P. 149-159Ecosystems adapted to low nitrogen (N) conditions such as Calluna-heathlands are especially sensitive to enhanced
atmospheric N deposition that affects many aspects of ecosystem functioning like nutrient cycling,
soil properties and plant-microbial-enzyme relationships. We investigated the effects of five levels of experimentally-
simulated N deposition rates (i.e., N fertilization treatments: 0, 10, 20 and 50 kg N ha− 1 yr− 1 for
3 years, and 56 kg N ha− 1 yr− 1 for 10 years) on: plant, litter, microbial biomass and soil nutrient contents,
soil extracellular enzymatic activities, and plant root ericoid mycorrhizal colonization. The study was conducted
in marginal montane Calluna-heathlands at different developmental stages resulting from management
(young/building-phase and mature-phase). Our findings revealed that many soil properties did not show a statistically
significant response to the experimental addition of N, including: total N, organic carbon (C), C:N
ratio, extractable N-NO3
−, available phosphorus (P), urease and β-glucosidase enzyme activities, and microbial
biomass C and N. Our results also evidenced a considerable positive impact of chronic (10-year) high-N
loading on soil extractable N-NH4
+, acid phosphatase enzyme activity, Calluna root mycorrhizal colonization
by ericoid fungi, Calluna shoot N and P contents, and litter N content and N:P ratio. The age of heathland
vegetation influenced the effects of N addition on ericoid mycorrhizal colonization, resulting in higher colonized
roots in young heathlands at the control, low and medium N-input rates; and in mature ones at the high
and chronically high N rates. Also, young heathlands exhibited greater soil extractable N-NO3
−, available P,
microbial biomass N, Calluna shoot N and P contents, and litter N content, compared to mature ones. Our
results highlighted that accounting for the N-input load and duration, as well as the developmental stage of
the vegetation, is important for assessing the effects of added N, particularly at the heathlands' southern distribution
limit.S