Soil Surface Sampling Approaches for Reliable Radiogenic Isotope Tracer Investigations

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The cosmogenic nuclide beryllium-7 (7Be) is successfully used as a suitable tracer isotope to detect fine-scaled surface changes caused by single precipitation events. Short radioactive half-life (53.3 days), high sorption to soil particles, and immobility at given pH values result in shallow 7Be depth distributions in surface soils. Precise sampling techniques, which are constrained by soil conditions, are needed to fulfill the demand of a reliable fine soil increment collection. Five commonly used sampling approaches for 7Be tracer application were tested on three soil substrates under moist and dry surface conditions: (i) manual approach, (ii) core tube, (iii) wood router, (iv) metal frame, and (v) scraper-plate. The accuracy of the intended 2-mm sampling depth was controlled by a terrestrial laser scanner (internal accuracy ± 0.2 mm). Moreover, sampling time and practicability in the field were evaluated. Smooth and moist surface conditions and the absence of any surface crusting facilitate the most precise increment collection of materials with low aggregate stability. Of all tested methods, core tubes and the metal frame technique were the best in terms of accuracy, time efficiency, and practicability. The selection of soil-dependent sampling techniques is essential for a successful application of the 7Be tracer method. An exact fine increment soil collection is impossible, but a range of acceptable deviations can be achieved.

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