Processes Governing Ammonia Emissions from Rice

2 PROCESSES GOVERNING AMMONIA EMISSIONS FROM RICE 

Urea broadcast into the ricefield floodwater is hydrolysed to ammonium, bicarbonate and hydroxyl ions the reaction is catalysed by the enzyme urease  

One mol of H+ is consumed in this reaction for every 2mol of NH4+ formed. In subsequent volatilization of NH3, 1 mol of H+ is produced for every mol of NH4+ converted to NH3  

Because urease activities are much greater in the soil than in the floodwater, the NH4+ is largely formed in the soil as the urea moves downward by mass flow and diffusion. The NH4+, H+ and other reactants will also move between the floodwater and soil-both upward and downward-with NH3 being lost from the floodwater by volatilization. The recovery of N in the crop therefore depends on the rate of movement of urea and its reaction products through the soil and on the rate at which the roots remove N from the downward moving pool. 

Rachhpal-Singh and Kirk (1993a,b) developed a model of these processes based on equations for the transport and reaction of urea, ammoniacal species (NH4+, NH3, NH4OH), carbonate species (H2CO3, HCOs , COs ) and mobile acid-base pairs (H2C03-HC03, HCOs -COs , NH4+-NH3, NH4+-NH4OH, H20-OH ). The equations are of the form of Equation (2.6) 

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