Short-Term Changes in Extractable Inorganic Nutrients during Storage of Tropical Rain Forest Soils

The nutrient status of tropical forests is commonly assessed by measuring inorganic nutrients extracted from soil, yet samples from remote research sites may be stored for prolonged periods of time before analysis. We assessed the influence of soil storage conditions on extractable nutrients in three lowland tropical forests soils from the Republic of Panama. The soils spanned a strong rainfall gradient and contained contrasting chemical and physical properties. Storage treatments were: (i) room temperature (22 degrees C in the dark), (ii) refrigerated (4 degrees C in the dark), (iii) air dried (10 d at 22 degrees C and 55% humidity), and (iv) frozen (-35 degrees C). Ammonium and NO(3) were extremely unstable and concentrations changed considerably within hours of sampling. Phosphate extracted by anion-exchange membranes also changed rapidly following sampling, although cations (Ca, K, and Mg) extracted in Mehlich-3 solution were less influenced by storage. Soil pH declined slowly in A samples during field-moist storage (4 and 22 degrees C). Freezing and air drying generally caused significant changes in extractable nutrients, although the effects varied among soils and nutrients. We therefore conclude that inorganic nutrients should be extracted from tropical forest soils within 24 h of sampling, and preferably on the day of sampling for N fractions, to ensure that values represent field conditions. Where this is not possible, rapid air drying or storage of field-moist samples may be acceptable for some measurements (e.g., PO(4), cations, pH), but are unlikely to provide realistic measurements of inorganic N.