Effects of Amorphous Silica on CO2 and N2O Emissions Mediated by Water-Filled Pore Space in Diverse Agricultural Soils

Silicon (Si) is abundant in the Earth’s crust; however, its amorphous form (ASi) is often depleted in agricultural soils. While ASi benefits plant nutrient uptake and growth, its effects on soil pore characteristics, such as water-filled pore space (WFPS), and regulating greenhouse gas (GHG) emissions remain poorly understood. We investigated the effect of ASi addition on soil bulk density, WFPS, and subsequent N ₂ O and CO ₂ emission dynamics in two soil types of differing texture: Luvisols (moderate silt and clay) and Arenosols (low silt and clay). In a first experiment, we assessed how varying ASi levels affected soil bulk density and WFPS. A second experiment investigated the impact of 1% ASi on N ₂ O and CO ₂ emissions. ASi addition altered soil bulk density, leading to a decrease in WFPS, especially at 10% ASi in Luvisols. In Arenosols, WFPS increased at 1% ASi before declining at higher rates. The 1% ASi addition increased CO ₂ and N ₂ O emissions in Luvisols but reduced both in Arenosols. These contrasting outcomes likely reflect a dual effect of ASi: in finer-textured Luvisols, ASi reduces bulk density and increases pore volume, which lowers WFPS under fixed water input, improves aeration, and enhances microbial respiration and nitrification, resulting in increased CO ₂ and N ₂ O emissions. In coarser-textured Arenosols, ASi may reduce macroporosity by clogging larger pores, resulting in higher WFPS and oxygen limitation, thereby decreasing emissions. Our findings suggest ASi has texture-dependent effects on soil properties and GHG emissions. These outcomes highlight the need for further field-based investigation under natural conditions.