Phosphorus speciation along a soil to kettle hole transect: Sequential P fractionation, P XANES, and 31P NMR spectroscopy

Phosphorus (P) is an essential element for all living organisms but can also be an important factor in the eutrophication of aquatic/marine ecosystems. Kettle holes are often situated in regions under intense agricultural land use where there is a high risk of nutrient transfer to larger waterbodies. The chemical speciation of soil P influences its rate of transfer from arable fields to aquatic environments. However, there is a paucity of information on the P speciation in kettle holes and their effect on the P cycle. Thus, we investigated the P composition of arable soils and kettle hole sediments in surface and subsurface layers along a transect of an agricultural field. Multiple P analyses were carried out including modified sequential Hedley P fractionation, P K-edge X-ray absorption near edge structure (XANES) spectroscopy, and 31P nuclear magnetic resonance (NMR) spectroscopy. The Pt concentrations ranged from 680 to 1123 mg kg−1 in the soils and 797 to 2591 mg kg−1 in the sediments. A predominance of the residual-P fraction, characterized as not extractable P, occluded P or stable forms of organic P (Po) was determined by sequential fractionation, ranging from 37 to 67 % followed by 3 to 38 % H2SO4-Pi and 3 to 16 % NaOH-Po of total P (Pt) in the soils and sediments. Analyses with P K–edge XANES spectroscopy assigned 61 to 100 % Fe- and Al-P, 0 to 14 % Ca-P, and 0 to 39 % Po in the arable soils and 46 to 74 % Fe- and Al-P, 14 to 43 % Ca-P, and 0 – 38 % Po in the adjoining kettle hole sediments. Solution 31P NMR spectra on alkaline extracts revealed a broad signal in the phosphomonoester region, which was the most abundant form of organic P across all samples. Besides, the proportion of ‘complex’ phosphomonesters (broad signal) to that of total extractable P was about 2-fold greater in the kettle hole sediments than in soils. Complex forms of organic P are known to be strongly associated with organic matter (OM) and appear to accumulate on a decadel timeframe. Therefore, it is desirable that kettle hole sediments are conserving P (and carbon), which will help reduce the transfer of P from agricultural fields to other, more vulnerable aquatic/marine ecosystems.