Effects of tree species on topsoil properties and nitrogen cycling in natural forest and tree plantations of northern Iran

• Tree species induce a specific signature in terms of litter quality which changes the properties of the topsoil.
• Soil net N mineralization rate was significantly higher under Alnus subcordata.
• The N-fixing species performed better than other species in improving soil nutrient availability.
• Soil biological properties were affected by variations in soil chemistry under different tree species.

After 15 years, the effects of Alnus subcordata (AS), Populus deltoides (PD), Taxodium distichum (TD) plantations and a mixed natural forest dominated by Quercus castaneifolia, Carpinus betulus and Parrotia persica (QC-CB-PP) on litter quality and soil fertility were assessed in northern Iran. Sixteen samples per stand were taken from the forest floor and top 10 cm of soil. Litter quality differed among the tree species, showing the highest N concentration (1.85%) and lowest C (39.43%) in the AS stand. Sand, clay and water content did not differ for the tree species, but soil bulk density were highest in AS (1.59 g cm− 3) ≈ PD (1.56 g cm− 3) and silt content peaked in QC-CB-PP (47%). Soil pH (7.16), EC (0.29 dS m− 1), total N (0.34%), available P (24.6 mg kg− 1), available K (337.4 mg kg− 1), available Ca (256.3 mg kg− 1), available Mg (57.4 mg kg− 1), earthworm density (3.5 individuals m− 2), earthworm biomass (42 mg m− 2) and microbial respiration (0.48 mg CO2-C g soil− 1 day− 1) were significantly higher in the AS stand. In contrast, higher contents of soil organic C (2.58%) and C/N ratio (28.70) were found in the TD stand. The fine root biomass varied among the study sites in the ranked order of QC-CB-PP (94.2 g m− 2) > PD (65.1 g m− 2) ≈ AS (64.6 g m− 2) > TD (36.9 g m− 2). Negative nitrification rates were observed in all forest types. Net soil ammonification and N mineralization rates were significantly higher in the AS (0.46 and 0.31 mg kg− 1 d− 1, respectively) than in the PD (0.22 and 0.08 mg kg− 1 d− 1, respectively), QC-CB-PP (− 0.07 and − 0.24 mg kg− 1 d− 1, respectively), and the TD (− 0.07 and − 0.26 mg kg− 1 d− 1, respectively) sites. Our study reveals the differential impacts of tree species on the physical, chemical, and biological properties of the topsoil. In particular, planting N2-fixing Alnus subcordata can help increase soil fertility in northern Iran.