Abstract
Which factors are the most important determinants of weed species composition in organic rice fields? Which species can be associated with the extreme values of the most important background variables? To answer these questions, we surveyed the weed flora along with 15 management and 3 environmental variables in 42 organically managed rice fields in eastern Hungary. Filamentous algae, Chara vulgaris , Chara braunii , Schoenoplectus mucronatus , Elatine triandra , and Echinochloa crus-galli were the most dominant weeds. Using a minimal adequate model containing 4 terms with significant net effects, 48.3% of the total variation in weed species data could be explained. Farm holding size (correlated with sowing type, shift crop, organic manure and water depth) was found to be the most important explanatory variable, which was followed by hand weeding, tillage type (correlated with tillage depth, shift crop and supplementary nutrients) and rice cover. Farm holding size was negatively associated with most of the aquatic plants suggesting its adverse impact on farmland biodiversity. Hand weeding appeared to be efficient against Rumex stenophyllus and Echinochloa crus-galli , but it was tolerated by Cyperus difformis . Perennial weeds like Persicaria amphibia and Bolboschoenus planiculmis were more abundant in sites without soil inversion and Lemna aequinoctialis seemed to be most tolerant for the shading effect of crop canopy. The responses of weed species to the studied variables provide new information about the assembly rules of aquatic plant communities, and our findings also can be used to optimise non-chemical weed control strategies in organic rice production.