Labile soil organic matter changes related to forest floor quality of tree species mixtures in Oriental beech forests

Labile soil organic matter, which is an important supply of energy for soil micro-organisms, has been suggested as a sensitive indicator of soil organic matter (SOM). In this study, the effects of forest vegetation on forest floor thickness and quality, labile SOM fractions and soil properties were examined in five different forest ecosystems, such as beech-hornbeam-alder, beech-hornbeam, beech-alder, beech-oak, and pure beech in the Hyrcanian region, northern Iran. Each forest type was replicated three times. In each plot, six forest floors and soil samples were taken in a 30 × 30 cm area. Pure beech had always a higher forest floor thickness than the other studied forest types. Increase in tree diversity in beech forest type reduced forest floor C:N ratio, more than three-fold, compared to pure beech. Based on our data, the influence of forest type on labile SOM was generally higher in the 0–10 cm soil layer than in the 10–20 cm layer. Beech-hornbeam-alder and beech-alder forest types, with high quality of forest floor and more values of N fractions in labile SOM, increased soil biological activities and fertility whereas pure beech and beech-oak forest types, with lower quality of forest floor and more values of C fractions in labile SOM reduced soil biological activities and fertility. Based on the result, the abundance of beech influenced nutrient cycling via forest floor quality. This led to an increase in labile SOM and subsequent soil fertility through the admixture of broadleaf species into beech forests.