Soil nitrogen isotopic composition of the Xifeng loess-paleosol sequence and its potential for use as a paleoenvironmental proxy

Abstract

The isotopic composition of soil nitrogen (soil δ15N values) has been widely used to explore the soil biogeochemical cycle and study land vegetation conditions. However, the δ15N values in the loess-paleosol sequence have not been thoroughly studied. In this study, we provide the first report of δ15N values from the Xifeng loess-paleosol profile, which spans the last 130 ka. The δ15N values show great variation (2.1‰–6.1‰) in the Xifeng loess sequence. According to these results, the δ15N values have a similar pattern with changes in the magnetic susceptibility (MS) and exhibit significant differences within the glacial-interglacial cycle over the last 130 ka. We hypothesize that climate conditions and land vegetation conversion could be reflected by the changes in the δ15N values within the glacial-interglacial cycle. (1) The δ15N values are positively correlated with the reconstructed mean annual temperature (MAT) and mean annual precipitation (MAP) on the glacial-interglacial time scale. Moreover, it appears that the MAT may be more effective than the MAP for the δ15N changes in the Xifeng loess-paleosol sequence. (2) Vegetation changing from grasses to shrubs may cause 15N depletion during the glacial period. In addition, increasing grass groups may cause 15N enrichment of the soil during the interglacial stage in the Xifeng loess-paleosol sequence. Because nitrogen is one of the most significant nutrient elements for vegetation, paleovegetation changes may be the main controlling factor for δ15N values in the loess-paleosol section. Therefore, δ15N values have the potential for use as a proxy in paleoenvironmental reconstructions of the Chinese Loess Plateau.