Retention of inorganic nitrogen by epiphytic bryophytes in a tropical montane forest

We developed and evaluated a model of the canopy of a tropical montane forest at Monteverde, Costa Rica, to estimate inorganic nitrogen (N) retention byepiphytes from atmospheric deposition. We first estimated net retention of inorganic N by samples of epiphytic bryophytes, epiphyte assemblages, vascular epiphyte foliage, and host tree foliage that we exposed to cloud water and precipitation solutions. Results were then scaled up to the ecosystem level using a multilayered model of the canopy derived from measurements of forest structure and epiphyte mass. The model was driven with hourly meteorological and event-based atmospheric deposition data, and model predictions were evaluated against measurements of throughfall collected at the site. Model predictions were similar to field measurements for both event-based and annual hydrologic and inorganic N fluxes in throughfall. Simulation of individual events indicated that epiphytic bryophytes and epiphyte assemblages retained 33-67 percent of the inorganic N deposited in cloud water and precipitation. On an annual basis, the model predicted that epiphytic components retained 3.4 kg N ha/yr, equivalent to 50 percent of the inorganic N in atmospheric deposition (6.8 kg N ha/yr). Our results indicate that epiphytic bryophytes play a major role in N retention and cycling in this canopy by transforming highly mobile inorganic N (ca. 50% of atmospheric deposition is NO3-) to less mobile (exchangeable NH4+) and recalcitrant forms in biomass and remaining litter and humus.