Aims <p>Increased nitrogen (N) deposition is expected to enhance soil acidification, especially in tropical N-rich soil. In contrast, N-poor tropical soils do not always exhibit soil acidification, and additional insights are required to gain a mechanistic understanding of this process. However, recent efforts to reduce atmospheric N due to pollution control have reduced the field study opportunities.</p> Methods <p>We investigated variations in soil acidity in N-poor tropical soils under increased or reduced N deposition using an experimental platform established in 2019.</p> Results <p>Short-term (4 years) N treatments had no significant effect on soil acidity, as evidenced by no significant changes in the soil pH, exchangeable cations, and acid neutralizing capacity (ANC). Additionally, a lagged effect was observed for the soil pH and exchangeable cation concentrations under reduced N deposition. The stability of soil acidity was presumably explained by (i) the dominance of Ca²⁺ and Mg²⁺, which exhibited a strong buffering capacity to maintain stable soil acidity despite N deposition, and (ii) the N-poor status of the soil, which facilitated plant uptake of the added N, thereby minimizing its accumulation and potential acidifying effects.</p> Conclusions <p>Our findings demonstrate that neither increased nor reduced N deposition had a significant effect on soil acidity, providing experimental evidence that tropical N-poor soils with high exchangeable Ca²⁺ and Mg²⁺ possess strong buffering capacity despite N deposition. This research enhances our understanding of the mechanisms underlying soil acidification and suggests that N deposition is not always related to soil acidification in N-poor tropical soils.</p>