Two-year comparison of nitrogen uptake and yield in ‘Takanari’ and ‘Koshihikari’ rice grown under organic field conditions in Shonai region, Northeast Japan

It is plausible that under organic rice farming conditions, with Monochoria vaginalis as the dominant weed, the Indica rice ‘Takanari’ (Tak) outperform the Japonica rice ‘Koshihikari’ (Kos) in terms of nitrogen (N) uptake and biomass production. However, how N uptake, biomass, and yield in Tak and Kos are affected by weeds under organic rice farming paddy fields across multiple growing seasons remains unclear. To investigate this, we conducted a two-year field experiment (2022 and 2023) at Yamagata University Farm, Tsuruoka, Japan. Tak and Kos were transplanted individually (four seedlings/hill) or interplanted as Tak + Kos (2 + 2 seedlings/hill), with weeding and no-weeding as the main treatments. Rice and weed biomass and N uptake, as well as rice yield, were measured at harvest, with significant differences observed among all treatments. Weeding practices and growing seasons significantly affected on biomass and N uptake in both Tak and Kos. In 2023, aboveground rice biomass under weeding conditions was significantly lower than that in 2022, whereas no significant difference was observed under no-weeding conditions. Tak consistently showed higher biomass, N uptake, and yield compared with Kos across all treatments, including both seasons, weeding practices, and planting modes. The aboveground biomass ratios between Tak and Kos in interplanting mode were higher than those in individual planting mode under both weeding conditions across both years. These results suggest that Tak exhibited stronger competitive ability in terms of N uptake, biomass, and yield when interplanted with Kos. In conclusion, this two-year organic rice farming field experiment indicates that Tak may sustain higher N uptake and yield compared with Kos under both weeding and no-weeding conditions.