Soil Property and Fertilizer Additive Effects on Ammonia Volatilization from Urea

Core Ideas Total exchange capacity was the best soil predictor of volatilization from urea. Urease activity and pH were not effective predictors of volatilization from urea. NBPT effectively reduces ammonia volatilization compared with nonamended urea. NBPT efficacy was reduced in more acidic soils. Calcium salt of maleic‐itaconic copolymer did not reduce ammonia volatilization. Urea efficiency can be substantially reduced through nitrogen loss as ammonia (NH₃–N), which is controlled by soil properties and environmental conditions. A laboratory incubation measured the effects of a range of soil properties on ammonia volatilization over 7 d from 168 kg N ha–¹ as surface‐applied urea 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) (urea + NBPT/NPPT) and urea plus a commercial formulation of calcium salt of maleic‐itaconic copolymer (MIP) from 79 soils across the United States. Total exchange capacity (TEC), 0.01 mol L–¹ CaCl₂ pH (pH), soil organic matter (SOM), hydroxide buffering capacity (OHBC), clay content, and urease activity were measured as predictors. Generalized regression models identified that TEC, clay content, OHBC, and SOM accounted for most variation in NH₃–N losses in urea (R² = 0.69) and urea + MIP (R² = 0.67). Total exchange capacity was the strongest predictor of volatilization; greater TEC resulted in lower NH₃–N losses. pH and TEC accounted for the most variation in NH₃–N loss among soils fertilized with urea + NBPT/NPPT (R² = 0.58). Volatilization increased with lower pH in urea + NBPT/NPPT, indicating that NBPT efficacy decreases in acidic soils, potentially due to faster chemical degradation. The addition of NBPT/NPPT to urea significantly reduced volatilization from 24.5 to 6.3% of applied N (P < 0.0001), whereas urea + MIP did not reduce volatilization (P = 0.9707).