Effects of Soil Properties and Fertilizer Additives on Ammonia Volatilization from Urea.

The efficiency of urea can be substantially reduced through atmospheric N loss as ammonia gas (NH₃–N), which is controlled by interrelated soil properties and environmental conditions. A laboratory incubation measured the effects of a wide range of soil properties on ammonia volatilization over 7 days from 168 kg ha^-1 of N as surface-applied urea (U) and the efficacy of two fertilizer additives: urea plus a commercial formulation of N-(n-butyl) thiophosphoric triamide (NBPT) + N-(n-propyl) thiophosphoric triamide (NPPT) (U + NBPT) and urea plus a commercial formulation of calcium salt of maleic-itaconic copolymer (U + MIP) from 79 soil surface samples across the United States. Total exchange capacity (TEC), 1:1 CaCl₂ pH (pH), soil organic matter (SOM), hydroxide buffering capacity (OHBC), clay content, and urease activity (UA) were measured as predictors. Generalized regression models identified that TEC, clay content, buffering capacity, and SOM accounted for most variation in NH₃–N losses in U (R² = 0.69) and U + MIP (R² = 0.67). Total exchange capacity was the strongest predictor of volatilization, and greater TEC resulted in lower NH₃–N losses. Interestingly, pH and TEC accounted for most of the variation in U + NBPT (R² = 0.58). Volatilization increased  with lower soil pH in U + NBPT, indicating that NBPT efficacy decreases in acidic soils, potentially due to more rapid chemical degradation. Volatilization was significantly reduced (P < 0.0001) by 18 percentage points in U + NBPT compared to U, and U + MIP did not reduce volatilization (P = 0.9707).