Woody plant encroachment allocated more photosynthetic carbon belowground through soil pores in grasslands

• Shrub encroachment allocated more photosynthates to belowground through shrubs. • Shrub encroachment slowed assimilated 13 C to shoots by shrubs of grasslands. • Photosynthetic C was allocated to soils first through macropores (>150 μm Ø) • Much bacteria and G - of shrub-encroached grasslands leads to a reduction in SOC. Shrub encroachment is a significant ecological challenge for grassland ecosystems worldwide. However, few studies have investigated the allocation of photosynthetic C to soils, and the role of soil pore structure in regulating root-derived C input in shrub-encroached grasslands remains unclear. Therefore, we aimed to investigate the characteristics of the allocation of photosynthetic, root-derived C and the role of soil pore structure in C allocation. To this end, we conducted an in situ 13 C labeling experiment in typical shrub-encroached grasslands, using native grasslands as the controls, and performed X-ray computed tomography (CT) scanning to determine the soil pore structure. The results showed that a substantially higher proportion of 13 C was allocated belowground (32.60 % in roots and 19.46 % in soil) in shrub-encroached grasslands than that in native grasslands (5.21 % in roots and 9.46 % in soil) 14 days after labeling. Shrubs allocated more photosynthetic C belowground compared with that allocated by herbs. Additionally, photosynthetic C transport from shrubs to rhizosphere soils was slower than that from herbs. Photosynthetic C was allocated to rhizosphere soils first through macropores (>150 μm Ø) and then through smaller pores (<150 μm Ø). In summary, shrub encroachment enhanced the stability of newly assimilated carbon by increasing photosynthetic carbon input and slowing soil carbon turnover. However, long-term carbon pool measurements revealed that soil C stocks of grasslands were twice as high as those in shrub-encroached soils. Therefore, compared with native grasslands, shrubs may be more suitable for short-term soil C sequestration in shrub-encroached grasslands. However, in the long term, the greater abundance of bacteria and Gram-negative bacteria in shrub-encroached grasslands will enhance mineralization and lead to infertile soils.