Effects of seabird nitrogen input on biomass and carbon accumulation during 50 years of primary succession on a young volcanic island, Surtsey
Primary succession is known to be a slow process, especially in cold climates with a short growing season, and depends usually on seed rain and/or nitrogen availability and water retaining capacity. Consequently Surtsey, the youngest volcanic island in Iceland is expected to be still in an early phase of primary succession after only 50 years of development. However, a seabird colony was established on the island 27 years ago, and that might have given the primary succession process a kick start, by increasing nutrient input rate and altering soil characteristics. We measured total ecosystem stocks of biomass, carbon (C) and nitrogen (N) on permanent survey plots both outside and inside the seabird colony. This enabled us to test the effect of seabird activities on biomass and carbon accumulation and to clarify the role of N, which is generally the first limiting element in northern ecosystems, in the primary succession. After only 27 years since colonization, the seabird influence had shifted the ecosystem from a desert ecosystem (~ 0.01 DW ha-1 outside the colony) to a lush grassland ecosystem (> 0.5 t DW ha-1 inside the colony) in terms of biomass. We argue that the main driver of this drastic change was the N input in the form of bird feces. Firstly, the N accumulation rate outside the seabird colony was very low (1.1 kg ha-1 y -1) and agreed closely with the annual atmospheric N deposition rate in Iceland (1.0 – 2.5 kg ha-1 y-1). Inside the seabird colony, the N accumulation rate had increased to 40 - 65 kg ha-1 y-1. The major part of this increase could be assigned to bird input, since no significant nitrogen fixing activity has been found on Surstey. Secondly, total ecosystem biomass and C stocks showed a strong positive relationship with total ecosystem nitrogen stock (r2 = 0.88 and 0.98 respectively), especially inside the seabird colony. This study demonstrated that the slow process of biomass and carbon accumulation during primary succession in northern ecosystems can be greatly accelerated by external nutrient inputs.