Changes in inorganic N and CO2 evolution in soil induced by l-methionine-sulphoximine

Short-term (up to 48 h) incubation assays were conducted in the presence of l-methionine-dl-sulphoximine (MSX), an inhibitor of glutamine synthetase, to assess its effect on exchangeable NH4+  N, NO3−N production, and CO2 evolution. A sandy-clay-loam (Pistoia) and a sandy (Romola) soil were moistened and either amended with glucose (200 μmol glucose-C g−1 soil), glucose + (NH4)2SO4 [50 μmol N g−1 soil as (NH4)2SO4] or left unamended. In the two unamended soils, NH4+N concentration was increased by the highest MSX level (1 μmol g−1 soil), while the lowest inhibitor concentration (0.5 μmol g−1 soil) had less influence. NH4+N concentrations were higher with 1 μmol MSX than without the inhibitor in the glucose-only-treated Pistoia soil in the 0–12 h period; thereafter the opposite situation was observed. Probably the CO2 evolution increased as a result of inhibitor mineralization after 12 h. In the glucose-treated Romola soil both MSX concentrations were generally effective in increasing NH4+N concentrations with respect to the same amendment without the inhibitor. These increases were probably due to glutamine synthetase inhibition by MSX and not to the presence of mineralization of the inhibitor because CO2 evolution was only slightly increased at 48 h by MSX. Probably, as a result of this inhibition, NO3−N was used as an alternative N source in the glucose-amended Romola soil. The inhibitor had no significant effect on NH4+N concentration when both soils were amended with glucose + (NH4)2SO4 probably because, in the presence of high NH4+N concentrations, NH3 assimilation occurred more through glutamate dehydrogenase than through glutamine synthetase-glutamate synthase enzymes. NO3−N concentrations were decreased by MSX in the glucose-amended Romola but not in the glucose-amended Pistoia soil.