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

Abstract

Silicon (Si) is abundant in the Earth's crust; however, its amorphous form (ASi) is often depleted in agricultural soils due to crop uptake and removal with harvest. 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 N2O and CO2 emission dynamics in two soil types of differing texture: Luvisols (moderate silt and clay) and Arenosols (low silt and clay). In a first experiment (Experiment I), soils were amended with 0% (control), 1%, 5%, and 10% ASi (w/w, relative to soil dry weight) to assess effects on bulk density and WFPS under fixed water input. A second experiment (Experiment II) investigated the effect of 0% (control) and 1% ASi (w/w, relative to soil dry weight) on N2O and CO2 emissions. The addition of ASi altered soil bulk density, leading to a decrease in WFPS. This was in particular the case at 10% ASi addition to Luvisols. In Arenosols, WFPS first increased at 1% ASi before finally declining at higher rates as well. In experiment II, 1% ASi addition reduced CO2 emissions in Arenosols by ~42-45% and N2O by 8-44%, but increased CO2 in Luvisol by ~47% and N2O emissions by ~18-57%. The contrasting responses were texture-dependent, with ASi affecting soil physical properties and associated N2O and CO2 emissions differently in Luvisols and Arenosols, consistent with inferred effects on pore structure and water retention. As these results are derived from controlled incubation conditions using disturbed soil cores under constant temperature and fixed water input, further field-based investigations are needed to assess the in-situ effects of a broader applicability of ASi.