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 (δ ¹⁵N and δ ¹³C) 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 δ ¹⁵N 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 ¹⁵N values of soil organic nitrogen (SON) increased from 0.8‰ to 5.7‰ and δ ¹³C 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, δ ¹⁵N values of SON (mean 4.9‰) were likely affected by microbial assimilation of ¹⁵N-depleted ${\mathrm{NO}}_3^{-}$ . The variations in δ ¹⁵N 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.

Keywords: 15N enrichment factor; Land-use types; Red soil; Soil organic nitrogen; δ15N composition.