Background and aims <p>Restoring degraded ecosystems is a key strategy for capturing carbon (C). The C fixation and allocation within plant-soil-microbe in ecosystems with varying degrees of degradation remain poorly understood.</p> Methods <p>We used two pioneer trees (<i>Albizia julibrissin</i> and <i>Platycladus orientalis</i>) to restore lightly, moderately, and heavily degraded lands over a two-year period, and traced C assimilation and distribution within the plant-soil-microbe systems using in situ ¹³CO₂ pulse labeling.</p> Results <p>Seeding pioneer-tree seeds on degraded lands can rapidly restore vegetation and accumulate biomass. Land degradation level and tree species significantly affected plant biomass and root-shoot ratio. The biomass accumulation, net photosynthetic rate, and ¹³C uptake in the leaves of nitrogen-fixing tree <i>A. julibrissin</i> were consistently greater than those of non-nitrogen-fixing tree <i>P. orientalis</i> across all land degradation levels. The biomass root-to-shoot ratio of both tree species increased with the degree of land degradation. The belowground ¹³C content / aboveground ¹³C content ratio and total ¹³C content in the leaves, stems, roots, and whole plant body were not affected by degradation level. The translocation of recently assimilated C to soil was significantly affected by degradation level, with the higher ¹³C content in the rhizosphere and bulk soil of lightly and moderately degraded lands than in heavily degraded land.</p> Conclusion <p>Our study highlights the stability of internal allocation of newly-fixed photosynthetic C in plants under various environments, and suggests that selecting nitrogen-fixing pioneer species is an effective strategy for promoting vegetation recovery and C sequestration.</p>