Six years of different fertilization regimes shift weed community and competition with winter oilseed rape

Context or problem: Weed composition and diversity responses to fertilization, and the effects of weed-crop competition on crop yield are not well understood, despite the importance for designing weed management strategies in sustainable crop production. Objective or research question: The aim of the study was to determine weed community shifts and weed-oilseed rape competition as influenced by different fertilizations. Methods: Field investigations were conducted at the two final growing seasons of oilseed rape, after six years of different fertilization regimes, including NPK fertilization, with 180 kg N ha-1, 60 kg P2O5 ha-1 and 62 kg K ha-1; NP fertilization, no K; NK, no P; PK, no N; and control, no fertilization. Oilseed rape yield, oilseed rape and weed species biomass and nutrient uptake, as well as weed species density and diversity were examined each growing season. Results: Different fertilization regimes led to distinct patterns in weed community composition. NPK fertilization enhanced oilseed rape yield and growth, resulting in the lowest weed density due to intensified crop-weed competition particularly for light. Intensified crop-weed competition under high fertilization favored the dominance of a few species. In contrast, nutrient omission treatments (i.e., NP, NK, PK and control) increased the abundance of some weed species that were otherwise less competitive. The weed species Beckmannia syzigachne dominated all treatments but clearly benefited from N omission, showing the highest biomass and density under PK and control treatments. It was suppressed under NP and NPK treatments in terms of absolute biomass, but represented a greater proportion of total weed biomass than other species, indicating a higher competitive ability. A further evaluation of the relative importance of different weed species in weed-crop competition revealed that the dominant weed species Beckmannia syzigachne was the most detrimental in terms of density, biomass and nutrient uptake. In contrast, the impacts of some other species (e.g., Mazus japonicus and Ixeris polycephala) was negligible in this study. Conclusions: Overall, the results highlighted that NPK balanced fertilization increased oilseed rape yield with improved nutrient uptake. However, the yield gain achieved did not coincide with a diverse weed community, which calls for more attention to be paid on biodiversity besides high crop yield. Suppression of targeted weed species that are competitive and dominant might be suggested to maintain weed diversity and mitigate yield loss.