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Consequences of the physical nature of the parent material for pedogenesis, nutrient availability, and succession in temperate rainforests

Turner, BenjaminL., Wells, Andrew, Andersen, KellyM., Condron, LeoM.
Plant and soil 2018 v.423 no.1-2 pp. 533-548
Entisols, rain forests, trees, sand, phosphorus, nitrogen content, Spodosols, ecosystems, tree ferns, Nothofagus menziesii, age of soil, chronosequences, gravel, mineral soils, decline, soil formation, ectomycorrhizae, rivers, nitrogen, nutrient availability, organic horizons, stand basal area, New Zealand
BACKGROUND AND AIMS: Soil development follows a predictable pattern of nutrient availability over hundreds to thousands of years, which drives changes in the composition and productivity of associated plant communities. However, our understanding of the influence of soil forming factors such as parent material on ecosystem development is constrained by the scarcity of appropriate chronosequences. METHODS: We studied a chronosequence of coastal beach ridges at the mouth of the Arawhata River in Westland, New Zealand. The Arawhata chronosequence is formed of coarse gravel, so comparison with the nearby Haast chronosequence, formed of sand of the same lithological origin, presents a rare opportunity to isolate the influence of the physical nature of the parent material on ecosystem development. RESULTS: Soils at Arawhata are Entisols with a shallow organic horizon over unconsolidated rounded coarse gravel, although old soils demonstrate features of podsolization and are morphologically similar to Spodosols. The fine earth fraction is a small proportion of the total volume, but increased in surface mineral soils from 5% to 20% over approximately 2000 years of pedogenesis. Corresponding soil phosphorus concentrations in the fine-earth fraction declined markedly, from >900 mg P kg⁻¹ in young soils to 135 mg P kg⁻¹ in older soils. However, on a profile basis the fine-earth fraction at Arawhata contained little phosphorus (10 g P m⁻² on a 300-year-old soil) compared to the same-aged soil at Haast (253 g P m⁻²). Total nitrogen concentrations declined in surface mineral soil but not in the organic horizon, so nitrogen to phosphorus ratios increased with pedogenesis only in the organic horizon. Tree basal area at Arawhata was lower than on similar-aged soils at Haast, and the tree community on the youngest undisturbed ridge at Arawhata included Nothofagus menziesii, an ectomycorrhizal species that appears only on old infertile soils at Haast. Similarly, tree ferns were rare on the youngest soil at Arawhata and became more abundant along the chronosequence, yet at Haast they were most abundant on the phosphorus-rich young soils and decreased with soil age. CONCLUSIONS: Collectively, these results suggest that ecosystem development along the Arawhata chronosequence represents ‘backward succession’, because phosphorus stocks in the fine earth increase during the early stages of pedogenesis, even as mass-based concentrations decline. We conclude that the nature of the parent material plays a key role in determining the basal area and composition of tree communities during succession in the region, although further studies are required to isolate the relative importance of chemical and physical characteristics of the substrate.