The isotopic dilution technique was used to determine gross N mineralisation and immobilisation rates of two soils (Pistoia, sand-clay-loam, and Romola, sandy) in the presence of L-methionine-sulphoximine (MSX), an inhibitor of glutamine synthetase (GS) enzyme. We hypothesized that a complete inhibition of this enzyme would block N immobilisation and thus would allow us to determine gross mineralisation rates from net mineralisation rates. Soils were treated with 15N-labelled ammonium sulphate at 10.1 atom% (10 μg N g-1 soil) plus unlabelled potassium nitrate (10 μg N g-1 soil), with or without MSX (1 μmol g-1 soil) and incubated at 25°C; selected variables were determined at 0, 6, 24, and 48 h of incubation. The fate of MSX was followed by its recovery from the two soils by extraction, derivatization and HPLC analysis. The addition of MSX to both soils increased gross and net mineralisation; the immobilisation data suggested that MSX had, to some extent, inhibited N immobilisation, although its effectiveness appeared to decrease with time in the Pistoia soil. This behaviour was consistent with the greater rate of MSX disappearance in this soil, probably due to adsorption on soil colloids (the Pistoia soil had 19% clay compared to 6% for Romola). The correspondence between the N in the unrecovered MSX and the additional cumulative gross N mineralisation following MSX addition in Romola soil strongly suggested that MSX was also microbially degraded. Inhibition of N immobilisation by MSX was inferred from: (1) The ratio 15N excess in the microbial biomass to the mean NH4+ abundance throughout; (2) the mass balance of the inorganic N pool. The second method of calculation suggested that, with the inhibition of NH4+ immobilisation by MSX, soil microorganisms switched to immobilising NO3/-. The premise for using MSX was that, in its presence, net mineralisation would equal gross mineralisation in the untreated soil. This was not the case because MSX was only partially effective at blocking immobilisation, MSX was itself mineralised releasing N, and the soil microorganisms appeared to switch to NO3-N as an N source.
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