Increase of soil organic carbon stock by iron slag-based silicate fertilizer application in paddy soils

Abstract

Silicate fertilizer derived from iron slag (hereafter, silicate fertilizer) as a soil amendment has been added to Korean and Japanese rice paddies to improve soil pH and supply nutrients to the crop. However, the consequences of silicate fertilizer application to soil organic carbon (SOC) stocks remain unclear. Hypothetically, silicate fertilizer might deplete SOC by increasing soil pH and stimulating microbial activity or increase SOC by improving rice productivity and biomass. To investigate this, we first quantified the relationship between plant-available silicate (SiO2) and soil organic matter concentrations using national soil survey data. We found a highly positive correlation between the two parameters, suggesting that long-term silicate fertilizer application might promote SOC sequestration. We then examined the effect of silicate fertilizer and lime (CaCO3) addition on soil respiration for 30 days under flooded conditions. Both materials increase soil pH but affect soil respiration in different ways: respiration increased significantly with lime but decreased significantly with silicate fertilizer application. This demonstrates that silicate fertilizer simultaneously improves pH while minimizing C loss through soil respiration. Finally, in a field study spanning two years, we ascertained the impact of silicate fertilizer (1.5 Mg ha-1) on annual SOC stock changes under rice cropping. Silicate fertilizer application increased net primary production (NPP) of rice plants by around 20% over control plots that did not receive silicate but decreased respired C loss by 16–20%. As a result, silicate fertilizer application increased the SOC stock by 0.96–1.1 Mg C ha−1 year−1 over the control. Our findings show that iron slag-based silicate fertilizer can promote C sequestration in soil organic matter while simultaneously improving soil properties and rice productivity.