Post-agricultural restoration of soil organic carbon pools across a climate gradient.

• Post-agricultural restoration increased SOC due to the build-up of non-protected SOC. • Post-agricultural restoration shifts SOC composition towards more easily available C. • Physico-biochemically protected SOC was hampered by restoration in temperate soils. • Within SOC composition, convergent and divergent self-restoring trends coexist. Post-agricultural natural restoration is a worldwide strategy for eco-environmental sustainability. However, it is unclear how it affects soil organic carbon (SOC) pools and composition among soil types across climate gradient. Here, we investigated 23-year post-agricultural restorations of SOC in three soils: Luvic Phaeozem, Calcaric Cambisol and Ferralic Cambisol typical for mid-temperate, warm-temperate and subtropical zones, respectively. Six SOC fractions with different protection mechanisms (non-protected, physically, chemically, biochemically, physico-chemically and physico-biochemically) were separated. Compared with pre-restoration in 1990, post-agricultural restoration rebuilt SOC similarly (+68–+91%) among the three soils despite of different SOC background. Compared with continuous cultivation, post-agricultural restoration increased total SOC pools in all the three soils (+33–+60%) mainly because of the increments of non-protected pool (coarse particulate organic C, cPOC). However, the pure physically, chemically, and biochemically protected SOC fractions were less sensitive to post-agricultural restoration. The physico-biochemically protected SOC was hampered by restoration in the two temperate soils but remained stable in the subtropical soil, suggesting a divergent self-restoring trend. Positive correlations of the total SOC and most fractions with wetness (precipitation/temperature ratio) demonstrated the climate dependency of SOC. In conclusion, post-agricultural natural restoration builds up SOC pool mainly due to the cPOC increment and shifts SOC composition towards more easily available C in three soils across the climatic gradient. [ABSTRACT FROM AUTHOR]