Effects of atmospheric nitrogen deposition on the functioning, structure, and composition of Calluna vulgaris (L:) Hull heathlands in Cantabrian mountains (NW Spain): a critical load approach = Efectos de las deposiciones atmosféricas de nitrógeno en el funcionamiento, estructura, y composición de los brezales de Calluna vulgaris (L.) Hull de la cordillera cantábrica (NO España): una aproximación a la carga crítica

Abstract

Efectos de las deposiciones atmosféricas de nitrógeno en el funcionamiento de los brezales de Calluna vulgaris (L.) Hull de la cordillera cantábrica.= Calluna-vulgaris-heathlands located at the southern-most distribution area (Cantabrian Mountains, NW Spain) are currently threatened by global change drivers such as atmospheric nitrogen (N) depositions that affect their functioning, structure and plant species composition. The main aim of the present Doctoral Thesis was to evaluate the effects of different atmospheric N loads in Cantabrian Mountains heathlands at two different life-cycle stages (young- and mature-phase).

Field measured bulk inorganic N deposition rates in the Cantabrian Mountains ranged between 2.8 kg N ha-1 yr-1 and 4.6 kg N ha-1 yr-1, with higher N deposition rates during the wet period. The prevailing N chemical form in atmospheric depositions is from oxidized compounds (NOY). The distribution and fate of these airborne N inputs in the Calluna-heathland were traced with 15N isotope. These heathlands were able to retain ∼72% of total N inputs in the short-term (< 1 year). The soil organic horizon immobilized ∼47% of the N inputs. During the second year, ∼95% of N inputs was likely lost by denitrification fluxes during the winter-spring period. Leaching losses were negligible over two years, suggesting that these heathlands were not N saturated.

Furthermore, we aimed to evaluate the effects of different levels of experimentally simulated N deposition on the functioning, structure and composition of Cantabrian Calluna-heathlands. The increase in atmospheric N loads resulted in a subsequent increase in soil ammonium (NH4+) with long-term N inputs (10 years); although soil nitrate (NO3-), total N, organic carbon (C), and available phosphorus (P) pools remained unchanged. Besides, Calluna shoot N and P contents and litter N content also increased, which was accordingly reflected in higher N:P ratios. The greatest P demand by Calluna biomass from P-poor soils was supplied by the increase in acid phosphatase enzymatic activity to mineralize organic P, and by the extent of Calluna roots colonized by mycorrhizal fungi to support the higher P uptake. There were no changes in soil microbial biomass C and N contents in response to higher N availability, as well as in urease (N cycle) and β-glucosidase (C cycle) enzymatic activities associated to microbial nutrient demand. The lack of response of several soil properties and microbial nutrient contents could be due to extreme climatic conditions and short period of physiological activity in these montane areas, indicating that montane heathlands are highly resistant to increased N loads. Furthermore, plant community composition and plant species richness remained unchanged in response to enhanced N inputs, likely due to the resistant and resilient mechanisms of heathland dominant vegetation. However, at life-form level, percentage of cover of N-tolerant graminoids and forbs species increased with higher N availability, while the cover of bryophytes and lichens declined. There was a stimulation of Calluna vulgaris vital rates (shoot growth and flowering) in response to short-term N inputs, but the first symptoms of N saturation were observed under long-term N inputs (10 years) through a declined trend in these vital rates. The heathland life-cycle stage conditioned the response to enhanced N loads through stronger N-related effects in young stands compared to mature ones. In Cantabrian heathlands, the N critical load was set in the range of 10-20 kg N ha-1 yr-1 for both young and mature stands, based mainly on changes in Calluna flowering and shoot N content at 14.6 kg N ha-1 yr-1.