Soil carbon changes in paddy fields amended with fly ash

Abstract Increasing soil carbon (C) sequestration in the agricultural sector is an important strategy for mitigating climate change; however, conventional best management practices such as crop residue retention and organic fertilizer application do not always increase soil C content due to C loss by cultivation. In this context, application of fine-textured minerals such as coal fly ash (FA) may be effective in increasing soil C sequestration by enhancing plant biomass production and protecting soil C from being lost. We conducted a three-year field experiment in a paddy field with three levels of FA application (0, 5, and 10% by soil weight) in combination with the following four nitrogen (N) treatments: no input, and applications of urea, pig manure compost (compost) and hairy vetch ( Vicia Villosa Roth.) green manure (vetch). Across the three seasons, rice grain yield was in the order of vetch = urea > compost > no input, reflecting the effect of N availability in each treatment. Application of FA (particularly at 10%) reduced the total rice plant biomass by hampering tillering. However, FA application did not reduce grain yield due to increased individual grain weight. In spite of decreased rice residue incorporation into the soil, FA application increased the soil C content at the end of the third season regardless of the N source, driven by reduced soil C loss. We conclude that the application of mineral soil amendments such as FA is effective in enhancing soil C sequestration without decreasing rice yield in paddy fields.