6 Immobilization transforms mineral nitrogen into soil organic matter. Activity of soil microbes is mainly stimulated by ammonium. Immobilized nitrogen is not immediately available for plant uptake, but needs to be mineralized first. Mineralization of soil organic matter (and manure) releases ammonium into the soil. 7 Ammonia volatilization occurs when ammonium is converted to ammonia and lost to the atmosphere. A high soil pH level favours and temperature favour conversion of ammonium to ammonia. If conversion takes place at the soil surface, losses are highest8 Leaching of nitrate occurs mainly during winter and fallow periods when rainfall washes residual and mineralized nitrates below the root zone. Accurate fertilization increases N use efficiency and reduces the risk of leaching during the growth period and afterwardsNitrogen transformations when applying Urea1 Application of fertilizers, containing mineral nitrogen as urea. Organic fertilizers contain mostly complex forms of N and ammonium.2 Hydrolysis of Urea by soil enzymes converts urea into ammonium and CO2 gas. Depending on temperature, hydrolysis takes a day to a week. The soil pH around the urea granules strongly increases during the process, favouring ammonia volatilization.3 Ammonia volatilization occurs when ammonium is converted to ammonia and lost to the atmosphere. A high soil pH level favours and temperature favour conversion of ammonium to ammonia. If conversion takes place at the soil surface, losses are highest. These conditions are met when urea is spread and not immediately incorporated.4 Nitrification by soil bacteria converts ammonium into nitrate in between a few days and a few weeks. Nitrous oxide and nitric oxide are lost to the atmosphere during the process. 5 Uptake of nitrate is rapid due to the high particle mobility. Most plants therefore prefer nitrate over ammonium.6 Uptake of ammonium is slower than that of nitrate. Ammonium is bound to clay particles in the soil and roots have to reach it. Most of the ammonium is therefore nitrified before it is taken-up by plants.7 Denitrification is favoured by lack of oxygen (water logging). Soil bacteria convert nitrate and nitrite into gaseous nitrous oxide, nitric oxide and nitrogen. These are lost to the atmosphere.6 Immobilization transforms mineral nitrogen into soil organic matter. Activity of soil microbes is mainly stimulated by ammonium. Immobilized nitrogen is not immediately available for plant uptake, but needs to be mineralized first. Mineralization of soil organic matter (and manure) releases ammonium into the soil. ConclusionAmmonium Nitrate is taken-up directly by plants while urea first needs to be converted. Ammonium Nitrate therefone offers a higher nitrogen use efficiency than urea. Ammonium Nitrate thus reduce losses and environmental impact.
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