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RESEARCH PRODUCT

Reduction of nitrate leaching: what is the contribution of the residual weed flora?

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International audience; Integrated crop protection tolerates residual weeds if they are not harmful for crop production. Weeds are often solely considered as a pest, causing crop yield loss, harvest pollution, diseases … However, they may also provide beneficial services for agroecosystems, promoting plant biodiversity and feeding other organisms potentially beneficial to crop production (pollinators, carabids…). The role of residual weed flora as nitrate catchers, during summer fallow, has however rarely been assessed. Yet, some weed species are known for their high potential to take up soil nitrogen. The present study aimed to (1) develop an indicator to account for the potential beneficial role of weed floras as nitrate catchers at the field scale, and (2) calculate this indicator from the outputs of a weed dynamics model (FlorSys) in order to analyse whether and in which situations weed floras contribute to reducing potential nitrate leaching, while limiting negative impacts on crop production. A weed flora was considered to potentially reduce nitrate leaching if it contains species with a high ability to take up soil nitrogen and if these species are abundant in the field. So, the indicator that we developed to account for the potential role of weed floras as nitrate catchers depended on both: • Weed species ‘nitrophily’ (referred in ecological databases): the more nitrophilic a species is, the more its growth depends on soil nitrogen availability, and therefore the more it potentially takes up soil nitrate; • The cumulated leaf area of all plants per species, accounting for species abundancy and associated nitrogen sink strength. For a given field, this indicator was calculated considering all the species present in the weed flora, during the period running from the harvest of the preceding crop to the beginning of water drainage. This indicator was used to predict the potential of nitrate catching by weed floras in cropping systems from six regions from France and one from Spain which were simulated with the FlorSys model. Data analysis (under progress) will determine trade-offs between this indicator and other indicators of weed harmfulness for crop production and weed contribution to plant functional biodiversity. It will help to identify in which production situations residual weed floras can play a significant role in preventing nitrate leaching, while minimising negative weed impacts, such as crop yield loss and harvest pollution by weed seeds.