Phosphorus solubility and sorption in frozen, air-dried and field-moist soil

The effects of freezing on soil phosphorus (P) chemistry are poorly understood, although freezing is habitual for many soils at middle and high latitudes. We studied the effects of various freezing treatments on the solubility and sorption of P in an incubation experiment on two coarse and two fine-textured cultivated surface soils in Finland. Air-drying was included in the experimental arrangement because freezing and drying have similar features. Compared with field-moist soils stored at +5°C in the dark, freezing had few effects on P extractability by water or on sorption properties of P studied with a Q/I plot technique. Air-drying, by contrast, increased almost systematically the equilibrium concentration of P estimated with Q/I plots, water-soluble organic carbon, and the extractability of P, aluminium, iron and manganese in the soils. The results imply that drying destroys organomineral complexes. The breakdown of these complexes releases P, while simultaneously exposing new surfaces on which P could sorb. Because of the considerable impact of drying on the behaviour of P, air-drying of soil samples should be avoided in studies of the chemistry of P in soil. Freezing seems to be a safe way of storing mineral soil for such studies, but it may significantly alter the P conditions of soils rich in organic matter.