Changes in soil phosphorus fractions and ecosystem dynamics across a long chronosequence in Hawaii

Ecology, July, 1995 by Timothy E. Crews, Kanehiro Kitayama, James H. Fownes, Ralph H. Riley, Darrell A. Herbert, Dieter Mueller-Dombois, Peter M. Vitousek

INTRODUCTION

Walker and Syers (1976) put forth a model of soil phosphorus transformations during soil development that provides a useful starting point for investigating P dynamics and C-N-P interactions at different stages in soil development [ILLUSTRATION FOR FIGURE 1 OMITTED]. The model suggests that all soil P is in the primary mineral form (mainly as calcium apatite minerals) at the beginning of soil development, which also coincides with the onset of primary succession. With time, the primary mineral-P slowly dissolves and is either taken up by organisms, thus entering the organic-phosphorus ([P.sub.o]) pool, or is sorbed onto secondary mineral surfaces (Tiessen and Stewart 1985). This sorbed inorganic P (termed "non-occluded P" by Walker and Syers) remains labile for a period of time and can be desorbed in response to a diffusion gradient around a plant root or microbe (Mattingly 1975). Sorbed P that is not taken up by organisms eventually can become surrounded or occluded by Fe or Al hydrous oxides, or allophane (an amorphous alumino-silicate) in the case of certain young volcanic ash soils, rendering the P largely unavailable to the biota (Uehara and Gillman 1981, Nanzyo et al. 1993). The opportunities for occlusion of inorganic P increase greatly later in soil development as secondary silicate minerals dissolve, giving way to Fe and Al oxides that have a strong affinity for P (Fox et al. 1991). Phosphorus taken up by organisms ([P.sub.o]) may be cycled back to the inorganic soil pool, or may become incorporated into chemically recalcitrant decomposition products (also [P.sub.o]) that remove P from biotic cycling for long periods.

The Walker and Syers model of P transformations through time has provided a powerful theoretical context for evaluating the dominant soil processes controlling P cycling during particular stages of pedogenesis (Singleton and Lavkulich 1987, Lajtha and Schlesinger 1988, Birkeland et al. 1989, Walker 1989, Chapin et al. 1994). The model has also contributed to ecosystem theory regarding C, N, P, and S interactions during ecosystem development (McGill and Cole 1981, Tate and Salcedo 1988). However, testing the model itself, and its ecosystem-level implications, has not been straightforward due to the difficulty of finding sites that form an unambiguous chronosequence that includes very distinct stages of pedogenesis (Yaalon 1975).

The Hawaiian islands provide an exceptional chronosequence for evaluating soil P transformations with time, and relationships between soil P and contemporary ecosystem processes. In this study we addressed the following questions: Do changes in soil P fractions as well as total N and C across a broad chronosequence agree with predictions made by the Walker and Syers model? Is phosphorus availability to the biota at its lowest early in soil development, when the majority of soil P is held in apatite minerals, and late in soil development, when most phosphorus is irreversibly bound to sesquioxides (sensu Walker and Syers 1976)? Does biologically available N roughly track available P dynamics as would be expected if biologically available P regulates N accumulation and transformation rates (Cole and Heil 1981, Vitousek and Howarth 1991)? Do decomposition rates respond to variation in site fertility across the chronosequence, resulting in feedbacks that could intensify or relax nutrient limitation (Vitousek 1982)? Does soil nitrous oxide flux correspond to N availability at different stages of soil development (Robertson and Tiedje 1984)? Does variation in forest stature and plant species composition from site to site correspond to changes in soil properties associated with pedogenesis (Mueller-Dombois 1986)?

THE CHRONOSEQUENCE

The Hawaiian archipelago is well suited for chronosequence studies because of how it was formed (Sherman and Ikawa 1968). Basaltic magma has erupted from a stationary convective plume or "hotspot" in the mantle for millions of years, creating large volcanic islands. With the northwest movement of the Pacific Plate, volcanoes are carried away from the hotspot and become inactive, while new volcanoes are formed in their place (Moore and Clague 1992). While the ages of the Hawaiian volcanoes vary continuously from presently active (the island of Hawaii), to [greater than]4 x [10.sup.6] yr (the island of Kauai), parent material composition has remained relatively constant reflecting the convective plume of mantle material (Clague and Dalrymple 1987). By matching elevation, precipitation, slope position, and disturbance history it is possible to put together a sequence of sites that varies mainly in soil age.

The chronosequence we used consisted of a total of six sites, all located in montane rainforest on the islands of Hawaii, Molokai, and Kauai ([ILLUSTRATION FOR FIGURE 2 OMITTED], Table 1). The six sites were Thurston (300 yr old), Olaa (2100 yr), Laupahoehoe (20 000 yr), Kohala (150 000 yr), Kolekole (1.4 x [10.sup.6] yr), and Kokee (4.1 x [10.sup.6] yr). It was not always possible to conduct all measurements at each of the six sites, so some components of our study involve four or five of the sites. Below we describe the soil chronosequence using Jenny's (1941) five state factors of soil formation: parent material, time, climate, organisms, and topography.