Natural and anthropogenic consequences of tropical forest soils in Northern Peru using environmental radionuclides as radiotracers

Abstract

Environmental radionuclides ( 40 K, 137 Cs, 210 Pb, 226 Ra) were determined in soils under semi-arid stands, a transition area from thorn to deciduous forest in El Angolo, and heath stands in the Alto Mayo in Northern Peru in order to elucidate their activity level in soil, and to trace underground environment in remote areas of little anthropogenic intervention. Anthropogenic 137 Cs was only found in the uppermost portion of the soils in small amount, whereas the 210 Pb activity was exceptionally high at the soil surfaces in a tropical stand of Alto Mayo. The results suggest a fallout nuclide, 210 Pb, having been transported from local source(s) not to be specified yet.

Depth profiles of potassium and its isotope ratio (40K/K) in several forest soils

“…3,4 However, in our previous study depth profiles of 40 K in some soils were not regarded as homogeneous distribution. 5,6 We also found that radio-potassium was present only in the uppermost portion of a tropical forest soil in Alto Mayo in northern Peru where the bed rock is acidic sand stone with little potassium content. 6 It was not known how trees could survive in severe environment with poor nutrient.…”

“…5,6 We also found that radio-potassium was present only in the uppermost portion of a tropical forest soil in Alto Mayo in northern Peru where the bed rock is acidic sand stone with little potassium content. 6 It was not known how trees could survive in severe environment with poor nutrient. Those findings may partly be ascribed to biological activities, such as root uptake and recycling of nutrients, and bioturbation by moving micro (and macro) organisms in soil as well as labile properties of potassium, especially in surface soils.…”

“…Three points (P1-0, P1-1 and P1-2) around a big ceiba tree were selected to compare soil properties in the same geological background. 6 In the observation site in Sapporo soil sampling was carried out in different seasons to investigate seasonal variation of the soil properties including environmental radionuclides ( 226 Ra, 137 Cs and 210 Pb) other than 40 K. Radon ( 222 Rn) and CO 2 (and its carbon isotopes,  13 C and  14 C) have also been monitored in this site since 2002. 5,10,11,17 It is interested to note in Table 1 that the amount of soil organic matter varied in winter-high and summer-low under a deciduous stand in Sapporo.…”

“…6 However, it should be noted that absolute values of the potassium content (and also 40 K activity concentration) were different at individual points of similar geological background. This is probably due to biological activities like root uptake and local population of micro (and macro) organisms.…”

“…5,6,14 The soils were dried at 110 ºC and passed through a 2 mm sieve. A set of data on several soil properties (pH, humidity and loss on ignition) was obtained using standard methods.…”