Alkaline pretreatment enhances the silicon release efficiency of silicon-rich biochar

Silicon (Si)-accumulating plants alleviate the problem of low soil Si availability due to high weathering by absorbing labile Si from soil phytoliths. However, high Si contents do not indicate high Si release. It is necessary and important to investigate the efficient Si release from Si-accumulating plant residues.

Rice husk and bamboo leaves were prepared and transformed into rice husk biochar (RHB), bamboo leaf biochar (BLB) and their alkali-modified biochar (ARHB, ABLB), which were then applied to highly weathered soils (high-availability Si (HASi) and low-availability Si (LASi) soils). Explore efficient Si release pathways and evaluate their potential as efficient Si fertilizers.

The first-order model demonstrated that the biochar Si pools were divided into a rapidly released Si pool C₁ and a slowly released Si pool C₂ and that alkali-modified biochar had a greater Si solubilization capacity. Soil incubation experiments showed that the available Si in ARHB- and ABLB-treated soils was significantly greater than that in RHB- and BLB-treated soils, with a greater effect in LASi soil. In a rice growth pot experiments, biochar addition increased the grain, straw, and root Si concentrations in LASi soil independently of the amount added (from 0.4% to 0.8%). Conversely, for HASi, Si-rich biochar had no effect on the straw Si concentration.

The alkaline pretreatment of Si-rich biomass can improve its Si release efficiency, leading to increased labile Si content in the soil and improved soil Si availability. Therefore, alkali-modified Si-rich biochar can better sustain Si availability in soils characterized by high weathering and severe desilication.