Nutrient availability and substrate type limit benthic denitrification in a tropical Andean stream

In order to quantify the role of denitrification in a larger nitrogen budget for Rio Las Marías, Venezuela, we measured denitrification rates using the acetylene-block technique with chloramphenicol inhibition.  We separated dominant substrate types (silt and organic matter, gravel, and rocks and cobble) into large chambers for a total of 96 four-hour assays.  We added nitrate and glucose to a subset of assays, using target concentrations of 100 mg NO₃-N L^-1 and 40 mg C L^-1 above background, to estimate potential rates.  Field and potential denitrification rates were highly variable within each substrate treatment; and we found no significant differences among field rates of the three substrate types.  However, nutrient-amended silt and organic matter denitrification rates were significantly higher than the rest of the field and nutrient substrate assays (p < 0.0001).  Scaled by the frequency of different substrate types, denitrification in Rio Las Marías is responsible for removing 1.82 µg N₂O-N m^-2 hr^-1 (assuming no denitrification on boulders and woody debris), with the potential to remove up to 17.46 µg N₂O-N m^-2 hr^-1 with increased nutrient availability.  There are few published studies of denitrification in tropical freshwater systems, but rates in Rio Las Marías were lower than those reported for temperate streams.  When compared to nitrogen lost via fish harvest (approximately 15 mg m^-2 day^-1), denitrification was responsible for less than 0.3% of nitrogen loss.  Escalating sedimentation and nutrient-loading from human activities along the riparian zone have the potential to increase future contributions of denitrification to total nitrogen losses.