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

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 14 C‐radiolabeled substrates to soil and subsequent tracking of their fate with time by trapping evolved 14 CO 2 in a strong alkali trap. However, in calcareous soils, 14 CO 2 and H 2 14 CO 3 produced by microbial decomposition may become trapped as Ca(H 14 CO 3 ) 2 The aim of this study was to develop and validate an experimental procedure for the accurate quantification of 14 C‐labeled substrate degradation rates in calcareous soils. Conventional methods for determining 14 C‐labeled substrate decomposition rates in calcareous soils are inaccurate due to incomplete recovery of 14 CO 2 Up to 49% of the 14 CO 2 produced during microbial degradation of 14 C‐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 14 CO 2 was trapped. We describe a simple, accurate, and reliable method, which includes a postincubation HCl addition, for the accurate determination of 14 CO 2 ‐evolution and substrate degradation in calcareous soils.