Urea Hydrolysis in Pine Tree Substrate Is Affected by Urea and Lime Rate

Pine tree substrate (PTS) has relatively high C:N ratio that requires the application of more N fertilizer than used with other substrates due to immobilization. If PTS were pre-charged with urea, then growers would not have to add extra N to compensate for immobilization, however, the rate of urea hydrolysis as influenced by urea and lime rate is unknown in PTS. The objective of this experiment was to determine how urea and lime rate influence urea hydrolysis rate in PTS. Approximately 18-month-old PTS made from loblolly pine trees (Pinus taeda L.) was amended with 0 or 1.0 kg·m^-3 dolomitic limestone in factorial combination with urea (46% N) rates of 0, 0.5, 1.0, 1.5, or 2.0 mg·g^-1 dry wt. Urea hydrolysis was quantified by the detection of NH₄-N in the substrate solution at 0, 48, 96, and 144 h. Substrate pH values were also measured. At 144 h, substrate solutions were incubated with jackbean urease to determine the remaining urea amount. After 48 h there was no increase in the amount of NH₄-N detected in solutions for the 0 and 1.0 kg·m^-3 lime treatments and for all urea rates. However, NH₄-N amounts at 48 h for the 0, 0.5, 1.0, 1.5, or 2.0 mg·g^-1 urea dry weight treatments were 0, 0.06, 0.12, 0.17, and 0.22 mg·g^-1 dry weight substrate for PTS without lime, respectively; NH₄-N amounts for the 1.0 kg·m^-3 lime treatment were 0, 0.08, 0.17, 0.25, and 0.34 mg·g^-1 dry weight substrate, respectively. Initial substrate pH values for the 0 and 1.0 kg·m^-3 lime treatments were 4.5 and 5.6, respectively. After 48 h, the 0 kg·m^-3 lime treatment pH values for the 0, 0.5, 1.0, 1.5, or 2.0 mg urea rate treatments were 4.6, 5.4, 6.2, 7.0, and 7.6, respectively; the 1.0 kg·m^-3 lime treatment pH values for the. 0, 0.5, 1.0, 1.5, or 2.0 mg urea rate treatments were 5.9, 6.5, 7.1, 7.6, and 7.9, respectively. Samples treated with jackbean urease had less than 2% of the initial urea amount. However, only 13% of the total amount of urea N added to PTS was detected as NH₄-N in the unlimed treatment after 144 h (for all urea rates); detected amounts for the 1.0 kg·m^-3 lime treatment ranged from 10% to 16%. The large difference in the amount of unrecovered NH₄-N may be explained by microbial N consumption.