Denitrification

Denitrification loss of N

1. Essential Elements: Forms, Occurance , Functions and Deficiency Symptoms Krishna Aryal Asst. Professor (Soil Science) [email protected] 1

2. 2 Denitrification Reduction process in which bacterial reduction of NO 3 - and NO 2 - takes place leading to the release of NO, N 2 O and N 2 gases. Occurs in anaerobic conditions( when soil becomes water logged) such as rice field • Two forms of denitrification – Assimilatory Denitrification: NO 3 - converts into NH 4 + (Preserved) – Dissimilatory Denitrification: NO 3 - …NO 2 - …. NO…N 2 O & N 2 (lost)

3. 3 Dissimilatory denitrification is common than the assimilatory form. Facultative anaerobic bacteria belonging to the genera Pseudomonas , Bacillus denitrificans and Paracoccus are responsible for this. Autotrophs like Thiobacillus denitrificans also brings about denitrification.

4. 4 Ecological conditions for Denitrification Anaerobic conditions Optimum temp. 37 o c Nitrate (electron acceptor); Carbon (energy source) pH 5-9 (optimum at pH 7) Unspecific process……………..unspecific organisms

5. 5 Nitrate Leaching All applied N fertilizer sources eventually convert completely to the nitrate-N form. Urea can convert to nitrate-N in <2 wks in late spring; Anhydrous ammonia converts more slowly to nitrate-N because of its initial toxic effects on the soil microbes responsible for the conversion of ammonium-N to nitrate-N. This form of N is Soluble in nature not held tightly by soil particles leached from the soil profile with excessive rains, especially on lighter textured soils. (flooding)

6. 6 Ways of reducing Nitrate Leaching Increasing water holding capacity through organic matter addition Split application of N fertilizer Controlled irrigation Using nitrification inhibitors like: Natural nitrification inhibitors -Neem cake ( Azadiracta indica ) Chemical nitrification inhibitors - N - serve or nitrapyrin (2 chloro – 6 tri chloromethyl pyridine) ,DCD (Dicyandiamide) ,AM (2 amino, 4 chloromethyl pyrimidine) ,ATC (4-amino 1,2, 4 triazole) , Sulphathiozole ,Thiourea ,Hydroquinone Calcium carbide (CaC2) Phenyl phosphorodiamidate (PPD) Slow release N fertilizers like neem coated urea, sulphur coated urea.

7. 7 Nitrogen immobilization Conversion of inorganic N (NH 4 + or NO 3 - ) to organic N and it is basically the reverse of N mineralization. The Microorganisms accumulate NH 4 + and NO 3 – in the form of protein, nucleic acid and other complexes. C:N >30:1…………….immobilization C:N 20 to 30:1………neither immobilization nor release of N C:N ratio <20:1……….mineralization occurs.

8. 8 Ammonium Fixation • Certain clay minerals, particularly vermiculite & illite , are capable of fixing NH 4 + by replacement of K + for inner-layer cations in the expanded lattices of clay minerals. • Fixed NH 4 + can be replaced by cations that can expand clay lattice(Ca 2+ , Mg 2+ , Na + & H + ) & not by cations that contract lattice (K+)

9. 9 • K + restricts NH 4 + fixation since K + can also fills fixation sites. • Consequently, it has been suggested that K fertilzer application prior to the NH 4 + application is a practical way of reducing NH 4 + fixation where it is a problem in the field. Order of fixation: Vermiculite> Fine grained micas> Smectite

10. 10 Factors affecting ammonium fixation Type and amount of clay: NH 4 + fixation increases with increase in the content particularly 2:1 type of clay minerals like vermiculite, fine grained mica and smectite. Moisture content of the soil: The moisture content and temperature of the soil will affect the fixation of NH 4 + . Freezing and drying increases the fixation. Alternate cycles of wetting and drying; freezing and thawing are believed to contribute to the stability of recently fixed NH 4 + Amount of K + : The presence of K + will restrict NH 4 + fixation since K + also fills the fixation sites.

11. 11 Depth of the soil: Fixation of NH 4 + is generally higher in sub soil than surface soil due to higher clay content and lower rate of nitrification. Organic matter content : Higher the organic matter content more is the NH 4 + fixation. Population of nitrifying bacteria : Nitrifying bacteria oxidize NH 4 + to nitrite and nitrates thus reducing the chances for NH 4 + fixation.

12. 12 N Volatilization (NH 4 + …………….NH 3 + H + ) • When NH 4 + forming fertilizers ( eg. Urea) are applied to acid or neutral soil (usually at surface), sol pH increases around soil granules during hydrolysis as H + in consumed in reaction. • Under such condition, urease enzyme directly reduce NH 4 + to the NH3, which is subjected to loss to atmosphere. • Forms of N-fertilizer susceptible to volatilization losses include dry urea or liquid urea-ammonium-nitrate (UAN) sol surface-applied without incorporation.

13. 13 • Risk is greatest with warm sunny days after application, & surface soil pH levels > 7.0; risk is also high on light textured soils with low buffer capacity. • Volatilization loss is higher if fertilizer is broadcasted than subsurface applied or incorporation.

14. 14 Functions of N in plant It’s an essential constituent of metabolically active compounds like :amino acids, proteins, nucleic acids, porphyrins, flavins, purines and pyrimidine nucleotides, flavin nucleotides, enzymes, coenzymes and alkaloids. Being a constituent of RNA and DNA, N is responsible for cell division and reproduction. (transfer of genetic code.) Constituent of chlorophyll molecule and thus N containing chlorophyll fixes atmospheric CO 2 through photosynthesis.

15. 15 Imparts dark green colour & vegetative growth vigorously to the plants. It increases crop yields. Improves the quality by increasing protein content.

16. 16 Deficiency Symptoms of N Since N is mobile nutrient deficiency symptoms first appear on the older/mature leaves. However symptoms spread rapidly to the young leaves when plant deficit in N. Chlorosis of older leaves.( leaves turn green to yellow) In severe N-deficiency, the leaves will turn brown and die

17. 17 In maize, the lower leaves usually fire or turn brown beginning at the leaf tip & progressing along the mid-rib until the entire leaf is dead and appears 'v' shape. Plants become stunted. N deficient plants mature early & crop quality & yield are reduced.

18. 18 Flowering and fruit setting are adversely affected hence; size and quality of the fruit are poor. Few tillers, slender stalks, short heads & grains with low protein content in cereals. Leaf curling and small tubers in potato. Rapid crop yield decline

19. 19 Excess of N It is also harmful. High N produces succulence in plants & enhances their sensitivity to water & temperature stress. High N-plants also become susceptible to lodging, pathogen & pests attack.

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