Long-Term Effect of Lime Application on Quantity and Quality of Soil Organic Carbon in Double Rice Cropping System

Lime application is an effective measure for improving rice yield and alleviating soil acidity, whereas its long-term effects on the sequestration and stability of soil organic carbon (SOC) remain unclear in paddy fields. Here, we report on the first 10-year long-term experiment to examine the impact of lime application on the quantity and quality of SOC in an acidic paddy field with double rice cropping. Lime was applied every 4 years with and without rice straw incorporation. Size and density fractionation and solid-state 13C nuclear magnetic resonance spectroscopy were employed to examine the physical fractions and chemical composition of SOC, respectively. The results showed that lime application had no significant effect on either the total SOC concentration or stocks. Compared to the non-lime control, lime application led to a 60.0% decrease in the free particulate organic carbon (fPOC) concentration but a significant 17.9% increase in the concentration of occluded particulate organic carbon (oPOC) while reducing the concentration of mineral-associated organic carbon (MAOC) by 5.3%. Chemical composition analyses revealed a 5.1% reduction in the content of alkyl carbon (C) and a 6.8% decrease in the ratio of Alkyl C to O-Alkyl C. Lime application and straw retention had a significant interactive effect on the composition of SOC. Under straw removal, lime application increased the oPOC concentration by 56.6%, while no significant effect was observed under straw return. Lime application had no significant effect on the MAOC concentration under straw removal, whereas it reduced this concentration by 9.8% under straw return. Under straw removal, lime application reduced the proportion of Alkyl C by 9.5%, while no significant effect was observed under straw return. Therefore, we conclude that although the total SOC stocks are not altered, long-term lime application reduces the content of MAOC and Alkyl C in the acidic paddy soil, suggesting that long-term liming may reduce SOC stability.