The Effects of Stand Development Stages on Soil Carbon and Nitrogen Stocks in Black Pine, Scots Pine and Fir Stands in Türkiye

The influence of stand development stages on soil organic carbon (SOC) and total nitrogen (TN) stocks was examined in black pine (Pinus nigra Arnold.), Scots pine (Pinus sylvestris L.), and Kazdağı fir (Abies nordmanniana subsp. equi-trojanı (Aschers. & Sint. ex Boiss) Coode et Cullen), differing in the mean tree diameters in which reproduction stages (RS)=<8 cm, sapling or pole stages (SPS)= 8-19.9 cm, large pole stages (LPS)= 20-35.9 cm, and medium wood stages (MWS)=36-51.9 cm in three tree species located in northwestern Turkey. A total of 216 soil samples were collected and analyzed for pH, organic matter, bulk density, maximum water holding capacity, carbon, and nitrogen concentrations, and the SOC and TN stocks were calculated. SOC and TN stocks varied significantly among the four stand development stage classes. The SOC stock at 0-30 cm increased significantly due to an increase in the diameter of black stands (BPLPS and BPMWS). SOC stocks in all stand development stages peaked in the large pole (44.94 Mg/ha) and declined as the sapling or pole (37.71 Mg/ha) was replaced by medium wood stands (30.17 Mg/ha), and a low point (27.94 Mg/ha) was found in the reproduction stages of stand development for Scots pine. The TN stock at a soil depth of 0-30 cm ranged from 1.66 to 6.46 Mg/ha. The highest TN stock was observed in the SPSPS (6.46 Mg/ha) and FirRS (5.48 Mg/ha), and the lowest was observed in the BPLPS (1.66 Mg/ha) stands. The results illustrate that soil was the main storage of C and N in all different stand development stages of tree species.