Interaction effects of environmental factors on soil nitrogen fractions based on a novel sequential extraction method

Abstract

Soil nitrogen (N) is controlled by several environmental factors. However, it is unclear how these factors and their interactions affect soil N fractions (SNFs) because of the lack of systematic N extraction methods. The Harbin–Dali belt, a transect more than 3,300 km long across China covering various climates, topographies, soil parent materials and vegetation types, was taken as the study area, and the interactive effects on SNFs in largely undisturbed woodlands and grasslands were explored using three SNFs: labile N, moderately stable N and stable N. The soil N fractions were obtained through a novel sequential extraction method that used distilled H2O, 0.5 M CaCl2, 1 M HCl, 6 M HCl, mixed acid (1 M HCl and 5 M HF; MA), concentrated HCl (cHCl) and concentrated H2SO4 as extractive agents to successively separate H2O‐, CaCl2‐, 1 M HCl‐, 6 M HCl‐, MA‐, cHCl‐ and RES‐N fractions. The results showed that increasing mean annual temperature (MAT) directly decreased the concentrations of H2O‐, CaCl2‐ and 1 M HCl‐N, suggesting that warming may reduce available N. The interactions between MAT, parent material, aspect and aridity index also contributed to the three SNFs. Vegetation and its interactions with parent material primarily controlled 6 M HCl‐, cHCl‐ and RES‐N, and climate and topography had indirect effects during these processes. Particularly, more cHCl‐ and RES‐N were accumulated in areas with warm humid climates and rich vegetation species than in other environments. Moreover, soil labile N (H2O‐, CaCl2‐ and 1 M HCl‐N) and cHCl‐N exhibited potential as sensitive indicators for reflecting the changes in MAT and vegetation, respectively. Therefore, the various interactions between climate and other environmental factors controlled the changes in most SNFs. This novel sequential extraction method thus offers a reliable analytical approach to measuring SNFs. Highlights Soil nitrogen fractions (SNFs) were fractionated via a novel sequential extraction method. Increasing MAT decreased the H2O‐, CaCl2‐ and 1 M HCl‐N concentrations. Interactions of climate, parent material and vegetation affected 6 M HCl‐, MA‐, cHCl‐ and RES‐N. H2O‐, CaCl2‐, 1 M HCl‐ and cHCl‐N are sensitive SNFs that reflect environmental changes.