REGULATION OF UREASE PRODUCTION IN SOIL BY MICROBIAL ASSIMILATION OF NITROGEN

Studies of the effects of different forms of N on urease production in soils amended with organic C showed that although microbial activity, as measured by CO2 production, was stimulated by the addition of NH4+ or NO3- to C-amended soils (200-mu-mol glucose-C g-1 soil), urease production was repressed by these forms of N. The addition of L-methionine sulfoximine, an inhibitor of inorganic N assimilation by microorganisms, relieved the NH4+ and NO3- repression of urease production in C-amended Soil. The addition of sodium chlorate, an inhibitor of NO3- reduction to NH4+ by microorganisms, relieved the NO3- repression of urease production, but did not eliminate the repression associated with NH4+. These observations indicate that microbial production of urease in C-amended soils is not directly repressed by NH4+ or NO3-, but by products formed by microbial assimilation of these forms of N. This conclusion is supported by our finding that the biologically active L-isomers of alanine, arginine, asparagine, aspartate, and glutamine, repressed urease production in C-amended soil, whereas the D-isomers of these amino acids had little or no influence on urease production. This work suggests that urease synthesis by soil microorganisms is controlled by the global N regulon.