Warming may have profound effects on nitrogen (N) cycling by changing plant N demand and underground N supply. However, large uncertainty exists regarding how warming affects the integrated N dynamic in tropical forests. We translocated model plant‐soil ecosystems from a high‐altitude site (600 m) to low‐altitude sites at 300 and 30 m to simulate warming by 1.0°C and 2.1°C, respectively, in tropical China. The effects of experimental warming on N components in plant, soil, leaching, and gas were studied over 6 years. Our results showed that foliar δ15N values and inorganic N (NH4‐N and NO3‐N) leaching were decreased under warming, with greater decreases under 2.1°C of warming than under 1.0°C of warming. The 2.1°C of warming enhanced plant growth, plant N uptake, N resorption, and fine root biomass, suggesting higher plant N demand. Soil total N concentrations, NO3‐N concentrations, microbial biomass N and arbuscular mycorrhizal fungal abundance were decreased under 2.1°C of warming, which probably restricted bioavailable N supply and arbuscular mycorrhizal contribution of N supply to plants. These changes in plants, soils and leaching indicated more closed N cycling under warming, the magnitude of which varied over time. The closed N cycling became pronounced during the first 3 years of warming where the sustained reductions in soil inorganic N could not meet plant N demand. Subsequently, the closed N cycling gradually mitigated, as observed by attenuated positive responses of plant growth and less negative responses of microbial biomass N to warming during the last 3 years. Overall, the more closed N cycling under warming could facilitate ecosystem N retention and affect production in these tropical forests, but these effects would be eventually mitigated with long‐term warming probably due to the restricted plant growth and microbial acclimation.

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Over the past 30 years, global surface temperature has increased by approximately 0.2°C per decade (IPCC, 2018). Clearly, the increase in temperature can affect tree growth and mortality (Bowman et al., 2014; Liu et al., 2016), influencing forest structure and composition in tropical forests (Zhou et al., 2013, 2014). The response of trees to warming depends on whether they can adjust their phenotypic traits; for example, many tree species maintain enhanced growth by increasing photosynthetic capacity (Drake et al., 2015). However, few studies address leaf hydraulic and economic traits, combined with related phenotypic plasticity, in long-term field warming experiments (Grady et al., 2013). Leaf hydraulic traits, such as leaf hydraulic conductance (K leaf), stomatal conductance (g s) and stomatal anatomical traits, play a vital role in regulating leaf gas exchange. K leaf measures water flow efficiency through the leaf, defining leaf water transport capacity (Locke et al., 2013). Under warmer conditions, higher K leaf may increase