Nitrogen fertilization promotes wheat–weed coexistence through niche differentiation and fitness equalization

Nitrogen fertilization is commonly applied to increase crop yields in agroecosystems. Beyond its direct benefits, nitrogen fertilization may also influence crop production indirectly by altering crop–weed competition. Moreover, nitrogen fertilization can simultaneously threaten the biodiversity of both natural and managed communities, including weed diversity in wheat fields. Such diversity losses may weaken resistance to antagonists (e.g. pests and pathogens), compromising agroecosystem health and crop productivity. However, how nitrogen fertiliation affects crop‐weed coexistence in agroecosystems, and the mechanisms underlying these effects, remain unclear.

Here, we conducted a competition experiment between wheat and two dominant weeds (i.e. Avena fatua L. and Echinochloa crus‐galli (L.) P. Beauv.) by growing a total of 2490 individuals in a response‐surface design with and without nitrogen fertilization. At the end of growing season after 8 months, we harvested one‐third individuals to measure individual seed production to parametrize competitive population models, allowing to quantify niche and fitness differences and to predict competitive outcomes. We then measured various functional traits at individual level on the remaining two‐thirds at their peak growth, including plant height, three leaf traits (e.g. leaf area, specific leaf area), five root traits (root area, specific root area) with and without nitrogen fertilization.

We found that nitrogen fertilization facilitated coexistence between wheat and weeds, but the underlying processes differed for the two weed species with distinct functional strategies. Specifically, fertilization increased the niche differences between wheat and E. crus‐galli , resulting from amplified root area and length differences between them. In contrast, nitrogen addition reduced the competitive advantage of wheat over A. fatua and increased the likelihood of their coexistence by increasing A. fatua 's seed mass, specific root area, and length while reducing wheat height.

Synthesis . These findings demonstrate that nitrogen addition can affect coexistence through both niche and fitness difference‐related processes and reveal the effect of nitrogen fertilization on plant coexistence is highly species‐specific. Our study highlights the importance of trait plasticity for mediating coexistence under varying nutrient conditions and offers practical recommendations for selecting wheat varieties that support sustainable weed management under varying fertilization regimes to balance crop production and biodiversity conservation.