Distinct effects of long-term canopy and understory nitrogen addition on soil silicon forms in a subtropical forest

• Different N − addition approaches exhibit distinct effects on soil Si forms. • CaCl 2 − Si is positively correlated with non − biogenic Si forms. • Canopy N − addition could reflect the real impacts of atmospheric N deposition on soil Si dynamics in forests. Investigating the impacts of chronic atmospheric nitrogen (N) deposition on soil silicon (Si) forms is critical for predicting terrestrial Si biogeochemistry, but the influence of N deposition on soil Si forms in subtropical forests is vague. Here, we evaluated the impacts of 12 year’s canopy and understory N-addition on soil Si forms in a subtropical forest. Our results revealed divergent effects of N-addition approaches on topsoil Si forms. Canopy N-addition led to significant increases in Si pools extracted with CaCl 2 (plant-available/mobile Si), acetic acid (adsorbed Si) and oxalate (Si associated with pedogenic oxides). This was attributed to the increase in poorly crystalline pedogenic oxides (evidenced by an increase in oxalate-Fe), which might provide enhanced adsorption sites for Si. In contrast, understory N-addition significantly decreased soil pH and increased soil organic carbon, which enhanced amorphous/biogenic forms of Si (Si extracted with H 2 O 2 (organic-matter-associated Si), Na 2 CO 3 (amorphous Si), and physically-extracted biogenic silica). Interestingly, mobile Si was positively correlated with amorphous Si and physically-extracted biogenic silica only in deeper soil, but with adsorbed Si and organic-matter-associated Si in both soil depths. This suggests that biogenic Si may not be the only source of plant-available Si and that sorption/adsorption processes are also critical for regulating Si mobility. These results demonstrate different mechanisms by which canopy and understory N-addition treatments affect soil Si forms, and highlight the necessity and importance to unbiasedly assess the effects of atmospheric N deposition on Si biogeochemical cycle in forest ecosystems, taking canopy processes into consideration.