C and N cycle under beech and hornbeam tree species in the Iranian old-growth forests

Abstract

Beech and hornbeam are two of the most common species in old-growth temperate forests. However, how these two species affect soil carbon (C), soil nitrogen (N) and soil microbial and enzymatic activities is still not well understood. Thus, the aim of the present study was to quantify the effects of beech and hornbeam on soil chemical, biochemical and biological features at Makarod forest, Northern Iran, by sampling either forest floor or mineral soil (30 × 30 × 15 cm) under individual trees of both species. Based on our data, hornbeam showed higher concentrations of N, phosphorous (P), potassium (K), calcium (Ca) and magnesium (Mg), but lower total thickness, C and C: N ratio in the forest floor. Top mineral soil under hornbeam had alkaline condition (pH > 7), higher electrical conductivity (EC), total N, N in micro- and macro-aggregates, N sequestration, available P, K, Ca, Mg, humic acid, fine root biomass, total earthworm (i.e. epigeic, anecic and endogeic) density and biomass, total nematode, acarina and protozoa densities. Hornbeam trees increased soil basal respiration and substrate induced respiration (0.50 and 1.47 mg CO2 g−1) compared to beech. The two studied species did not show any significant difference in soil microbial biomass C, metabolic quotient, microbial ratio, C availability index, particulate organic C and dissolved organic C, whereas, higher values of soil microbial biomass N, particulate organic N and dissolved organic N (66.97 mg kg−1, 0.52 g kg−1 and 38.53 mg kg−1, respectively) were again found under hornbeam. Except for arylsulfatase, all the other soil enzyme activities (i.e. urease, acid phosphatase and invertase) were almost two-fold higher under hornbeam (24.55 μg NH4+–Ng−1 2 h−1, 498.30 μg PNP g−1h−1 and 296.2 μg Glucose g−1 3 h−1, respectively). As well, higher values of ammonification rate were measured under hornbeam (0.38 mg kg−1 d−1), whereas beech significantly increased the nitrification rate (−0.16 mg kg−1 d−1). Net N mineralization rate was about three-fold higher under hornbeam (0.14 mg kg−1 d−1) than under beech. Our results indicate that differences in functional traits between beech and hornbeam caused changes in microbial community and in its activity, underlying a higher forest floor quality, higher soil fertility and more active microbial populations under the latter.