Phosphorus Speciation Evolution Across a Climate Gradient in Semi-Arid Prairie Soils.

Climate factors (precipitation and temperature) affect P speciation and thus bioavailability in soils. A few studies addressed how P speciation changes across climate gradients based on sequential chemical extraction method, but the conclusions are inconsistent. Here, we used the modified Hedley chemical sequential extraction (CSE) and P K-edge X-ray absorption near-edge structure (XANES) spectroscopy to characterize soil P speciation changes in surface soils (0 – 10 cm) across a climate gradient located in Inner Mongolia, China with mean annual temperature (MAT) of 0.78 - 5.08 ^oC and mean annual precipitation (MAP) of 161 - 370 mm. Results show that the total P along the transect ranged from 179 to 436 mg/kg with no correlation with either  MAP or MAT. CSE results show that the percentage of the extracted organic P and Ca-P are 6.6 - 32.4% and 17.2 - 42.3%, respectively. XANES results show that the organic P, Ca-P, Al-P, and Fe-P fractions accounted for 28 - 52%, 16 - 34%, 3 - 44%, and 0 - 25%, respectively. The Ca-P proportions from both methods were comparable, but the organic P proportions from the XANES analysis were much higher than those from the CSE. The difference is caused by that a large proportion of organic P was not extracted and remained in the residuals, which is confirmed by the XANES analysis on the residuals showing that up to 65% organic P in the residuals with the rest being Fe-P. A Pearson correlation analysis between the proportions of the XANES-derived P speciation and the MAT/MAP show that both Al-P and Fe-P fractions were negatively correlated to MAP and MAT while the organic P fraction has a positive correlation with MAP and MAT. Correlations between the Ca-P fraction with MAP or MAT were not observed. The ongoing solution ³¹P NMR spectroscopic analysis will provide further information on the interaction between each P species and the climate factors. This work provides insights into how climate affects soil P cycling.