Long-term application of mineral fertilizer affects weed flora in winter wheat monoculture (Triticum aestivum L.)

Mineral fertilizers influence weed flora, but their effects are difficult to isolate due to agricultural practices and crop rotations. This study investigates weed community composition and structure under long-term mineral fertilization in an herbicide-managed winter wheat (Triticum aestivum L.) monoculture. The trial, established in 1896 in Gembloux, Belgium, includes NK, NP, NPK, PK, and Control treatments. Since 1975, winter wheat has been grown continuously. Weed flora was assessed through field surveys (realized flora) and seed bank analysis (potential flora) during the 2020-2021 study period. The realized flora (i.e. plant species present at the time of observation) surveyed before the first weeding operation comprised 12 species, with no significant effect of treatments on species richness. The dominant species were common to all treatments (Matricaria chamomilla, Papaver rhoeas, and Polygonum aviculare), with Equisetum arvense additionally dominant in the Control treatment. However, total weed density was higher in Control and PK plots compared to N-fertilized plots, likely due to reduced wheat growth and increased light availability allowing for the development of both spring- and autumn-germinating species. Distinct community structures emerged, with Control and PK plots having different and less nitrophilous communities than N-fertilized plots. The potential flora (i.e. viable seeds and vegetative organs present on or in the soil) contained 30 species, with about 75% overlapping with the realized flora. Spring or summer germinators were present. Differences in community structure were less pronounced in the potential than in the realized flora. This study highlights that even in low-diversity, herbicide-managed fields, long-term chemical fertilizers impact weed density and composition, driven by species responding to nitrogen treatments. Long-term nutrient limitations, hindering crop growth, promote spring-germinating weed development, influencing community structure and density in winter wheat.