Urea Hydrolysis in Soils Along a Toposequence: Influences of Chemical Conditions.

Urea fertilizer has been linked to harmful algal blooms in estuaries and ocean waters. The objective of this research is to determine the effects of soil chemical conditions on urea hydrolysis in toposequences of soils from Maryland in the Mid-Atlantic region of the USA. Each transect included an active agricultural field, a grassed field edge, and a site adjacent to a surface water. Soil was sampled from two depths: 0-15 cm and a depth representative of the B horizon between 30-80 cm, depending upon the site. The soils were sieved to 4 mm, stored at field-sampled moisture content, and maintained at room temperature (22˚C).  Soil samples were mixed with a urea solution and the disappearance of urea was measured over time to determine a rate of hydrolysis at each site. An initial study indicated that rates of urea hydrolysis did not change significantly with storage for up to six months. Surface soil samples hydrolyzed urea more rapidly than B-horizon samples, with an average rate of -0.84 mg urea-N/kg soil/hour at one site and -0.88 mg urea-N/kg soil/hour at a second site. B-horizon soils had lower rates of hydrolysis: -0.19 mg urea-N/kg soil/hour and -0.53 mg urea-N/kg soil/hour. Preliminary data showed that a decrease in pH reduced urea hydrolysis in all soils or resulted in no measurable change. For example, urea hydrolysis in an agricultural field surface soil with a native pH_s of 5.3 decreased by 50% when pH was reduced to 4.6. The effects of CaCO₃ addition to soils had mixed results; it decreased urea hydrolysis in some surface soils and increased urea hydrolysis in some subsoils. Knowledge of soil chemical conditions affecting urea hydrolysis will help us understand whether agricultural fertilizer timing or placement would help reduce urea leaching from agricultural fields.