Phosphorus and its reactions in terrestrial soils and lake sediments

The P status in some soils and sediments and the fate of P in various sorption and desorption tests was studied. The aim was to elucidate some factors possibly controlling the availability of soil P and the eutrophication risk owing to eroded soil material being carried into surface waters. Also the factors affecting the significance of bottom deposits in the P budget of lakes were discussed.The occurrence and reactions of various inorganic P forms were investigated by a modified CHANG and JAcKSON fractionation procedure which proved a suitable method also for sediment studies for limnological and paleolimnological purposes. In the sorption and desorption experiments the activities of the fractions varied. The extent of retention and the distribution of sorbed P in various fractions was closely related to amorphous oxides of Al and Fe in soils and sediments. In nature, the secondary phosphates seem to participate more actively in desorption processes than the acid soluble P, supposed to represent apatite-P. However, the ratio of secondary phosphates to their corresponding sorption components appeared primarily to control the extent of desorption. Further, the net release of P under desorptive conditions was dependent on the effectiveness of the resorption reactions. In addition to soil and sediment properties the environmental factors also affect the sorption and desorption of P. The immediate effect of temperature on sorption by different soils varied, but seemed to be generally quite unessential. A high content of active organic matter or oxygen depleted conditions may lower the sorption or even cause some desorption, while high concentrations of inorganic salts may enhance retention. Thus, the eutrophication risk owing to the eroded soil material is dependent also on conditions in the recipient ‘water.