ABSTRACT Acid rain and nitrogen deposition are important environmental stressors affecting soil nitrogen cycling in subtropical forests, whereas fine‐root decomposition represents a major pathway for soil organic nitrogen (SON) input and transformation. However, how substrate changes in different root orders interact with soil environmental shifts to regulate SON fractions remains unclear. Here, we conducted three complementary decomposition experiments in a subtropical Chinese fir ( Cunninghamia lanceolata ) plantation to disentangle the effects of fine‐root substrate changes, soil environmental changes, and their combined in situ effects on SON fractions and depolymerase activities. The results showed that acid rain and nitrogen addition altered the initial chemical composition of fine roots before decomposition: total carbon in higher‐order roots decreased by 11.2%–14.4%, whereas lignin and cellulose contents increased mainly in lower‐order roots. Under a common soil environment, fine‐root substrate origin and root order jointly differentiated SON fractions and stimulated most depolymerase activities. Higher‐order roots tended to retain more acid‐hydrolyzable organic N under CK‐ and N‐derived substrates, whereas lower‐order roots showed stronger SON accumulation and enzyme responses under acid‐rain‐related substrates. By contrast, under a common fine‐root source, nitrogen addition promoted the accumulation of some acid‐hydrolyzable organic nitrogen fractions, while acid rain increased acid‐hydrolyzable amino acid nitrogen and modified enzyme responses. Under in situ conditions, fine‐root substrate changes and soil environmental shifts showed a synergistic effect, increasing some acid‐hydrolyzable organic nitrogen fractions by 16%–32% relative to the control. Model analyses further identified soil pH, total nitrogen, and protease activity as key predictors of SON transformation. These findings indicate that SON fraction changes under acid rain and nitrogen deposition are jointly regulated by fine‐root substrate quality and soil environmental conditions. Distinguishing the functional differentiation between lower‐order absorptive roots and higher‐order transport roots provides a more accurate understanding of root‐soil interactions and soil nitrogen cycling in Chinese fir plantations.
Zhang et al. (Thu,) studied this question.