Analysis of potentially mobile phosphorus in arable soils using solid state nuclear magnetic resonance.
In many intensive agroecosystems continued inputs of phosphorus (P) over many years can significantly increase soil P concentrations and the risk of P loss to surface waters. For this study we used solid-state 31P nuclear magnetic resonance (NMR) spectroscopy, high-power decoupling with magic angle spinning (HPDec-MAS) NMR, and cross polarization with magic angle spinning (CP-MAS) NMR to determine the chemical nature of potentially mobile P associated with aluminum (Al) and calcium (Ca) in selected arable soils. Three soils with a range of bicarbonate-extractable Olsen P concentrations (40-102 mg P kg(-1)) were obtained from a long-term field experiment on continuous root crops at Rothamsted, UK, established in 1843 (sampled 1958). This soil has a threshold or change point at 59 mg Olsen P kg(-1), above which potentially mobile P (as determined by extraction with water or 0.01 M CaCl2) increases much more per unit increase in Olsen P than below this point. Results showed that CaCl2 and water preferentially extracted Al-P and Ca-P forms, respectively, from the soils. Comparison among the different soils also indicated that potentially mobile P above the threshold was largely present as a combination of soluble and loosely adsorbed (protonated-cross polarized) P forms largely associated with Ca, such as monetite (CaHPO4) and dicalcium phosphate dihydrate (CaHPO4-2H2O), and some Al-associated P as wavellite. The findings of this study demonstrate that solid-state NMR has the potential to provide accurate information on the chemical nature of soil P species and their potential mobility.
McDowell
R W
RW
USDA-ARS Pasture Systems and Watershed Management Research Laboratory, University Park, PA 16802-3702, USA. richard.mcdowell@agresearch.co.nz
Condron
L M
LM
Mahieu
N
N
Brookes
P C
PC
Poulton
P R
PR
Sharpley
A N
AN
D000383:Agriculture; D017753:Ecosystem; D004785:Environmental Pollutants; D008279:Magnetic Resonance Imaging / Q000379:methods*; D010758:Phosphorus / Q000032:analysis / Q000737:chemistry*; D012987:Soil; D012989:Soil Pollutants; D012995:Solubility; D014873:Water Pollutants
D004785:Environmental Pollutants; D012987:Soil; D012989:Soil Pollutants; D014873:Water Pollutants; D010758:Phosphorus