Background

A multitude of studies analyzed the dynamics of soil organic carbon (SOC), but their methodology does not provide sufficient understanding of the differential impact of artificial and natural afforestation on SOC dynamic.

Methods and aims

We investigated the SOC dynamics following artificial (AR) and natural (NQ) afforestation on abandoned farmland (AF) in China’s Loess Plateau in an attempt to evaluate the effects of these afforestation methods. We characterized soil structure and stoichiometry using stable isotope carbon and radiocarbon models. We aim to compare SOC dynamics, clarify SOC sources under different afforestation, examine comparability of the study areas and ascertain how soil aggregate size classes control SOC dynamics.

Results

The 0-10 cm and 10-20 cm SOC stocks were significant higher in NQ than AR and AF. At other depths, there is no significant difference among the three land-use systems. Total topsoil SOC stocks, C:N and C:P of differently sized soil aggregates significantly increased following afforestation. Carbon isotope analysis results indicated that the SOC decomposition rate and the new SOC input rate were lower under NQ than AR.

Conclusions

Afforestation can lead to SOC accumulation in soil depths up to 1m mainly because the topsoil (0-20 cm) changes significantly. SOC resources are mainly from macroaggregate formation provided by fresh plant residues. The comparability of study sites is validated, so the“space-for-time substitution” method is applicable in this study.