Stable nitrogen and carbon isotope compositions in plant-soil systems under different land-use types in a red soil region, Southeast China

Abstract

Background Stable N isotope compositions in plant-soil systems have been widely used to indicate soil N transformation and translocation processes in ecosystems. However, soil N processes and nitrate (${\mathrm{NO}}_{3}^{-}$) loss potential under different land-use types are short of systematic comparison in the red soil region of Southeast China. Methods In the present study, the stable N and C isotope compositions (δ15N and δ13C) of soil and leaf were analyzed to indicate soil N transformation processes, and the soil to plant 15N enrichment factor (EF) was used to compare soil ${\mathrm{NO}}_{3}^{-}$ loss potential under different land-use types, including an abandoned agricultural land, a natural pure forest without understory, and a natural pure forest with a simple understory. Results The foliar δ15N value (−0.8‰) in the abandoned agricultural land was greater than those of the forest lands (ranged from −2.2‰ to −10.8‰). In the abandoned agricultural land, δ15N values of soil organic nitrogen (SON) increased from 0.8‰ to 5.7‰ and δ13C values of soil organic carbon (SOC) decreased from −22.7‰ to −25.9‰ with increasing soil depth from 0–70 cm, mainly resulting from SON mineralization, soil organic matter (SOM) decomposition, and C4 plant input. In the soils below 70 cm depth, δ15N values of SON (mean 4.9‰) were likely affected by microbial assimilation of 15N-depleted ${\mathrm{NO}}_{3}^{-}$. The variations in δ15N values of soil profiles under the two forests were similar, but the EF values were significant different between the pure forest with a simple understory (−10.0‰) and the forest without understory (−5.5‰). Conclusions These results suggest that soil to plant 15N enrichment factor have a great promise to compare soil ${\mathrm{NO}}_{3}^{-}$ loss potential among different ecosystems.