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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Procedure for Determining the Biodegradation of Radiolabeled Substrates in a Calcareous Soil

School of Agricultural and Forest Sci., Univ. Wales-Bangor, Gwynedd, LL57 2UW, UK

Corresponding author (Lena.Strom{at}planteco.lu.se)

Calcareous soils are frequently typified by a low availability of plant nutrients due to poor solubility of these elements at high pH. Calcicole plants have recently been shown to release organic acids in response to the nutrient deficient conditions prevailing in these soils. It has been speculated, however, that the efficiency of this nutrient mobilization mechanism may be significantly reduced by microbial degradation of the organic acids. In conventional methods, root exudate degradation is typically determined by the addition of ¹⁴C-radiolabeled substrates to soil and subsequent tracking of their fate with time by trapping evolved ¹⁴CO₂ in a strong alkali trap. However, in calcareous soils, ¹⁴CO₂ and H₂¹⁴CO₃ produced by microbial decomposition may become trapped as Ca(H¹⁴CO₃)₂. The aim of this study was to develop and validate an experimental procedure for the accurate quantification of ¹⁴C-labeled substrate degradation rates in calcareous soils. Conventional methods for determining ¹⁴C-labeled substrate decomposition rates in calcareous soils are inaccurate due to incomplete recovery of ¹⁴CO₂. Up to 49% of the ¹⁴CO₂ produced during microbial degradation of ¹⁴C-labeled organic acids (malate, oxalate, citrate) was trapped as carbonate in this calcareous soil (pH 7.58). For an acid soil (pH 4.32) no detectable amount of ¹⁴CO₂ was trapped. We describe a simple, accurate, and reliable method, which includes a postincubation HCl addition, for the accurate determination of ¹⁴CO₂-evolution and substrate degradation in calcareous soils.