Abstract: The retention of atmospheric N in plant and soil is a key pathway of ecosystem nitrogen (N) sequestration and sustainable supply. Stable isotope tracing techniques with ¹⁵NO₃^- and ¹⁵NH₄⁺ can be used to quantify the fate of the two inorganic forms of deposited N. Globally, the main characteristics of isotopic N tracing studies are applying trace amount of ¹⁵N (mostly lower than 250 mg ¹⁵N·m^-2), short experimental duration (mostly shorter than 48 months), and scarcely comparing the fate of NO₃^- and NH₄⁺. The retention of atmospheric-deposited NO₃^- and NH₄⁺ in ecosystems may depend on plant N uptake preference, N competition between soil microbes and plants, and differences in abiotic versus biotic fixations. Several studies demonstrate that continuously cycling   microbial biomass N is the main place for the turnover and fixation of exogenous N, that microbes preferentially take up NH₄⁺ rather than NO₃^-, that most plant species prefer to absorb NO₃^- of which being transferred to root surface and assimilated more quickly, and that soil aggregation can substantially modulate N retention in plant-soil systems and soil N saturation process. Future studies should be strengthened in systematically investigating the spatial and temporal patterns of atmospheric-deposited reactive N distributing and stabilizing in various ecosystem components and the retention mechanisms of deposited N in different soil aggregates. These studies would provide scientific evidence and supportive data sets for improving ecosystem N-cycling theory and optimizing N-cycling models.

Key words: natural ecosystem, soil aggregate, nitrogen uptake, nitrogen addition form, nitrogen stable isotope.