Can afforestation with Cupressus lusitanica restore soil C and N stocks depleted by crop cultivation to levels observed under native systems?

This study quantified changes in soil organic carbon (SOC) and total nitrogen (STN) stocks from the conversion of natural forest to crop field followed by afforestation of these fields. Soil (0–100 cm), leaf and fine roots were collected in the natural forest, the adjacent to 50-year-old crop fields converted from the natural forest, and 5, 8, and 17-year-old Cupressus lusitanica plantation sites (hereafter P5, P8 and P17, respectively) established on the converted crop fields in Southern Ethiopia. Soil and leaf pH, soil texture, bulk density, fine root mass, and SOC and STN contents and stocks were determined. The results showed that soil pH was lower and clay fraction was higher in P17 site than natural forest. Leaf pH of Cupressus lusitanica in plantation sites was lower than those of trees in natural forest. Fine root mass was greater in the plantation sites than the crop field. Soil bulk density (0–40 cm) was higher in the crop fields than the natural forest but there was no significant difference between crop field and plantation sites. SOC and STN stocks (0–100 cm) were higher in the natural forest than the crop field and all plantation sites and there was no significant difference between the crop field and all plantation sites. Overall, SOC and STN stocks decreased by 22.9% and 40.3%, respectively, in conversion of natural forest to crop field. However, after 17 years of afforestation, the crop field showed no change of stocks. The results suggest that afforestation in agricultural lands may not guarantee, or take a long period, to restore SOC and STN to the original natural forest level.