Nitrate Over Ammonium: Limited inorganic N niche partitioning between wheat and weeds regardless of fertilization treatment

Fertilization is a crucial agricultural practice that influences biogeochemical cycles and ecosystem functions, and it plays a central role in widespread wheat and weed coexistence. However, it remains unclear how wheat and weeds coexist under N-limited conditions and how plant N uptake strategy change after N fertilization. Wheat (Triticum aestivum L.), and two weeds (wild oats (Avena fatua), and barnyard grass (Echinochloa crusgalli)) were selected as targeted plant species. We grew them alone, and after about seven months, we labeled these plants with ¹⁵NH₄Cl or ¹⁵KNO₃ for 2 h to quantify their NH₄⁺ and NO₃⁻ uptake, and measured root length, root area, specific root length, specific root area, specific root volume, and root tissue density. We found that fertilization led to a more resource-acquisitive nutrient acquisition traits in wheat (i.e., increased specific root area and specific root volume (P < 0.05)), without altering weed root traits. Across three species, the increased NH₄⁺ and NO₃⁻ uptake after fertilization were not mediated by root traits, but by the direct effect of fertilization. Additionally, both wheat and weeds predominantly preferred NO₃⁻ than NH₄⁺ regardless of fertilization or not, indicating a limited niche differentiation for wheat-weed coexistence. These findings can improve our understanding of the mechanisms of species coexistence in agricultural systems, particularly with regards to N uptake strategies among crops and weeds.