Nitrogen addition increases aboveground silicon and phytolith concentrations in understory plants of a tropical forest

Purpose: Silicon (Si) is a beneficial element for plants and plays important roles in the biogeochemical cycle of mineral elements. Yet, few studies have focused on the impact of nitrogen (N) deposition on plant Si uptake and the Si biocycle. Methods: We designed an experiment investigating canopy and understory N addition in a tropical forest to comprehensively assess the response of the Si cycle to N addition focusing on understory plants and topsoil. After six years of treatment, we compared the effects of different treatments on the concentrations of Si-related variables in the leaves of dominant understory species, and topsoil. The foliar elemental stoichiometry, plant water-use efficiency, and soil properties were also assessed to explore potential mechanisms underpinning an N-regulated biological feedback loop of Si. Results: The concentrations of SiO₂ and phytoliths in the leaves of the understory plants increased under N addition, while those of phytolith and plant-available Si in the topsoil were not altered. The significant accumulation of Si in the leaves and the unaltered plant-available Si concentrations in the soil were associated with enhanced plant Si uptake and decreased soil phytolith return due to increased phosphorus (P) limitation, soil acidification, and decreased leaf C:N ratios caused by N addition. Conclusion: Our findings suggest that N addition affected plant phytolith production and its coupling with the Si biocycle in the forest which may stimulate plant Si accumulation and decrease soil Si return. Nitrogen enrichment will alter the biological Si cycle in forest ecosystems at regional and global scales.