CHANGES IN THE CHEMISTRY OF SOIL SOLUTION AND ACETIC-ACID EXTRACTABLE-P FOLLOWING DIFFERENT TYPES OF FREEZE-THAW EPISODES

Freezing significantly influences the amount and chemical form of extr actable soil phosphorus and, therefore, has important consequences reg arding plant nutrient availability. Much of the current evidence relie s on field-based observations and tends to be rather contradictory. Th e present work reports the specific effects various freezing episodes have on phosphorus availability. Experimental treatments included the freezing rate and the time soil remained frozen together with the numb er of freeze/thaw cycles. These laboratory incubations used two soils, an iron humus podzol and a peaty podzol, sub-samples of which were eq uilibrated at three different soil-moisture contents. Concentrations o f phosphorus in soil solutions and acetic acid extracts (Ac-P) were me asured. Freezing had the greatest effect on the organic (Op) horizon o f the peaty podzol, where the amounts of total dissolved phosphorus (T DP) and Ac-P increased from 0.58 to 3.21 and from 11.8 to 19.3 mg P kg -1, respectively. TDP in soil solution was subdivided into three broad categories, dissolved (molybdate) reactive (MRP), organic (DOP) and c ondensed (DCP). The rather variable effect that freeze/thaw episodes h ad on these fractions highlighted the problems associated with attribu ting the 'freezing effect' to any single process.