Changes in canopy processes following whole-forest canopy nitrogen fertilization of a mature spruce-hemlock forest

Most experimental additions of nitrogen to forest ecosystems apply the N to the forest floor, bypassing important processes taking place in the canopy, including canopy retention of N and/or conversion of N from one form to another. To quantify these processes, we carried out a large-scale experiment and determined the fate of nitrogen applied directly to a mature coniferous forest canopy in central Maine (18-20 kg N ha(-1) y(-1) as NH4NO3 applied as a mist using a helicopter). In 2003 and 2004 we measured NO3-, NH4+, and total dissolved N (TDN) in canopy throughfall (TF) and stemflow (SF) events after each of two growing season applications. Dissolved organic N (DON) was greater than 80% of the TDN under ambient inputs; however NO3- accounted for more than 50% of TF N in the treated plots, followed by NH4+ (35%) and DON (15%). Although NO3- was slightly more efficiently retained by the canopy under ambient inputs, canopy retention of NH4+ as a percent of inputs increased markedly under fertilization. Recovery of less than 30% of the fertilizer N in TF suggested that the forest canopy retained more than 70% of the applied N (> 80% when corrected for N which bypassed tree surfaces at the time of fertilizer addition). Results from plots receiving N-15 enriched NO3- and NH4+ confirmed bulk N estimations that more NO3- than NH4+ was washed from the canopy by wet deposition. The isotope data did not show evidence of canopy nitrification, as has been reported in other spruce forests receiving much higher N inputs. Conversions of fertilizer-N to DON were observed in TF for both (NH4+)-N-15 and (NO3-)-N-15 additions, and occurred within days of the application. Subsequent rain events were not significantly enriched in N-15, suggesting that canopy DON formation was a rapid process related to recent N inputs to the canopy. We speculate that DON may arise from lichen and/or microbial N cycling rather than assimilation and re-release by tree tissues in this forest. Canopy retention of experimentally added N may meet and exceed calculated annual forest tree demand, although we do not know what fraction of retained N was actually physiologically assimilated by the plants. The observed retention and transformation of DIN within the canopy demonstrate that the fate and ecosystem consequences of N inputs from atmospheric deposition are likely influenced by forest canopy processes, which should be considered in N addition studies.