Regulation of urease production in soil by microbial assimilation of nitrogen

Summary 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 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.