Nitrogen−stimulated rhizosheath organic acid release enhances phytolith concentrations in leaves of dominant trees in a subtropical forest

Abstract

Background and aims Plants take up and use silicon (Si) as a beneficial nutrient for growth and/or survival, which can affect terrestrial Si biogeochemistry and its links with the carbon cycle. Soil nutrients, such as nitrogen (N) and phosphorus (P), and their ratios can affect rhizosphere processes, with potential impacts on soil-plant Si dynamics. How atmospheric N deposition affects plant Si acquisition and Si cycling in subtropical forests remains, however, unknown. Methods We performed a field experiment in a P-limited subtropical forest to explore the effects of 9-year canopy and understory N addition at two rates on Si biogeochemical cycle and its underlying mechanism. Results We found that N addition increased concentrations of rhizosphere organic acids, leaf N:P ratios, and concentrations of phytoliths in leaves of the dominant tree species. Nitrogen addition also increased the concentrations of plant-available Si at 0–5 cm depth, but not deeper. Concentrations of plant-available Si were negatively correlated with soil amorphous Si (exacted with NaOH), but positively correlated with Si associated with organic materials (extracted with H₂O₂) at 0–5 cm depth. Conclusions The increase in plant-available Si and resulting leaf phytolith concentrations following N addition was possibly explained by the increase in rhizosphere organic acids under strongly P-limited conditions, with cascading effects on silicates weathering and release of soluble Si from poorly-available forms. Overall, our results suggest that atmospheric N deposition increases the biological Si cycle in subtropical P-limited forest ecosystems.