Determination and depletion kinetics of L-methionine-sulphoximine in soil

L-Methionine-sulphoximine (MSX) is an inhibitor of glutamine synthetase act ivity which may be used in short-term soil incubation assays for studying i ts effects on soil N cycling. In order to monitor the fate of MSX in soil a reversed-phase HPLC method equipped with UV-detection has been developed. The fate of the inhibitor was assessed by its recovery after incubation in five contrasting soils. There was a marked decrease of MSX concentration in all cases. The first order kinetic model f(x) = A.(k(r)+k(f).exp(-x.(k(r) + k(r))))/(k(f) + k(r)) was fitted to recovery values and it showed the rat e of MSX loss was a time-dependent equilibrium process. The effect of clay minerals in reducing the concentration of MSX in soil solution through adso rption and binding processes was also assessed in two of the five soils aft er their amendment with mined clay (either kaolinite or montmorillonite). A nalytical results showed that the amount of MSX recovered was related negat ively to soil CEC values (r = -0.904) and total C (r = -0.931) and N (r = - 0.952) content. The weak ability of MSX to block N immobilization in colloi d-rich soils could therefore be due to MSX adsorption-desorption by soil co lloids which considerably reduce the inhibitor concentration. (C) 1999 Else vier Science Ltd. All rights reserved.