Nitrogen (N) is a key element regulating forest productivity and feedback to climate warming. Sustained climate warming may have profound effects on N cycling in N-rich tropical forests. However, large uncertainty exists regarding how warming affects the integrated N dynamic in tropical forests, especially on the scale of long-term warming.

To address the uncertainty related to N cycling, researchers from South China Botanical Garden of Chinese Academy of Sciences conducted a field warming experiment by translocating model plant-soil ecosystems from a high-altitude site (600 m) to low-altitude sites at 300 m and 30 m to simulate warming by 1.0°C and 2.1°C, respectively, in tropical China, and investigated N components in plant, soil, leaching, and gas over 6 years.

Results showed that warming decreased foliar δ¹⁵N values and inorganic N (NH₄-N and NO₃-N) leaching. 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 depressed under warming, which probably restricted bioavailable N supply and arbuscular mycorrhizal contribution of N supply to plants. The significant responses of each N component to warming gradually decreased during the last 3 years, probably due to the restricted plant growth and microbial acclimation.

Overall, warming can lead to more closed N cycling in N-rich tropical forests, which was associated with the larger plant N demand but lower underground N supply. This finding suggests that warming may slow down N losses caused by high N deposition in N-rich tropical forests, thus contributing to forest N retention, but this effect may decrease with long-term warming.

This study systematically reveals the response of N cycling to long-term and sustained warming in N-rich tropical forests, providing a new insight in understanding and predicting the forest N budgets, productivity, and ecosystem feedback to climate warming.

This work has been published online in Global Change Biology, entitled "Warming leads to more closed N cycling in N-rich tropical forests". For further reading, please refer to: https://doi.org/10.1111/gcb.15432.