Atmospheric nitrogen deposition: what are the impacts on silicon dynamics in a subtropical forest?

Background and aims

Plants take up and use silicon (Si) as a beneficial nutrient for growth or survival, which can affect terrestrial Si biogeochemistry and its links with the carbon cycle. While nitrogen (N) deposition can impact many processes in terrestrial ecosystems (soil biogeochemical processes, plant productivity, etc.), how it affects plant Si cycling in subtropical forests remains unknown.

Methods

We performed a field experiment in a subtropical forest to explore the effects of nine − years of canopy and understory N addition at three rates (0, 25 and 50 kg N ha⁻¹ yr⁻¹) on Si biogeochemical cycling. We quantified different soil Si pools, and phytolith concentrations in the vegetation.

Results

In topsoil, soil pH decreased in all N − addition treatments. In CN50 and UN50, concentrations of CaCl₂ − Si (mobile Si) and NaOH − Si (amorphous Si) increased and decreased, respectively. H₂O₂ − Si (fragile phytoliths associated with SOM) concentrations increased following N addition. The rates of carboxylate exudation were significantly enhanced, especially under high − rate N addition. Leaf phytolith concentrations significantly increased in all N − addition treatments, except CN25. Concentrations of CaCl₂ − Si were negatively correlated with those of NaOH − Si and positively correlated with those of H₂O₂ − Si in topsoil. Rhizosheath organic acids, soil pH and CaCl₂ − Si were dominant factors affecting leaf phytoliths.

Conclusions

Nitrogen addition decreased soil pH, increased the soil H₂O₂ − Si pool, and stimulated carboxylate release of fine roots which, together, enhanced Si availability and plant phytolith accumulation. Our results show that atmospheric N deposition affects Si dynamics in plant-soil systems in subtropical forests.