Ecosystems 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 50kgNhayr for 3years, and 56kgNhayr for 10years) 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-NO, 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-NH, 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-NO, 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.

Nitrogen (N) deposition has been identified as one of the main traits of terrestrial ecosystems, affecting their structure and functioning. Among terrestrial ecosystems, heathlands are characterised by low nutrient status and highly affected by N deposition. However, few studies have been developed under natural field conditions to evaluate the amount of N deposition in these ecosystems. Therefore, a field experiment was carried out to investigate the inorganic N inputs in mountainous heathlands of NorthWestern Spain. Two study sites (La Majúa and San Isidro) were selected on the south side of the Cantabrian Mountains, as a representative monitoring N-sensitive ecosystems. Three replicated bulk collectors and one rain gauge were installed at each study site to collect monthly rainwater samples over three years. Wet bulk N deposition was different between the study areas (2.81 kg N ha-1 year-1 in La Majúa and 4.56 kg N ha-1 year-1 in San Isidro), but showed the same temporal pattern, with the highest N deposition rate observed in April and the lowest in August-September. In general, annual wet bulk NO3-N deposition was higher than NH4+-N, reflecting the prevailing NOX emission sources. Depositions of NO3-N were mostly originated by NOX emissions from northern highly industrialized and populated areas of North Spain. The lower NH4+/NO3-ratio in rainwater observed in the study area could be due to the decline of traditional land uses (livestock grazing, heath burning and croplands) associated with NHY emissions. Despite the low rates of N deposition observed in this study, N-sensitive heathlands in NorthWestern Spain could be threatened by N depositions.

Elevated atmospheric nitrogen (N) deposition is a major driver of change, altering the structure/functioning of nutrient-poor Calluna vulgaris-heathlands over Europe. These effects amply proven for north-western/central heathlands may, however, vary across the ecosystem's distribution, especially at the range limits, as heathlands are highly vulnerable to land-use changes combined with present climate change. This is an often overlooked and greatly understudied aspect of the ecology of heathlands facing global change. We investigated the effects of five N-fertilisation treatments simulating a range of N deposition rates (0, 10, 20, and 50 kg N ha yr for 1 year; and 56 kg N ha yr for 9 years) on the Calluna-plants, the plant functional groups, species composition and richness of two life-cycle stages (building/young- and mature-phase) of Calluna-heathlands at their rear-edge limit. Our findings revealed a dose-related response of the shoot length and number of flowers of young and mature Calluna-plants to the addition of N, adhering to the findings from other heathland locations. However, cumulative high-N loading reduced the annual growth and flowering of young plants, showing early signs of N saturation. The different plant functional groups showed contrasting responses to the cumulative addition of N: annual/perennial forbs and annual graminoids increased with quite low values; perennial graminoids were rather abundant in young heathlands but only slightly augmented in mature ones; while bryophytes and lichens strongly declined at the two heathland life-cycle stages. Meanwhile there were no significant N-driven changes in plant species composition and richness. Our results demonstrated that Calluna-heathlands at their low-latitude distribution limit are moderately resistant to cumulative high-N loading. As north-western/central European heathlands under high-N inputs broadly experienced the loss of plant diversity and pronounced changes in plant species dominance, rear-edge locations may be of critical importance to unravel the mechanisms of heathland resilience to future global change.

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Management strategies developed in heathlands, such as burning, produce changes in plant community composition and rejuvenation of Calluna vulgaris plants, which affect their carbon sequestration capacity (Kopittke et al., 2013).In this paper we aim to evaluate the impact of management strategies (prescribed burning) in the framework of N deposition on the regulating ecosystem services (capacity of carbon sequestration) provided by Cantabrian heathlands. We try to answer the following questions: 1) Does prescribed burning have a positive effect on carbon sequestration in heathlands? 2) Does increased N deposition have the potential to alter carbon storage after burning in heathlands? 3) Which is the main ecosystem compartment for carbon storage in heathlands?