Phytolith formation and its contribution to soil organic carbon sequestration in an Inner Mongolia grassland under long-term nitrogen deposition

Nitrogen (N) deposition can enhance plant growth but may also disrupt soil nutrient balances and cause soil acidification, potentially affecting silicon (Si) uptake. Si is vital for phytolith formation and the sequestration of phytolith-occluded carbon (C) (PhytOC). Understanding how N deposition influences these dynamics is crucial, given their roles in C sequestration and ecosystem functions. A long-term (2014–2020) N addition experiment was conducted in the Inner Mongolia grassland, applying six N rates (0, 2, 5, 10, 20, and 50 g N m⁻² yr⁻¹). Sampling occurred in 2015, 2019, and 2020 to investigate the impact of N deposition on PhytOC production in vegetation and the ratio of soil PhytOC to soil organic C (PhytOC-to-SOC ratio). Our results indicate that N addition reduced plant phytolith content. However, it enhanced plant PhytOC production by stimulating aboveground net primary productivity (ANPP). Additionally, soil PhytOC-to-SOC ratio gradually increased after long-term N addition, likely due to the soil acidification rather than N-induced effects on ANPP. Long-term N addition more effectively boosted PhytOC production in plant than short-term exposure, favoring Si-rich species like Leymus chinensis, which thrive and gradually dominate under high N conditions. These results suggest that community productivity and Si-rich species are key drivers of plant PhytOC production, while soil pH regulates PhytOC sequestration by influencing soil phytolith solubility during preservation. These also emphasize the importance of considering PhytOC in future assessments of C pool storage and stability, especially in the context of N deposition and ecosystem management practices such as fertilization.