Soil development and nitrogen turnover, in montane rainforest soils on Hawaii

Soil nutrients and nitrogen turnover were measured in two soil chronosequences on the Kilauea and Mauna Loa volcanoes, Hawai'i. Organic carbon and nitrogen accumulated rapidly in sites derived from volcanic ash and more slowly in sites derived from pahoehoe; a minimum of 22 kg N* ha-l yr-I were added to tephra sites early in soil development. Substantial nitrogen fixation by an unknown organism or organisms is probably responsible for most of the nitrogen accumulated on ash. Nitrogen turnover increased with soil age in both absolute (mg N/M2) and relative (g N/g of soil N) terms in both chronosequences. In comparison with other localities, net nitrogen mineralization potentials were extremely low in all of the sites except the two oldest ash sites. Nitrogen availability is probably not related to most of the forms of 'ohi'a dieback occurring in Hawai'i, but low nitrogen availability is observed in sites susceptible to the dryland form of dieback which occurs early in primary succession. SOIL CHRONOSEQUENCE STUDIES HAVE BEEN AN IMPORTANT MEANS FOR UNDERSTANDING THE REGULATION OF NUTRIENT accumulation and availability in natural terrestrial ecosystems. Using this approach, Dickson and Crocker (1953), Crocker and Major (1955), Olson (1958), and Van Cleve et al. (1971) demonstrated increases in soil nitrogen in chronosequences developing after mud flows, glacial recession, sand dune stabilization, and flood plain deposition respectively. Robertson and Vitousek (1981) showed that the availability of soil nitrogen (measured by net release during soil incubations) also increased with soil age on sand dunes. T. W. Walker and coworkers (summarized in Walker and Syers 1976), demonstrated progressive decreases in soil phosphorous pool sizes and availability in four separate chronosequences, with the declines reaching a "terminal steady state" of very low phosphorus availability and reduced plant productivity in some sites. Walker further suggested that symbiotic nitrogen fixers grow well on the high-phosphorus, low-nitrogen substrates early in soil development, but that the combination of increased nitrogen and decreased phosphorus later in soil development largely excludes symbiotic nitrogen fixers (Stevens and Walker 1970, Walker and Syers 1976). Low nitrogen availability should thus be more likely to limit the growth of plants unable to fix nitrogen in relatively young soils and phosphorus should be more limiting in relatively old soils. Montaine forests on the island of Hawai'i offer an excellent opportunity to examine the relationships among soil development, nutrient accumulation, and primary succession (Sherman and Ikawa 1968). Frequent eruptions have occurred from both the kilauea and Mauna Loa volcanoes, and the deposits from these eruptions can be dated by historical records since 1790 or by 14C dating of plant remains (Lockwood and Lipman 1980). Moreover, the importance of soil-forming factors other than time (climate, relief, organisms, and parent materialJenny 1941, 1980) can be minimized through the appropriate location of sample sites. The islands are in the trade winds, and climate is highly predictable from elevation and slope exposure. Both Kilauea and Mauna Loa are active shield volcanoes, so relief is gentle and relatively constant. The isolation of Hawai'i has led to a relatively impoverished flora and fauna (Mueller-Dombois 198 la). There are several woody plants in the area, but only one dominant tree species ('ohi'a Metrosideros collina ssp. polymorpha) usually forms the upper canopy. It is present at all ages and on all variations of substrate in the study area. Finally, three "parent materials" with different textures but similar chemistries are produced by these volcanoes: volcanic ash (tephra), pahoehoe (smooth ropy lava flows), and 'a'a (clinker-type lava flows). Primary plant succession in 'ahi'a forests has been described by Atkinson (1970), Eggler (1971), Smathers and Mueller-Dombois (1974), and Mueller-Dombois I Present address: Department of Biological Sciences, Stanford University, Stanford, California 94305 U.S.A. 268 BIOTROPICA 15(4): 268-274 1983 This content downloaded from 157.55.39.35 on Fri, 02 Sep 2016 05:11:21 UTC All use subject to http://about.jstor.org/terms (198 1b). There are three unusual features to the primary succession on Hawai'i. First, the sites are low in dominant tree species diversity. Second, no nitrogen fixers symbiotic with vascular plants are present in the early stages of soil development in the rain forest zone. Native nitrogen fixers (especially Acacia koa) are present at both higher and lower elevations but not within the area studied. Third, primary succession in the montane rain forest appears to proceed through a phase of biomass accumulation to a rather rapid dieback of the dominant 'ahi'a trees, followed (most often) by 'ahi'a regeneration (Mueller-Dombois et al. 1980). Several forms of dieback are recognized, including wetland, bog formation, 'ahi'a displacement, gap formation, and dryland dieback (Mueller-Dombois 1981b). The mechanisms responsible for these diebacks are not fully known, but they appear to occur naturally in primary succession rather than as an effect of an introduced pathogen (Mueller-Dombois 1981 b). This study was designed to examine the accumulation, availability, and turnover of soil nitrogen in soil chronosequences developed on tephra and pahoehoe. We tested the generalization that nitrogen availability and nitrogen turnover increase from very low levels early in primary succession and evaluated the possibility that nitrogen availability could affect the susceptibility of the 'ohi'a forest to dieback.