Isotopic constraints on nitrate sources and cycling in Antarctic soils

While nitrate (NO₃^-) serves as a vital nutrient for soil organisms, the lack of knowledge on its sources and cycling in Antarctic soils does not allow to predict the possible effects of climate and environmental changes on the functioning of terrestrial ecosystems. Thus, the NO₃^- and its stable isotopes of Δ¹⁷O_NO3, δ¹⁵N_NO3, and δ¹⁸O_NO3 were analyzed in Antarctic soils, collected in coastal ice-free areas of East Antarctica and the tip of the northern Antarctic Peninsula. The soil NO₃^- concentrations and isotopic values of Δ¹⁷O_NO3, δ¹⁵N_NO3, and δ¹⁸O_NO3 were comparable across the latitudinal gradient except for soils collected beneath moss cushions in the Fildes Peninsula. The proportional calculation of Δ¹⁷O_NO3 indicated that 93–97% of soil NO₃^- derived from nitrification process, whereas atmospheric deposition probably contributed to the NO₃^- budget for a fraction <10 %. The linear relationship between δ¹⁸O_NO3 and δ¹⁵N_NO3 suggested the occurrence in Antarctic soils of denitrification process and active cycling between NO₃^- and nitrite (NO₂^-). The NO₃^- assimilation in soil likely occurred, but at minimal rates. These findings provide new insights in the biogeochemical cycle of nitrogen (N) in Antarctic terrestrial ecosystems and seem to indicate that despite climatic and environmental changes along the wide latitudinal gradient, the sources and cycling of NO₃^- maintain a rather consistent pattern along coastal ice-free areas. The noteworthy increase of NO₃^- concentrations and δ¹⁵N_NO3, δ¹⁸O_NO3, and Δ¹⁷O_NO3 values in soils collected under moss in the Fildes Peninsula could also suggest that the warming and the increased availability of liquid water will likely favor the developments of soil biotic communities and changes in the biogeochemical cycle of N in Antarctic terrestrial ecosystems.