The ecological mechanisms shaping plants assemblages are multiple and scale-dependent. While the differentiation of resource patches may govern assembly rules at intermediate scales, the role dispersal processes is expected to decrease and the one of species interaction to increase from large to fine spatial scale. In this study, we investigate the contribution of these three processes to weed assembly based on a hierarchical sampling design conducted in 454 arable fields of the LTSER Zone Atelier ‘Plaine & Val de Sèvre’ from 2014 to 2016 . Using multiplicative diversity partitioning, the total field weed diversity was decomposed into four metrics, corresponding to differences in species richness between sampling scales (subplots, plots, field core-margin) and mean subplot richness. Between-plot difference in species richness was the main component of weed diversity, but its contribution decreased when the total field weed diversity increased while between-subplot difference in species richness and mean subplot richness increased. Even if plots within field core corresponded to similar resources patches (according to Ellenberg indices), we showed that the dispersion of plant height within subplots increased with the total field diversity. Furthermore, field core weed richness strongly increased with the richness of field margin, and to a lesser extent with the cover of organic fields within 1000m. All together, these results demonstrate that the increase of field weed diversity is related to small-scale (i.e. subplot) community diversification due mainly to niche-partitioning and short-distance dispersal from margin, and provide new insights on the assembly rules of weed communities.