Belowground allocation and fate of tree assimilates in plant–soil–microorganisms system: ¹³C labeling and tracing under field conditions

Although forests account for only 27% of the total land area, they store approximately 80% of the aboveground carbon (C) and 40% of soil C globally. However, there is currently little information regarding the input and distribution of photoassimilates of trees in plant–soil systems. To quantify the belowground C input and allocation to plant–soil pools, we pulse labeled 5-year-old pioneer (Populus davidiana) and climax (Quercus wutaishanica) species with ¹³CO₂ under field conditions. The ¹³C allocation dynamics were traced in the leaves, branches, roots, and soil microorganisms, rhizosphere and bulk soil under Populus davidiana and Quercus wutaishanica over 21 days. ¹³C recovery (% of assimilated ¹³C) in the leaves of Populus davidiana and Quercus wutaishanica decreased from nearly 90% at 6 h after labeling to 40% and 45% at 21 days, respectively. Continuous assimilate allocation from above- to belowground increased ¹³C recovery in roots from 0.4% at 6 h after labeling to 9.5% in Populus davidiana and from 1.5% to 15% in Quercus wutaishanica at 21 days after labeling. The recently assimilated C was detected in the soil immediately after labeling. The ¹³C amounts in the bulk and rhizosphere soil of the climax species Quercus wutaishanica was two-fold greater than that under the pioneer species Populus davidiana. The total belowground net C input (including that in roots) by Populus davidiana and Quercus wutaishanica was 109 and 283 g C m⁻² yr⁻¹ (top 20 cm of soil), respectively, including rhizodeposition of 4.2 and 28 g C m⁻² yr⁻¹. Consequently, the belowground C allocation and soil C sequestration increase from pioneer to climax tree species.