Early changes in soil organic carbon following afforestation of former agricultural land

Afforestation of less productive, risky and degraded agricultural land is one of the methods which is recommended for practical agriculture to increase the carbon sequestration. In this study, we have attempted to determine the effect of afforestation of agricultural land (warm, mildly dry climatic region of the Czech Republic) on the soil organic carbon (C_ox) concentrations in the mineral soil. Two soil types (Haplic Chernozem and Haplic Cambisol) were afforested. Both an indirect estimation (loss-on-ignition method) as well as chromsulfuric acid mixture oxidation were used to determine the organic carbon content in the soil samples and the methods were compared. In the case of the Haplic Chernozem, the C_ox concentration at a depth of 0–10 cm after 1–3 years of afforestation with pedunculate oak or Scots pine significantly decreased (P < 0.01 and P < 0.004, respectively) with the stand age. Similar to the case of the Haplic Chernozem, the C_ox concentration in the Haplic Cambisol also significantly decreased in the variants with Scots pine (P < 0.003) or a mixture of forest tree species (P < 0.006); no significant (P > 0.05) decrease was found in the case of a mixture of forest tree species on the Haplic Chernozem or with Douglas fir on the Haplic Cambisol. Significantly higher (P < 0.05) C_ox concentrations were typically found in the case of 1-year-old stands compared to 2-year-old or 3-year-old stands. A higher C_ox loss than the quantity of residues returned to the soils may be the reason the soil C_ox concentration significantly (P < 0.00001 and P < 0.000001) decreased for the control agricultural plots (Haplic Chernozem and Haplic Cambisol). The carbon stock in the upper 10 cm of the 5-year-old stands was higher on the Haplic Chernozem and lower on the Haplic Cambisol compared to the control agricultural plots.