Objectives Acidification and inadequate available silicon were widespread constraints limiting high and stable rice yields in the red paddy soils of southern China. This study investigated the effects of continuous application of organic and inorganic silicon sources on soil silicon fractions, soil properties, and their interactions in red paddy soil, aiming to provide a theoretical basis for the coordinated improvement of soil fertility in red paddy fields.

Methods Red paddy soil was collected from the Jiangxi Academy of Red Soil and Germplasm Resources. A two-year pot experiment with four consecutive rice crops was conducted in 2023 and 2024. The silicon treatments included sodium silicate (NaSi), rice straw (S), rice husk biochar (RBC), and bamboo leaf biochar (BBL). Soil samples were collected one day before the harvest of each early and late rice crop. Three vertical and evenly distributed soil cores were taken from each pot using a soil auger. The contents of soil available Si and different Si fractions, soil aggregate composition, soil organic carbon, alkali-hydrolyzable nitrogen, available phosphorus, available potassium, cation exchange capacity (CEC), exchangeable K and Na, and amorphous Fe and Al were analyzed.

Results The application of exogenous silicon increased the contents of soil available silicon, mobile Si, adsorbed Si, organically bound Si, and oxide/hydroxide-bound Si to varying degrees, thereby enhancing soil silicon availability and improving aggregate stability. Sodium silicate showed a significantly stronger effect on increasing available silicon content and silicon availability than the three silicon-rich organic materials, with the increase in available silicon rising from 62.42% in the first rice crop season to 267.6% in the fourth rice crop season. Compared with CK, continuous application of sodium silicate increased the mean weight diameter (MWD) of soil aggregates and the proportion of aggregates >0.25 mm (R_0.25). Soil pH increased by 1.35–2.40 units, while available potassium and exchangeable K increased by 1.27–4.59 times and 1.31–2.29 times, respectively. Amorphous Fe and amorphous Al increased by 12.14%–19.96% and 6.43%–8.82%, respectively.The three organic silicon sources, namely S, RBC, and BBL, increased soil organic carbon, alkali-hydrolyzable nitrogen, and available phosphorus to varying degrees. In the late rice season of 2024, the MWD values under S and RBC were significantly higher than those under BBL and NaSi. The effects of the three organic silicon sources on increasing soil organic carbon followed the order BBL > S > RBC, whereas their effects on alkali-hydrolyzable nitrogen followed the order S > RBC > BBL. The effects of S and RBC on available phosphorus were similar, while BBL had no significant effect on available phosphorus. The three organic silicon sources had no significant effects on available potassium, exchangeable K, or exchangeable Na. Correlation analysis showed that mobile Si and adsorbed Si were extremely significantly positively correlated with soil pH, exchangeable K and Na, amorphous Al, and available silicon (P<0.01), and significantly positively correlated with amorphous Fe (P<0.05). Oxide/hydroxide-bound Si was extremely significantly positively correlated with available phosphorus, alkali-hydrolyzable nitrogen, and CEC (P<0.01).

Conclusions Continuous application of exogenous silicon increased soil pH, available Si content, and Si availability in red paddy soil, while also enhancing soil structural stability. Sodium silicate rapidly and cumulatively increased available Si content and Si availability, and improved available potassium content; however, it did not increase soil organic carbon and even reduced available phosphorus content. Although Si-rich organic materials were less effective than sodium silicate in improving soil available Si supply capacity, they showed greater advantages in increasing soil organic carbon, alkali-hydrolyzable nitrogen, and available phosphorus contents, as well as improving soil structural stability. Therefore, the combined application of organic and inorganic silicon sources is recommended to improve silicon nutrition and overall soil fertility in red paddy soils.