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

Abstract

Summary-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 NH: -N, NOT-N production, and CO2 evolution. A sandy-clay-loam (Pistoia) and a sandy (Romola) soil were moistened and either amended with glucose (200 pmol glucose-c g-I soil), glucose + (NH&SO., [50 ymol N g -' soil as (NH&SO41 or left unamended. In the two unamended soils, NH:-N concentration was increased by the highest MSX level (1 pmol g-I soil), while the lowest inhibitor concentration (0.5 pmol g-' soil) had less influence. NH:-N concentrations were higher with I pmol MSX than without the inhibitor in the glucose-only-treated Pistoia soil in the (r12 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 NH:-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, Nor-N was used as an alternative N source in the glucose-amended Romola soil. The inhibitor had no significant effect on NH:-N concentration when both soils were amended with glucose + (NH&SO4 probably because, in the presence of high NH:-N concentrations, NH, assimilation occurred more through glutamate dehydrogenase than through glutamine synthetase-glutamate synthase enzymes. NOT-N concentrations were decreased by MSX in the glucose-amended Romola but not in the glucose-amended Pistoia soil.