Isotopic Signatures of Nitrogen in Selected Soils from Croatia

The mobility of nitrogen (N) in the environment is conditioned by its cycling between atmospheric, terrestrial, and marine ecosystems. It is a key element for global biogeochemistry, and although isotope analysis has been an integral part of many studies over the past eighty years, the complexity of the nitrogen cycle hinders a correct and detailed understanding of the mechanisms behind its processes. It could be argued that the interpretation of the isotopic signatures of nitrogen in soils is still in its infancy. In Croatia, such research has recently begun and is driven by a need for the comprehensive study of nitrogen isotopes in terrestrial ecosystems. The aim of this study was to compare the abundance of the 15 N isotope in soils from continental and coastal parts of Croatia with different types of land use (arable land/crop production, meadows, forests, orchards, ski slopes, urban soil/city roads) and to authenticate the nitrogen origin in soils in relation to different soil management practices. This research was based on 27 soil samples collected at 11 locations in Croatia. The samples differed according to soil type, land use, applied mineral and organic nitrogen fertilization, and climatic condition at each specific location. The determination of δ 15 N T (T—total nitrogen) values in bulk samples was performed in duplicate with the IRMS (Isotope Ratio Mass Spectrometry) method using an IsoPrime100-Vario PYRO Cube (OH/CHN Pyrolyser/Elemental Analyzer). The results reveal that the mean δ 15 N abundance in soils according to different land use declines in the following order: crop production (+5.66 ± 1.06‰) > apple orchard (+5.60 ± 0.10‰) > city road (+4.33 ± 0.38‰) > meadow (+3.71 ± 0.85‰) > ski slope (+2.20 ± 0.10‰) > forest (+2.15 ± 1.86‰). The individual values were in the range from 0.00 ± 0.10‰ in the forest soil in continental Croatia to +7.19 ± 0.07‰ in the vegetable garden (crop production) soil in coastal Croatia. Among the investigated soil properties and weather conditions, PCA analysis identified close correlations between P 2 O 5 content and δ1 15 N abundance in arable soils, as well as between soil reaction (pH) and mean annual temperatures, while high C/N ratio values explained the isotopic distribution in non-arable soils (city roads and forests). Despite the long-term application of mineral nitrogen fertilizers, the results represent nitrogen of organic origin in the arable soils (crop production), which partly confirms the sustainable management of those agroecosystems.