Aims <p>Plants are capable of detecting the genetic identity of neighboring individuals, responding through either advantages of kin recognition (kin selection theory) or competitions under resource limitations (niche partitioning theory). While morphological responses such as root allocation and biomass changes are well studied, little is known about how kin recognition affects functional traits like nitrogen (N) uptake strategies.</p> Methods <p>We conducted a microcosm experiment using <i>Brassica juncea</i> individuals from six maternal families and applied <i>in situ</i> ¹⁵N labelling to test whether kin recognition influences plant growth and N uptake strategies. Each family was paired both with siblings (same maternal family) and strangers (different maternal families), with two seedlings per pot. Biomass responses and N uptake of inorganic N (NH₄⁺ and NO₃^–) were quantified.</p> Results <p>Plants grown with strangers showed increases in biomass response ratio (49% in shoots and 79% in roots) but did not significantly differ in absolute biomass compared to sibling-grown plants. In contrast, N uptake rates, particularly for NO₃^–, were significantly higher in sibling-grown plants. Sibling-grown plants exhibited a stronger uptake preference for NO₃^– (76%), whereas stranger-grown plants showed a shift toward NH₄⁺. The magnitude of kin recognition effects on N uptake varied across maternal families, with one single family showing a strong genotype-specific response.</p> Conclusion <p>Our study demonstrates that kin recognition in <i>B. juncea</i> modulates N uptake strategies, with sibling-grown plants preferentially for NO₃^– uptake while stranger-grown plants shift toward NH₄⁺. These effects vary among maternal families, revealing genotype-specific responses in nutrient foraging behavior.</p>