Forest Soil Carbon Efflux Evaluation Across China: A New Estimate With Machine Learning

Forest soil respiration (Rs) plays an important role in the carbon balance of terrestrial ecosystems. China's forest occupies a large part of the world's forest. However, due to the lack of integrated observation data and appropriate upscaling methodologies, substantial uncertainties exist in the Rs evaluation, which limits our understanding of the carbon balance. Here, we re-evaluated the total soil carbon effluxes across China by combining field observations from 634 published annual Rs with a machine learning technique (i.e., Random Forest (RF)). Our results revealed that the combination of systematic measurements with the RF model allowed a definite estimate. The average annual Rs was 776.9 g C m(-2) yr (-1), ranging from 411.5 to 1,770.7 g C m(-2) yr (-1). Total forest soil carbon effluxes amounted to 1.17 Pg C yr -1 in China. Geographically, annual Rs showed a clear spatially increasing trend from northeast to southwest. Forest type is an important factor in determining the soil respiration rate. Bamboo and Evergreen broadleaf forests were higher than other types of forests. These results provide a unique insight into the magnitudes and mechanisms of soil CO2 emissions in China's forest ecosystems.

Automated summary

Forest soil respiration is re-evaluated in China, revealing a total carbon efflux of 1.17 Pg C yr -1 and an average annual soil respiration of 776.9 g C m(-2) yr (-1). The spatial pattern of soil respiration shows an increasing trend from northeast to southwest, with bamboo and evergreen broadleaf forests having higher respiration rates compared to other forest types.