Urea Hydrolysis in Pine Forest Floor: Effect of Water Potential.

Previous research has shown that the water potential of the forest floor plays a significant role on the magnitude of ammonia volatilization losses from urea applied to loblolly pine forest floor.  The controlling factor of water potential is likely due to its effect on urea hydrolysis, but data on this effect are limited.  Consequently, field and laboratory studies were carried out to evaluate the effect of water potential of the forest floor on urea hydrolysis.   In seven field studies, urea at 200 kg N/ha was surface applied to 10-cm ID cylinders inserted into a pine forest floor, with cylinders extracted and analyzed for urea at 3, 14, 21, and 29 d after urea application. In laboratory studies, samples of new needles, old needles, or humified layer were equilibrated to water potentials ranging from -1 to -50 MPa, with the amount of urea hydrolyzed measured at the end of 3 to 6 days.  The percentage of urea hydrolyzed in the field studies ranged from 17 to 92%, with lower values observed at lower water potentials.  Laboratory studies showed very little urea hydrolysis in any of the three forest floor layers at water potentials below -10 MPa. These results suggest that urea applied to pine forest floor will undergo very little hydrolysis, and therefore little ammonia loss, when the water potential of the forest floor is below – 10 MPa.