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N-fixing trees as a source of nitrate for tropical streams
High abundance of nitrogen fixing trees in tropical areas has been hypothesized to drive higher stream nitrate concentrations compared to temperate streams. However, few studies have examined annual variation in concentration and isotopes of nitrate in tropical watersheds. We examined the del15N and del18O in NO3 for one year in throughfall and stream water in three sites in the Salto watershed in La Selva Biological Station, Costa Rica. del15N of throughfall was more depleted (-4.15) than stream water (4.65 - 4.90), while there was little difference in del18O across all sites (2.05-2.99). Stream water del15N and del18O did not change from a first order stream site to a third order stream, even though the third order stream receives solute rich interbasin groundwater inputs. There were no differences in throughfall or stream del15N and del18O in dry vs wet season. Evaluation of the proportion of N lost via denitrification for this watershed using isotopic models suggest that N-fixation in tropical watersheds can sustain high N exports.
Primary Presenter: Marcelo Ardon Sayao, NC State University (mlardons@ncsu.edu)