganic nitrogen is also added directly to soil in precipitation, or as fertilisers.

9 Nitrosomonas spp.under electron microscopy (39,000X). Nitrobacter sp.

10 Leaching  major loss mechanism - involves two forms of nitrogen - ammonium (NH 4 ) and nitrate (N0 3 )  NH 4 is held by the clay and OM with only small amounts in soil solution  NH 4 nitrogen not easily washed out  but NH 4 N broadcast on warm, moist soils rapidly changed to nitrate & then leached - reduced by application in bands  NO 3 moves more easily - more subject to leaching  mainly on sandy soils in high rainfall areas, or under irrigated conditions  fine textured clay soils are able to hold more moisture and allow much less movement of water and nitrate down through the soil

11 Assimilation by plants and animals  NO 3 & NH 4 assimilated into tissue of algae and higher plants  converted to organic forms, such as amino acids and proteins  animals ingest - converting them into own body compounds

12 Ammonification of decaying organic matter  Organic N exists in materials formed from animal, human, and plant activities - manures, sewage waste, compost, and decomposing roots or leaves  transformed into organic soil material called humus  process absorbs inorganic nitrogen in a process called “immobilisation”  plants cannot use organic N but microbes convert organic N into inorganic forms – “mineralisation” - that plants can then use

13  decomposed by micro-organisms such as detritivores in the process of ammonification  positive charged NH 4 can be adsorbed and fixated on to the negatively charged soil clay particles or taken up directly by plants  although fixation of atmospheric N is essential part of the N cycle, ammonification and then nitrification are predominant methods by which organic N is prevented from returning to the atmosphere

14 C: N Ratio  decomposition of OM requires N and leads to temporary immobilisation of the nitrogen  C/N ratio used to indicate speed at which OM decomposes  low C/N indicates rapid decomposition - nutrients bound up in the organic fraction released relatively quickly  as C/N ratio increases, speed of decomposition decreases  finally, at high C/N ratios, OM organic matter is stable – decomposes slowly  adding nutrients - especially N - to soils of high C/N ratio is likely to result in their being locked up by being bound into the organic matter until the C/N ratio falls to a value in line with the local environment

15  plant growth stunted if C:N ratio too high – good value is 30:1  in warm areas - speed of cycling is high – OM lower  C/N ratio in warmer soils higher than that in cooler soils  allophane (volcanic soils) supports higher OM levels than other clays, so the C/N ratio in volcanic soils is likely to be lower, making them relatively better store-houses of nutrients than other soils in the same climatic zone  adding compost increases the stable OM fraction - high C/N ratio gives it slow breakdown rate - helps to improve soil structure and moisture-holding capacity

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17 Denitrification and volatisation  some N returns to atmosphere as denitrifying bacteria break down nitrates or nitrites to obtain oxygen & release gaseous N 2 and N 2 O - escape (volatilise) into the atmosphere  especially in water-logged, anaerobic and poorly drained soils  deplete soil fertility and reduce agricultural productivity

18  Thiobacillus denitrificans, Micrococcus denitrificans, & some species of Serratia, Pseudomonas, and Achromobacter  Pseudomonas aeruginosa can reduce the amount of fixed N by 50 percent  denitrification needed in N cycle or would accumulate in oceans  N is lost from plants and soil via other routes such as erosion, runoff, volatilisation of ammonia, and leaching

19 Pseudomonas sp.

20 Volatilisation of fertilisers  N fertilisers containing urea or ammonium may also be lost through volatilisation as an ammonia gas into the atmosphere  Ammonium nitrate (34-0-0) and ammonium sulphate (21-0-0) less subject to ammonium volatilisation than urea [CO(NH 2 ) 2 ] - (46-0-0)  losses greater from alkaline than from acid soils  losses greater from dry rather than wet soils  losses from sandy soils greater than from heavier soils  losses greater at higher temperatures than low

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