The mean concentration of NH4 in the precipitation (5 µM) was approximately equal to that of NO3. The combined input of these two nitrogen sources represents an appreciable input of nitrogen into this ecosystem each year (approximately 4 kg N/ha/yr). Ammonium concentrations decrease upon interaction with the canopy and may reflect NH4 uptake by epiphytes during the period when no foliage is present on the trees.
Ammonium concentrations decreased by a factor of approximately three in the A horizon soil solutions following tree removal; however, concentrations in the AB horizon were significantly larger than those measured in oak canopy plots. Ammonium concentrations were considerably less than NO3 concentrations in comparable soil horizons due to transformation of NH4 to NO3 (nitrification), retention of NH4 on the cation exchange capacity, and plant uptake of NH4. As a result, there is little loss of nitrogen in the form of NH4 from the rooting zone.