Adverse density-dependent effects on competition outcomes (e.g., fecundity) are stronger within species than between them, promoting species coexistence. However, empirical evidence on the resource competition process (e.g., nitrogen, N) and on how plant root traits and fertilization alter this process remains unclear. We conducted a response-surface design competition experiment involving wheat and weeds (Avena fatua L. or Echinochloa crus-galli (L.) P. Beauv., i.e., WB and WO pair) across 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), and fertilization treatment. After seven months, root morphology traits were measured and NH4+ and NO3− uptake was studied using short-term 15N labeling. Without fertilization, high wheat density reduced wheat's root area (αii = −0.130 and −0.147 in WB and WO pairs) and length (−0.110; −0.120) while increasing specific root length (0.040; 0.062) and area (0.019; 0.035) to mitigate a reduction in NH4+ (−0.142) than NO3− (−0.205). Weeds reduced root length and NH4+ uptake. With fertilization, increasing wheat density decreased wheat and weed root length (αii = −0.117, αij = −0.238 in WB pair; αii = −0.117, αij = −0.181 in WO pair) and area (αii = −0.126, αij = −0.283; αii = −0.152, αij = −0.206), NH4+ (αii = −0.281; αij = −0.224), and NO3− uptake (αii = −0.079; αij = −0.326). Weeds also increased specific root length (0.220 and 0.079) and area (0.054 in the WO pair). These N reduction and root plasticity were not observed with increased weed density, suggesting that maintaining weed presence did not reduce wheat's N uptake. Our research indicated that weed management strategies should consider density-dependent N competition and root plasticity within agricultural systems.
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