Biological Fixation Of Canopy Epiphytes In Montane Moist Evergreen Broad-Leaved Forest In Ailao Mountains, SW China

Biological fixation is a main resource of nitrogen in natural ecosystem. There are three categories of nitrogen fixing microorganisms:free-living, symbiotic, and symbiotic free-living nitrogen fixing microorganisms.Ailao Mountain National Nature Reserve (NNR) has an area of 504 km2, and contains the largest tract of natural broad-leaved forest in China. The forest is dominated by tree species in the families of Fagaceae, Lauraceae, and Magnoliaceae, and lacks any significantsymbiotic N-fixing plants. The forest canopy is covered with thick layers of epiphytes dominated by mosses and lichens. My objective is to demonstrate and quantify if the canopy epiphytes can fix nitrogen from atmosphere.We chose C2H2 reduction and 15N natural abundance methods to determine the nitrogen fixation potentials in the forest. The C2H2 reduction method is commonly used in N2 fixation studies due to its high precision and low costs. To reduce the variability of measurements, we incubated epiphyte samples for a period of 1-4 days to obtain realistic estimates of N2 fixation potentials. We took measurements for epiphyte samples taken in both dry and wet seasons and from both top and sub-canopy layers.Nitrogen fixing potentials of canopy epiphytes varied between dry and wet seasons and between top and sub-canopies. Peak N2 fixing potential occurred at the transition period between dry and wet seasons. N2 fixing potential tended to be low near the end of each of the dry and wet season, reaching zero at the end of dry season. Rewetting of epiphytes can rapidly restore N2 fixing capacity. Nitrogen fixing potentials did not differ between epiphyte samples, or among host tree species.Our N data showed that epiphytes N2 fixing rate did not differ between the top and sub-canopy layers, among plots, or among different sampling trees. This is in contrary with data from the C2H2 reduction estimate probably due to the difficulties in selecting non-N2 fixing reference trees.Nitrogen fixing potentials for the subtropical most forest in Ailao Mountains ranged between 0.027 kg/ha/year and 2.24 kg/ha/year, which the C2H4 production rates was 6.95×10-7 ml/g/h-5.63×10-5 ml/g/h, and it declined to 0 when the epiphytes were close to air dry in the dry season. We found 39 species of mosses and lichens in the forest canopy, greater than 37 species recorded in literature.Our results indicate that the canopy epiphytes in the subtropical moist forest of Ailao Mountains can fix a significant amount of atmospheric nitrogen. This finding suggests a new nitrogen source for the forest ecosystem, thus can have profound impact on the studies of nitrogen cycling.