Nitrate Δ17O tracer method for determining gross nitrification rates

Abstract

Gross nitrification rate (GNR) can reflect the actual status of nitrification process but is difficult to constrain. To develop a practical method for assessing GNR, Δ17O-NO3− tracers were applied to container systems under greenhouse conditions to test their feasibility. Two treatments with double- (15NΔ17O3−, negative Δ17O value) or single-labeled (NΔ17O3−, positive Δ17O value) fertilizer nitrate salts were implemented to constrain nitrification fractions and GNRs in soilless media container systems. The nitrification fractions calculated from the Δ17O-NO3− tracer method concurred with those from the 15NO3− tracer method, with small deviations and a linear regression slope close to 1. This suggested the Δ17O-NO3− tracer inherently works in the same way as the well-established 15NO3− tracer. GNRs in the soilless media container systems ranged from 250.5(±39.1) to 861.5(±275.6) mg N/(m2‧d), generally higher than in natural ecosystems. The discrepancies in GNRs between the two treatments were generally smaller than discrepancies in GNRs between replicate container systems, indicating the applicability of either negatively-labeled or positively-labeled Δ17O-NO3− tracers. Commercial nitrate salts mined from the Atacama and Kumtag Deserts are proposed for use as Δ17O-NO3− tracers for GNR estimation due to their abundant availability, low cost and distinct discrimination from nitrified NO3−.