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.