Iron fertilization and soil carbon sequestration in rice paddies

Iron (Fe) fertilization of the ocean mitigates global warming by sequestering carbon dioxide (CO2) in phytoplankton, but the effect of Fe fertilization on carbon (C) sequestration in arable soils remains unknown. Iron is often added to rice paddies as blast furnace slag (BFS), a byproduct of steel manufacturing used as a silicon (Si) fertilizer to improve productivity. However, BFS also contains large amounts of Fe oxides, which might promote C sequestration by forming complexes with organic matter. To investigate this, we first analyzed data from a national survey of soils from South Korea to estimate the effect of continuous Fe addition via BFS on soil organic C (SOC) stocks. This revealed a strong positive correlation between SOC and extractable Fe and available Si concentrations, indicating that periodic silicate fertilizer application contributed to an increase in SOC stock. Second, to isolate the effect of Fe addition on SOC stocks, we conducted an incubation test with BFS enriched with Fe oxides (0–5%, wt wt−1). Soil respiration was significantly reduced by silicate fertilizer application, and this effect was strengthened with the Fe-enriched fertilizer. Finally, to verify the effect of Fe addition on SOC stock changes in the field, we added three different Fe-enriched silicate fertilizers to rice paddies and quantified SOC stock changes by net ecosystem C budget (NECB) estimation. Silicate fertilizer significantly increased net primary production (NPP) by 18–20% over the control, and this effect was strengthened with increasing Fe addition. Silicate fertilizer application decreased soil respiration by 15–30% over the control, and this effect was strengthened further by Fe enrichment. As a result, silicate fertilizer application during rice cultivation increased the SOC stock by 0.65–0.68 Mg C ha−1 over the control and by 0.90–0.96 Mg C ha−1 for Fe-enriched fertilizer. In conclusion, the positive effect of BFS addition on SOC stock is related in part to the role of Fe oxides, primarily through the suppression of soil respiration. Fe-enriched silicate fertilizer therefore provides a management strategy to increase SOC stocks and crop productivity in rice paddies.