Variations in nitrogen isotopic values among various particle-sized fractions in modern soil in northwestern China

Ratios of stable nitrogen isotopes in organic matter derived from plants and preserved in soil are potential tracers for nitrogen cycles in natural ecosystems and valuable for evaluation of climate change. However, the relationship between nitrogen isotopic compositions in surface soil and in plant litter during the decomposition process from plant litter to soil organic matter is not well understood. By using nitrogen isotopic analysis of soil particle-sized fractions, nitrogen isotope discrimination between plant litter and surface soil organic matter in various modern ecosystems in northwestern China was conducted. The results of our study indicate that: (1) in general, the nitrogen isotopic compositions of particle-sized fractions from surface soil are different, and δ¹⁵N values increase from plant litter to fine soil organic matter; (2) the δ¹⁵N values in the soil particle-sized fractions become larger with increasing relative humidity and temperature, and the largest variation in the δ¹⁵N values is from -5.9‰ to -0.3‰; and (3) under a controlled climate, significant nitrogen isotope differences in δ¹⁵N values (Δδ¹⁵N_plant-soil) between plant litter and bulk soil organic matter were observed, with the values of 1.52 to 4.75 at various sites. Our results suggested that comparisons of Δδ¹⁵N values between bulk soil and the particle-sized fractions of soil could reveal the effect of humidity on transferring process of nitrogen from plant to soil in arid and semi-arid ecosystems.