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

Abstract

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°C in the dark), (ii) refrigerated (4°C in the dark), (iii) air dried (10 d at 22°C and 55% humidity), and (iv) frozen (−35°C). Ammonium and NO3 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 all samples during field‐moist storage (4 and 22°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., PO4, cations, pH), but are unlikely to provide realistic measurements of inorganic N.

Stability of hydrolytic enzyme activity and microbial phosphorus during storage of tropical rain forest soils

“…The stability of hydrolytic enzyme activities and microbial phosphorus is in marked contrast to the rapid changes in extractable inorganic nutrients in these soils (Turner and Romero, 2009). In particular, both inorganic nitrogen (ammonium and nitrate) and phosphate concentrations changed markedly within hours of sampling when stored at either 4 C or 22 C. This means that nutrients must be extracted from soil within 24 h of sampling to ensure that values are representative of those in the field.…”

“…In particular, both inorganic nitrogen (ammonium and nitrate) and phosphate concentrations changed markedly within hours of sampling when stored at either 4 C or 22 C. This means that nutrients must be extracted from soil within 24 h of sampling to ensure that values are representative of those in the field. If it can be demonstrated that enzyme activities reflect nutrient demand at a site, such assays will provide a more robust measure of nutrient status than timesensitive extractions of inorganic nutrients (Turner and Romero, 2009). …”

“…Soil properties are shown in Table 1. Information on short-term changes in inorganic nutrients in the soils was reported previously and indicated that both inorganic nitrogen and phosphorus concentrations changed rapidly (within hours) following sampling for both refrigerated and room temperature storage (Turner and Romero, 2009). …”

“…Soil sampling and storage treatments were described in detail previously (Turner and Romero, 2009). Briefly, the Albrook soil was sampled on 5 June 2006, while the Pipeline Road and Fort Sherman soils were sampled on 22 August 2007.…”

“…Air-dried samples were analyzed for pH and the concentrations of clay and total C, N, and P (methods described in Turner and Romero, 2009). Soil properties are shown in Table 1.…”