Predicting soil pH changes in response to application of urea and sheep urine

Volatilization of NH₃ following urea application or livestock urine deposition can result in significant loss of N to the environment. Urea hydrolysis to NH₄ ⁺ results in an increase in pH, which in turn promotes transformation of NH₄ ⁺ to NH₃ . Accurately predicting changes in soil pH following urea (or urine) application will allow successful simulation of NH₃ volatilization. The magnitude of the pH change depends on the soil's pH buffering capacity (pHBC). However, as actual pHBC values are not generally available, pHBC proxies (e.g., cation exchange capacity) have been used in modeling studies. In a 34-d laboratory incubation study, we measured soil pH and mineral N (NH₄ ⁺ and NO₃ ^- ) following a large application of urea (800 mg N kg^-1 soil) to four soils with a range of pHBC values. In a second incubation, pH changes and mineral N dynamics were monitored in soil treated with sheep urine (773 mg N kg^-1 soil) in the absence and presence of the nitrification inhibitor dicyandiamide. In both incubations, pH changes associated with urea hydrolysis and subsequent nitrification of NH₄ ⁺ were predicted well using measured pHBC data. Our results confirmed that pHBC is base-type dependent (values greater when measured using KOH than NH₄ OH). Soil pHBC is easily measured, and the use of a measured value (determined using NH₄ OH) can improve model simulations of pH in the field and, potentially, lead to improved estimates of NH₃ loss from animal-deposited urine patches and urea-treated soil.