Disentangling the effect of nitrogen input and weed control on crop-weed competition suggests a potential agronomic trap in conventional farming

Weeds are commonly assumed to induce yield loss because of resource competition. However, there is growing empirical evidence that the picture is much more complex, because fertilizer inputs, by modifying the level of resources and weeding by removing some weed species, affect the outcome of crop-weed interaction. We assess how two important crop production inputs - nitrogen (N) fertilization and weed-control - affect a fundamental non-chemical means of weed management, the outcome of crop-weed competition, in real field conditions in 56 winter cereal fields, of which 23 were organically farmed. We used a factorial design with two levels (absence/ presence) of nitrogen input and weed control inputs, but in our case, the "control" (i.e., presence level) for both practices was the usual practice of the farmer (which therefore varied). We found that crop aboveground biomass and grain yield were positively related to the amount of Ntotal (N soil plus N fertilizer), while weed species assemblages were negatively affected, showing lower species richness and weed abundance (i.e., number of plants). We also detected a contrast between farming systems: conventional fields (CF), managed with higher amount of total N and weed control, showed higher crop biomass and grain yield, and lower weed abundance compared to organically farmed fields (OF). Importantly, the findings showed that the outcome of the competition between crops and weeds was largely in favor of the crop plants in CF fields, even in the absence of weed control. In OF fields, the outcome of the competition between weeds and crop plants was still largely in favor of the crop, but at a lesser extent than in CF fields. The patterns were similar in unfertilized plots, though weed control in CF fields was more effective at low amounts of N, suggesting that more intense weed control is required in N-rich fields to maintain crop production. Overall, we argue that these results may underlie an agronomic trap: while an increased supply of nitrogen generally increases crop yield, it also benefits to weeds, requiring more efficient weed control.