Bias in calculating gross nitrification rates in forested catchments using the triple oxygen isotopic composition (Δ17O) of stream nitrate

Abstract

A novel method has been proposed and applied in recent studies to quantify gross nitrification rate (GNR) in forested catchments using the triple oxygen isotopic composition (Δ17O) of stream nitrate. However, the equations used in these calculations assume that the Δ17O value of nitrate consumed through assimilation or denitrification in forest soils is equal to the Δ17O value of stream nitrate. The GNR estimated from the Δ17O value of stream nitrate was significantly higher than the GNRs in our simulated calculations for a forested catchment where the soil nitrate had Δ17O values higher than those the stream nitrate. Given that most reported soil nitrate in forested catchments showed Δ17O values higher than those of the stream nitrate, we concluded that the GNR estimated from the Δ17O value of stream nitrate was, to an extent, an overestimate of the actual GNR.