NUTRIENT AVAILABILITY IN FOREST SOILS AS MEASURED WITH ANION-EXCHANGEMEMBRANES

Classical chemical extraction methods to assess nutrient availability are often inadequate to detect in situ temporal and spatial soil-nutri ent changes produced by fertilization. The objective of this work was to develop and evaluate an in situ soil test suitable for estimating t he soil nutrient status in both climax and fertilized forests. Simulta neous extraction of soil phosphate, nitrate, and sulfate by mean of an ion-exchange membranes (AEM) placed in situ is described. These anions were analyzed simultaneously with a Dionex chromatograph. Membranes w ere placed al soil depths of 2.5, 10, 20, and 30 cm, in natural and P- fertilized forests with minimal disturbance of the soil. Seasonal patt erns of ion sorption presumably reflect the effect of a combination of several factors: soil temperature and moisture, microbial and roof ac tivities, and ion mobility. ion sorption by the AEM was strongly affec ted by soil water regime. All nutrient ions sorbed by the AEM diminish ed in summer. Phosphate concentration in the soil varied significantly with parent material, depth, season, and as expected, P fertilization . Nitrate and sulfate contents at the different soil subhorizons were not affected by P fertilization. The different anion levels found at d ifferent soil subhorizons are a consequence of both the decreasing of the soil organic matter content and hence enzymatic activities and an increase in soil fixation phenomena at depth.