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

l-Methionine-sulphoximine (MSX) is an inhibitor of glutamine synthetase activity which may be used in short-term soil incubation assays for studying its 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·(kr+kf·exp(−x·(kf+kr)))/(kf+kr) was fitted to recovery values and it showed the rate of MSX loss was a time-dependent equilibrium process. The effect of clay minerals in reducing the concentration of MSX in soil solution through adsorption and binding processes was also assessed in two of the five soils after their amendment with mined clay (either kaolinite or montmorillonite). Analytical results showed that the amount of MSX recovered was related negatively 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 colloid-rich soils could therefore be due to MSX adsorption–desorption by soil colloids which considerably reduce the inhibitor concentration.