Soil organic carbon and rice yield stability under unfavorable conditions: Contrasting responses across soil types in long-term Japanese paddy experiments

Background Soil organic carbon (SOC), a widely used indicator of soil organic matter status, is known to improve average crop yield, but its role in contributing to yield stability under environmental stress remains poorly understood, particularly in paddy ecosystems. Purpose This study aimed to quantify the effects of long-term organic matter application on SOC accumulation, rice productivity, and yield stability across diverse environmental conditions in Japan. Methods We used a dataset from 14 nationwide long-term experiments (LTEs) under continuous rice monoculture spanning 11–54 years. Of these, 11 sites were on Lowland soils and 3 on Andosols. We compared three treatments: inorganic fertilizer only (NPK), NPK with rice straw (RS), and NPK with compost (COM). To evaluate potential associations between SOC concentration and yield stability, we analyzed SOC accumulation rates, mean grain yield, long-term yield variability, and relative yield during objectively defined “low-yield years” (years in which NPK yields fell by >1 SD below the long-term mean). Results While NPK plots in Lowland soils frequently exhibited SOC depletion, continuous COM application significantly increased SOC in most sites across both soil types, with SOC accumulation rates ranging from 0.083 to 0.531 g C kg −1 yr −1 in Lowland soils and from 0.166 to 0.514 g C kg −1 yr −1 in Andosols. Organic amendments generally enhanced mean grain yields, although the magnitude of the response varied among sites. Based on mean treatment-to-NPK yield ratios, yields were on average 3.6% higher in RS and 6.3% higher in COM plots in Lowland soils, with corresponding yield advantages of 7.2% and 9.4% in Andosols. No significant relationship was observed between SOC concentration and long-term yield variability. In contrast, SOC concentration was positively associated with relative yield during low-yield years in Lowland soils ( R ² = 0.15, p < 0.05). Although the relationship explained only a limited proportion of the observed variation, its positive direction was generally maintained across alternative analytical approaches, suggesting that higher SOC concentrations were associated with a modest reduction in yield decline under unfavorable conditions. Site-specific comparisons further showed that the relative yield advantage of organic amendment treatments tended to be greater during low-yield years than during normal years, particularly in Lowland soils. No statistically significant relationship was observed in Andosols, possibly reflecting their inherently high SOC concentrations and other unique physicochemical properties associated with volcanic ash soils. Conclusions This study provides empirical evidence of a positive association between SOC concentration and yield stability in Lowland paddy soils, although the magnitude of the relationship was modest. Continuous organic matter inputs represent an important pathway for building and maintaining SOC and, alongside other factors, may contribute to the maintenance of rice productivity under unfavorable conditions, particularly in Lowland soils.