Effect of flooding irrigation and intermittent irrigation patterns on weed community diversity in late rice fields

Abstract: Water-saving irrigation technique for rice is an essential agricultural practice to save water and increase crop yields. Weeds are important components of paddy fields, and play an important role in maintaining the ecological sustainability of rice-based ecosystems. In order to reveal the effects of water-saving irrigation on weed community composition and species diversity of rice fields, this study investigated weed community dynamics in rice field during different growth periods of rice. A field experiment was conducted in 2014 at Jiangxi Central Irrigation Experiment Station during the whole growth period of late rice, with two different irrigation levels (flooding irrigation and intermittent irrigation) and three repeats for each treatment. Using the methods of community ecology, three quadrates with an area of 1 m2 were set in each sampling plot to investigate weed species, plants, coverage in every quadrate. Weed density, dominant weed specie, importance value, weed community number and diversity were analyzed. The diversity of weeds was measured using 4 frequently-used indexes: Margalef species richness index, Simpson dominance index, Shannon-Wienner diversity index and Pielou eveness index. The results showed that 14 and 11 species of weeds were found in the rice fields with intermittent irrigation and flooding irrigation, respectively. Cyperus iria, Lindernia antipoda and C.difformis, which occurred in intermittent irrigation fields, did not grow in the rice fields with flooding irrigation. Weed density was suppressed in the intermittent irrigation fields compared to flooding irrigation rice fields. Intermittent irrigation reduced the weed density in all growth periods by 1.2%-53.2%, and by 27.8% on average, which inhibited weeds growth effectively and helped to control the infestation of weeds. However, intermittent irrigation had the same dominant weed species with the flooding irrigation in rice field. Dominant weed species in the experiment were Sagittaria pygmaea, Leptochloa chinensis, Lemna minor and Astragalus sinicus with the importance value being 0.39, 0.20, 0.17, and 0.09 respectively. With intermittent irrigation, the importance value of Sagittaria pygmaea classified as Broad-leaved weeds was reduced by 9.76%, and that of Lemna minor classified as monocotyledonous weeds was reduced by 65.38%. Our experiment showed that intermittent irrigation decreased the proportion of dominant weeds of Sagittaria pygmaea, Lemna minor in the community, avoided severely dominant weed such as Lemna minor outbreaking, and then made the distribution of weed community much more balanced. In addition, xerophytic weeds with high density dominated in the rice field with intermittent irrigation pattern, while aquatic weeds were dominant weeds in fields with flooding irrigation. Intermittent irrigation increased the density of drought weeds by 277.51% and reduced the density of aquatic weeds community from 344.81 down to 63.98 plants per m2 by 81.44% decline on average during the rice growth period. In the intermittent irrigation pattern, Margalef index, Simpson index, Shannon-Wienner index, and Pielou index were all higher than that in the flooding irrigation, increasing by 27.01%, 14.31%, 9.09% and 3.48% respectively. Moreover, weed community structure indexes were significant different during the milky stage between the 2 irrigation patterns. Weed diversity indexes were increased obviously in intermittent irrigation fields, which were beneficial to improve paddy ecological conservation. This study provided valuble data support for setting watering-saving irrigation system considering controlling the harm of dominant weeds and protecting weeds diversity. It was noted that this study included the experimental datum of one year in late rice field. Further studies about the effect of flooding irrigation and intermittent irrigation patterns on weed diversity without herbicides in a longer period should be considered.