Nitrogen Isotopic Fractionation Related to Nitrification Capacity in Agricultural Soils

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 NH+4. δ15N values of NH+4 increased with enhanced nitrification over time in the four upland soils with NH+4 addition, while those of NO+4 decreased consistently to the minimum and thereafter increased. δ15N values of NH+4 showed a significantly negative linear relationship with NH+4-N concentration, but a positive linear relationship with NO+4-N concentration. The apparent isotopic fractionation factor calculated based on the loss of NH+4 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 NH+4 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.