Carbon and nitrogen stocks and their mineralization potentials are higher under organic than conventional farming practices in Japanese Andosols

This study aimed at understanding whether and how long-term organic rice farming affects soil carbon (C) and nitrogen (N) stocks and their mineralization potentials in submerged rice paddies in Tochigi, Japan. An incubation experiment was carried out to assess the impacts of internal nutrient cycling after organic farming (OF) for 4–5 years (4OF), 8–9 years (8OF), and 12 years (12OF), compared with a conventional rice field (CF). Soil samples were collected at 0–15 cm and 15–20 cm in flooded rice fields after harvest in October 2013. pH and bulk density at 0–15 cm were significantly lower in 12OF fields than in CF fields (by 0.22 unit pH and 17.5%, respectively). Compared with CF, 12OF fields showed significant differences in soil organic carbon (SOC) and total nitrogen (TN), but 4OF and 8OF fields did not. In 8OF fields, the C decomposition (Co) and N mineralization (No) potentials were significantly higher (by 34.0% and 35.6%, respectively, at 0–15 cm, and by 67.1% and 24.5% at 15–20 cm) than in CF fields. Similarly, in 8OF fields at 0–15 cm, the Co:SOC and No:TN ratios were 19.8% and 23.2% higher, respectively, than in CF fields. Co, No, Co:SOC, and No:TN in 12OF fields were higher than those in CF fields, demonstrating the effects of prolonged organic rice farming. Additionally, in 12OF fields, C and N stocks were significantly higher (by 15.5% and 17.2%, respectively, at 0–15 cm, and by 4.8% and 12.1% at 15–20 cm) than in CF fields. Our findings suggest that long-term organic rice farming increases soil C and N stocks as well as C and N mineralization in Japanese Andosols.