Abstracts

Johannes Friedl₁, Clemens Scheer₁, Johanna Trappe₂, David W. Rowlings_1, Peter R. Grace₁

¹Institute for Future Environments, Queensland University of Technology, Brisbane, QLD 4000, Australia. johannes.friedl@qut.edu.au,

² University of Münster, Germany

Abstract

Nitrogen turnover and related denitrification losses are a major uncertainty when estimating N loss and replacement from agro-ecosystems, due to methodological constraints quantifying N₂ and laborious analytical procedures. We present a novel, simplified incubation assay that combines the ¹⁵N gas flux method with the ¹⁵N pool dilution method, to quantify denitrification losses as a function of N turnover. This assay was tested using a pasture soil from sub-tropical Australia. N-fertiliser (35 µg g^-1 soil) was applied either as a single (NH₄¹⁵NO₃^–) or double (¹⁵NH₄¹⁵NO₃) labelled treatment at 10 atom %, with a third treatment (NH₄¹⁵NO₃^–) at 60 atom % to quantify N₂ emissions. Gross rates of N mineralisation, nitrification and related N₂ and N₂O emissions were measured during 48 hours of incubation at 80% WFPS. Gross N production and gross N consumption was consistent with the directly measured N pool sizes, with denitrification losses (N₂+N₂O) at 7.0 + 1.4 µg N g^-1 soil accounting for 62% of the calculated NO₃^– consumption. N turnover was dominated by mineralisation and nitrification, increasing the NO₃^– pool by a factor of 3. High NO₃^– concentrations shifted the N₂:N₂O ratio towards N₂O, with 60 % of denitrification losses emitted as N₂O. More than 25% of the applied ¹⁵N fertiliser was lost via denitrification, showing the significance of denitrification as a major pathway of N loss from agro-ecosystems. The simplified incubation assay proved to be an efficient tool to quantify N pools and emissions, and as such is an effective method to establish comprehensive datasets of denitrification losses linked to N turnover from agro-ecosystems.