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

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.