EGU2020-3692
https://doi.org/10.5194/egusphere-egu2020-3692
EGU General Assembly 2020
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Nitrite isotope characteristics in 15N-labelled and non-labelled agricultural soil

Dominika Lewicka-Szczebak1 and Reinhard Well2
Dominika Lewicka-Szczebak and Reinhard Well
  • 1Centre for Stable Isotope Research and Analysis, Georg-August-Universität Göttingen, Göttingen, Germany (dominika.lewicka@uni-goettingen.de)
  • 2Thünen Institute of Climate-Smart Agriculture, Braunschweig, Germany (reinhard.well@thuenen.de)

Nitrite (NO2-) is a crucial compound in the complex N soil cycle. As an intermediate of nearly all N transformations its isotopic signature may provide precious information on the active pathways and processes. NO2- analyses have been already applied in 15N tracing studies increasing their interpretation perspectives. Natural abundance NO2- isotope studies in soils were so far not applied and this study aims at testing if such analyses are useful in tracing the soil N cycle.  

We conducted laboratory soil incubations with parallel natural abundance and 15N treatments accompanied by analyses of soil N compounds (NO3-, NO2-, NH4+) and released N gases (N2O and N2). Water content was varied during the experiment from 55 to 86% water-filled pore space. NO2- was immediately extracted and analysed with the denitrifier method for selective nitrite reduction with Stenotrophomonas nitritireducens.

NO2- content varied in the wide range from 0.6 to 6.6 μmol kg-1 soil, whereas NO3- content was one order higher and quite stable from 1.3 to 1.7 mmol kg-1 soil. Similarly, the δ15N(NO2-) varied largely from -16.7 to +8.8‰, whereas δ15N(NO3-) was very stable from 3.5 to 5.9‰. The δ15N(NO2-) was correlated with NO2- content. Applying Keeling plot the isotopic signature for the NO2- input of -11.7‰ was determined. When related to δ15N(NO3-) this gives the ε(NO2-/ NO3-) of -16.2‰, which is within the literature data for NO3- to NO2- reduction step of denitrification.

The parallel 15N treatment was used to provide interpretation for the natural abundance isotope nitrite dynamics. We observed a sudden drop in 15N abundance in NO2- (a15N(NO2-)) after water addition to the soil from 14.7 to 3.2 at%, whereas  15N abundance in NO3- (a15N_NO3-) showed only slight decrease from 14.2 to 13.1 at%. This indicates an incorporation of another source of unlabelled NO2- for the wet part of the experiment. In natural abundance isotopes this change was also reflected in higher Δ15N(NO2-/ NO3-); for the wet part of the experiment it was even positive with +1.6‰, whereas for the dry part it was lower with -5.9‰. This additional nitrite source is most probably oxidation of organic N, which will be clarified by further studies, including detailed analysis with the 15N Ntrace model.

The observed changes in nitrite isotope characteristics were not reflected in N2O. Whereas a15N(NO2-) droped to 3.2 at%, for a15N(N2O) still 13.6 at% were found. The pool derived N2O fraction calculated with the 15N gas flux method showed that the entire N2O originated from NO3- in the wet part of the experiment. This shows that NO2- pools originating from different pathways must be isolated and not the entire NO2- pool undergoes further reduction to N2O.

Natural abundance nitrite isotope studies may provide a new important tool in constraining the N soil cycling.

How to cite: Lewicka-Szczebak, D. and Well, R.: Nitrite isotope characteristics in 15N-labelled and non-labelled agricultural soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3692, https://doi.org/10.5194/egusphere-egu2020-3692, 2020.

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