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The primary succession on the 50 year old volcanic island of Surtsey, Iceland, has been intensively studied. Initial soil development and other belowground processes are important drivers of primary succession but frequently overseen. A Leymus arenarius and Honckenya peploides dominated plant community has formed a relatively stable successional sere on the island, where external inputs of nutrients remain low. These plants have had a stable <10% aboveground surface cover during the past 20 years, but less is known about their belowground development. We investigated the organic matter (carbon) output and input processes (soil respiration, ecosystem respiration and photosynthesis) of the community and how they were affected by soil temperature, soil water content, vegetation and age of L. arenarius dunes. We found that both soil respiration and root stocks have increased substantially from 1987, when an earlier study was conducted. The same pattern was found when different aged L. arenarius dunes were studied. L. arenarius had a stronger effect on the soil respiration fluxes than its surface cover might indicate, through its much higher photosynthesis rates than H. peploides. The study furthermore illustrated how water stress may temporally limit belowground processes in this coastal community. INTRODUCTION The study was conducted within the sparse Leymus arenarius and Honckenya peploides plant community that dominates the eastern part of Surtsey, where windblown tephra and sand have mostly levelled the underlying lava. This area is at present found at an earlier successional stage (sere) than areas which have received additional nutrient inputs from a dense seagull colony on the SW part of the island (Magnússon et al. 2014) or from organic matter washed upon the shore by the surf and possibly from a seal colony on the low ness (Figure 1). The eastern part was, however, the first area of the main (higher) island to be colonised by vascular plants, as L. arenarius and H. peploides, as well as Mertensia maritima, seedlings were found there in 1968 (Fridriksson et al. 1972). From this earliest colonisation, only H. peploides persisted, ast the two other species did not successfully colonise until 1973 (Fridriksson 1978). The plant species in this community are all commonly found in costal habitats in Iceland (Magnússon et al. 2014) and most form smaller aboveground dense cushions (e.g. H. peploides) or dunes (L. arenarius) (Figure 2). The average plant surface cover in this area was 8.5% in 1987 (Fridriksson 1992) and the average surface cover of areas outside the seagull colony was found to be more or less unchanged in 2012, or only 7.1% (Magnússon et al. 2014), i.e. the aboveground vegetation succession has more or less halted after the initial colonisation phase.

Ecosystem CO2 flux rates in relation to vegetation type and age of Leymus arenarius dunes on Surtsey