Effects of Afforestation on Soil Organic Carbon and Nitrogen Stocks in the Long Term in Semi-Arid Regions of Türkiye

The black pine (Pinus nigra Arn.) is among the most preferred tree species for afforestation in Türkiye. This study aims to examine the effects of afforestation carried out in 1968, 1973, 1985, 1996, and 2002 on soil properties, especially soil organic carbon (SOC) and nitrogen (N) in a semi-arid region of Türkiye. Soil texture, electrical conductivity (EC), reaction (pH), Cation Exchange Capacity (CEC), lime (CaCO3) concentration, N, and inorganic-organic C contents were determined for each afforestation site. Although afforestation significantly increases SOC and TN stocks, the stand’s age did not affect the dynamics of the SOC stocks. But early stages of afforestation increased N stocks by more than 500%–600% compared to older ones. Our results show that afforestation combined with soil preparation increases the SOC and N contents, and soil tilling without plantation accelerates this process in the initial stages of afforestation. Rather than planting only one tree species, a plantation that mixes broadleaves and conifers with other annual and perennial plants may be more suitable for long-term C sequestration and the use of assisted natural succession in the revegetation of degraded arid and semi-arid regions as an alternative to large-scale afforestation should be paid more attention in the future.