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

• 31P nuclear magnetic resonance spectroscopy was used on kettle hole sediments. • More calcium-bound phosphorus was determined in kettle hole sediments than in soils. • Kettle hole sediments contain more complex phosphomonoesters than arable soils. • Kettle hole sediments can lower the risk of leaching phosphorus to aquatic ecosystems. 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 P t 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 (P o) was determined by sequential fractionation, ranging from 37 to 67 % followed by 3 to 38 % H 2 SO 4 -P i and 3 to 16 % NaOH-P o of total P (P t) 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 % P o in the arable soils and 46 to 74 % Fe- and Al-P, 14 to 43 % Ca-P, and 0 – 38 % P o 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. [ABSTRACT FROM AUTHOR]