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), a nd bulk precipitation (BP) were measured above the canopy, and through fall (TF; n = 20) was collected beneath an epiphyte-laden canopy of a tropical montane forest (TMF) for 1 y at Monteverde, Costa Rica. Total deposition (cloud + wet + dry) of inorganic ions to the canopy was es timated using a sodium (Na+) mass balance technique. Annual BCWP and B P depths were 2678 mm and 1792 mm for events where mean windspeeds (u) greater than or equal to 2 m s(-1) and 4077 mm and 3191 mm for all ev ents, respectively. Volume-weighted mean pH and concentrations of nitr ate-N (NO3--N) and ammonium-N (NH4+-N) were 4.88, 0.09 and 0.09 mg l(- 1) in BCWP, and 5.00, 0.05 and 0.05 mg l(-1) in BP, respectively. Clou d 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 c anopy was 0.49, 3.4 and 3.4 kg ha(-1) y(-1), respectively. Mean TF dep th was 1054 +/- 83 mm (mean +/- S.E.) for events where u greater than or equal to 2 m s(-1), and 2068 +/- 132 mm for all events. Volume-weig hted mean pH and concentrations of NO3--N and NH4+-N in TF were 5.72, 0.04 mg l(-1), and 0.07 mg l(-1), 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(-1) y(-1), and percent net retention of these ions by the canop y was 92 +/- 2, 80 +/- 6, and 61 +/- 6%, respectively. Phosphate, pota ssium calcium and magnesium were leached from the canopy. Seasonal dat a suggest that biomass burning increased concentrations of NO3- and NH 4+, in cloud water and precipitation at the end of the dry season. Reg ardless, a large majority of the inorganic N in atmospheric deposition was retained by the canopy at this site.