Between ice and ocean; soil development along an age chronosequence formed by the retreating Breidamerkurjökull glacier, SE-Iceland

As glaciers in Iceland retreat they expose new surfaces for plant succession and soil formation. In the foreland of Breidamerkurjökull glacier, we used a high time resolution chronosequence to study vegetation succession; morphological, physical and chemical changes in soils; and the impact of avifauna on the vegetation and soils developing on the moraines. In total, 38 sampling sites were distributed along moraines deposited in 1890, 1930, 1945, 1960, 1982, 1994, 2004 and 2012, representing 8 age groups. On moraines, where the influence of seabirds was apparent, soils were collected from bird hummocks, one for each age group. Soil samples were also collected from nearby birch (Betula pubescens Ehrh.) woodlands, representing soils in a potentially future mature ecosystem. The results show that mosses and grasses dominated and characterized the vegetation on the moraines with the exception of the two youngest moraines that were non- or sparsely vegetated. An AC horizon sequence slowly formed yet the A horizon remained thin (<2.5cm). Bulk density decreased with time since deglaciation, from 1.2 g cm^-3 down to 0.8-1.0 g cm^-3 in the oldest moraines. Similarly, soil pH (H₂O) decreased with time, reaching a steady state at the value of 6 after 67yrs. LOI (loss on ignition), SOC (soil organic carbon), and total N (nitrogen) concentrations along with pH (NaF) increased over the time span investigated. The increase in LOI, SOC and N concentrations occurred slowly during the first few decades, rates increased after 50yrs since deglaciation but eolian input appeared to reduce the rates again in the 122yr-old moraine. The highest SOC and N concentrations were reached in the 82yr-old moraine (1.4% SOC, 0.07% N) and the highest NaF values of 9.3 after 67yrs. The lack of shrubs and dwarf shrubs indicated that the long distance from seed sources retarded vegetation succession within the Breidamerkurjökull moraines, which in turn affected the soil development on the moraines, although perhaps to a lesser degree than expected. The soil in the bird hummocks was characterized by dense root system and thick A (or O) horizons (<14 cm) generated by the dense growth of grasses and herbs, accompanied by considerably much higher SOC and N values (18% SOC, 1.1% N after 67yrs) when compared to the surrounding moraine soils. Additionally, bulk density was much lower within the bird hummocks (0.26-0.75 g cm^-3) and soil pH (H₂O) was generally lower, ranging between 5.2 and 6.2 depending on moraine age. This study shows that the point-centered input by avifauna into the terrestrial ecosystem is an important factor in locally enhancing vegetation growth and the rates of soil formation increasing the heterogeneity within the moraine soils.