Nutrient availability in forest soils as measured with anion‐exchange membranes

Abstract

Classical chemical extraction methods to assess nutrient availability are often inadequate to detect in situ temporal and spatial soil‐nutrient changes produced by fertilization. The objective of this work was to develop and evaluate an in situ soil test suitable for estimating the soil nutrient status in both climax and fertilized forests. Simultaneous extraction of soil phosphate, nitrate, and sulfate by means of anion‐ex‐change membranes (AEM) placed in situ is described. These anions were analyzed simultaneously with a Dionex chromatograph. Membranes were placed at soil depths of 2.5, 10, 20, and 30 cm, in natural and P‐fertilized forests with minimal disturbance of the soil. Seasonal patterns of ion sorption presumably reflect the effect of a combination of several factors: soil temperature and moisture, microbial and root activities, and ion mobility. Ion sorption by the AEM was strongly affected by soil water regime. All nutrient ions sorbed by the AEM diminished 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 different 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.