Large-scale afforestation is regarded as an effective natural climate solution. However, afforestation-induced changes in soil organic C (SOC) are poorly quantified due to the paucity of large-scale sampling data. Here, we provide the first comprehensive assessment of the afforestation impact on SOC stocks with a pairwise comparative study of samples from 619 control-and-afforested plot pairs in northern China. We found context-dependent effects of afforestation on SOC: afforestation increases SOC density (SOCD) in C-poor soils but decreases SOCD in C-rich soils, especially in deeper soil. Thus, the fixed biomass/SOC ratio assumed in previous studies could overestimate the SOC enhancement by afforestation. By extrapolating the sampling data to the entire region, we estimate that afforestation increased SOC stocks in northern China by only 234.9 ± 9.6 TgC over the last three decades. The study highlights the importance of including pre-afforestation soil properties in models of soil carbon dynamics and carbon sink projections.
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The data that support the findings of this study are available from the corresponding authors upon reasonable request.
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This study was supported by the National Key R&D Program of China grant no. 2017YFA0604702, the Strategic Priority Research Program (A) of the Chinese Academy of Sciences grant no. XDA20050101 and National Natural Science Foundation of China grant no. 41988101.
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Extended Data Fig. 2 Robustness test of the negative correlation between ΔSOCD and SOCD_c across six tree species groups.
The same methods are used with Extended Data Fig. 1.
The central lines in the box-whisker plots indicate the medians, and the bottom and top edges of the boxes indicate the 25th and 75th percentiles, respectively. The maximum whisker lengths are specified as 1.5 times the interquartile range, and outliers are marked using+. Independent sample t-tests with false discovery rate correction were conducted to compare the data of each group with 0. p > 0.05 for all five groups. A one-way ANOVA (post hoc LSD test) was also used to test the difference between groups (p = 0.07).
80% of the samples are randomly selected to train the model and the remaining are used for test, which is repeated for 10 times to avoid contingency.
The uncertainty is from standard errors between BRT models run for 100 times.
Three profiles were dug in each plot.
The bars indicate the frequency distribution of the coefficient of variations for soil organic carbon densities between three profiles in each plot.
Extended Data Fig. 8 Uncertainties of the thresholds estimated from bootstrapping method across six tree species and all groups pooled together.
Error bars indicate the 95% confidence intervals.
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Hong, S., Yin, G., Piao, S. et al. Divergent responses of soil organic carbon to afforestation. Nat Sustain 3, 694–700 (2020). https://doi.org/10.1038/s41893-020-0557-y
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