Measuring Nitrogen Losses from Lowland Rice Using Bulk Aerodynamic and Nitrogen‐15 Balance Methods

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Abstract
Management practices, designed to reduce ammonia (NH3) volatilization and total N loss from flooded rice fields, after application of urea, were assessed in the dry season at Mabitac, Philippines. The assessment was made in replicated 4.8‐m × 5.2‐m plots by determining NH3 and total fertilizer N losses with bulk aerodynamic and 15N balance methods, respectively. The bulk aerodynamic method was first evaluated by comparison of ammoniacal‐N concentrations, pH values, and equilibrium NH3 concentrations for a 50‐m diam, circular area and small plots receiving the same fertilizer treatment. Good agreement was obtained so the bulk aerodynamic method was used to determine NH3 loss from the small plots receiving the different management treatments. Ammonia loss was high when urea was broadcast into floodwater 10 d after transplanting—48 and 56% of the applied N was volatilized as NH3 during the first 8 d after application of 53 and 80 kg N ha−1, respectively. Total N losses were 60 and 59%, respectively. Ammonia loss was reduced slightly (to a mean of 43% of the applied N for the two application rates) by broadcasting the fertilizer into 0.05‐m‐deep floodwater and incorporating it into the soil by harrowing before transplanting. Removal of the floodwater before applying urea onto saturated soil and incorporation by harrowing reduced NH3 loss to a mean of 9% and total fertilizer N loss to a mean of 33%. Fertilizer N recovery by rice plants was significantly increased by incorporation without surface water.

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