Monday, November 13, 2006 - 2:45 PM

Transformations of Soil Organic Phosphorus During Ecosystem Development.

Benjamin Turner1, Leo Condron2, Sarah Richardson3, Duane Peltzer3, and Victoria Allison3. (1) Smithsonian Tropical Research Inst., Unit 0948, APO, AA 34002-0948, (2) Agriculture and Life Sciences, Lincoln Univ, PO Box 84, Canterbury, 8150, New Zealand, (3) Landcare Research, PO Box 69, Lincoln, Canterbury, 8152, New Zealand

Changes in soil phosphorus during pedogenesis influence ecosystem development and lead to a decline in forest productivity in undisturbed landscapes. Much of the soil phosphorus in the late stages of ecosystem development occurs in organic forms, although this is rarely assigned ecological significance. However, soil organic phosphorus occurs in a variety of chemical forms that differ markedly in their bioavailability, so changes in its composition during pedogenesis may offer a partial explanation for corresponding shifts in tree community composition. We investigated this by assessing the chemical nature of soil organic phosphorus along the 120,000 year Franz Josef glacial chronosequence, New Zealand. Soil organic phosphorus was extracted in an alkaline EDTA solution and analyzed by solution phoshorus-31 nuclear magnetic resonance spectroscopy. Major changes were detected in the concentrations of all forms of organic phosphorus during pedogenesis, with a rapid accumulation of inositol phosphates, DNA, phospholipids, and phosphonates in the first 500 years, followed by a gradual decline to low values in older soils characterized by intense phosphorus limitation. However, there were also clear changes in the contribution of the various compounds to the total organic phosphorus. The inositol phosphates, conventionally considered recalcitrant in soil, declined markedly in the late stages of the sequence, indicating their bioavailability under phosphorus limited conditions. In contrast, the proportion of DNA in the soil organic phosphorus increased continually throughout the sequence, suggesting its incorporation into recalcitrant humic structures. The changes in soil organic phosphorus composition coincided with marked changes in the forest community, with a decine in species diversity and a shift from evergreen angiosperms to conifers (Podocarpaceae). In summary, these results challenge the conventional understanding of the dynamics of soil organic phosphorus, and indicate a link between its chemical nature and the species composition of forests.