Short communication

Use of an amino acid mixture to estimate the mineralization capacity of grassland soils

Persistence of Bacillus thuringiensis parasporal protein-crystal in an arable soil (pH 5.2) incubated at −0.01 MPa at 25°C, was monitored by radioisotope labelling (evolution of ¹⁴CO₂ from soil with or without (±) organic supplement, inoculated with ¹⁴C-labelled crystal) and by bioassay (loss of insecticidal activity toward susceptible insect larvae, in soil ± CaCO₃ ± organic supplement). The protein-crystal was decomposed by soil microorganisms. The extent of decomposition was significantly reduced (P < 0.05) by addition of a soluble organic supplement to soil. Insecticidal activity fell rapidly at exponential rates in all soil treatments. The loss rate did not diner significantly between soil and soil + CaCO₃ (t_0.5 = 2.7 days), but was significantly reduced in soil + organic supplement (t_0.5 = 5.2 days), and in soil + CaCO₃ + organic supplement (t_0.5= 5.8 days).