Wheat density, not weed density, drives negative density dependence in nitrogen uptake and root plasticity in wheat–weed communities

It is known that the negative density-dependent effects in competition outcomes (i.e., survival, growth, and fecundity) are stronger within species than between them, allowing diverse species to coexist. However, empirical evidence on resource competition process (e.g., nitrogen (N)) and how plant root traits mediation and their alteration by fertilization remains unclear. We conducted a response-surface design competition experiment involving wheat and weeds (Avena fatua L. or Echinochloa crusgalli (L.) P. Beauv.) with varying densities (4, 8, 12, 16 individuals per pot) and proportions (wheat:weed 0:1, 0.25:0.75, 0.5:0.5, 0.75:0.25, 1:0), with and without fertilization. After seven months, we measured plant root length and area, specific root length, area, and volume, root tissue density, and NH4+ and NO3- uptake using short-term 15N labeling. We found that without fertilization, high wheat density reduced wheat’s root area and length while increasing specific root length and area to mitigate a reduction in NH4+ than NO3-. Weeds reduced root length and NH4+ uptake (from a to b). Under fertilization, increasing wheat density decreased wheat and weed root length and area, NH4+ and NO3- uptake. Weeds also increased specific root length and area, buffering NH4+ reduction. These N reduction and root plasticity patterns were not observed with increased weed density, suggesting that maintaining weed presence did not reduce wheat’s N uptake. Wheat had strong intraspecific competition for N, while weeds showed strong interspecific competition from wheat, challenging long-term coexistence due to wheat’s competitive advantage in N absorption. Our research suggests that weed management strategies should consider density-dependent N competition and root plasticity within agricultural systems.