Nitrogen isotope enrichment factor as an indicator of denitrification potential in top and subsoil in the Apače Valley, Slovenia

Abstract

Denitrification is still a poorly explained process in soil nitrogen cycles. Nitrogen isotope analyses, in combination with conventional soil-science methods (i.e. rate measurements), permit the tracing and quantification of several turn-over and transfer processes in soils. The aim of the presented investigations is to determine the 15N isotope enrichment factor (ε) in a laboratory experiment during denitrification in the topsoil and subsoil layer of a Eutric Fluvisol (FAO classification) from the Apače Valley (NE Slovenia). Intact soil cores taken from depths of 0.10–0.20 m and 0.90–1.00 m were incubated under anaerobic conditions at a temperature of 18°C. A nitrate dose of 116 mg N-NO3/L was added to the topsoil layer and 58 mg N-NO3/L to the subsoil material. Glucose (250 mg) was added to each soil column. After the initial 24 h of incubation, water samples were taken every 8 h. The isotope enrichment factor for denitrification in the upper soil-layer columns was –7.60 ± 1.28‰ and for subsoil columns –34.91 ± 1.77‰ (n = 3). The isotope enrichment factor close to zero demonstrated that microorganisms easily consumed abundant substrate in the topsoil layer. Added nitrate and glucose exceeded the capacity of the microbial population in the subsoil and the process was limited by a low microbial population for which a high ε is characteristic.