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Millennial‐Scale Phosphorus Transformations during Diagenesis in a Subtropical Peatland

The record of ecosystem history preserved in wetland soils is often used to set ecosystem restoration targets but is complicated by diagenetic changes. We examined changes in the forms of soil P in a 4000-yr-old subtropical peat deposit in Florida using sequential fractionation and solution ³¹P nuclear magnetic resonance spectroscopy. By combining information on changes in soil P composition with ¹⁴C dates, our aim was to estimate the long-term rate of P storage in a subtropical peatland. Diagenesis caused a decline in labile compounds and an accumulation of recalcitrant compounds. For example, stable P forms determined by sequential fractionation increased from 23% of total P in surface peat to 48 to 73% of total P at a depth of 1.5 m. In contrast, fulvic-acid bound P declined with increasing depth at a rate of 37 to 65 μg P kg⁻¹ yr⁻¹. Solution ³¹P nuclear magnetic resonance (NMR) spectroscopy showed that the majority of the extractable organic P occurred as phosphodiesters, which declined markedly with depth, indicating recycling of labile organic P in surface sediments. Carbon-14 dating at two soil coring stations indicated a long-term P accretion rate of 1.71 mg P m⁻² yr⁻¹ at both stations, a rate approximately 20-fold lower than previous estimates of long-term P accretion in this wetland. Recently deposited material undergoes rapid biogeochemical transformations that complicate associations between soil properties and extrinsic factors such as nutrient loading. We conclude that studies of P accretion based only on approximately <200-yr-old peat will tend to overestimate long-term sequestration rate of the ecosystem.