Nitrogen Dynamics of Soils  Introduction  Forms and Role of Nitrogen in Soils & Plants  Nitrogen Fixation  Distribution of Nitrogen (N-Cycle)  Nitrogen Transformations  Fate of Nitrogen in Soils  Nitrogen Fertilizer and Management

Introduction Nitrogen is an essential plant nutrient It is mainly in organic forms in soil In it’s ionic form, it is very mobile in soils and plants It is also responsible for some environmental problems.

1. Forms of Nitrogen taken by plants Nitrogen in the soil solution is in three forms NO 3 - NH 4 + SON NO 3 - and NH 4 + ions behave differently in soils and are differently preferred by plants. NO 3 - exchange with HCO 3 - and OH - at root surface –increase pH NH 4 + exchange with H + at root surface –reduce pH

Component of essential plant compounds Amino acids ~ building blocks of proteins, enzymes nucleic acid ~ hereditary control Chlorophyll ~ photosynthesis Plants respond to good available N by having deep green color of leaves Increased protein content increased plumpness of grains Increased plant productivity in general 2. Role of nitrogen in plants

Nitrogen is quite mobile (easily translocated) within plants, so available N is sent to newest foliage first Deficiency exhibits the following: Pale yellowish green color (chlorosis) in older leaves Have stunted growth and thin stems (low shoot:root ratio) Plants mature more quickly than healthy plants Protein content is low and sugar content is high There is reduced productivity in general 3. Deficiency of nitrogen in plants

When too much N is applied, the following may occur: Excessive vegetative growth (but weak plant stems) that lead to lodging with rain or wind Delays maturity and cause plants to be prone to diseases General decline of plant product quality Environmental aspects (build up of nitrates) 4. Oversupply of nitrogen in plants

Nitrogen Fixation Biological Fixation Biological fixation which is by far more important than lightening is catalyzed by enzyme nitrogenase. N 2 + H + +6e - = 2NH 3 + H 2 NH 3 is formed which combines with organic acids to form amino acids and ultimately protein. NH 3 + Organic Acids → Amino Acids → Proteins The two types of biological fixation are: 1.Symbiotic fixation 2.Non symbiotic fixation N-Fixation is the process of converting the inert dinitrogen gas of the atmosphere to nitrogen containing organic compounds that becomes available to all forms of live N-Fixation is accomplished in nature biologically by a) certain microorganisms, and b) by lightening.

Symbiotic Fixation (Legumes & Non legumes) Legumes and bacteria enter into symbiotic relationship. The bacteria (Rhizobium) infects the root hair and cortical cells inducing the formation of root nodules that serve as the site of nitrogen fixation. Plant provides carbohydrate Some non legumes have also been observed to develop nodules that form site of N fixation. Some nitrogen fixation have even been described as fixation without nodules.

Nonsymbiotic Nitrogen Fixation Certain free living organisms present in soil that are not directly associated with higher plants are able to fix nitrogen. Because these organisms are not directly associated with higher plants, the transformation is referred to as nonsybiotic or free living N- fixation Some heterotrophs e.g. Azotobacter Autotrophs e.g. Photosynthetic bacteria

Distribution of Nitrogen (The Nitrogen Cycle)

Nitrogen Transformations Mineralization Conversion of organic forms of N into inorganic forms (NO 3 - and NH 4 + ) Immobilization Conversion of inorganic N forms (NO 3 - and NH 4 + ) to organic N Nitrification Conversion of NH 4 + ions into NO 3 - Denitrification Conversion of NO 3 - to N gas (NO, NO 2, N 2 O) Volatilization Transformation of NH 4 + ions into ammonia gas

Fate of SON, NH 4 + and NO 3 - in Soils a) Soluble organic Nitrogen (SON) Not much information is available on this N form Comprise of amino sugars and amino acids It is taken up directly by plant roots OR, it is leached and carried in groundwater to streams where they cause environmental problems

b) NH 4 + Like other positively charged ions NH 4 + is attracted to negatively charged soil colloids NH 4 + can be fixed by 2:1 minerals because of its unique size NH 4 + can volatilize into NH 3 gas NH 4 + can also be converted into nitrates (NO 3 - ) directly (nitrosomonas bacteria) OR through an intermediary step (NO 2 - ) (nitrobacter bacteria)

c) NO 3 - Plants utilize NO 3 - directly NO 3 - is negatively charged and so is not adsorbed by the negative charges that dominate most soils. This makes it move down freely with drainage water causing several environmental problems (eutrophication and hypoxia). Nitrate can also be converted to gaseous forms of nitrogen by series of reduction reactions. Denitrification

Nitrogen Fertilizer Fertilizers supply nitrogen in soluble forms such as nitrate or ammonium, or as urea. Nitrate or ammonium from fertilizer are taken up by plants and participate in the N cycle in exactly the same way as nitrate or ammonium derived from organic matter mineralization or other sources. Fertilizer-N has much more concentration in time and space than N from other sources.

Management of Soil Nitrogen Objectives of good N management 1. Maintain adequate N supply 2. Regulate the soluble forms of N to ensure enough is readily available 3. Minimizing leakage from soil-plant system Strategies for Achieving the Objectives:  Taking into account N contribution from other sources so as not to oversupply N  Improving efficiency with which fertilizer is applied  Improving crop response knowledge  Avoiding overly optimistic goals of meeting crop needs that are higher than possible