Kinship Modulates Carbon Allocation and Phosphorus Acquisition in Chinese Fir–AMF Networks Under Neighbor P Limitation

Phosphorus (P) deficiency in forest soils is a key constraint on the sustainable management and productivity of Chinese fir (<i>Cunninghamia lanceolata</i>) plantations. This study investigated how P limitation alters the reciprocal exchange of "photosynthetic carbon and mineral phosphorus" between Chinese fir and arbuscular mycorrhizal fungi (AMF) when the focal plant grows adjacent to neighbors with different degrees of relatedness. An indoor pot experiment simulating heterogeneous P supply was conducted using clonal seedlings of Chinese fir No. 36 as the focal plant, with Chinese fir No. 36, Chinese fir No. 41, and <i>Schima superba</i> as neighboring plants to establish three two-plant combinations: a kin pair (No. 36 + No. 36), a close-kin pair (No. 36 + No. 41), and an unrelated-kin pair (No. 36 + <i>S. superba</i>). <i>Funneliformis mosseae</i> was inoculated into the shared root-zone room connecting the two plants, and the neighbor was subjected to a gradient of P limitation (sufficient P, low P, and zero P). Meanwhile, the focal No. 36 plant received ¹³CO₂ pulse labeling to form a "Chinese fir-AMF-P-limited neighbor" symbiotic network in which No. 36 served as the ¹³C donor. AMF colonization, seedling growth, and changes in ¹³C enrichment and P concentration in plant tissues of the focal plant were quantified. Neighbor P limitation significantly increased AMF colonization in roots and whole-plant P concentration of the focal Chinese fir. Following ¹³CO₂ pulse labeling, whole-plant ¹³C enrichment of the focal plant increased significantly under the neighbor zero P treatment, suggesting enhanced carbon allocation under severe neighbor P limitation. Moreover, under the neighbor zero P treatment, focal plants grown with an unrelated-kin neighbor showed significant increases in stem P concentration (1.86 g·kg^-1) and stem atom% ¹³C (1.50%), whereas focal plants grown with a kin neighbor exhibited a significant increase in root Atom% ¹³C (1.29%). These patterns indicate that neighbor relatedness may modulate carbon allocation and P acquisition within the mycorrhizal network: in the kin context, the focal plant tended to allocate more photosynthetic carbon belowground and may partially subsidize the AMF carbon demand (i.e., a higher C reward), coinciding with a relatively weaker P accumulation in its own tissues; in contrast, in the unrelated kin context, carbon allocation shifted toward stems and was associated with strengthened P accumulation in stem tissues. Overall, the results highlight the dynamic nature of AMF-mediated carbon-nutrient reciprocity across hosts of contrasting relatedness and provide new insights into how mycorrhizal networks may facilitate plant adaptation to nutrient limitation.