Ammonium nutrition enhances rhizosphere mobilization and uptake of silicon in white lupin grown in low phosphorus soil

While nitrogen (N) supply can enhance plant silicon (Si) accumulation, the mechanisms by which different forms of N affect Si mobilization in the rhizosphere are not well understood. This study aims to elucidate how pH changes induced by ammonium (NH₄⁺) and nitrate (NO₃⁻) affect Si availability in the rhizosphere, especially under low phosphorus (P) conditions.

White lupin (Lupinus albus) plants were grown in non-fertilized low-P soil, supplied with a low dose of N, either as NH₄⁺ or NO₃⁻, with or without supply of monosilicic acid. We measured Si levels in various rhizosphere soil pools, along with different plant and rhizosphere soil parameters.

The addition of NH₄⁺ significantly lowered rhizosphere pH and decreased both Si adsorbed to pedogenic Fe/Mn oxides and amorphous phytogenic Si, resulting in higher concentrations of plant available Si in the white lupin rhizosphere. This led to greater Si uptake and improved plant growth compared to both the –N and + NO₃⁻ treatments. The supply of Si further enhanced these effects, with NH₄⁺ showing a consistently different pattern of influence compared to NO₃⁻. Additionally, –N white lupin plants accumulated more P than those treated with N, while Si supply significantly improved P acquisition across all treatments.

Our study demonstrates that rhizosphere acidification induced by NH₄⁺ nutrition can significantly enhance Si mobilization from the rhizosphere soil in the absence of Si supply and reduce Si adsorption when Si is applied. These findings may have practical implications for improving both Si mobilization in the rhizosphere and the effectiveness of Si fertilizers.