Soil Development and Nutrient Availability Along a 2 Million-Year Coastal Dune Chronosequence Under Species-Rich Mediterranean Shrubland in Southwestern Australia

Soil chronosequences provide valuable model systems to investigate pedogenesis and associated effects of nutrient availability on biological communities. However, long-term chronosequences occurring under seasonally dry climates remain scarce. We assessed soil development and nutrient dynamics along the Jurien Bay chronosequence, a 2 million-year sequence of coastal dunes in southwestern Australia. The chronosequence is significant because it occurs in a Mediterranean climate and supports hyperdiverse shrublands within a global biodiversity hotspot. Young soils formed during the Holocene (< 6,500 years old) are strongly alkaline and contain abundant carbonate, which is leached from the profile within a few thousand years. Middle Pleistocene soils (ca 120,000-500,000 years old) are yellow decalcified sands with residual iron oxide coatings on quartz grains over a petrocalcic horizon that occurs at increasing depth as soils age. Early Pleistocene soils (> 2,000,000 years old) are completely leached of iron oxides and consist of bleached quartz sand several meters deep. Changes in soil organic matter and nutrient status along the Jurien Bay chronosequence are consistent with patterns observed along other long-term chronosequences and correspond closely to expectations of the Walker and Syers (1976) model of biogeochemical change during pedogenesis. Organic carbon and nitrogen (N) accumulate rapidly to maximum amounts in intermediate-aged Holocene dunes and then decline as soils age. In contrast, total phosphorus (P) declines continuously along the chronosequence to extremely low levels after 2 million years of pedogenesis, eventually representing some of the lowest P soils globally. Ratios of soil organic carbon to P and N to P increase continuously along the chronosequence, consistent with a shift from N limitation on young soils to extreme P limitation on old soils. Phosphorus fractionation by sequential extraction reveals a rapid decline in primary and non-occluded phosphate and an increase in organic and occluded P as soils age. Concentrations of extractable (that is, readily bioavailable) N and P, as well as exchangeable cations, are greatest in Holocene dunes and decline to low levels in Pleistocene dunes. Extractable micronutrient concentrations were generally very low and varied little across the chronosequence. We conclude that the Jurien Bay chronosequence is an important example of changing patterns of nutrient limitation linked to long-term soil and ecosystem development under a Mediterranean climate.