Acidity Balances in Ricefields

The removal of fertilizer N in the crop as NH4+ does not lead to acidification. Hydrolysis of urea fertilizer—by far the main form of N fertilizer used in wetland rice, together with ammonium bicarbonate in some countries—consumes 1 mol of H+ per mol of NH4+ formed (Table 7.1, Process 1). So although absorption of N as NH4+ leads to a net export of H+ from the roots to balance the resulting excess intake of cations over anions (Table 7.1, Process 5), this acidity is matched by the H+ consumed in urea hydrolysis. Likewise there is no net generation of acidity as a result of NH3 volatilization, although 1 mol of H+ is left behind per mol of NH4+ converted to NH3 (Table 7.1, Process 3). 

Nitrification of the NH4+ does not cause net acidification, whether the NOs is absorbed by the crop or denitrified. Two mol of H+ are formed per mol of NH4+ nitrified, resulting in a net addition of one H+ per urea-N hydrolysed. If the NOs is absorbed by the crop, 1 mol of OH is exported from the roots per mol of NOs absorbed, or if it is denitrified, lmolH+ is formed per mol of N denitrified (Table 7.1, Process 4). 

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