Atmospheric deposition and net retention of ions by the canopy in a tropical montane forest, Monteverde, Costa Rica

Meteorological variables, bulk cloud water and precipitation (BCWP), and bulk precipitation (BP) were measured above the canopy, and throughfall (TF; n = 20) was collected beneath an epiphyte-laden canopy of a trop- ical montane forest (TMF) for 1 y at Monteverde, Costa Rica. Total deposition (cloud + wet + dry) of inorganic ions to the canopy was estimated using a sodium (Na+) mass balance technique. Annual BCWP and BP depths were 2678 mm and 1792 mm for events where mean windspeeds (u) > 2 m s-1, and 4077 mm and 3191 mm for all events, respectively. Volume-weighted mean pH and concentra- tions of nitrate-N (NO3--N) and ammonium-N (NH4+-N) were 4.88, 0.09 and 0.09 mg 1-1 in BCWP, and 5.00, 0.05 and 0.05 mg 1-1 in BP, respectively. Cloud water and mist deposition to the canopy was estimated to be 356 mm. Estimated deposition of free acidity (H+), NO3--N, and NH4-N to the canopy was 0.49, 3.4 and 3.4 kg ha-' y-1, respectively. Mean TF depth was 1054 ? 83 mm (mean ? S.E.) for events where u > 2 m s-', and 2068 ? 132 mm for all events. Volume-weighted mean pH and concentrations of NO3- -N and NH4+-N in TF were 5.72, 0.04 mg I-', and 0.07 mg I1-, respectively. Mean fluxes of H+, NO3- -N, and NH4+-N in TF were 0.04 + 0.01, 0.6 ? 0.2 and 1.3 ? 0.2 kg ha-' y-', and percent net retention of these ions by the canopy was 92 ? 2, 80 ? 6, and 61 + 6%, respectively. Phosphate, potas- sium, calcium and magnesium were leached from the canopy. Seasonal data sug- gest that biomass burning increased concentrations of NO3- and NH,' in cloud water and precipitation at the end of the dry season. Regardless, a large majority of the inorganic N in atmospheric deposition was retained by the canopy at this site.