Quantitative assessment of the effects of biochar amendment on photosynthetic carbon assimilation and dynamics in a rice–soil system

Biochar amendment has been proposed as a promising means to increase carbon (C) sequestration and simultaneously benefit plant productivity. However, quantifying the assimilation and dynamics of photosynthetic C in plant-soil systems under biochar addition remains elusive. This study established two experimental factors involving biochar addition and nitrogen (N) fertilization to quantitatively assess the effect of biochar on photosynthetic C fate in a rice plant-soil system. The rice plants and soil samples were collected and analyzed after 6-h pulse labeling with ¹³ CO₂ at the tillering, jointing, heading and ripening stages. Biochar did not affect the proportions of photoassimilated carbon-13 (¹³ C) allocations in plant-soil systems. Nevertheless, biochar enhanced the ¹³ C contents in the shoot, root, and soil pools, especially when combined with N fertilization, and biochar increased the cumulative assimilated ¹³ C contents in the shoot, root, and soil pools by 23%, 14% and 20%, respectively, throughout the whole growth stage. Moreover, biochar addition significantly enhanced the N use efficiency (NUE) by c. 23% at the heading and ripening stages. In summary, biochar increases the content of photoassimilated C in plant-soil systems by improving plant productivity via enhancing NUE, thus resulting in a higher soil C sequestration potential.