Pedogenesis, nutrient dynamics, and ecosystem development: the legacy of T.W. Walker and J.K. Syers

Abstract

Almost four decades ago, T.W. Walker and J.K. Syers published a paper in the journal Geoderma that transformed our understanding of nutrient availability and limitation in terrestrial environments. The paper, entitled The fate of phosphorus during pedogenesis ,d istilled data from four New Zealand soil chronosequences to show that soil nutrients followed predictable but fundamentally different patterns during long-term ecosystem development. Specifically, Walker and Syers observed that nitrogen is absent from most parent materials and enters ecosystems through biological nitrogen fixation. As a result,nitrogenconcentrations are low in young soils but increase rapidly during the early stages of ecosystem development. In contrast, they argued that phosphorus is derived almost exclusively from the parent material, so phosphorus concentrations are greatest in young soils but decline continuously during pedogenesis as phosphorus is lost in runoff at a greater rate than it is replenished by bedrock weathering. Importantly, Walker and Syers also demonstrated that the decline in total phosphorus occurs in parallel with chemical transformations of the phosphorus remaining in the soil. These changes include a decline in primary mineral phosphate (principally apatite) and an accumulation of phosphorus in organic and secondary mineral forms associated with metal oxides. The changes in soil nutrients predicted by the Walker and Syers model have important ecological consequences, because nutrient availability shapes the productivity, composition, and diversity of biological communities (Vitousek 2004). A key prediction of the model, now supported by a number of different lines of evidence, is that the nutrient most limiting to primary production varies during ecosystem development, with nitrogen limitation on young, weakly weathered soils, co-limitation by nitrogen and phos