No-weeding suppressed methane emissions by Takanari and Koshihikari rice varieties under organic farming

To determine the difference in greenhouse gas emissions between two rice (Oryza sativa L.) varieties; Takanari (Tak) and Koshihikari (Kos), under organic rice cultivation with or without weeds management, a pot experiment was conducted at Tsuruoka, Yamagata, Japan. The Tak and Kos varieties were transplanted individually and interplanted as Tak +Kos, with the main treatments of weeding (w) and no-weeding (nw). Methane (CH4), nitrous oxide (N2O), and plant night respiration (CO2 emission) fluxes were measured once weekly or biweekly during the rice growth period. As the results, N2O fluxes were not detected but CH4 fluxes varied with the rice growth stage for all treatments. Compared with the Kos(w), Tak(w) and Tak + Kos(w) significantly decreased total seasonal CH4 emissions by 12.4 and 19.1, respectively. This was attributed to higher rhizospheric CH4 oxidation and the offset of potential methanogenic activities because of the larger root biomass and a higher harvest index for both Tak(w) and Tak +Kos(w) compared with Kos(w). Conversely, Kos(nw) significantly reduced cumulative CH4 emissions by 31.7 and 48.6 compared with Tak(nw) and Tak +Kos(nw), respectively. This was ascribed to high rhizospheric CH4 oxidation promoted by the presence of significantly higher weed biomass for Kos(nw) at 65.0 and 44.0 compared with Tak(nw) and Tak +Kos(nw), respectively. On average, the no-weeding practice significantly decreased cumulative CH4 emissions by 50.5 compared with the weeding practice, with a significantly high (P<0.001) interaction between weed management and variety differences observed. The CH4 emissions and plant biomass under variety interplanting (Tak +Kos) were not significantly different to Tak individual planting, but were significantly different to Kos individual planting, which implied variety Tak had strong competition to variety Kos. More studies on appropriate variety choice and pairs, and the effect of aquatic weeds presence in no-weeding organic paddy fields are necessary to estimate CH4 emission and attenuation.