Nitrogen turnover and N2:N2O partitioning from agricultural soils – a simplified incubation assay

Johannes Friedl1, Clemens Scheer1, Johanna Trappe2, David W. Rowlings1, Peter R. Grace1

1Institute for Future Environments, Queensland University of Technology, Brisbane, QLD 4000, Australia. [email protected],

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.