Low δ18O Values of Nitrate Produced from Nitrification in Temperate Forest Soils

Analyses of δ18O of nitrate (NO3 –) have been widely used in partitioning NO3 – sources. However the δ18O value of NO3 – produced from nitrification (microbial NO3 –) is commonly estimated using the δ18O of environmental water and molecular oxygen in a 2:1 ratio. Here our laboratory incubation of nine temperate forest soils across a 1500 m elevation gradient demonstrates that microbial NO3 – has lower δ18O values than the predicted using the 2:1 ratio (by 5.2–9.5‰ at low elevation sites), in contrast to previous reports showing higher δ18O values (up to +15‰) than their predicted values. Elevated δ18O values of microbial NO3 – were observed at high elevation sites where soil was more acidic, perhaps due to accelerated O-exchange between nitrite, an intermediate product of nitrification, and water. Lower δ18O of microbial NO3 – than the predicted and from previous observations suggests that the contribution of anthropogenic N inputs, such as fertilizer and atmospheric deposition, to a given ecosystem and the progress of denitrification in nitrogen removal are greater than we know. More than half of the δ18O of stream NO3 – lower than the predicted value along the elevation gradient also indicate the impropriety using the 2:1 ratio for differentiating NO3 – sources.