Low δ18O values of nitrate produced from nitrification in temperate forest soils.

Analyses of δ(18)O of nitrate (NO(3)(-)) have been widely used in partitioning NO(3)(-) sources. However the δ(18)O value of NO(3)(-) produced from nitrification (microbial NO(3)(-)) is commonly estimated using the δ(18)O 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 NO(3)(-) has lower δ(18)O 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 δ(18)O values (up to +15‰) than their predicted values. Elevated δ(18)O values of microbial NO(3)(-) 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 δ(18)O of microbial NO(3)(-) 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 δ(18)O of stream NO(3)(-) lower than the predicted value along the elevation gradient also indicate the impropriety using the 2:1 ratio for differentiating NO(3)(-) sources.