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Using X-ray absorption spectroscopy to study phosphorus in soil development
Phosphorus (P) is an essential plant nutrient that is critical for productive ecosystems. Phosphorus form, or “speciation,” is an important factor influencing nutrient availability, and the amount of bioavailable or plant-accessible P in soils. As soil genesis and development occurs, the availability of plant-accessible P and other nutrients changes: primary mineral P from the parent material weathers, and P may become associated with secondary minerals or organic matter. Phosphorus K-edge X-ray absorption near edge structure (XANES) spectroscopy was used to investigate evolution of P speciation along two soil sequences: The marine terraces of the “Ecological Staircase” in Mendocino County, CA, which form a soil chronosequence, and the climosequence in Kohala, Hawaii. Previous research demonstrated a decrease in bioavailable P across the “Ecological Staircase,” without identifying causes of this P limitation. We used phosphorus XANES spectra to provide insight on the molecular speciation and retention mechanisms of P in soil, and to distinguish between mineral P and adsorbed or organic P, which have differing bioavailability. At the Ecological Staircase, XANES spectra indicated that P speciation in an older terrace, characterized by the presence of a “Pygmy Forest”, was dominated by Fe-Phytate P (an organic form of P) complex associated with oxide minerals in soils. It is unclear if this chemical change in P speciation caused the nutrient limitation that led to the Pygmy Forest; other factors, such as soil pH, and an iron-stained clay hardpan may also limit plant development and nutrient availability to the trees in the Pygmy Forest. To further understand P speciation and soil development, a P K-edge XANES study of 46 sites along the well-characterized Kohala climosequence indicated, as previous studies have shown, that the arid sites were rich in primary mineral P and the semi-arid and wet sites were dominated by P associated with Fe oxides, such as hematite. No sudden changes in the phosphorus species were observed across the climosequence, but increased moisture appears to induce gradual changes in soil P species. XANES spectroscopy provides insights into changes in P speciation throughout soil development, and changes in P speciation may help to explain the variation of ecological features across a landscape. This work explored the relationship between chemical factors (i.e., P speciation) and other factors (e.g., physical barriers such as a hardpan, or climatic variations) on the ecology of two well-characterized sites.