Song Ge;Zhao Xu;Wang ShenQiang;Xing GuangXi

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China)

nh+4 addition;nitrification;nitrogen isotope fractionation;reaction kinetics

A laboratory-based aerobic incubation was conducted to investigate nitrogen (N) isotopic fractionation related to nitrification in five agricultural soils after application of ammonium sulfate ((NH₄)₂SO₄). The soil samples were collected from a subtropical barren land soil derived from granite (RGB), three subtropical upland soils derived from granite (RQU), Quaternary red earth (RGU), Quaternary Xiashu loess (YQU) and a temperate upland soil generated from alluvial deposit (FAU). The five soils varied in nitrification potential, being in the order of FAU > YQU > RGU > RQU > RGB. Significant N isotopic fractionation accompanied nitrification of NH₄⁺. delta ¹⁵N values of NH₄⁺ increased with enhanced nitrification over time in the four upland soils with NH₄⁺ addition, while those of NO₃^- decreased consistently to the minimum and thereafter increased. delta ¹⁵N values of NH₄⁺ showed a significantly negative linear relationship with NH₄⁺-N concentration, but a positive linear relationship with NO₃^--N concentration. The apparent isotopic fractionation factor calculated based on the loss of NH₄⁺ was 1.036 for RQU, 1.022 for RGU, 1.016 for YQU, and 1.020 for FAU, respectively. Zero- and first-order reaction kinetics seemed to have their limitations in describing the nitrification process affected by NH₄⁺ input in the studied soils. In contrast, N kinetic isotope fractionation was closely related to the nitrifying activity, and might serve as an alternative tool for estimating the nitrification capacity of agricultural soils.