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

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 ( δ 15 N and δ 13 C) of soil and leaf were analyzed to indicate soil N transformation processes, and the soil to plant 15 N 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 δ 15 N 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, δ 15 N values of soil organic nitrogen (SON) increased from 0.8‰ to 5.7‰ and δ 13 C 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 C 4 plant input. In the soils below 70 cm depth, δ 15 N values of SON (mean 4.9‰) were likely affected by microbial assimilation of 15 N-depleted ${\mathrm{NO}}_{3}^{-}$. The variations in δ 15 N 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 15 N enrichment factor have a great promise to compare soil ${\mathrm{NO}}_{3}^{-}$ loss potential among different ecosystems.