A decreasing carbon allocation to belowground autotrophic respiration in global forest ecosystems

Belowground autotrophic respiration (RA(soil)) depends on carbohydrates from photosynthesis flowing to roots and rhizospheres, and is one of the most important but least understood components in forest carbon cycling. Carbon allocation plays an important role in forest carbon cycling and reflects forest adaptation to changing en-vironmental conditions. However, carbon allocation to RA(soil) has not been fully examined at the global scale. To fill this knowledge gap, we first used a Random Forest algorithm to predict the spatio-temporal patterns of RA(soil) from 1981 to 2017 based on the most updated Global Soil Respiration Database (v5) with global environmental variables; calculated carbon allocation from photosynthesis to RA(soil) (CA(B)) as a fraction of gross primary produc-tion; and assessed its temporal and spatial patterns in global forest ecosystems. Globally, mean RA(soil) from forests was 8.9 +/- 0.08 Pg C yr(-1) (mean +/- standard deviation) from 1981 to 2017 and increased significantly at a rate of 0.006 Pg C yr(-2), paralleling broader soil respiration changes and suggesting increasing carbon respired by roots. Mean CA(B) was 0.243 +/- 0.016 and decreased over time. The temporal trend of CA(B) varied greatly in space, reflecting uneven responses of CAR to environmental changes. Combined with carbon use efficiency, our CA(B) results offer a completely independent approach to quantify global aboveground autotropic respiration spatially and temporally, and could provide crucial insights into carbon flux partitioning and global carbon cycling under climate change. (C) 2021 Elsevier B.V. All rights reserved.