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RESEARCH PRODUCT

Which nitrogen fertilization techniques and crop traits to promote biological weed regulation by competition?

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International audience; Background and AimsThe necessary reduction of herbicide use may result in a residual weed flora in arable fields. Moreover, the useof mineral fertilizer must be reduced for environmental reasons. Thus, crop-weed competition for nitrogen mightincrease.Nitrogen competition among plants is driven by soil nitrogen availability and plants traits related to nitrogen nutrition.Selecting species for their nitrogen nutrition traits and adapting nitrogen fertilization (rate, date, number ofnitrogen applications, straw burial or export) could change competitive relationships between plants. Weeds withhigh nitrogen nutrition requirements are often hard to manage and could be hampered by such changes. Thisstudy aimed to identify which traits and which nitrogen fertilization options can reduce weed harmfulness whilemaintaining crop production in the long term.Material and MethodsA maize monoculture from Aquitaine (France) was simulated over 15 years and 5 weather repetitions using FLORSYS(Gardarin et al. (2012), Munier-Jolain et al. (2013), Colbach et al. (2014)). This process-based model simulatescrop growth and weeds dynamic over the years with a daily time step from cropping system, pedoclimate andspecies traits. We simulated numerous combinations of nitrogen fertilization (nitrogen input date and rate, strawburial vs export) and initial soil nitrogen content with 3 maize varieties (1 actual, 2 virtual) differing by their traitvalues related to nitrogen nutrition. The effects of fertilization, soil nitrogen supply and maize variety on weeddisservice and service indicators were studied (Figure 1B).Key resultsThe simulations showed that service indicators were often positively correlated to disservice indicators (Figure 1A).The sensitivity analysis revealed that maize variety and initial soil nitrogen content were the variables that affectedthe most weed impact indicators. The interactions between maize variety and nitrogen fertilization techniqueswere strong.Compared to the actual maize variety, one virtual variety improved weed (dis)services except nitrate leaching(Figure 1B). The other virtual variety had the opposite effect. The effect of nitrogen application date was differentfor all 3 varieties, except for bee food which decreased with the lateness of application, regardless of the variety.For the actual variety, increasing nitrogen rate decreased weed harmfulness over the long term, and increasedweed services (except bee food). The 2 virtual varieties showed the opposite for weed services and a similar behavioras actual variety on disservices.Increasing the initial soil nitrogen content decreased current but increased future weed harmfulness (Figure 1B).On the long term (beyond 15 years), a high initial soil nitrogen content might increase the weed soil seed bank andthus decrease grain yield loss.Further analyses and simulations will be carried out to (1) disentangle the effects of fertilization techniques frommaize variety and initial soil nitrogen content and (2) identify which weed traits drive weed (dis)services in thesemaize-based cropping systems.ConclusionsThe preliminary results of these simulations of maize cropping systems showed that choosing varieties based ontheir nitrogen nutrition traits can reduce weed disservices. However, it was almost impossible to conciliate reductionof weed harmfulness and increase of weed services in these systems.