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

We sequentially extracted soil samples using the following extraction agents: anion exchange resin (0.4 g Biorad 1-X8, 20-50 mesh resin beads, [Cl.sup.-] form in 86 mesh [[approximately equal to]190-[[micro]meter] openings] polyester bags), 0.5 mol/L NaHC[O.sub.3] adjusted to pH 8.5 with 5 mol/L NaOH, 0.1 mol/L NaOH, a second round of 0.1 mol/L NaOH after sample sonication at 175 W for 2 min, 1 mol/L HCl, concentrated HCl (hot) (see Tiessen and Moir 1993), and a final digestion of remaining soil materials in concentrated [H.sub.2]S[O.sub.4] (boiling) with repeated additions of a 30% [H.sub.2][O.sub.2] solution. The Olaa soil fractionations, which were carried out at a later time than for the other soils, involved the same procedure except that we excluded the sonicated NaOH and hot HCl steps. Extractions took 16 h in a home-built, end-over-end soil rotisserie. We collected supernatant by centrifuging samples at 1.7 x [10.sup.4] m/[s.sup.2] (3200 rpm) for 5 min in a clinical centrifuge, followed by filtering samples through a 0.45-[[micro]meter] micropore filter. After filtration, we recovered as much of the soil from the filter as possible by gently scraping with a clean spatula, and then rinsing the soil from the spatula back into the centrifuge tube with the subsequent extract. We determined total P(organic inorganic) in the NaHC[O.sub.3], NaOH, sonicated NaOH, and hot, concentrated HCl extracts by potassium persulfate digestion. Organic phosphorus fractions were estimated by subtracting inorganic P from the total P measured in each of these extracts. We adjusted the pH of final solutions to [approximately equal to]4.5 using p-nitrophenol or a pH meter, and added standard acid molybdate reagents (Olsen and Sommers 1982) to develop color in the samples, and measured P at 712 nm on a spectrophotometer.

The P fractionation scheme was designed to extract the most labile P first, with subsequent extracts removing P that is decreasingly available to the biota. In brief, labile, inorganic P ([P.sub.i]) was extracted by the anion exchange resin and NaHC[O.sub.3]. Labile, organic phosphorus ([P.sub.o]) was extracted by NaHC[O.sub.3] and NaOH. Conventionally, the bicarbonate fraction is considered to be the most labile [P.sub.o]; however this assumption is now being questioned as it has been shown not to extract relatively labile Po associated with cellulose (H. Tiessen, personal communication). Non-occluded [P.sub.i] was extracted from surfaces of Fe and Al minerals with NaOH. Calcium phosphates (presumably apatites) were removed with 1 mol/L HCl. Occluded [P.sub.i] was extracted with NaOH after sonication with hot concentrated HCl, and the final [H.sub.2]S[O.sub.4]/[H.sub.2][O.sub.2] digest. Protected and recalcitrant Po was extracted in the sonicated NaOH and hot concentrated HCl extracts. For thorough reviews of the P pools believed to be extracted with each step, as well as background references for the different fractions, see Stewart and McKercher (1982), Sharpley et al. (1987), Tiessen (1991), and Tiessen and Moir (1993). We realize that the chemical extractions and digests of this and other phosphorus fractionations represent approximations of theoretical pools, but they have proved useful in practice (Tiessen et al. 1984, 1992, Lajtha and Schlesinger 1988, Frossard et al. 1989, Schoenau et al. 1989).