Organic matter and nitrogen accumulation and nitrogen fixation during early ecosystem development in Hawaii

We used a chronosequence comprised of 10 y, 52 y and 142 y old 'a'a lava flows on Mauna Loa, Hawaii, to determine the accumulation of organic matter and nitrogen and rates of nitrogen fixation through time. The mass of organic matter (live and dead biomass and soil organic matter) on the 1984, 1942 and 1852 lava flows was 0.6, 2.2 and 7.6 kg m(-2), respectively, while total N was 4.8, 10.9 and 85.7 g m(-2). We estimated the total rates of nitrogen fixation for the three different aged ecosystems using an acetylene reduction assay calibrated with N-15 incubations. While mean rates of total N fixation remained largely constant across the three sites - between 2.0 and 3.1 kg ha(-1) y(-1) - the most important sources of N fixation changed. On the 10 y flow, the most important fixer was the pioneering cyanolichen, Stereocaulon vulcani. After 52 years of ecosystem development, the most important N fixer was a cyanoalga, while after 142 years, the predominant N fixers were heterotrophic bacteria associated with leaf litter, twigs and detritus. The total amount of N accumulated after 52 years of ecosystem development was equivalent to cumulative inputs through biological N fixation. After 142 years, however, cumulative inputs from N fixation could only account for between 27-59% of the total nitrogen accrued. We used fertilizer additions of all essential nutrients other than N to test whether the availability of lithophilic nutrients regulated rates of N fixation in early ecosystem development. Rates of nitrogen fixation by the lichen, S. vulcani, approximately doubled when fertilized on the 1984 and 1942 flows. Rates of N-fixation by heterotrophic nitrogen fixing bacteria on leaf litter of Metrosideros polymorpha also increased significantly when fertilized with lithophilic nutrients. These findings suggest that weathering rates of lava in part regulate rates of nitrogen fixation in these young ecosystems.