Abstract: Aims The objective of this study was to explore the effects of long-term nitrogen (N) addition on leaf litter production and nutrient return by the dominant tree species in broadleaf evergreen forests of subtropical regions. Methods A long-term N addition manipulation experiment with three levels (0 kg·hm-2·a-1, CK; 20 kg·hm-2·a-1, LN; and 40 kg·hm-2·a-1, HN) was conducted to examine the effects of N addition on leaf litter production, contents of leaf litter carbon (C), N, and phosphorus (P), as well as their return within Michelia wilsonii forests located on the western margin of the Sichuan basin. Important findings (1) Significant inter-annual variations were observed in the production of M. wilsonii leaf litter, as well as in the mean contents of C, N, and P and their annual return, but N addition did not affect these inter-annual dynamics. (2) The HN treatment significantly increased the annual leaf litter production of M. wilsonii by 24.9%. Additionally, the effect of N addition on the average annual litter production decreased as the duration of N addition increased. (3) The HN treatment resulted in a significant increase in the mean annual content of C in by 3.4%. The effects of HN treatment on leaf litter N and P content exhibited a turning point during the 2019-2020 interval, coinciding with notable alterations in the annual average N content in leaf litter. Specifically, the annual average N content was suppressed in both 2018 and 2019, but then experienced a significant increase in 2020. In contrast, a general downward trend was observed in the P content from 2016 to 2019. (4) N addition significantly increased the annual return of C, N, and P in leaf litter, with the promotional effect intensifying as the level of N addition increased. However, the effect of N addition on the annual P return showed a significant linear decline trend over the study period. These findings highlight that high N additions significantly promoted the productivity of M. wilsonii forests and augment carbon and nutrient inputs within the ecosystem Distinct effects of short-term versus long-term N inputs on the N cycling processes of M. wilsonii forest ecosystems were observed. Additionally, the future exogenous N addition may potentially inhibit the leaf litter yield and P return of M. wilsonii.

Key words: long-term nitrogen addition, leaf litter yield, content of carbon, nitrogen and phosphorus in leaf litter, element return, inter-annual dynamics