Little is known about the effects of air-drying and freezing on the transformation of phosphorus (P) fractions in soils. It is important that the way in which soils respond to such perturbations is better understood as there are implications for both P availability and loss to surface waters from soils. In this study, the effects of air-drying and freezing were investigated using two soils, one being a forest soil (FS) high in organic matter and the other being a sterile soil (SS) low in organic matter. Soil P was fractionated using a modified Hedley fractionation method to examine the changes of phosphorus fractions induced by air-drying and freezing. Generally, there were no significant differences of total phosphorus among the three treatments (CV% < 10%). Compared with field moist soils, freezing the soil evoked few changes on phosphorus fractions except that the resin-P increased in FS soil. On the contrary, air-drying significantly changed the distribution of phosphors fractions for both soils: increased the labile-P (especially resin-P) and organic-P (NaHCO 3 -Po, NaOH-Po and Con.HCl-Po) at the expense of NaOH-Pi and occlude-P (Dil.HCl-P and Con.HCl-Pi). Resin-P significantly increased by 31% for SS soil and by 121% for FS soil upon air-drying. The effect of air-drying seemed to be more pronounced in the FS soil with high organic matter content. These results indicated that drying seem to drive the P transformation form occlude-P to labile-P and organic-P and accelerated the weathering of stable P pool. This potentially could be significant for soil P supply to plants and P losses from soils to surface waters under changing patterns of rainfall and temperature as predicted by some climate change scenarios.

There are few researches on the characteristics of phosphorus (P) in the newly born wetland soils from Yellow River Delta of China. Meanwhile, whether the wetland soils can protect the aquatic ecosystem from excessive P input or not and the risk of P release from the soils to the water quality have generated considerable interests. Through isothermal batch static equilibrium sorption experiments followed by six subsequent extraction steps, the present study assessed the variations in P sorption and desorption properties of the newly born wetland soils in Yellow River Delta. It is found that the P sorption isotherms were nonlinear and the amount of P sorption reached a steady state as P initial concentration added increased in the studied soils. Though the capacity of P retention differed strongly among the studied zone, the soils can protect the aquatic ecosystem from excessive P input to a certain extent degree. From the desorption experiments, it is found that the newly sorbed P was easily desorbed in higher initial added concentration and the desorption P significantly decreased with successive extraction in all samples. The release of P from soil proposed a significant contribution to the water quality and eutrophication of coastal zone. Correlation and regression analyses showed that the amorphous and free Fe/Al oxides, correlated with clay content in the studied soils, were the crucial chemical factors ascribed to the soil P retention and release capacity.

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